Clamping elements- these are mechanisms directly used to secure workpieces, or intermediate links of more complex clamping systems.
Most simple view universal clamps are those that are activated by keys, handles or handwheels mounted on them.
To prevent the movement of the clamped workpiece and the formation of dents on it from the screw, and also to reduce the bending of the screw when pressing on a surface not perpendicular to its axis, swinging shoes are placed on the ends of the screws (Fig. 68, α).
Combinations of screw devices with levers or wedges are called combination clamps and, a variety of which are screw clamps(Fig. 68, b), The device of the clamps allows you to move or rotate them so that you can more conveniently install the workpiece in the fixture.
In Fig. 69 shows some designs quick release clamps. For small clamping forces, a bayonet device is used (Fig. 69, α), and for significant forces, a plunger device is used (Fig. 69, b). These devices allow the clamping element to be moved a long distance from the workpiece; fastening occurs as a result of turning the rod through a certain angle. An example of a clamp with a folding stop is shown in Fig. 69, v. Having loosened the handle nut 2, remove the stop 3, rotating it around its axis. After this, the clamping rod 1 is retracted to the right at a distance h. In Fig. 69, d shows a diagram of a high-speed device lever type. When turning the handle 4, the pin 5 slides along the bar 6 with an oblique cut, and the pin 2 slides along the workpiece 1, pressing it against the stops located below. Spherical washer 3 serves as a hinge.
The large amount of time and significant forces required to secure the workpieces limit the scope of use of screw clamps and, in most cases, make quick-release clamps preferable. eccentric clamps . In Fig. 70 shows disk (α), cylindrical with L-shaped clamp (b) and conical floating (c) clamps.
Eccentrics are round, involute and spiral (along the Archimedes spiral). IN clamping devices Two types of eccentrics are used: round and curved.
Round eccentrics(Fig. 71) are a disk or roller with the axis of rotation shifted by the eccentricity size e; the self-braking condition is ensured when the ratio D/е≥ 4.
The advantage of round eccentrics is the ease of their manufacture; The main disadvantage is the variability of the lifting angle α and clamping forces Q. Curvilinear eccentrics, the working profile of which is carried out along an involute or an Archimedes spiral, have a constant angle of elevation α, and, therefore, ensure a constant force Q when clamping any point of the profile.
Wedge mechanism used as an intermediate link in complex clamping systems. It is simple to manufacture, easily placed in the device, and allows you to increase and change the direction of the transmitted force. At certain angles, the wedge mechanism has self-braking properties. For a single-bevel wedge (Fig. 72, a) when transmitting forces at a right angle, the following relationship can be accepted (with ϕ1 = ϕ2 = ϕ3 = ϕ where ϕ1…ϕ3 are friction angles):
P = Qtg (α ± 2ϕ),
where P is the axial force; Q - clamping force. Self-braking will take place at α<ϕ1 + ϕ2.
For a two-skew wedge (Fig. 72, b) when transmitting forces at an angle β>90, the relationship between P and Q at a constant friction angle (ϕ1 = ϕ2 = ϕ3 = ϕ) is expressed by the following formula:
P = Qsin(α + 2ϕ)/cos (90° + α - β + 2ϕ).
Lever Clamps used in combination with other elementary clamps, forming more complex clamping systems. Using the lever, you can change the magnitude and direction of the transmitted force, as well as simultaneously and uniformly secure the workpiece in two places. In Fig. Figure 73 shows diagrams of the action of forces in single-arm and double-arm straight and curved clamps. The equilibrium equations for these lever mechanisms are as follows; for a single-arm clamp (Fig. 73, α):
direct double-arm clamp (Fig. 73, b):
curved clamp (for l1 where p is the friction angle; ƒ - friction coefficient. Centering clamping elements are used as installation elements for the external or internal surfaces of rotating bodies: collets, expanding mandrels, clamping bushings with hydroplastic, as well as membrane cartridges. The cone angle of the compression sleeve is made 1° less or greater than the collet cone angle. The collets ensure installation eccentricity (runout) of no more than 0.02...0.05 mm. The base surface of the workpiece should be processed according to the 9th...7th accuracy grade. Expanding mandrels various designs (including designs using hydroplastic) are classified as mounting and clamping devices. Diaphragm cartridges used for precise centering of workpieces along the outer or inner cylindrical surface. The cartridge (Fig. 75) consists of a round membrane 1 screwed to the faceplate of the machine in the form of a plate with symmetrically located protrusions-cams 2, the number of which is selected within the range of 6...12. A pneumatic cylinder rod 4 passes inside the spindle. When the pneumatics are turned on, the membrane bends, pushing the cams apart. When the rod moves back, the membrane, trying to return to its original position, compresses the workpiece 3 with its cams. Rack and pinion clamp(Fig. 76) consists of a rack 3, a gear 5 sitting on a shaft 4, and a handle lever 6. By rotating the handle counterclockwise, lower the rack and clamp 2 to secure the workpiece 1. The clamping force Q depends on the value of the force P applied to the handle. The device is equipped with a lock, which, by jamming the system, prevents the reverse rotation of the wheel. The most common types of locks are: Roller lock(Fig. 77, a) consists of a drive ring 3 with a cutout for roller 1, which is in contact with the cut plane of the roller. 2 gears. Drive ring 3 is attached to the handle of the clamping device. By rotating the handle in the direction of the arrow, rotation is transmitted to the gear shaft through roller 1*. The roller is wedged between the bore surface of the housing 4 and the cut plane of the roller 2 and prevents reverse rotation. Direct Drive Roller Lock the moment from the driver to the roller is shown in Fig. 77, b. Rotation from the handle through the leash is transmitted directly to the 6th wheel shaft. Roller 3 is pressed through pin 4 by a weak spring 5. Since the gaps in the places where the roller touches ring 1 and shaft 6 are selected, the system instantly jams when the force is removed from handle 2. By turning the handle in the opposite direction, the roller wedges and rotates the shaft clockwise . Conical lock(Fig. 77, c) has a conical sleeve 1 and a shaft with a cone 3 and a handle 4. The spiral teeth on the middle neck of the shaft are engaged with the rack 5. The latter is connected to the actuator clamping mechanism. At a tooth angle of 45°, the axial force on shaft 2 is equal (without taking into account friction) to the clamping force. * Locks of this type are made with three rollers located at an angle of 120°. Combination