The purpose of clamping devices is to ensure reliable contact of the workpiece with the installation elements and to prevent its displacement and vibration during processing. Figure 7.6 shows some types of clamping devices.
Requirements for clamping elements:
Reliability in operation;
Simplicity of design;
Ease of maintenance;
Should not cause deformation of workpieces and damage to their surfaces;
The workpiece should not be moved during its fastening from the installation elements;
Fastening and detaching workpieces must be done with minimum cost labor and time;
The clamping elements must be wear-resistant and, if possible, replaceable.
Types of clamping elements:
Clamping screws, which are rotated with keys, handles or handwheels (see Fig. 7.6)
Fig.7.6 Types of clamps:
a – clamping screw; b – screw clamp
Fast acting clamps shown in fig. 7.7.
Fig.7.7. Types of quick release clamps:
a – with a split washer; b – with a plunger device; c – with folding stop; g – with a lever device
Eccentric clamps, which are round, involute and spiral (along the Archimedes spiral) (Fig. 7.8).
Fig.7.8. Types of eccentric clamps:
a – disk; b – cylindrical with an L-shaped clamp; g – conical floating.
Wedge clamps– the wedging effect is used and is used as an intermediate link in complex clamping systems. At certain angles, the wedge mechanism has the property of self-braking. In Fig. Figure 7.9 shows the calculated diagram of the action of forces in the wedge mechanism.
Rice. 7.9. Calculation diagram of forces in the wedge mechanism:
a- one-sided; b – double-skewed
Lever Clamps used in combination with other clamps to form more complex clamping systems. Using the lever, you can change both the magnitude and direction of the clamping force, as well as simultaneously and uniformly secure the workpiece in two places. In Fig. Figure 7.10 shows a diagram of the action of forces in lever clamps.
Rice. 7.10. Diagram of the action of forces in lever clamps.
Collets They are split spring sleeves, the varieties of which are shown in Fig. 7.11.
Rice. 7. 11. Types of collet clamps:
a – with a tension tube; b – with a spacer tube; V - vertical type
Collets ensure concentricity of workpiece installation within 0.02...0.05 mm. The base surface of the workpiece for collet clamps should be processed according to accuracy classes 2…3. Collets are made of high-carbon steels of type U10A with subsequent heat treatment to a hardness of HRC 58...62. Collet cone angle d = 30…40 0 . At smaller angles, the collet may jam.
Expanding mandrels, the types of which are shown in Fig. 7.4.
Roller lock(Fig. 7.12)
Rice. 7.12. Types of roller locks
Combination clamps– combination of elementary clamps various types. In Fig. 7.13 shows some types of such clamping devices.
Rice. 7.13. Types of combined clamping devices.
Combination clamping devices are operated manually or by power devices.
Guide elements of devices
When performing some machining operations (drilling, boring), the rigidity cutting tool And technological system in general it turns out to be insufficient. To eliminate elastic pressing of the tool relative to the workpiece, guide elements are used (guide bushings when boring and drilling, copiers when processing shaped surfaces, etc. (see Fig. 7.14).
Fig.7.14. Types of conductor bushings:
a – constant; b – replaceable; c – quick-change
Guide bushings are made of steel grade U10A or 20X, hardened to a hardness of HRC 60...65.
Guide elements of devices - copiers - are used when processing shaped surfaces complex profile, whose task is to guide the cutting tool along the processed surface of the workpiece to obtain the specified accuracy of the trajectory of their movement.
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
At machining The following applies to the workpiece:
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 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, he must have minimum 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 fixture clamping devices 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 clamping devices manual drive(single 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 uniformly distributed load q, directly acting on the workpiece and created either as a result atmospheric pressure, or by means of 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
| 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 |
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 are powered by 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
To reduce the time for installation, alignment and clamping of parts, it is advisable to use special (designed for processing a given part) clamping devices. It is especially advisable to use special devices when producing large batches of identical parts.
Special clamping fixtures may have screw, eccentric, pneumatic, hydraulic or air-hydraulic clamping.
Single device diagram
Since devices must quickly and reliably secure the workpiece, it is preferable to use such clamps when clamping of one workpiece in several places is simultaneously achieved. Ha fig. 74 shows a clamping device for a body part, in which the clamping is carried out simultaneously by two clamps 1 And 6 on both sides of the part by tightening one nut 5 . When tightening the nut 5 pin 4 having a double bevel in the die 7 , through traction 8 affects the bevel of the die 9 and presses it with a nut 2 sticking 1 sitting on a pin 3 . The direction of the clamping force is shown by arrows. When unscrewing the nut 5 springs placed under clamps 1 And b, lift them, freeing the part.
