Main Points of Machine Tool Fixture Design

Main Points of Machine Tool Fixture Design

Machine tool fixtures are an important part of the machining process system. In order to ensure the processing requirements of a certain process of the workpiece, the workpiece must be in an accurate relative position relative to the cutting or forming movement of the tool on the machine tool. When a batch of workpieces are clamped and processed by a fixture, this requirement is achieved through the fixture. To achieve this requirement, it must meet three conditions: ① a group occupies the correct machining position of the workpiece in the fixture; ② the jig clamping the exact position on the machine; ③ the exact position of the tool relative to the fixture. There are three relationships involved here: parts are relative to fixtures, fixtures are relative to machine tools, and parts are relative to machine tools. The final accuracy of the workpiece is obtained by the part relative to the machine tool. Therefore, " positioning " also involves a three-layer relationship: the positioning of the workpiece on the fixture, the positioning of the fixture relative to the machine tool, and the positioning of the workpiece relative to the machine tool is indirectly guaranteed by the fixture. After the workpiece is positioned, the workpiece must be fixed by a certain device to generate a clamping force to keep the workpiece in an accurately positioned position. Otherwise, the position will change or cause vibration due to the cutting force, inertial force and other forces during the machining process. , Destroys the original accurate positioning and cannot guarantee the processing requirements. This kind of device that generates clamping force is the clamping device.

One fixture

1. Overview of machine tool fixtures

1. The concept of machine tool fixtures

The machine tool fixture is a device used to clamp the workpiece (and guide the tool) on the machine tool. Its function is to position the workpiece so that the workpiece can obtain the correct position relative to the machine tool and the tool, and to clamp the workpiece reliably.

2. Classification of machine tool fixtures

Machine tool fixtures can be divided into general fixtures, special fixtures, modular fixtures, general adjustable fixtures and group fixtures according to their scope of use. Machine tool fixtures can also be classified according to the machine tool used and the power source that generates the tightening force. According to the machine tool used, the fixtures can be divided into lathe fixtures, milling machine fixtures, drilling fixtures (drilling molds), boring machine fixtures (boring molds), grinder fixtures and gear machine fixtures, etc. According to the power source that generates the tightening force, the fixtures can be divided into Manual fixtures, pneumatic fixtures, hydraulic fixtures, electric fixtures, electromagnetic fixtures and vacuum fixtures, etc.

3. The composition of machine tool fixtures

(1) Positioning components

(2) Clamping device

(3) Tool setting, guiding element or device

(4) Connecting components

(5) Folder specific

(6) Other components and devices

2. Positioning benchmark

The datum is used to determine the point, line, and surface on which the geometric relationship between the geometric elements on the production object is based. The datum used for positioning in processing is called the positioning datum. Sometimes, the point, line, and surface used as the reference do not necessarily exist on the workpiece (such as the center line of the hole and the center plane of symmetry, etc.), and its function is embodied by some specific surface (such as the cylindrical surface of the inner hole), which reflects the reference The active surface is called the base surface.

Third, the positioning of the workpiece in the fixture

( 1 ) Principle of six-point positioning

Any unpositioned workpiece has six degrees of freedom in the spatial rectangular coordinate system. The task of workpiece positioning is to limit all or part of the freedom of the workpiece according to the processing requirements. The six-point positioning principle of the workpiece refers to the method of using six supporting points to respectively limit the six degrees of freedom of the workpiece, so that the workpiece can be positioned in space.

 ( 2 ) Support points and positioning components

The figure above shows the degree of freedom restricted by the positioning elements in the common positioning methods and the equivalent number of supporting points

( 3 ) Complete positioning and incomplete positioning

The positioning in which the six degrees of freedom of the workpiece are completely restricted is called complete positioning. According to processing requirements, positioning that allows one or several degrees of freedom without being restricted is called incomplete positioning.

( 4 ) Under-positioning and over-positioning

According to the processing requirements of the process, the degree of freedom of the workpiece should be restricted but the unrestricted positioning is called under-positioning. When determining the workpiece positioning plan, under positioning is absolutely not allowed. The positioning of the same degree of freedom of the workpiece with two or more support points repeatedly restricted is called over positioning. Under normal circumstances, try to avoid over-positioning.

There are generally two ways to eliminate over-positioning and its interference: one is to change the structure of the positioning element to eliminate the degree of freedom that is repeatedly restricted; the other is to improve the position between the workpiece positioning base surface and the working surface of the fixture positioning element Accuracy to reduce or eliminate interference caused by over positioning.

Four, modular fixtures and accompanying fixtures

( 1 ) Modular fixture 

Modular fixture is a special fixture assembled from a set of pre-manufactured standard components and parts.

( 2 ) Accompanying fixture

The accompanying fixture is a mobile fixture used on automatic lines in mass production.

 Analysis and Calculation of Position Error of Second Fixture

The so-called positioning error refers to the position error of the processing surface relative to the process reference caused by the positioning of the workpiece. Because for a batch of workpieces,

The position of the tool is fixed after adjustment, that is, the position of the processed surface is unchanged relative to the positioning datum, so the positioning error is the process

The maximum amount of variation of the datum in the machining dimension direction.

 (I)  Introduction &

① △ total ≤δ 

Among them, △ is the sum of errors caused by various reasons, δ is the tolerance of the workpiece to be processed, △ always includes the clamping error of the fixture on the machine tool, the positioning error and the clamping error of the workpiece in the fixture, the adjustment error of the machine tool, and the process The elastic deformation and thermal deformation error of the system, the manufacturing error and wear error of the machine tool and the cutting tool, etc.

② △ set + ω≤δ  where, [omega] except for the positioning error, the sum of the error caused by other factors, look-up table is determined according to the precision machining economy.

Therefore, we know from ① and ② : △定≤δ-ω  (it is the formula for checking whether the processed workpiece is qualified or not)

 Or: △定≤1/3δ (also the formula for checking whether the processed workpiece is qualified or not)

(2)  Composition of positioning error

1. Definition : Positioning error is the maximum position change of the machining surface in the direction of machining requirements relative to the working datum caused by the positioning of the workpiece in the fixture due to inaccurate positioning.

  2. Composition of positioning error:

The positioning error caused by the inconsistency between the positioning reference and the process reference is called the reference non-coincidence error, that is, the maximum variation of the process reference relative to the positioning reference in the processing dimension direction, which is not represented by △ .

The positioning error caused by the manufacturing error of the positioning reference surface and the positioning element itself is called the reference position error, that is, the maximum variation of the relative position of the positioning reference in the processing dimension direction, expressed by the △ base. Therefore:

  △定=△ no +△ base

In addition, two points are clear:

① Only use the adjustment method to process a batch of parts to produce positioning error, and use the trial cutting method to produce no positioning error;

②The positioning error is a limit value (there is a range). 

(3)  Analysis and calculation of positioning error

⑴ Positioning error when the workpiece is positioned on a plane

Positioning datum: flat surface  ; working surface of positioning element: flat surface   ====> Easy to process and smooth, good contact ===>

So △ Base =0 △定=△ No (Note: If it is a rough surface, there is still △ Base)

⑵ Positioning error when positioning the cylindrical surface outside the workpiece (taking V -shaped block as an example)