clamping devices are a combination of elementary clamps of various types. They are used to increase the clamping force and reduce the dimensions of the device, as well as to create greater ease of control. Combination clamping devices can also provide simultaneous clamping of a workpiece in several places. Types of combined clamps are shown in Fig. 78. The combination of a curved lever and a screw (Fig. 78, a) allows you to simultaneously secure the workpiece in two places, uniformly increasing the clamping forces to a given value. A conventional rotary clamp (Fig. 78, b) is a combination of lever and screw clamps. The swing axis of lever 2 is aligned with the center of the spherical surface of washer 1, which relieves pin 3 from bending forces. The clamp with an eccentric shown in Fig. 78 is an example of a high-speed combined clamp. At a certain lever arm ratio, the clamping force or stroke of the clamping end of the lever can be increased. In Fig. 78, d shows a device for securing a cylindrical workpiece in a prism using a hinge lever, and in Fig. 78, d - diagram of a high-speed combined clamp (lever and eccentric), providing lateral and vertical pressing of the workpiece to the supports of the device, since the clamping force is applied at an angle. A similar condition is provided by the device shown in Fig. 78, e. Hinge-lever clamps (Fig. 78, g, h, i) are examples of high-speed clamping devices actuated by turning the handle. To prevent self-release, the handle is moved through the dead position to stop 2. The clamping force depends on the deformation of the system and its rigidity. The desired deformation of the system is set by adjusting pressure screw 1. However, the presence of a tolerance for size H (Fig. 78, g) does not ensure constant clamping force for all workpieces of a given batch. Combined clamping devices are operated manually or by power units. Clamping mechanisms for multiple fixtures must provide the same clamping force in all positions. The simplest multi-place device is a mandrel on which a package of blanks “rings, disks” is installed, secured along the end planes with one nut (sequential clamping force transmission scheme). In Fig. 79, α shows an example of a clamping device operating on the principle of parallel distribution of clamping force. If it is necessary to ensure the concentricity of the base and machined surfaces and to prevent deformation of the workpiece, elastic clamping devices are used, where the clamping force is uniformly transmitted by means of a filler or other intermediate body to the clamping element of the device within the limits of elastic deformations). Conventional springs, rubber or hydroplastic are used as an intermediate body. A parallel clamping device using hydroplastic is shown in Fig. 79, b. In Fig. 79, in shows a device of mixed (parallel-series) action. On continuous machines (drum-milling, special multi-spindle drilling) workpieces are installed and removed without interrupting the feed movement. If the auxiliary time overlaps with the machine time, then various types of clamping devices can be used to secure the workpieces. In order to mechanize production processes, it is advisable to use Automatic clamping devices(continuous) driven by the feed mechanism of the machine. In Fig. 80, α shows a diagram of a device with a flexible closed element 1 (cable, chain) for securing cylindrical workpieces 2 on a drum milling machine when processing end surfaces, and in Fig. 80, 6 - diagram of a device for securing piston blanks on a multi-spindle horizontal drilling machine. In both devices, operators only install and remove the workpiece, and the workpiece is secured automatically. An effective clamping device for holding workpieces made of thin sheet material during finishing or finishing is a vacuum clamp. The clamping force is determined by the formula: where A is the active area of the device cavity limited by the seal; p = 10 5 Pa - the difference between atmospheric pressure and the pressure in the cavity of the device from which air is removed. Electromagnetic clamping devices used for securing workpieces made of steel and cast iron with a flat base surface. Clamping devices are usually made in the form of plates and chucks, the design of which takes as initial data the dimensions and configuration of the workpiece in plan, its thickness, material and the necessary holding force. The holding force of the electromagnetic device largely depends on the thickness of the workpiece; at small thicknesses, not all the magnetic flux passes through the cross section of the part, and some of the magnetic flux lines are scattered into the surrounding space. Parts processed on electromagnetic plates or chucks acquire residual magnetic properties - they are demagnetized by passing them through a solenoid powered by alternating current. In magnetic clamping In devices, the main elements are permanent magnets, isolated from one another by non-magnetic gaskets and fastened into a common block, and the workpiece is an armature through which the magnetic power flow is closed. To detach the finished part, the block is shifted using an eccentric or crank mechanism, while the magnetic force flow is closed to the device body, bypassing the part. Page INTRODUCTION The main group of technological equipment consists of devices for mechanical assembly production. In mechanical engineering, devices are auxiliary devices for technological equipment used when performing processing, assembly and control operations. GENERAL INFORMATION ABOUT DEVICES. In mechanical engineering, a variety of technological equipment is widely used, which includes fixtures, auxiliary, cutting and measuring tools. Rice. 1 – Classification of machine tools Non-separable special devices (NSD) are assembled from standard parts and general-purpose assembly units, as irreversible long-term devices. The structural elements of the layouts included in the system, as a rule, are used until they are completely worn out and are not reused. The layout can also be carried out by constructing a device from two main parts: a unified base part (UB) and a replaceable setup (SN). This design of the NSP makes it resistant to changes in the design of the workpieces being processed and to adjustments in technological processes. In these cases, only the replaceable adjustment is replaced in the fixture. MAIN ELEMENTS OF DEVICES The following equipment elements exist: Clamping elements of fixtures 1 Purpose of clamping elements Rice. 2 - Workpiece installation diagram Clamping devices are used in some cases to ensure correct installation and centering of the workpiece. In this case, they perform the function of installation and clamping devices. These include self-centering chucks, collet clamps, etc. 2 Types of clamping elements Rice. 3 – Schemes of screw clamps In Fig. Figure 4 shows some designs of quick-release clamps. For small clamping forces, a bayonet device is used (Fig. 4, a), and for significant