Single clamping fixtures are used for large parts, while for small parts it is more appropriate to use fixtures in which several workpieces can be installed and clamped at the same time. Such devices are called multi-seat.
Multi-person devices
Fastening several workpieces with one clamp reduces the time for fastening and is used when working on multi-place devices.
In Fig. 75 shows a diagram of a double device for clamping two rollers when milling keyways. Clamping is done with a handle 4
with an eccentric that simultaneously presses the clamp 3
and through traction 5
for sticking 1
, thereby pressing both workpieces against the prisms in the body 2
devices. The rollers are released by turning the handle 4
V reverse side. At the same time, the springs 6
pull back the clamps 1
And 3
.
In Fig. 76 shows a multi-seat device with a pneumatic piston power drive. Compressed air enters through a three-way valve either into the upper cavity of the cylinder, clamping the workpieces (the direction of the clamping force is shown by arrows), or into the lower cavity of the cylinder, releasing the workpieces.
The described device uses a cassette method for installing parts. Several workpieces, for example, in this case five, are installed in a cassette, while another batch of the same workpieces is already processed in the cassette. After processing is completed, the first cassette with milled parts is removed from the device and another cassette with blanks is installed in its place. The cassette method allows you to reduce the time for installing workpieces.
In Fig. 77 shows the design of a multi-position clamping device with a hydraulic drive.
Base 1
drive is fixed on the machine table. In a cylinder 3
the piston moves 4
, in the groove of which a lever is installed 5
, rotating around an axis 8
, fixedly fixed in the eyelet 7
. Lever arm ratio 5 is 3:1. At 50 oil pressure kg/cm 2 and piston diameter 55 mm force at the short end of the lever arm 5
reaches 2800 kg. To protect against chips, a fabric casing 6 is placed on the lever.
Oil flows through a three-way control valve into the valve 2
and further into the upper cavity of the cylinder 3
. Oil from the opposite cavity of the cylinder through a hole in the base 1
enters the three-way valve and then goes to the drain.
When turning the handle three way valve in the clamping position, oil under pressure acts on the piston 4
, transmitting the clamping force through the lever 5
fork lever 9
clamping device that rotates on two axle shafts 10
. Finger 12
, pressed into lever 9, turns the lever 11
relative to the point of contact of the screw 21
with the device body. In this case, the axis 13
lever moves the rod 14
to the left and through the spherical washer 17
and nuts 18
transfers the clamping force to the clamp 19
, rotating around an axis 16
and pressing the workpieces to a stationary jaw 20
. The clamping size is adjusted using nuts 18
and screw 21
.
When turning the handle of the three-way valve to the release position, the lever 11
will turn in the opposite direction, moving the rod 14
to the right. In this case the spring 15
removes the stick 19
from blanks.
IN Lately pneumohydraulic clamping devices are used, in which compressed air coming from the factory network with a pressure of 4-6 kg/cm 2 presses on the piston of the hydraulic cylinder, creating an oil pressure of about 40-80 in the system kg/cm 2. Oil with such pressure, using clamping devices, secures the workpieces with great force.
An increase in the pressure of the working fluid allows, with the same clamping force, to reduce the size of the vice drive.
Rules for selecting clamping devices
When choosing the type of clamping fixtures, the following rules should be followed.
The clamps must be simple, fast-acting and easily accessible for actuating them, sufficiently rigid and not loosen spontaneously under the action of a cutter, from vibrations of the machine or due to random reasons, and must not deform the surface of the workpiece and cause it to spring back. The clamping force in the clamps is counteracted by a support and, if possible, should be directed so as to help press the workpiece against the supporting surfaces during processing. To do this, the clamping fixtures should be installed on the machine table so that the cutting force generated during the milling process is absorbed by the stationary parts of the fixture, for example, the stationary jaw of a vice.
In Fig. 78 shows diagrams for installing the clamping device.
When milling against feed and counter-clockwise rotation cylindrical cutter The clamping force should be directed as shown in Fig. 78, a, and with right rotation - as in Fig. 78, b.
When milling with an end mill, depending on the feed direction, the clamping force should be directed, as shown in Fig. 78, in or fig. 78, city
With this arrangement of the device, the clamping force is opposed by a rigid support and the cutting force helps to press the workpiece against the supporting surface during processing.
Clamping elements hold the workpiece workpiece from displacement and vibrations arising under the influence of cutting forces.
