Eccentric clamps, as opposed to screw clamps, are quick-acting. It is enough to rotate the handle of such a clamp less than 180° to secure the workpiece.
The operation diagram of the eccentric clamp is shown in Figure 7. When the handle is turned, the radius of rotation of the eccentric increases, the gap between it and the part (or lever) decreases to zero; The workpiece is clamped by further “compacting” the system: eccentric - part - fixture.
Figure 7 - Scheme of eccentric clamp operation
To determine the main dimensions of the eccentric, you should know the magnitude of the workpiece clamping force Q, optimal angle rotation of the handle for clamping the workpiece ρ, tolerance for the thickness of the workpiece being fixed δ.
If the angle of rotation of the lever is unlimited (360°), then the magnitude of the cam eccentricity can be determined by the equation
where S 1 is the installation gap under the eccentric, mm;
S 2 - eccentric power reserve, taking into account its wear, mm;
Tolerance for workpiece thickness, mm;
Q – workpiece clamping force, N ;
L - clamping device rigidity, N /mm(characterizes the amount of spin of the system under the influence of clamping forces).
If the angle of rotation of the lever is limited (less than 180°), then the amount of eccentricity can be determined by the equation
The radius of the outer surface of the eccentric is determined from the condition of self-braking: the angle of rise of the eccentric, made up by the clamped surface and the normal to the radius of its rotation, must always be less than the friction angle, i.e.
(f=0.15 for steel),
Where D And R- the diameter and radius of the eccentric, respectively.
The clamping force of the workpiece can be determined by the formula
Where R - force on the eccentric handle, N (usually accepted ~ 150 N );
l - handle length, mm;
– friction angles between the eccentric and the part, between the trunnion and the eccentric support;
R 0 - eccentric rotation radius, mm.
To approximate the clamping force, you can use the empirical formula Q12 R(at t=(4- 5) R and P=150 N) .
a, b - for pressed flat workpieces; b- for fastening flat workpieces using a swinging beam; G- for tightening the shells using a flexible clamp
Figure 8 - Examples of eccentric clamps of various designs
TaskNo. 3 “Calculation of eccentric clamp parameters”
Using the tutor’s input data, select and calculate the parameters of the eccentric clamp (Figure 7), if the product must be pressed with force Q, clamping device rigidity L, the angle of rotation of the lever is unlimited, the installation gap under the eccentric S 1, the power reserve of the eccentric, taking into account its wear S 2, tolerance for the thickness of the workpiece, the welder is right-handed .
Calculate the diameter of the eccentric.
Determine the length of the eccentric handle l.
Draw a sketch of the clamp. Select the material from which the clamp should be made.
Table 4 - Problem options
|
Q, kN |
L, N/mm |
S 1 , mm |
S 2 , mm |
Eccentric clamps are easy to manufacture and for this reason they are widely used in machine tools. The use of eccentric clamps can significantly reduce the time for clamping a workpiece, but the clamping force is inferior to threaded clamps.
Eccentric clamps performed in combination with and without clamps.
Consider an eccentric clamp with a clamp.
Eccentric clamps cannot work with significant tolerance deviations (±δ) of the workpiece. For large tolerance deviations, the clamp requires constant adjustment with screw 1.
| Eccentric calculation |
The materials used for the manufacture of the eccentric are U7A, U8A With
heat treatment to HR from 50....55 units, steel 20X with carburization to a depth of 0.8... 1.2 With hardening HR from 55...60 units.
Let's look at the eccentric diagram. The KN line divides the eccentric into two? symmetrical halves consisting, as it were, of 2 x wedges screwed onto the “initial circle”.
The eccentric rotation axis is shifted relative to its geometric axis by the amount of eccentricity “e”.
Section Nm of the lower wedge is usually used for clamping.