forces, a plunger device is used (Fig. 4, b). These devices allow the clamping element to be moved a long distance from the workpiece; fastening occurs as a result of turning the rod through a certain angle. An example of a clamp with a folding stop is shown in Fig. 4, c. Having loosened the handle nut 2, remove the stop 3, rotating it around its axis. After this, the clamping rod 1 is retracted to the right at a distance h. In Fig. 4, d shows a diagram of a high-speed lever-type device. When turning the handle 4, the pin 5 slides along the bar 6 with an oblique cut, and the pin 2 slides along the workpiece 1, pressing it against the stops located below. Spherical washer 3 serves as a hinge. Rice. 4 - Quick release clamp designs The large amount of time and significant forces required to secure the workpieces limit the scope of application of screw clamps and, in most cases, make high-speed eccentric clamps preferable. In Fig. Figure 5 shows disk (a), cylindrical with L-shaped clamp (b) and conical floating (c) clamps. Rice. 5 – Various clamp designs Rice. 6 – Diagram of a round eccentric The advantage of round eccentrics is the ease of their manufacture; the main disadvantage is the inconsistency of the elevation angle a and the clamping forces Q. Curvilinear eccentrics, the working profile of which is carried out according to an involute or Archimedes spiral, have a constant elevation angle a, and, therefore, ensure constancy of the force Q when clamping any point in the profile. Where P is the axial force; P = Q sin (a + 2j/cos (90°+a-b+2j). Lever clamps are used in combination with other elementary clamps to form more complex clamping systems. Using the lever, you can change the magnitude and direction of the transmitted force, as well as simultaneously and uniformly secure the workpiece in two places. Fig. 7 – Diagrams of a single-bevel wedge (a) and a double-bevel wedge (b) Figure 8 shows diagrams of the action of forces in single-arm and double-arm straight and curved clamps. The equilibrium equations for these lever mechanisms are as follows: Rice. 8 - Schemes of the action of forces in single-arm and double-arm straight and curved clamps Centering clamping elements are used as installation elements for the external or internal surfaces of rotating bodies: collets, expanding mandrels, clamping bushings with hydroplastic, as well as membrane cartridges. Rice. 10 – Membrane cartridge diagram A rack and pinion clamp (Fig. 11) consists of a rack 3, a gear 5 sitting on a shaft 4, and a handle lever 6. By rotating the handle counterclockwise, lower the rack and clamp 2 to secure the workpiece 1. The clamping force Q depends on the value force P applied to the handle. The device is equipped with a lock, which, by jamming the system, prevents the reverse rotation of the wheel. The most common types of locks are: Rice. 11 - Rack and pinion clamp The roller lock (Fig. 12, a) consists of a drive ring 3 with a cutout for roller 1, which is in contact with the cut plane of the gear shaft 2. Drive ring 3 is attached to the handle of the clamping device. By rotating the handle in the direction of the arrow, rotation is transmitted to the gear shaft through roller 1. The roller is wedged between the bore surface of the housing 4 and the cut plane of the roller 2 and prevents reverse rotation. Rice. 12 – Schemes of various lock designs A roller lock with direct transmission of torque from the driver to the roller is shown in Fig. 12, b. Rotation from the handle through the leash is transmitted directly to the 6th wheel shaft. Roller 3 is pressed through pin 4 by a weak spring 5. Since the gaps in the places where the roller touches ring 1 and shaft 6 are selected, the system instantly jams when the force is removed from handle 2. By turning the handle in the opposite direction, the roller wedges and rotates the shaft clockwise . Rice. 13 - Types of combined clamps In Fig. 13, d shows a device for securing a cylindrical workpiece in a prism using a hinge lever, and in Fig. 13, d - diagram of a high-speed combined clamp (lever and eccentric), providing lateral and vertical pressing of the workpiece to the supports of the device, since the clamping force is applied at an angle. A similar condition is provided by the device shown in Fig. 13, e. Rice. 14 - Clamping mechanisms for multiple devices Conventional springs, rubber or hydroplastic are used as an intermediate body. A parallel clamping device using hydroplastic is shown in Fig. 14, b. In Fig. 14, c shows a device of mixed (parallel-series) action. Rice. 15 - Automatic clamping devices An effective clamping device for holding workpieces made of thin sheet material during finishing or finishing is a vacuum clamp. The clamping force is determined by the formula Q=Ap, The main purpose of clamping devices is to ensure reliable contact of the workpiece with the mounting elements and to prevent its displacement and vibration during processing. Clamping devices are also used to ensure correct positioning and centering of the workpiece. In this case, the clamps perform the function of mounting and clamping elements. These include self-centering chucks, collet clamps and other devices. The workpiece may not be secured if a heavy (stable) part is being processed, compared to the weight of which the cutting forces are insignificant; the force generated during the cutting process is applied in such a way that it does not disturb the installation of the part. During processing, the following forces can act on the workpiece: Cutting forces, which can be variable due to different processing allowances, material properties, dullness of the cutting tool; Weight of the workpiece (with the part in a vertical position); Centrifugal forces resulting from a displacement of the center of gravity of a part relative to the axis of rotation. The following basic requirements apply to fixture clamping devices: When securing the workpiece, its position achieved by installation must not be violated; The clamping forces must exclude the possibility of movement of the part and its vibration during processing; Deformation of the part under the action of clamping forces should be minimal. The crushing of the base surfaces should be minimal, so the clamping force should be applied so that the part is pressed against the mounting elements of the fixture with a flat base surface, and not a cylindrical or shaped one. Clamping devices must be fast-acting, conveniently located, simple in design and require minimal effort from the worker. Clamping devices must be wear-resistant, and the most wearable parts must be replaceable. The clamping forces must be directed towards the supports so as not to deform the part, especially a non-rigid one. Materials: steel 30ХГСА, 40Х, 45. The working surface must be processed in 7 square meters. and more precisely. Terminal designation: Clamping device designation: P – pneumatic H – hydraulic E – electric M – magnetic EM – electromagnetic G – hydroplastic In