Classification of clamping elements
The clamping elements of devices are divided into simple and combined, i.e. consisting of two, three or more interlocked elements.
Simple ones include wedge, screw, eccentric, lever, lever-hinge, etc. - called clamps.
Combined mechanisms are usually designed as screw-type
lever, eccentric-lever, etc. and are called tacks.
When to use simple or combined
mechanisms in arrangements with mechanized drive
(pneumatic or other) they are called mechanisms - amplifiers. Based on the number of driven links, the mechanisms are divided: 1. single-link - clamping the workpiece at one point;
2. two-link - clamping two workpieces or one workpiece at two points;
3. multi-link - clamping one workpiece at many points or several workpieces simultaneously with equal forces. By degree of automation:
1. manual - working with a screw, wedge and others
buildings;
2. mechanized, in
are divided into
a) hydraulic,
b) pneumatic,
c) pneumohydraulic,
d) mechanohydraulic,
d) electric,
e) magnetic,
g) electromagnetic,
h) vacuum.
3. automated, controlled from the working parts of the machine. They are driven by the machine table, support, spindle and centrifugal forces of rotating masses.
Example: centrifugal-energy chucks for semi-automatic lathes.
Requirements for clamping devices
They must be reliable in operation, simple in design and easy to maintain; should not cause deformation of the workpieces being fixed and damage to their surfaces; fastening and unfastening of workpieces should be carried out with a minimum expenditure of effort and working time, especially when securing several workpieces in multi-place devices; in addition, clamping devices should not move the workpiece during the process of securing it. Cutting forces should, if possible, not be absorbed by clamping devices. They should be perceived as more rigid installation elements of devices. To improve processing accuracy, devices that provide a constant clamping force are preferred.
Let's take a short excursion to theoretical mechanics. Let's remember what is the coefficient of friction?
If a body of weight Q moves along a plane with a force P, then the reaction to the force P will be a force P 1 directed in the opposite direction, that is
slip.
Friction coefficient
Example: if f = 0.1; Q = 10 kg, then P = 1 kg.
The coefficient of friction varies depending on the surface roughness.
Method for calculating clamping forces
First case
Second case
The cutting force P z and the clamping force Q are directed in the same direction
In this case Q => O
The cutting force P g and the clamping force Q are directed in opposite directions, then Q = k * P z
where k is the safety factor k = 1.5 finishing k = 2.5 roughing.
Third case
The forces are directed mutually perpendicularly. The cutting force P counteracts the friction force on the support (installation) Qf 2 and the friction force at the clamping point Q*f 1, then Qf 1 + Qf 2 = k*P z
G 
de f, and f 2 - sliding friction coefficients Fourth case
The workpiece is processed in a three-jaw chuck
In this direction, P tends to move the workpiece relative to the cams.
Calculation of threaded clamping mechanisms First case
Flat head screw clamp From equilibrium condition 
where P is the force on the handle, kg; Q - clamping force of the part, kg; R cp - average thread radius, mm;
R - radius of the supporting end;
Helix angle of thread;
Friction angle in threaded connection 6; 
- self-braking condition; f is the friction coefficient of the bolt on the part;
0.6 - coefficient taking into account the friction of the entire surface of the end. The moment P*L overcomes the moment of the clamping force Q, taking into account the friction forces in screw pair and at the end of the bolt.
Second case
■ Bolt clamp with spherical surface
With increasing angles α and φ, the force P increases, because in this case, the direction of the force goes up the inclined plane of the thread.
Third case
This clamping method is used when processing bushings or disks on mandrels: lathes, dividing heads or rotary tables on milling machines, slotting machines or other machines, gear hobbing, gear shaping, radial drilling machines, etc. Some information from the directory:
The Ml6 screw with a spherical end with a handle length L = 190 mm and a force P = 8 kg, develops a force Q = 950 kg
Clamping with a screw M = 24 with a flat end at L = 310 mm; P = 15kg; Q = 1550mm
Clamp with hex nut Ml 6 and wrench L = 190mm; P = 10kg; Q = 700kg.
Eccentric clamps are easy to manufacture for this reason we found wide application V 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 are made 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
M
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
Condition for self-braking of the eccentric. = 12Р
about chyazhima with expentoik
G
de 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 a 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.
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. At reverse stroke The rod 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 fluid pressures should be created or compressed air With different sides 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.
Let us consider the diagram of the action of forces in a single-skew, most often used in devices, wedge mechanism
Let's construct a force polygon.