Considering the mechanism as a combined one consisting of a lever L and a wedge with friction on two surfaces on the axis and point “m” (clamping point), we obtain a force relationship for calculating the clamping force.
where Q is the clamping force
P - force on the handle
L - handle shoulder
r - distance from the eccentric rotation axis to the point of contact With
workpiece
α - angle of rise of the curve
α 1 - friction angle between the eccentric and the workpiece
α 2 - friction angle on the eccentric axis
To avoid the eccentric moving away during operation, it is necessary to observe the condition of self-braking of the eccentric
where α -
sliding friction angle at the point of contact with the workpiece ø -
friction coefficient
For approximate calculations of Q - 12P, consider the diagram of a double-sided clamp with an eccentric
 |
Wedge clamps
Wedge clamping devices are widely used in machine tools. Their main element is one, two and three bevel wedges. The use of such elements is due to the simplicity and compactness of the designs, speed of action and reliability in operation, the possibility of using them as clamping element, acting directly on the workpiece being fixed, and as an intermediate link, for example, an amplifier link in other clamping devices. Typically self-braking wedges are used. The condition for self-braking of a single-bevel wedge is expressed by the dependence
α > 2ρ
Where α - wedge angle
ρ - the angle of friction on the surfaces G and H of contact between the wedge and the mating parts.
Self-braking is ensured at angle α = 12°, however, to prevent vibrations and load fluctuations during the use of the clamp from weakening the workpiece, wedges with an angle α are often used<12°.
Due to the fact that decreasing the angle leads to increased
self-braking properties of the wedge, it is necessary when designing the drive to the wedge mechanism to provide devices that facilitate the removal of the wedge from the working state, since releasing a loaded wedge is more difficult than bringing it into the working state.
This can be achieved by connecting the actuator rod to a wedge. When rod 1 moves to the left, it passes path “1” to idle, and then, hitting pin 2, pressed into wedge 3, pushes the latter out. When the rod moves back, it also pushes the wedge into the working position by hitting the pin. This should be taken into account in cases where the wedge mechanism is driven by a pneumatic or hydraulic drive. Then, to ensure reliable operation of the mechanism, different pressures of liquid or compressed air should be created on different sides of the drive piston. This difference when using pneumatic actuators can be achieved by using a pressure reducing valve in one of the tubes supplying air or liquid to the cylinder. In cases where self-braking is not required, it is advisable to use rollers on the contact surfaces of the wedge with the mating parts of the device, thereby facilitating the insertion of the wedge into its original position. In these cases, it is necessary to lock the wedge.
It’s hard to imagine a carpentry workshop without a circular saw, since the most basic and common operation is longitudinal sawing of workpieces. How to make a homemade circular saw will be discussed in this article.
Introduction
The machine consists of three main structural elements:
- base;
- sawing table;
- parallel stop.
The base and the sawing table itself are not very complex structural elements. Their design is obvious and not so complicated. Therefore, in this article we will consider the most complex element - the parallel stop.
So, the rip fence is a moving part of the machine, which is a guide for the workpiece and it is along it that the workpiece moves. Accordingly, the quality of the cut depends on the parallel stop because if the stop is not parallel, then either the workpiece or the saw blade may become jammed.
In addition, the parallel stop of a circular saw must be of a rather rigid structure, since the master makes efforts to press the workpiece against the stop, and if the stop is displaced, this will lead to non-parallelism with the consequences indicated above.
There are various designs of parallel stops depending on the methods of attaching it to the circular table. Here is a table with the characteristics of these options.
| Rip fence design | Advantages and disadvantages |
| Two-point mounting (front and rear) | Advantages:· Quite rigid design, · Allows you to place the stop anywhere on the circular table (to the left or right of the saw blade); Does not require the massiveness of the guide itself Flaw:· To fasten it, the master needs to clamp one end in front of the machine, and also go around the machine and secure the opposite end of the stop. This is very inconvenient when selecting the required position of the stop and with frequent readjustment it is a significant drawback. |
| Single point mounting (front) | Advantages:· Less rigid design than when attaching the stop at two points, · Allows you to place the stop anywhere on the circular table (to the left or right of the saw blade); · To change the position of the stop, it is enough to fix it on one side of the machine, where the master is located during the sawing process. Flaw:· The design of the stop must be massive to ensure the necessary rigidity of the structure. |
| Fastening in the groove of a circular table | Advantages:· Fast changeover. Flaw:· Complexity of the design, · Weakening of the circular table structure, · Fixed position from the line of the saw blade, · Quite a complex design for self-production, especially from wood (made only from metal). |
In this article we will examine the option of creating a parallel stop design for a circular saw with one attachment point.
Preparing for work
Before you begin, you need to decide on the necessary set of tools and materials that will be needed during the work process.
The following tools will be used for work:
- Circular saw or you can use.
- Screwdriver.
- Grinder (Angle grinder).
- Hand tools: hammer, pencil, square.
During the work you will also need the following materials:
- Plywood.