individual production, manual drives are used: screw, eccentric, etc. In mass production, mechanized drives are used. 5. CLAMPING THE PART. INITIAL DATA FOR DRAFTING A SCHEME FOR CALCULATING THE CLAMPING FORCE OF THE PART. METHOD FOR DETERMINING THE CLAMPING FORCE OF A PART IN A DEVICE. TYPICAL DIAGRAMS FOR CALCULATING FORCE, REQUIRED CLAMPING FORCE. The magnitude of the required clamping forces is determined by solving the statics problem of the equilibrium of a rigid body under the influence of all forces and moments applied to it. Clamping forces are calculated in 2 main cases: 1. when using existing universal devices with clamping devices that develop a certain force; 2. when designing new devices. In the first case, the calculation of the clamping force is of a testing nature. The required clamping force, determined from the processing conditions, must be less than or equal to the force that the clamping device of the universal fixture used develops. If this condition is not met, then the processing conditions are changed in order to reduce the required clamping force, followed by a new verification calculation. In the second case, the method for calculating clamping forces is as follows: 1. The most rational part installation scheme is selected, i.e. the position and type of supports, places of application of clamping forces are outlined, taking into account the direction of cutting forces at the most unfavorable moment of processing. 2. In the selected diagram, arrows indicate all forces applied to the part that tend to disrupt the position of the part in the fixture (cutting forces, clamping forces) and forces that tend to maintain this position (friction forces, support reactions). If necessary, inertial forces are also taken into account. 3. Select the static equilibrium equations applicable to the given case and determine the desired value of the clamping force Q 1 . 4. Having accepted the fastening reliability coefficient (safety factor), the need for which is caused by inevitable fluctuations in cutting forces during processing, the actual required clamping force is determined: The safety factor K is calculated in relation to specific processing conditions where K 0 = 2.5 – guaranteed safety factor for all cases; K 1 – coefficient taking into account the state of the workpiece surface; K 1 = 1.2 – for rough surface; К 1 = 1 – for finishing surface; K 2 – coefficient that takes into account the increase in cutting forces from progressive dullness of the tool (K 2 = 1.0...1.9); K 3 – coefficient that takes into account the increase in cutting forces during intermittent cutting; (K 3 = 1.2). К 4 – coefficient taking into account the constancy of the clamping force developed by the power drive of the device; K 4 = 1…1.6; K 5 – this coefficient is taken into account only in the presence of torques tending to rotate the workpiece; K 5 = 1…1.5. Typical diagrams for calculating the clamping force of a part and the required clamping force: 1. The cutting force P and the clamping force Q are equally directed and act on the supports: At a constant value of P, force Q = 0. This scheme corresponds to broaching holes, turning in centers, and counterbore bosses. 2. The cutting force P is directed against the clamping force: 3. The cutting force tends to move the workpiece from the mounting elements: Typical for pendulum milling and milling of closed contours. 4. The workpiece is installed in the chuck and is under the influence of moment and axial force: where Q c is the total clamping force of all cams: where z is the number of jaws in the chuck. Taking into account the safety factor k, the required force developed by each cam will be: 5. If one hole is drilled in a part and the direction of the clamping force coincides with the direction of drilling, then the clamping force is determined by the formula: k M = W f R W = k M / f R 6. If several holes are drilled simultaneously in a part and the direction of the clamping force coincides with the direction of drilling, then the clamping force is determined by the formula: LECTURE 3 3.1. Purpose of clamping devices The main purpose of fixture clamping devices is to ensure reliable contact (continuity) of the workpiece or assembled part with the installation elements, preventing its displacement during processing or assembly. The clamping mechanism creates a force to secure the workpiece, determined from the condition of equilibrium of all forces applied to it During machining the workpiece is subject to: 1) forces and cutting moments 2) volumetric forces - workpiece gravity, centrifugal and inertial forces. 3) forces acting at the points of contact of the workpiece with the device - support reaction force and friction force 4) secondary forces, which include the forces that arise when the cutting tool (drills, taps, reamers) is removed from the workpiece. During assembly, the assembled parts are subject to assembly forces and reaction forces that arise at the points of contact of the mating surfaces. The following requirements apply to clamping devices:: 1) when clamping, the position of the workpiece achieved by basing should not be disturbed. This is satisfied by a rational choice of the direction and places of application of the clamping forces; 2) the clamp should not cause deformation of the workpieces fixed in the fixture or damage (crushing) of their surfaces; 3) the clamping force must be the minimum necessary, but sufficient to ensure a fixed position of the workpiece relative to the installation elements of the devices during processing; 4) the clamping force must be constant throughout the entire technological operation; the clamping force must be adjustable; 5) clamping and detaching the workpiece must be done with minimal effort and worker time. When using manual clamps, the force should not exceed 147 N; Average duration of fastening: in a three-jaw chuck (with a key) - 4 s; screw clamp (key) - 4.5…5 s; steering wheel - 2.5…3 s; turning the pneumatic and hydraulic valve handle - 1.5 s; by pressing a button - less than 1 s. 6) the clamping mechanism must be simple in design, compact, as convenient and safe as possible in operation. To do this, it must have minimum overall dimensions and contain a minimum number of removable parts; The clamping mechanism control device should be located on the worker's side. The need to use clamping devices is eliminated in three cases. 1) the workpiece has a large mass, in comparison with which the cutting forces are small. 2) the forces arising during processing are directed in such a way that they cannot disturb the position of the workpiece achieved during basing. 