When transmitting forces at right angles, we have the following relationship
+ pinning, - unpinning
Self-braking occurs at α
Collet clamps
The collet clamping mechanism has been known for a long time. Securing workpieces using collets turned out to be very convenient when creating automated machines because to secure the workpiece, only one translational movement of the clamped collet is required.
When operating collet mechanisms, the following requirements must be met.
The clamping forces must be ensured in accordance with the emerging cutting forces and prevent movement of the workpiece or tool during the cutting process.
The clamping process in the general processing cycle is an auxiliary movement, so the response time of the collet clamp should be minimal.
The dimensions of the clamping mechanism links must be determined from the conditions of their normal operation when securing workpieces of both the largest and smallest sizes.
The positioning error of the workpieces or tools being fixed should be minimal.
The design of the clamping mechanism should provide the least elastic pressure during the processing of workpieces and have high vibration resistance.
The collet parts and especially the collet must have high wear resistance.
The design of the clamping device must allow its quick change and convenient adjustment.
The design of the mechanism must provide protection for the collets from chips.
The practically minimum acceptable size for fastening is 0.5 mm. On
multi-spindle bar automatic machines, bar diameters, and
therefore, the collet holes reach 100 mm. Collets with a large hole diameter are used to secure thin-walled pipes, because... relatively uniform fastening over the entire surface does not cause large deformations pipes
The collet clamping mechanism allows you to secure workpieces various shapes cross section.
The durability of collet clamping mechanisms varies widely and depends on the design and correctness technological processes in the manufacture of mechanism parts. As a rule, clamping collets fail before others. In this case, the number of fastenings with collets ranges from one (breakage of the collet) to half a million or more (wear of the jaws). The performance of a collet is considered satisfactory if it is capable of securing at least 100,000 workpieces.
Classification of collets
All collets can be divided into three types:
1. Collets of the first type have a “straight” cone, the top of which faces away from the machine spindle.
To secure it, it is necessary to create a force that pulls the collet into the nut screwed onto the spindle. Positive traits This type of collet is structurally quite simple and works well in compression (hardened steel has a higher permissible stress in compression than in tension. Despite this, collets of the first type are currently of limited use due to disadvantages. What are these disadvantages:
a) the axial force acting on the collet tends to unlock it,
b) when feeding the bar, premature locking of the collet is possible,
c) when secured with such a collet, harmful effects on
d) there is unsatisfactory centering of the collet in
spindle, since the head is centered in the nut, the position of which is on
The spindle is not stable due to the presence of threads.
Collets of the second type have a “reverse” cone, the top of which faces the spindle. To secure it, it is necessary to create a force that pulls the collet into the conical hole of the machine spindle.
Collets of this type ensure good centering of the workpieces being clamped, since the cone for the collet is located directly in the spindle and cannot
jamming occurs, the axial working forces do not open the collet, but lock it, increasing the fastening force.
At the same time, a number of significant disadvantages reduce the performance of collets of this type. Due to the numerous contacts with the collet, the conical hole of the spindle wears out relatively quickly, the threads on the collets often fail, not ensuring a stable position of the rod along the axis when fastened - it moves away from the stop. Nevertheless, collets of the second type are widely used in machine tools.
3 Clamping elements of fixtures.doc
3. Clamping elements of fixtures3.1. Selecting the location of application of clamping forces, type and number of clamping elements
When securing a workpiece in a fixture, the following basic rules must be observed:
the position of the workpiece achieved during its basing should not be disturbed;
the fastening must be reliable so that the position of the workpiece remains unchanged during processing;
The crushing of the workpiece surfaces that occurs during fastening, as well as its deformation, must be minimal and within acceptable limits.
To ensure contact of the workpiece with the support element and eliminate its possible shift during fastening, the clamping force should be directed perpendicular to the surface support element. In some cases, the clamping force can be directed so that the workpiece is simultaneously pressed against the surfaces of two supporting elements;
In order to eliminate the deformation of the workpiece during fastening, the point of application of the clamping force must be selected so that the line of its action intersects the supporting surface of the supporting element. Only when clamping particularly rigid workpieces can the line of action of the clamping force be allowed to pass between the supporting elements.
The number of points of application of clamping forces is determined specifically for each case of workpiece clamping. To reduce the crumpling of the workpiece surfaces during fastening, it is necessary to reduce specific pressure at the points of contact of the clamping device with the workpiece by dispersing the clamping force.
