Fixtures for CNC machining are very important. Fixture design is generally in parts machining process in accordance with the specific requirements of one process. In the process of formulating the technology, we will take the workability of fixture into consideration, and when designing the fixture, modification suggestions can be put forward to the process if necessary. The design quality of fixture should be measured by whether it can steadily ensure the processing quality of the workpiece, improve production efficiency, reduce production costs, convenient chip removal, safe operation, easy maintenance and so on.
The basic principles of fixture design:
1.To meet the stability and reliability of workpiece positioning in the process
2.There is sufficient load or clamping force to ensure that the workpiece on the fixture processing process smoothly.
3.Meet the requirements of simple and fast operation in the clamping process
4.Fragile parts must be easily replaceable, and it is best not to use other tools when conditions are sufficient
5.Meet the reliability requirements of repeated positioning of fixture in the process of adjustment or replacement.
6.Avoid complex structure and high cost fixtures
7.Use as much standard parts as components as possible.
8.Form the systematization and standardization of products in-house.
Basic knowledge of fixture design
A good machine tool fixture must meet the following basic requirements:
1.Ensure the machining accuracy of workpiece, First of all, the positioning datum, positioning method and positioning element must be selected correctly, positioning error analysis should be carried out if necessary, attention should also be paid to the influence of the structure of other fixture parts on the machining accuracy to ensure that the fixture can meet the machining accuracy requirements of the workpiece.
2.The complexity of fixtures used to improve production efficiency should be adapted to the capacity situation, and various fast and efficient clamping mechanisms should be adopted as far as possible to ensure convenient operation, shorten the auxiliary time and improve production efficiency.
3.The structure of fixture with good process performance should be simple and reasonable, easy to manufacture, assembly, adjustment, inspection, maintenance and so on.
4.Good performance fixture should have enough strength and stiffness, operation should be simple, labor-saving, safe and reliable. Under the premise of objective conditions and economic applicability, pneumatic, hydraulic and other mechanical clamping devices should be used as far as possible to reduce the labor intensity of operators. Fixture should also be convenient for chip removal, When necessary, the chip removal structure can be set up to prevent the chip from destroying the positioning of the workpiece and damaging the tool, and prevent the chip from accumulating a lot of heat and causing the deformation of the process system.
5.Cost-efficient fixture should adopt standard components and standard structure as much as possible, and strive to have simple structure and easy to manufacture, so as to reduce the manufacturing cost. Therefore, the design should consider order volume and capacity situation, make technical and economic analysis of fixture scheme if necessary to improve the economic benefit of fixture in production.
The standardization of fixture design overview
Outline of auxiliary fixture design
1.Basic rules & steps of auxiliary fixture design.
Auxiliary fixture design included below contents:
a)Design order, FG drawing, blank drawing and processing route, related technical data, etc. To understanding the machining technical requirements for each process, such as positioning, clamping solution, processing contents of previous process, blank status, machining machine, cutting tools, inspection tools, machining allowance and so on.
b)Understand the requirements of auxiliary fixture for mass production;
c)Understand the mainly technical parameters, performance, specifications, accuracy and the contact size for operation table with auxiliary fixture.
d)The storage status of raw material to build auxiliary fixture
Problems to be considered in fixture design
Fixture design structure is simple generally, especially with the use of hydraulic fixture changing, the original mechanical structure is greatly simplified, if the design process is not considered in detail , unnecessary trouble will appear:
a)Working allowance of workpiece to be processed.If the blank size is too large, it will interfere with the fixture. So before the fixture design blank drawings must be available.
b)The chip removal smoothness of the fixture. Due to the limitation of machine tool processing space, the gap between parts and parts should be as small as possible, then the iron chip generated in the process of processing in dead corner deposit of the fixture will be ignored sometimes, including chip fluid flow is not smooth, which bring inconvenience to subsequent processing. Therefore, in initial design of fixture should take these problems into consideration, after all, fixture is used to improve efficiency and make operation convenient.
c)The overall openness of the fixture. Ignoring the fixture openness makes it difficult to install and mounted for the operator.Time-consuming and laborious, it is a design big taboos.
d)The basic theoretical of fixture design. Each fixture has to undergo countless clamping and unclamping actions, it could meet the requirements at beginning, but the fixture should have its accuracy lifespan, so do not design the fixture that contrary to the machining principle. Even if it is lucky they can be short use. While they must be poor lifespan and cyclic use. A good design should stand the temper of time.
e)Positioning components’ replace-ability. The positioning components are worn severely. So we have to consider about the fixture’s quick and convenient replacement. It is better not to design a larger fixture.
Accumulation of fixture design experience is very important. Sometimes, design is a individual thing, but it is related thing in actual application, so good design is a process of continuous accumulation and summarization.