3) the workpiece installed in the fixture is deprived of all degrees of freedom. For example, when drilling a hole in a rectangular strip placed in a box jig. 3.2. Classification of clamping devices The designs of clamping devices consist of three main parts: a contact element (CE), a drive (P) and a power mechanism (SM). The contact elements serve to directly transfer the clamping force to the workpiece. Their design allows the forces to be dispersed, preventing the workpiece surfaces from being crushed. The drive serves to convert a certain type of energy into initial force R and transmitted to the power mechanism. A force mechanism is required to convert the resulting initial clamping force R and in clamping force R z. The transformation is carried out mechanically, i.e. according to the laws of theoretical mechanics. In accordance with the presence or absence of these components in the fixture, clamping devices of fixtures are divided into three groups. TO first The group includes clamping devices (Fig. 3.1a), which include all of the main parts listed: a power mechanism and a drive, which ensures the movement of the contact element and creates the initial force R and, converted by the power mechanism into clamping force R z . In second group (Fig. 3.1b) includes clamping devices consisting only of a power mechanism and a contact element, which is actuated directly by the worker applying the initial force R and on the shoulder l. These devices are sometimes called manual clamping devices (one-off and small-scale production). TO third This group includes clamping devices that do not have a power mechanism, and the drives used can only conditionally be called drives, since they do not cause movement of the elements of the clamping device and only create a clamping force R z, which in these devices is the resultant of a uniformly distributed load q, directly acting on the workpiece and created either as a result of atmospheric pressure or through a magnetic force flux. This group includes vacuum and magnetic devices (Fig. 3.1c). Used in all types of production. Rice. 3.1. Clamping mechanism diagrams An elementary clamping mechanism is a part of a clamping device consisting of a contact element and a power mechanism. Clamping elements are called: screws, eccentrics, clamps, vice jaws, wedges, plungers, clamps, strips. They are intermediate links in complex clamping systems. In table 2 shows the classification of elementary clamping mechanisms. table 2 Classification of elementary clamping mechanisms Based on the source of drive energy (here we are not talking about the type of energy, but rather the location of the source), drives are divided into manual, mechanized and automated. Manual clamping mechanisms are operated by the muscular force of the worker. Motorized clamping mechanisms operate from a pneumatic or hydraulic drive. Automated devices move from moving machine components (spindle, slide or chucks with jaws). In the latter case, the workpiece is clamped and the processed part is released without the participation of a worker. 3.3. Clamping elements 3.3.1. Screw terminals Screw clamps are used in devices with manual fastening of the workpiece, in mechanized devices, as well as on automatic lines when using satellite devices. They are simple, compact and reliable in operation. Rice. 3.2. Screw terminals: a – with a spherical end; b – with a flat end; c – with a shoe. Legend: R and- force applied at the end of the handle; R z- clamping force; W– ground reaction force; l- handle length; d- diameter of the screw clamp. Calculation of screw EZM. With a known force P 3, the nominal diameter of the screw is calculated where d is the screw diameter, mm; R 3- fastening force, N; σ р- tensile (compressive) stress of the screw material, MPa Clamping elements must ensure reliable contact of the workpiece with the installation elements and prevent its disruption under the influence of forces arising during processing, fast and uniform clamping of all parts and not cause deformation and damage to the surfaces of the fastened parts. Clamping elements are divided into:
By design -
for screw, wedge, eccentric, lever, lever-hinge (combined clamping elements are also used - screw-lever, eccentric-lever, etc.). According to the degree of mechanization -
manual and mechanized with hydraulic, pneumatic, electric or vacuum drive. The clamping bellows can be automated. Screw terminals used for direct clamping or clamping through clamping bars, or holding one or more parts. Their disadvantage is that that fastening and unfastening the part requires a lot of time. Eccentric and wedge clamps, just like screw ones, they allow you to fasten the part directly or through clamping bars and levers. Circular eccentric clamps are the most widely used. An eccentric clamp is a special case of a wedge clamp, and to ensure self-braking, the wedge angle should not exceed 6-8 degrees. Cam clamps are made from high carbon or case hardened steel and heat treated to a hardness of HRC55-60. Eccentric clamps are fast-acting clamps because... required for clamping turn the eccentric at an angle of 60-120 degrees. Lever-hinged elements used as drive and reinforcing links of clamping mechanisms. By design, they are divided into single-lever, double-lever (single- and double-acting - self-centering and multi-link). Lever mechanisms do not have self-braking properties. The simplest example of lever-hinged mechanisms is the clamping bars of devices, levers of pneumatic cartridges, etc. Spring clamps used for clamping products with little effort that occurs when the spring is compressed. To create constant and high clamping forces, reduce clamping time, and implement remote control of clamps, use
pneumatic, hydraulic and other drives. The most common pneumatic drives are piston pneumatic cylinders and pneumatic chambers with an elastic diaphragm, stationary, rotating and swinging. Pneumatic actuators are driven
compressed air under a pressure of 4-6 kg/cm² If it is necessary to use small-sized drives and create large clamping forces, use hydraulic drives, the operating oil pressure in which. reaches 80 kg/cm². The force on the rod of a pneumatic or hydraulic cylinder is equal to the product of the working area of the piston in square cm and the pressure of the air or working fluid. In this case, it is necessary to take into account friction losses between the piston and the cylinder walls, between the rod and guide bushings and seals. Electromagnetic clamping devices They are made in the form of slabs and faceplates. They are designed for holding steel and cast iron workpieces with a flat base surface for grinding or fine turning. Magnetic clamping devices can be made in the form of prisms that serve to secure cylindrical workpieces. There are plates that use ferrites as permanent magnets. These plates are characterized by high holding force and smaller distance between poles.
Collets They are split spring sleeves, the design varieties of which are shown in Fig. 74 (α - with a tension tube; 6 - with a spacer tube; c - vertical type). They are made from high-carbon steels, for example, U10A, and are heat-treated to a hardness of HRC 58...62 in the clamping part and to a hardness of HRC 40...44 in the tail parts. Collet cone angle α = 30…40°. At smaller angles, the collet may jam.