- Solid pine.
- Steel tube with an internal diameter of 6-10 mm.
- Steel rod with an outer diameter of 6-10 mm.
- Two washers with an increased area and an internal diameter of 6-10 mm.
- Self-tapping screws.
- Wood glue.
Design of a circular saw stop
The entire structure consists of two main parts - longitudinal and transverse (meaning relative to the plane of the saw blade). Each of these parts is rigidly connected to the other and is a complex structure that includes a set of parts.
The pressing force is large enough to ensure the strength of the structure and securely fix the entire rip fence.
From a different angle.
The general composition of all parts is as follows:
- The base of the transverse part;
- Longitudinal part
- , 2 pcs.);
- The base of the longitudinal part;
- Clamp
- Eccentric handle
Making a circular saw
Preparation of blanks
A couple of points to note:
- flat longitudinal elements are made from, and not from solid pine, like other parts.
We drill a 22 mm hole in the end for the handle.
It is better to do this by drilling, but you can simply hammer it with a nail.
The circular saw used for work uses a homemade movable carriage made of (or, as an option, you can whip up a false table), which is not too bad to be deformed or damaged. We hammer a nail into this carriage in the marked place and bite off the head.
As a result, we get a smooth cylindrical workpiece that needs to be processed with a belt or eccentric sander.
We make a handle - it is a cylinder with a diameter of 22 mm and a length of 120-200 mm. Then we glue it into the eccentric.
Transverse part of the guide
Let's start making the transverse part of the guide. It consists, as mentioned above, of the following details:
- The base of the transverse part;
- Upper transverse clamping bar (with an oblique end);
- Lower transverse clamping bar (with an oblique end);
- End (fixing) strip of the transverse part.
Upper transverse clamping bar
Both clamping bars - upper and lower - have one end that is not straight 90º, but inclined (“oblique”) with an angle of 26.5º (to be precise, 63.5º). We have already observed these angles when cutting the workpieces.
The upper transverse clamping bar serves to move along the base and further fix the guide by pressing against the lower transverse clamping bar. It is assembled from two blanks.
Both clamping bars are ready. It is necessary to check the smoothness of the ride and remove all defects that interfere with smooth sliding; in addition, you need to check the tightness of the inclined edges; There should be no gaps or cracks.
With a tight fit, the strength of the connection (fixation of the guide) will be maximum.
Assembling the entire transverse part
Longitudinal part of the guide
The entire longitudinal part consists of:
- , 2 pcs.);
- The base of the longitudinal part.
This element is made from the fact that the surface is laminated and smoother - this reduces friction (improves sliding), and is also denser and stronger - more durable.
At the stage of forming the blanks, we have already sawed them to size, all that remains is to refine the edges. This is done using edge tape.
The edging technology is simple (you can even glue it with an iron!) and understandable.
The base of the longitudinal part
We also additionally fix it with self-tapping screws. Do not forget to maintain a 90º angle between the longitudinal and vertical elements.
Assembly of transverse and longitudinal parts.
Right here VERY!!! It is important to maintain an angle of 90º, since the parallelism of the guide with the plane of the saw blade will depend on it.
Installation of the eccentric
Installing the guide
It's time to attach our entire structure to the circular saw. To do this, you need to attach the cross stop bar to the circular table. Fastening, as elsewhere, is carried out using glue and self-tapping screws.
... and we consider the work finished - the circular saw is ready with your own hands.
Video
Video on which this material was made.
Easy to manufacture, with a high gain, a fairly compact eccentric clamp, which is a type of cam mechanisms, has another, undoubtedly, main advantage...
... – instantaneous performance. If in order to “turn on and off” a screw clamp it is often necessary to make at least a couple of turns in one direction and then in the other, then when using an eccentric clamp it is enough to turn the handle only a quarter turn. Of course, they are superior to eccentric ones in terms of clamping force and working stroke, but with a constant thickness of the fastened parts in mass production, the use of eccentrics is extremely convenient and effective. The widespread use of eccentric clamps, for example, in stocks for assembling and welding small-sized metal structures and elements of non-standard equipment, significantly increases labor productivity.
The working surface of the cam is most often made in the form of a cylinder with a circle or Archimedes spiral at the base. Later in the article we will talk about the more common and more technologically advanced round eccentric clamp.