This is achieved by using contact elements of appropriate design in clamping devices, which make it possible to distribute the clamping force equally between two or three points, and sometimes even disperse it over a certain extended surface. TO Number of clamping points largely depends on the type of workpiece, processing method, direction of the cutting force. For decreasing vibrations and deformations of the workpiece under the influence of the cutting force, the rigidity of the workpiece-device system should be increased by increasing the number of places where the workpiece is clamped and bringing them closer to the machined surface.
3.3. Determining the type of clamping elements
Clamping elements include screws, eccentrics, clamps, vice jaws, wedges, plungers, clamps, and strips.
They are intermediate links in complex clamping systems.
3.3.1. Screw terminals
Screw terminals 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. 3.1. Screw clamps: a – with a spherical end; b – with a flat end; c – with a shoe.
The screws can be with a spherical end (fifth), flat, or with a shoe that prevents damage to the surface.
When calculating ball heel screws, only friction in the thread is taken into account.
Where: L- handle length, mm; - average thread radius, mm; - thread lead angle.
Where: S– thread pitch, mm; – reduced friction angle.
Where: Pu150 N.
Self-braking condition: .
For standard metric threads, therefore all mechanisms with metric thread self-braking.
When calculating screws with a flat heel, friction at the end of the screw is taken into account.
For the ring heel:
Where: D – outside diameter support end, mm; d – inner diameter support end, mm; – friction coefficient.
With flat ends:
For shoe screw:
Material: steel 35 or steel 45 with a hardness of HRC 30-35 and thread accuracy of the third class.
^ 3.3.2. Wedge clamps
The wedge is used in the following design options:
Flat single bevel wedge.
Double bevel wedge.
Round wedge.
Rice. 3.2. Flat single bevel wedge.
Rice. 3.3. Double bevel wedge.
Rice. 3.4. Round wedge.
4) a crank wedge in the form of an eccentric or flat cam with a working profile outlined along an Archimedean spiral;
Rice. 3.5. Crank wedge: a – in the form of an eccentric; b) – in the shape of a flat cam.
5) a screw wedge in the form of an end cam. Here the single-bevel wedge is, as it were, rolled into a cylinder: the base of the wedge forms a support, and its inclined plane- screw cam profile;
6) self-centering wedge mechanisms (chucks, mandrels) do not use systems of three or more wedges.
^ 3.3.2.1. Wedge self-braking condition
Rice. 3.6. Condition of self-braking of the wedge.
Where: - friction angle.
Where: – friction coefficient;
For a wedge with friction only on an inclined surface, the self-braking condition is:
With friction on two surfaces:
We have: ; or: ;.
Then: self-braking condition for a wedge with friction on two surfaces:
For a wedge with friction on an inclined surface only:
With friction on two surfaces:
With friction only on an inclined surface:
^ 3.3.3.Eccentric clamps
Rice. 3.7. Schemes for calculating eccentrics.
Such clamps are fast-acting, but develop less force than screw clamps. They have self-braking properties. The main disadvantage: they cannot work reliably with significant variations in size between the mounting and clamping surfaces of the workpieces.
;
Where: ( - the average value of the radius drawn from the center of rotation of the eccentric to point A of the clamp, mm; ( - the average angle of elevation of the eccentric at the clamping point; (, (1 - sliding friction angles at point A of the clamp and on the eccentric axis.
For calculations we accept:
At l 2D calculation can be done using the formula:
Condition for eccentric self-braking:
Usually accepted.
Material: steel 20X, carburized to a depth of 0.81.2 mm and hardened to HRC 50...60.
3.3.4. Collets
Collets are spring sleeves. They are used to install workpieces on external and internal cylindrical surfaces.
Where: Pz– workpiece fixing force; Q – compression force of the collet blades; - friction angle between the collet and the bushing.
Rice. 3.8. Collet.
^ 3.3.5. Devices for clamping parts such as bodies of rotation
In addition to collets, for clamping parts with a cylindrical surface, expanding mandrels, clamping bushings with hydroplastic, mandrels and chucks with disc springs, membrane chucks and others are used.
Cantilever and center mandrels are used for installation with a central base hole of bushings, rings, gears processed on multi-cutter grinding and other machines.
When processing a batch of such parts, it is necessary to obtain high concentricity of the external and internal surfaces and a specified perpendicularity of the ends to the axis of the part.
Depending on the method of installation and centering of the workpieces, cantilever and center mandrels can be divided into the following types: 1) rigid (smooth) for installing parts with a gap or interference; 2) expanding collets; 3) wedge (plunger, ball); 4) with disc springs; 5) self-clamping (cam, roller); 6) with a centering elastic bushing.