Cam lock(Fig. 77, d) consists of a wheel shaft 2 on which an eccentric 3 is jammed. The shaft is driven into rotation by a ring 1 attached to the lock handle; the ring rotates in the housing bore 4, the axis of which is displaced from the shaft axis by a distance e. When the handle rotates in reverse, transmission to the shaft occurs through pin 5. During the fastening process, ring 1 is wedged between the eccentric and the housing.
CONTENT INTRODUCTION………………….…………………………………… ……..…….....2
GENERAL INFORMATION ABOUT DEVICES…………………………... …3 MAIN ELEMENTS OF DEVICES……………….………………...6
Clamping elements of devices……………………………….……. …..6
1 Purpose of clamping elements………………………………………...6
2 Types of clamping elements……………………………………….…..…. .7
REFERENCES………………………………… ……………………..17
The use of devices allows you to: eliminate the marking of workpieces before processing, increase its accuracy, increase labor productivity in operations, reduce production costs, facilitate working conditions and ensure its safety, expand the technological capabilities of equipment, organize multi-machine maintenance, apply technically sound time standards, reduce the number of workers necessary for production.
The frequent change of production facilities, associated with the increasing pace of technological progress in the era of scientific and technological revolution, requires technological science and practice to create structures and systems of devices, methods for their calculation, design and manufacture, ensuring a reduction in production preparation time. In mass production, it is necessary to use specialized, quickly adjustable and reversible fixture systems. In small-scale and individual production, the system of universal prefabricated (USP) devices is increasingly being used.
New requirements for devices are determined by the expansion of the fleet of CNC machines, the readjustment of which for processing a new workpiece comes down to replacing the program (which takes very little time) and replacing or readjusting the device for basing and securing the workpiece (which should also take little time) .
Studying the patterns of the influence of devices on the accuracy and productivity of operations performed will make it possible to design devices that intensify production and increase its accuracy. Work on the unification and standardization of fixture elements creates the basis for automated design of fixtures using electronic computers and automatic machines for graphic display. This speeds up the technological preparation of production.
TYPES OF DEVICES
Devices are additional devices used for machining, assembly and control of parts, assembly units and products. According to their purpose, devices are divided into the following types:
1. Machine tools used for installing and securing processed workpieces on machines. Depending on the type of machining, these devices, in turn, are divided into devices for drilling, milling, boring, turning, grinding machines, etc. Machine tools make up 80...90% of the total fleet of technological equipment.
The use of devices ensures:
a) increasing labor productivity by reducing the time for installing and securing workpieces with partial or complete overlap of auxiliary time by machine time and reducing the latter through multi-place processing, combining technological transitions and increasing cutting conditions;
b) increasing processing accuracy due to the elimination of alignment during installation and associated errors;
c) facilitating the working conditions of machine operators;
d) expanding the technological capabilities of equipment;
e) increasing work safety.
2. Devices for installing and securing a working tool, communicating between the tool and the machine, while the first type communicates the workpiece with the machine. Using devices of the first and second types, the technological system is adjusted.
3. Assembly devices for connecting mating parts into assembly units and products. They are used to fasten base parts or assembly units of an assembled product, ensure correct installation of the connected elements of the product, pre-assemble elastic elements (springs, split rings, etc.), as well as to make tension connections.
4. Inspection devices for intermediate and final inspection of parts, as well as for inspection of assembled machine parts.
5. Devices for capturing, moving and turning over workpieces and assembly units used in the processing and assembly of heavy parts and products.
According to their operational characteristics, machine tools are divided into universal ones, designed for processing a variety of workpieces (machine vices, chucks, dividing heads, rotary tables, etc.); specialized, intended for processing workpieces of a certain type and representing replaceable devices (special jaws for a vice, shaped jaws for chucks, etc.), and special, intended for performing certain operations of machining a given part. Universal devices are used in conditions of single or small-scale production, and specialized and special devices are used in conditions of large-scale and mass production.
Using a unified system of technological preparation of production, machine tools are classified according to certain criteria (Fig. 1).
Universal prefabricated devices (USF) are assembled from prefabricated standard elements, parts and high-precision assembly units. They are used as special short-term devices for a specific operation, after which they are disassembled, and the delivery elements are subsequently reused in new arrangements and combinations. Further development of the USP is associated with the creation of units, blocks, individual special parts and assembly units that ensure the layout of not only special, but also specialized and universal adjustment devices for short-term operation,
Collapsible fixtures (CDF) are also assembled from standard elements, but less precise, allowing local modification according to the seats. These devices are used as special long-term devices. After disassembling the elements, you can create new layouts.
Universal non-adjustment devices (UPD) for general purpose are most common in mass production conditions. They are used for securing workpieces from rolled profiles and piece workpieces. UBPs are universal adjustable housings with permanent (non-removable) basic elements (chucks, vices, etc.), included with the machine upon delivery.
Specialized adjustment devices (SAD) are used to equip operations for processing parts grouped according to design characteristics and basing schemes; the arrangement according to the assembly diagram is the basic design of the housing with interchangeable settings for groups of parts.
Universal adjustment devices (UND), like SNP, have permanent (body) and replaceable parts. However, the replacement part is suitable for performing only one operation for processing only one part. When switching from one operation to another, the devices of the UNP system are equipped with new replaceable parts (adjustments).
Aggregate means of mechanized clamping (ASMZ) are a set of universal power devices, made in the form of separate units, which, in combination with devices, make it possible to mechanize and automate the process of clamping workpieces.
The choice of device design largely depends on the nature of production. Thus, in mass production, relatively simple devices are used, designed mainly to achieve the specified accuracy of processing the workpiece. In mass production, high demands are placed on fixtures in terms of performance as well. Therefore, such devices, equipped with quick-release clamps, are more complex designs. However, the use of even the most expensive devices is economically justified.
installation - to determine the position of the workpiece surface being processed relative to the cutting tool;
clamping - for securing the workpiece being processed;
guides - to give the required direction to the movement of the cutting tool relative to the surface being processed;
fixture housings - the main part on which all fixture elements are located;
fastening - for connecting individual elements to each other;
dividing or rotary, - to accurately change the position of the workpiece surface being processed relative to the cutting tool;
mechanized drives - to create clamping force. In some devices, installation and clamping of the workpiece is performed by one mechanism, called installation-clamping.