The dimensions of eccentric round cams for machine tools are standardized in GOST 9061-68*. The eccentricity of the round cams in this document is set to 1/20 of the outer diameter to ensure self-braking conditions over the entire operating range of rotation angles at a friction coefficient of 0.1 or more.
The figure below shows the geometric diagram of the clamping mechanism. The fixed part is pressed against the supporting surface as a result of turning the eccentric handle counterclockwise around an axis rigidly fixed relative to the support.
The position of the mechanism shown is characterized by the maximum possible angle α , while the straight line passing through the axis of rotation and the center of the eccentric circle is perpendicular to the straight line drawn through the point of contact of the part with the cam and the center point of the outer circle.
If you turn the cam 90˚ clockwise relative to the position shown in the diagram, then a gap is formed between the part and the working surface of the eccentric equal in magnitude to the eccentricity e. This clearance is necessary for free installation and removal of the part.
Program in MS Excel:
In the example shown in the screenshot, based on the given dimensions of the eccentric and the force applied to the handle, the mounting size from the axis of rotation of the cam to the supporting surface is determined, taking into account the thickness of the part, the self-braking condition is checked, the clamping force and the force transfer coefficient are calculated.
The value of the friction coefficient “part - eccentric” corresponds to the case “steel on steel without lubrication”. The value of the friction coefficient “axle - eccentric” is selected for the “steel on steel with lubrication” option. Reducing friction in both places increases the power efficiency of the mechanism, but reducing friction in the area of contact between the part and the cam leads to the disappearance of self-braking.
Algorithm:
9. φ 1 =arctg (f 1 )
10. φ 2 =arctg (f 2 )
11. α =arctg (2*e /D )
12. R =D/ (2*cos (α ))
13. A =s +R *cos (α )
14. e ≤ R*f 1+ (d/2)* f 2
If the condition is met, self-braking is ensured.
15. F = P * L * cos(α )/(R * tg(α +φ 1 )+(d /2)* tg(φ 2 ))
1 6 . k = F/P
Conclusion.
The position of the eccentric clamp chosen for calculations and shown in the diagram is the most “unfavorable” from the point of view of self-braking and gain in strength. But this choice is not accidental. If in such a working position the calculated power and geometric parameters satisfy the designer, then in any other positions the eccentric clamp will have an even greater force transmission coefficient and better self-braking conditions.
When designing, moving away from the considered position towards reducing the size A if other dimensions are kept unchanged, it will reduce the gap for installing the part.
Increase in size A can create a situation where the eccentric wears out during operation and significant fluctuations in thickness s, when it is simply impossible to clamp the part.
The article has deliberately not mentioned anything so far about the materials from which the cams can be made. GOST 9061-68 recommends using wear-resistant surface-cemented steel 20X to increase durability. But in practice, an eccentric clamp is made from a wide variety of materials, depending on the purpose, operating conditions and available technological capabilities. The above calculation in Excel allows you to determine the parameters of clamps for cams made of any materials, just remember to change the values of the friction coefficients in the initial data.
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The eccentric clamp is an improved design clamping element. Eccentric clamps (ECC) are used for direct clamping of workpieces and in complex clamping systems.
Manual screw clamps are simple in design, but have a significant drawback - to secure the part, the worker must perform a large number of rotational movements with a key, which requires additional time and effort and, as a result, reduces labor productivity.
The above considerations force, where possible, to replace manual screw clamps with quick-release clamps.
The most widespread are also.
Although it is fast-acting, it does not provide high clamping force on the part, so it is used only for relatively small cutting forces.
Advantages:
- simplicity and compactness of design;
- widespread use of standardized parts in the design;
- ease of setup;
- ability to self-braking;
- speed (drive response time is about 0.04 min).
Flaws:
- the concentrated nature of the forces, which does not allow the use of eccentric mechanisms for securing non-rigid workpieces;
- the clamping forces with round eccentric cams are unstable and significantly depend on the size of the workpieces;
- reduced reliability due to intensive wear of the eccentric cams.
Rice. 113. Eccentric clamp: a - the part is not clamped; b - position with clamped part
Eccentric Clamp Design
Round eccentric 1, which is a disk with a hole offset relative to its center, is shown in Fig. 113, a. The eccentric is freely mounted on axis 2 and can rotate around it. The distance e between the center C of disk 1 and the center O of the axis is called eccentricity.