Rice. 3.9. Mandrel designs: A - smooth mandrel; b - mandrel with split sleeve.
In Fig. 3.9, A shows a smooth mandrel 2, on the cylindrical part of which the workpiece 3 is installed . Traction 6 , fixed on the pneumatic cylinder rod, when the piston with the rod moves to the left with the head 5 presses the quick-change washer 4 and clamps part 3 on a smooth frame 2 . The mandrel with its conical part 1 is inserted into the cone of the machine spindle. When clamping the workpiece on the mandrel, the axial force Q on the rod of the mechanized drive causes 4 between the ends of the washer , mandrel shoulder and workpiece 3 moment from the friction force, greater than the moment M cut from the cutting force P z. Dependence between moments:
;
Where does the force on the rod of a mechanized drive come from:
.
According to the refined formula:
.
Where: - safety factor; R z - vertical component of cutting force, N (kgf); D- outer diameter of the surface of the workpiece, mm; D 1 - outer diameter of quick-change washer, mm; d- diameter of the cylindrical mounting part of the mandrel, mm; f= 0.1 - 0.15- clutch friction coefficient.
In Fig. 3.9, b mandrel 2 shown with a split sleeve 6, on which the workpiece 3 is installed and clamped. Conical part 1 mandrel 2 is inserted into the cone of the machine spindle. The part is clamped and released on the mandrel using a mechanized drive. When compressed air is supplied to the right cavity of the pneumatic cylinder, the piston, rod and rod 7 move to the left and the head 5 of the rod with washer 4 moves the split sleeve 6 along the cone of the mandrel until it clamps the part on the mandrel. When compressed air is supplied to the left cavity of the pneumatic cylinder, the piston, rod; and the rod move to the right, head 5 with washer 4 move away from sleeve 6 and the part opens.
Fig.3.10. Cantilever mandrel with disc springs (A) and disc spring (b).
Torque from vertical cutting force P z should be less than the moment from the friction forces on cylindrical surface split bushing 6 mandrels. Axial force on the rod of a motorized drive (see Fig. 3.9, b).
;
Where: - half the angle of the mandrel cone, degrees; - friction angle on the contact surface of the mandrel with the split sleeve, deg; f=0.15-0.2- friction coefficient.
Mandrels and chucks with disc springs are used for centering and clamping along the inner or outer cylindrical surface of workpieces. In Fig. 3.10, a, b a cantilever mandrel with disc springs and a disc spring are shown respectively. The mandrel consists of a body 7, a thrust ring 2, a package of disc springs 6, a pressure sleeve 3 and a rod 1 connected to the pneumatic cylinder rod. The mandrel is used to install and secure part 5 along the inner cylindrical surface. When the piston with rod and rod 1 moves to the left, the latter, with its head 4 and sleeve 3, presses on the disc springs 6. The springs are straightened, their outer diameter increases and their inner diameter decreases, the workpiece 5 is centered and clamped.
The size of the mounting surfaces of the springs during compression can vary depending on their size by 0.1 - 0.4 mm. Consequently, the base cylindrical surface of the workpiece must have an accuracy of 2 - 3 classes.
A disc spring with slots (Fig. 3.10, b) can be considered as a set of two-link lever-joint mechanisms of double action, expanded by axial force. Having determined the torque M res on cutting force R z and choosing the safety factor TO, friction coefficient f and radius R mounting surface of the spring disc surface, we obtain the equality:
From the equality we determine the total radial clamping force acting on the mounting surface of the workpiece:
.
Axial force on the motorized actuator rod for disc springs:
With radial slots
;
Without radial slots
;
Where: - angle of inclination of the disc spring when clamping the part, degrees; K=1.5 - 2.2- safety factor; M res - torque from cutting force R z , Nm (kgf-cm); f=0.1- 0.12- coefficient of friction between the mounting surface of the disc springs and the base surface of the workpiece; R - radius of the mounting surface of the disc spring, mm; R z- vertical component of cutting force, N (kgf); R 1 - radius of the machined surface of the part, mm.
Chucks and mandrels with self-centering thin-walled bushings filled with hydroplastic are used for installation on the outside or inner surface parts processed on lathes and other machines.
On devices with a thin-walled bushing, the workpieces with their outer or inner surfaces are mounted on the cylindrical surface of the bushing. When the bushing is expanded with hydroplastic, the parts are centered and clamped.
The shape and dimensions of the thin-walled bushing must ensure sufficient deformation for reliable clamping of the part on the bushing when processing the part on the machine.