The main purpose of clamping devices is to ensure reliable contact of the workpiece with the mounting elements and prevent its displacement relative to them and vibration during processing. By introducing additional clamping devices, the rigidity of the technological system is increased and this results in increased processing accuracy and productivity, and a reduction in surface roughness. In Fig. Figure 2 shows a diagram of the installation of workpiece 1, which, in addition to the two main clamps Q1, is secured with an additional device Q2, which imparts greater rigidity to the system. Support 2 is self-aligning.
Clamping devices are not used when processing heavy, stable workpieces, compared with the mass of which the forces arising during the cutting process are relatively small and are applied in such a way that they cannot disturb the installation of the workpiece.
Clamping devices of devices must be reliable in operation, simple in design and easy to maintain; they should not cause deformation of the workpiece being fastened and damage to its surface, and should not move the workpiece during the process of its fastening. The machine operator should spend a minimum of time and effort on securing and detaching workpieces. To simplify repairs, it is advisable to make the most wearing parts of clamping devices replaceable. When securing workpieces in multiple fixtures, they are clamped evenly; with limited movement of the clamping element (wedge, eccentric), its stroke must be greater than the tolerance for the size of the workpiece from the installation base to the place where the clamping force is applied.
Clamping devices are designed taking into account safety requirements.
The location where the clamping force is applied is selected according to the conditions of greatest rigidity and stability of the fastening and minimal deformation of the workpiece. When increasing the processing accuracy, it is necessary to comply with the conditions of a constant value of the clamping force, the direction of which must be consistent with the location of the supports.
Clamping elements are mechanisms directly used to secure workpieces, or intermediate links in more complex clamping systems.
The simplest type of universal clamps are clamping screws, which are activated by keys, handles or handwheels mounted on them.
To prevent the movement of the clamped workpiece and the formation of dents on it from the screw, and also to reduce the bending of the screw when pressing on a surface not perpendicular to its axis, swinging shoes are placed on the ends of the screws (Fig. 3, a).
Combinations of screw devices with levers or wedges are called combined clamps, a type of which are screw clamps (Fig. 3, b). The clamping device allows you to move or rotate them so that you can more conveniently install the workpiece in the fixture.
Eccentrics are round, involute and spiral (along the Archimedes spiral). Two types of eccentrics are used in clamping devices: round and curved.
Round eccentrics (Fig. 6) are a disk or roller with an axis of rotation shifted by the eccentricity size e; the self-braking condition is ensured at the ratio D/e ? 4.
The wedge mechanism is used as an intermediate link in complex clamping systems. It is simple to manufacture, easily placed in the device, and allows you to increase and change the direction of the transmitted force. At certain angles, the wedge mechanism has self-braking properties. For a single-bevel wedge (Fig. 7, a) when transmitting forces at right angles, the following dependence can be accepted (with j1=j2=j3=j, where j1...j3 are the friction angles):
P=Qtg(a±2j),
Q - clamping force.
Self-braking will take place at a
for single-arm clamp (Fig. 8, a)
,
for direct double-arm clamp (Fig. 8, b)
,
for double-arm curved clamp (for l1
where r is the friction angle;
f is the friction coefficient.
The collets are split spring sleeves, the design varieties of which are shown in Fig. 9 (a - with a tension tube; b - with a spacer tube; c - vertical type). They are made from high-carbon steels, for example U10A, and are heat treated to a hardness of HRC 58...62 in the clamping part and to a hardness of HRC 40...44 in the tail parts. Collet cone angle a=30. . .40°. At smaller angles, the collet may jam. The cone angle of the compression sleeve is made 1° less or greater than the collet cone angle. The collets ensure installation eccentricity (runout) of no more than 0.02...0.05 mm. The base surface of the workpiece should be processed according to the 9th...7th accuracy grade.
Expanding mandrels of various designs (including designs using hydroplastic) are classified as mounting and clamping devices.
Diaphragm cartridges are used for precise centering of workpieces along the outer or inner cylindrical surface. The cartridge (Fig. 10) consists of a round membrane 1 screwed to the faceplate of the machine in the form of a plate with symmetrically located protrusions-cams 2, the number of which is selected in the range of 6...12. A pneumatic cylinder rod 4 passes inside the spindle. When the pneumatics are turned on, the membrane bends, pushing the cams apart. When the rod moves back, the membrane, trying to return to its original position, compresses the workpiece 3 with its cams.
The conical lock (Fig. 12, c) has a conical sleeve 1 and a shaft 2 with a cone 3 and a handle 4. The spiral teeth on the middle neck of the shaft are engaged with the rack 5. The latter is connected to the actuator clamping mechanism. At a tooth angle of 45°, the axial force on shaft 2 is equal (without taking into account friction) to the clamping force.
An eccentric lock (Fig. 12, d) consists of a wheel shaft 2 on which an eccentric 3 is jammed. The shaft is driven into rotation by a ring 1 attached to the lock handle; the ring rotates in the housing bore 4, the axis of which is displaced from the shaft axis by a distance e. When the handle rotates in reverse, transmission to the shaft occurs through pin 5. During the fastening process, ring 1 is wedged between the eccentric and the housing.
Combined clamping devices are a combination of elementary clamps of various types. They are used to increase the clamping force and reduce the dimensions of the device, as well as to create greater ease of control. Combination clamping devices can also provide simultaneous clamping of a workpiece in several places. Types of combined clamps are shown in Fig. 13.
The combination of a curved lever and a screw (Fig. 13, a) allows you to simultaneously secure the workpiece in two places, uniformly increasing the clamping forces to a given value. A conventional rotary clamp (Fig. 13, b) is a combination of lever and screw clamps. The swing axis of lever 2 is aligned with the center of the spherical surface of washer 1, which relieves pin 3 from bending forces. Shown in Fig. 13, in an eccentric clamp, is an example of a high-speed combination clamp. At a certain lever arm ratio, the clamping force or stroke of the clamping end of the lever can be increased.