A handle 3 is attached to the eccentric, by turning which the part is clamped at point A (Fig. 113, b). From this figure it can be seen that the eccentric works like a curved wedge (see shaded area). To prevent the eccentrics from moving away after clamping, they must be self-braking. The self-braking property of eccentrics is ensured by the correct choice of the ratio of the diameter D of the eccentric to its eccentricity e. The ratio D/e is called the eccentric characteristic.
With a friction coefficient f = 0.1 (friction angle 5°43"), the eccentric characteristic should be D/e ≥ 20, and with a friction coefficient f = 0.15 (friction angle 8°30") D/e ≥ 14.
Thus, all eccentric clamps, whose diameter D is 14 times greater than the eccentricity e, have the property of self-braking, i.e., they provide reliable clamping.
Figure 5.5 - Schemes for calculating eccentric cams: a – round, non-standard; b- made according to the Archimedes spiral.
Eccentric clamping mechanisms include eccentric cams, supports for them, trunnions, handles and other elements. There are three types of eccentric cams: round with a cylindrical working surface; curved, the working surfaces of which are outlined along an Archimedes spiral (less often - along an involute or logarithmic spiral); end
Round eccentrics
Due to ease of manufacture, round eccentrics are most widespread.
A round eccentric (in accordance with Figure 5.5a) is a disk or roller rotated around an axis displaced relative to the geometric axis of the eccentric by an amount A, called eccentricity.
Curvilinear eccentric cams (in accordance with Figure 5.5b) compared to round ones provide stable clamping force and a larger (up to 150°) rotation angle.
Cam materials
Eccentric cams are made of steel 20X, carburized to a depth of 0.8...1.2 mm and hardened to a hardness of HRCe 55-61.
Eccentric cams are distinguished by the following designs: round eccentric (GOST 9061-68), eccentric (GOST 12189-66), double eccentric (GOST 12190-66), eccentric forked (GOST 12191-66), eccentric double-bearing (GOST 12468-67) .
The practical use of eccentric mechanisms in various clamping devices is shown in Figure 5.7
Figure 5.7 - Types of eccentric clamping mechanisms
Calculation of eccentric clamps
The initial data for determining the geometric parameters of the eccentrics are: tolerance δ of the size of the workpiece from its mounting base to the place where the clamping force is applied; angle a of rotation of the eccentric from the zero (initial) position; required force FZ of clamping the part. The main design parameters of eccentrics are: eccentricity A; diameter dc and width b of the eccentric pin (axis); eccentric outer diameter D; width of the working part of the eccentric B.
Calculations of eccentric clamping mechanisms are performed in the following sequence:
Calculation of clamps with a standard eccentric round cam (GOST 9061-68)
1. Determine the move hTo eccentric cam, mm:
If the angle of rotation of the eccentric cam is not limited (a ≤ 130°), then
where δ is the tolerance of the workpiece size in the clamping direction, mm;
Dgar = 0.2…0.4 mm – guaranteed clearance for convenient installation and removal of the workpiece;
J = 9800…19600 kN/m – rigidity of eccentric EZM;
D = 0.4...0.6 hk mm – power reserve, taking into account wear and manufacturing errors of the eccentric cam.
If the angle of rotation of the eccentric cam is limited (a ≤ 60°), then
2. Using tables 5.5 and 5.6, select a standard eccentric cam. In this case, the following conditions must be met: Fz ≤ Fh max and hTo≤ h(dimensions, material, heat treatment and other technical conditions in accordance with GOST 9061-68. There is no need to check the standard eccentric cam for strength.
Table 5.5 - Standard round eccentric cam (GOST 9061-68)
Designation | Outer eccentric cam, mm | Eccentricity, | Cam stroke h, mm, not less | |||
Angle of rotation limited to a≤60° | Angle of rotation limited to a≤130° |
|||||
Note: For eccentric cams 7013-0171...1013-0178, the values of F3 max and Mmax are calculated based on the strength parameter, and for the rest - taking into account ergonomic requirements with a maximum handle length of L = 320 mm. |
||||||
3. Determine the length of the eccentric mechanism handle, mm
Values M max and P z max are selected according to table 5.5.
Table 5.6 - Round eccentric cams (GOST 9061-68). Dimensions, mm
Drawing - drawing of an eccentric cam
DIY eccentric clamp
The video will show you how to make a homemade eccentric clamp designed for fixing a workpiece. Do-it-yourself eccentric clamp.