When designing chucks and mandrels with thin-walled bushings with hydroplastic, the following is calculated:
main dimensions of thin-walled bushings;
sizes of pressure screws and plungers for devices with manual clamping;
plunger sizes, cylinder diameter and piston stroke for power-driven devices.
Rice. 3.11. Thin-walled bushing.
The initial data for calculating thin-walled bushings are the diameter D d holes or workpiece neck diameter and length l d holes or necks of the workpiece.
To calculate a thin-walled self-centering bushing (Fig. 3.11), we will use the following notation: D - diameter of the mounting surface of the centering sleeve 2, mm; h- thickness of the thin-walled part of the bushing, mm; T - length of the bushing support belts, mm; t- thickness of the bushing support belts, mm; - the greatest diametrical elastic deformation of the bushing (increase or decrease in diameter in its middle part) mm; S max- maximum gap between the mounting surface of the bushing and the base surface of the workpiece 1 in a free state, mm; l To- length of the contact section of the elastic bushing with the mounting surface of the workpiece after the bushing has been unclamped, mm; L- length of the thin-walled part of the bushing, mm; l d- length of the workpiece, mm; D d- diameter of the base surface of the workpiece, mm; d- hole diameter of the bushing support bands, mm; R - hydraulic plastic pressure required to deform a thin-walled bushing, MPa (kgf/cm2); r 1 - radius of curvature of the sleeve, mm; M res =P z r- permissible torque arising from the cutting force, Nm (kgf-cm); P z - cutting force, N (kgf); r is the moment arm of the cutting force.
In Fig. Figure 3.12 shows a cantilever mandrel with a thin-walled sleeve and hydroplastic. Workpiece 4 the base hole is installed on the outer surface of the thin-walled bushing 5. When compressed air is supplied to the rod cavity of the pneumatic cylinder, the piston with the rod moves in the pneumatic cylinder to the left and the rod through the rod 6 and lever 1 moves plunger 2, which presses the hydroplastic 3 . The hydroplastic evenly presses on the inner surface of the sleeve 5, the bushing opens; The outer diameter of the sleeve increases, and it centers and secures the workpiece 4.
Rice. 3.12. Cantilever mandrel with hydroplastic.
Diaphragm chucks are used for precise centering and clamping of parts processed on lathes and grinding machines. In membrane chucks, the parts to be processed are mounted on the outer or inner surface. Base surfaces parts must be processed according to 2nd accuracy classes. Diaphragm cartridges provide a centering accuracy of 0.004-0.007 mm.
Membranes- these are thin metal disks with or without horns (ring membranes). Depending on the effect on the membrane of the rod of a mechanized drive - pulling or pushing action - membrane cartridges are divided into expanding and clamping.
In an expanding membrane horn chuck, when installing the annular part, the membrane with horns and the drive rod bends to the left towards the machine spindle. In this case, the membrane horns with clamping screws installed at the ends of the horns converge towards the axis of the cartridge, and the ring being processed is installed through the central hole in the cartridge.
When the pressure on the membrane stops under the action of elastic forces, it straightens, its horns with screws diverge from the axis of the cartridge and clamp the ring being processed along the inner surface. In a clamping diaphragm open-end chuck, when the annular part is installed on the outer surface, the diaphragm is bent by the drive rod to the right of the machine spindle. In this case, the membrane horns diverge from the axis of the chuck and the workpiece is unclenched. Then the next ring is installed, the pressure on the membrane stops, it straightens and clamps the ring being processed with its horns and screws. Clamping membrane horn chucks with a power drive are manufactured according to MN 5523-64 and MN 5524-64 and with a manual drive according to MN 5523-64.
Diaphragm cartridges come in carob and cup (ring) types, they are made from steel 65G, ZOKHGS, hardened to a hardness of HRC 40-50. The main dimensions of the carob and cup membranes are normalized.
In Fig. 3.13, a, b shows the design diagram of the membrane-horn chuck 1 . A chuck pneumatic drive is installed at the rear end of the machine spindle. When compressed air is supplied to the left cavity of the pneumatic cylinder, the piston with rod and rod 2 moves to the right. At the same time, rod 2, pressing on the diaphragm 3, bends it, the cams (horns) 4 diverge, and part 5 unclenches (Fig. 3.13, b). When compressed air is supplied to the right cavity of the pneumatic cylinder, its piston with rod and rod 2 moves to the left and moves away from the membrane 3. The membrane, under the influence of internal elastic forces, straightens, cams 4 the membranes converge and clamp part 5 along the cylindrical surface (Fig. 3.13, a).