Hinge-lever clamps (Fig. 13, g, h, i) are examples of high-speed clamping devices actuated by turning the handle. To prevent self-release, the handle is moved through the dead position to stop 2. The clamping force depends on the deformation of the system and its rigidity. The desired deformation of the system is set by adjusting pressure screw 1. However, the presence of a tolerance for size H (Fig. 13, g) does not ensure a constant clamping force for all workpieces of a given batch.
Combined clamping devices are operated manually or by power units.
Clamping mechanisms for multiple fixtures must provide equal clamping force at all positions. The simplest multi-place device is a mandrel on which a package of blanks (rings, disks) is installed, secured along the end planes with one nut (sequential clamping force transmission scheme). In Fig. 14a shows an example of a clamping device operating on the principle of parallel distribution of clamping force.
If it is necessary to ensure the concentricity of the base and workpiece surfaces and to prevent deformation of the workpiece, elastic clamping devices are used, where the clamping force is uniformly transmitted by means of a filler or other intermediate body to the clamping element of the device (within the limits of elastic deformations).
On continuous machines (drum-milling, special multi-spindle drilling), workpieces are installed and removed without interrupting the feed movement. If the auxiliary time overlaps with the machine time, then various types of clamping devices can be used to secure the workpieces.
In order to mechanize production processes, it is advisable to use automated clamping devices (continuous action), driven by the feed mechanism of the machine. In Fig. 15, a shows a diagram of a device with a flexible closed element 1 (cable, chain) for securing cylindrical workpieces 2 on a drum milling machine when processing end surfaces, and in Fig. 15, b - diagram of a device for securing piston blanks on a multi-spindle horizontal drilling machine. In both devices, operators only install and remove the workpiece, and the workpiece is secured automatically.
where A is the active area of the device cavity limited by the seal;
p=10 5 Pa - the difference between atmospheric pressure and the pressure in the cavity of the device from which air is removed.
Electromagnetic clamping devices are used to secure workpieces made of steel and cast iron with a flat base surface. Clamping devices are usually made in the form of plates and chucks, the design of which takes as initial data the dimensions and configuration of the workpiece in plan, its thickness, material and the necessary holding force. The holding force of the electromagnetic device largely depends on the thickness of the workpiece; at small thicknesses, not all the magnetic flux passes through the cross section of the part, and some of the magnetic flux lines are scattered into the surrounding space. Parts processed on electromagnetic plates or chucks acquire residual magnetic properties - they are demagnetized by passing them through a solenoid powered by alternating current.
In magnetic clamping devices, the main elements are permanent magnets, isolated from one another by non-magnetic gaskets and fastened into a common block, and the workpiece is an armature through which the magnetic power flow is closed. To detach the finished part, the block is shifted using an eccentric or crank mechanism, while the magnetic force flow is closed to the device body, bypassing the part.BIBLIOGRAPHY
Automation of design and engineering work and technological
[Enter text]
preparation of production in mechanical engineering /Under the general. ed. O. I. Semenkova.
T. I, II. Minsk, Higher School, 1976. 352 p.
Anserov M: A. Devices for metal-cutting machines. M.:
Mechanical Engineering, 1975. 656 p.
Blumberg V. A., Bliznyuk V. P. Re-adjustable machine tools. L.: Mechanical Engineering, 1978. 360 p.
Bolotin Kh. L., Kostromin F. P. Machine tools. M.:
Mechanical Engineering, 1973. 341 p.
Goroshkin A.K. Devices for metal-cutting machines. M.;
Mechanical Engineering, 1979. 304 p.
Kapustin N. M. Acceleration of technological preparation of mechanical assembly production. M.: Mechanical Engineering, 1972. 256 p.
Korsakov V. S. Fundamentals of designing devices in mechanical engineering. M.: Mechanical Engineering, -1971. 288 p.
Kosov N.P. Machine tools for parts of complex shape.
M.: Mechanical Engineering, 1973, 232 p.
Kuznetsov V. S., Ponomarev V., A. Universal prefabricated devices in mechanical engineering. M.: Mechanical Engineering, 1974, 156 p.
Kuznetsov Yu. I. Technological equipment for machines with software
management. M.: Mechanical Engineering, 1976, 224 p.
Fundamentals of mechanical engineering technology./Ed. V. S. Korsakova. M.:
Mechanical engineering. 1977, p. 416.
Firago V.P. Fundamentals of design of technological processes and devices, M.: Mechanical Engineering, 1973. 467 p.
Terlikova T.F. and others. Fundamentals of designing devices: Textbook. manual for mechanical engineering universities. / T.F. Terlikova, A.S. Melnikov, V.I. Batalov. M.: Mechanical Engineering, 1980. – 119 p., ill.
Machine tools: Directory. In 2 volumes / ed. Advice: B.N. Vardashkin (pres.) and others - M.: Mashinostroenie, 1984. 4. Purpose of clamps and features of their designs depending on the device design
ELEMENTARY CLAMPING MECHANISMS SIMPLE SCREW Clamping screws
With split washer or strip
Bayonet or plunger
ECCENTRIC Round eccentrics
Curvilinear involute
Curvilinear according to the Archimedes spiral
WEDGE With a flat single bevel wedge
With support roller and wedge
With double bevel wedge
LEVER Single-arm
Double-armed
Curved double arms
COMBINED CENTERING CLAMPING ELEMENTS Collets
Expanding mandrels
Clamping sleeves with hydroplastic
Mandrels and chucks with leaf springs
Diaphragm cartridges
RACK AND LEVER CLAMPS With roller clamp and lock
With conical locking device
With eccentric locking device
COMBINED CLAMPING DEVICES Lever and screw combination
Combination of lever and eccentric
Articulating lever mechanism
SPECIAL Multi-place and continuous action