Rice. 3.13. Scheme of a membrane-horn chuck
Basic data for calculating the cartridge (Fig. 3.13, A) with horn-like membrane: cutting moment M res, tending to rotate the workpiece 5 in the cams 4 cartridge; diameter d = 2b base outer surface of the workpiece; distance l from the middle of the membrane 3 to the middle of the cams 4. In Fig. 3.13, V a design diagram of a loaded membrane is given. A round membrane rigidly fixed along the outer surface is loaded with a uniformly distributed bending moment M AND, applied along a concentric circle of a membrane of radius b base surface of the workpiece. This circuit is the result of superposition of two circuits shown in Fig. 3.13, g, d, and M AND =M 1 +M 3 .
In Fig. 3.13, V accepted: A - radius of the outer surface of the membrane, cm (selected according to design conditions); h=0.10.07- membrane thickness, cm; M AND - moment bending the membrane, Nm (kgf-mm); - cam expansion angle 4 membrane required for installation and clamping of the workpiece with the smallest maximum size, deg.
In Fig. 3.13, e the maximum expansion angle of the diaphragm cams is shown:
Where: - additional cam expansion angle, taking into account the tolerance for inaccuracy in manufacturing the mounting surface of the part; - the angle of expansion of the cams, taking into account the diametrical clearance necessary for the possibility of installing parts in the chuck.
From Fig. 3.13, e it is clear that the angle:
;
Where: - tolerance for inaccuracy in the manufacture of a part at an adjacent previous operation; mm.
The number of cams n of the membrane cartridge is taken depending on the shape and size of the workpiece. Friction coefficient between the mounting surface of the part and the cams 
. Safety factor. The tolerance on the size of the mounting surface of the part is specified in the drawing. Elastic modulus MPa (kgf/cm2).
Having the necessary data, the membrane cartridge is calculated.
1. Radial force on one jaw of a diaphragm chuck for transmitting torque M res
Powers P h cause a moment that bends the membrane (see Fig. 3.13, V).
2. When large quantities chuck jaw moment M P can be considered to act uniformly around the circumference of the membrane radius b and causing it to bend:
3. Radius A the outer surface of the membrane (for design reasons) are specified.
4. Attitude T radius A membranes to radius b mounting surface of the part: a/b = t.
5. Moments M 1 And M 3 in fractions of M And (M And = 1) found depending on m= a/b according to the following data (Table 3.1):
Table 3.1
| m=a/b | 1,25 | 1,5 | 1,75 | 2,0 | 2,25 | 2,5 | 2,75 | 3,0 |
| M 1 | 0,785 | 0,645 | 0,56 | 0,51 | 0,48 | 0,455 | 0,44 | 0,42 |
| M 3 | 0,215 | 0,355 | 0,44 | 0,49 | 0,52 | 0,545 | 0,56 | 0,58 |
6. Angle (rad) of the cams opening when securing a part with the smallest maximum size:
7. Cylindrical stiffness of the membrane [N/m (kgf/cm)]:
Where: MPa - modulus of elasticity (kgf/cm 2); =0.3.
8. Angle of greatest expansion of cams (rad):
9. The force on the rod of the motorized drive of the chuck, necessary to deflect the membrane and spread the cams when expanding the part, to the maximum angle:
.
When choosing the point of application and direction of the clamping force, the following must be observed: to ensure contact of the workpiece with the support element and eliminate its possible shift during fastening, the clamping force should be directed perpendicular to the surface of the support element; In order to eliminate the deformation of the workpiece during fastening, the point of application of the clamping force must be selected so that the line of its action intersects the supporting surface of the mounting element.
The number of points of application of clamping forces is determined specifically for each case of clamping a workpiece, depending on the type of workpiece, processing method, and direction of the cutting force. To reduce vibration and deformation of the workpiece under the influence of cutting forces, the rigidity of the workpiece-fixture system should be increased by increasing the number of workpiece clamping points by introducing auxiliary supports.
Clamping elements include screws, eccentrics, clamps, vice jaws, wedges, plungers, and strips. They are intermediate links in complex clamping systems. The shape of the working surface of the clamping elements in contact with the workpiece is basically the same as that of the installation elements. Graphically clamping elements are designated according to table. 3.2.
Table 3.2 Graphic designation clamping elements
Test tasks.
Task 3.1.
Basic rules when securing a workpiece?
Task 3.2.
What determines the number of clamping points of a part during processing?
Task 3.3.
Advantages and disadvantages of using eccentrics.
Task 3.4.
Graphic designation of clamping elements.
