How to make a machining accuracy control plan


How to make a machining accuracy control plan

☞ We will sort out and classify the solutions to the problem of geometrical size tolerances in machining in four aspects: workpieces, tools, fixtures, and machine tools, and put forward some suggestions.
In mechanical processing, the unity composed of machine tools, fixtures, tools and workpieces is called a process system. In the process of processing, due to the existence of various primitive errors in the process system, the correct geometric relationship between the workpiece and the tool is destroyed, and the geometric size is out of tolerance. Generally, the original errors that may occur during the processing are divided into two types, namely, the "static" original errors related to the initial state of the process system and the "dynamic" original errors related to the processing process. The following proposes solutions to the problem of out-of-tolerance geometric dimensions in machining parts from four aspects: workpieces, tools, fixtures, and machine tools.

1) The general principle of machining should be followed, that is, benchmark first, then others, face first, hole first, master then second, rough first then fine, coarse and fine separation.
2) Before the finishing step of the process, loosen the pressure plate, properly release the clamping stress, and lightly compress the workpiece after realigning it. In order to ensure processing safety and processing quality, it is recommended to obtain the best torque value for light compaction through experimental verification and perform constant torque compaction.
3) The point of action of the clamping force should be a solid point, and the point of action is deformed because it is not a real point. There are 3 solutions: 

①Decompose the original one clamping into two solid-point clamping, but pay attention to switching The sequence of clamping, if necessary, the workpiece needs to be re-aligned. 

②Set up the clamping process boss for the workpiece, and after all the procedures are completed, decide whether to remove the process boss according to whether it affects the assembly of the work piece.

③Auxiliary supports are provided at the weak points of the clamping structure (see Figure 1). Some hydraulic auxiliary supports on the current market can already achieve "zero deformation" support. To

Auxiliary support1

4) Ensure that the positioning surfaces of the workpiece and the fixture have a good flatness. The flatness self-check is shown in Figure 2. 

If the positioning plane is large, the fixture positioning surface can be replaced with a block form to avoid the clamping warping deformation caused by the workpiece and the large plane of the fixture; 

if the flatness of the workpiece cannot be guaranteed, copper skin, paper scraps and Warp fibers, etc., flatten the positioning surface and then compress it for processing.

Flatness self-check
      5) During the machining process, cutting fluid is used to dissipate heat in time to reduce the thermal deformation of the workpiece.
      6) The aging treatment is arranged before semi-finish machining to completely release the internal stress of the workpiece.
      7) When turning, for thin-walled parts, use soft jaws or split collars for clamping, or use end face compression instead of circumferential compression.

8) During turning, reverse deformation processing is performed according to the deformation, that is, the corresponding deformation amount is pre-processed in the opposite direction, 

and the reverse deformation amount and the deformation amount cancel each other after the processing is completed.

9) When turning large irregular workpieces, counterweight should be carried out according to the position of the center of mass of the workpiece to reduce the centrifugal force.

Tool aspect

1) Before drilling, boring and reaming, if there is a half-wall structure in the machining part, the half-wall part must be removed first; 

if the machining part is a blank surface, the blank surface must be spot-faced first to ensure uniform force during cutting.

2) Before machining, check the radial and axial runout of the tool in accordance with relevant standards.
       3) Tool holders are prohibited from bumping and scratching, and special tools are used to clean the taper holes of            the machine spindle regularly.
       4) Control the length of the tool, as far as possible to ensure that the tool has a sufficient diameter, to ensure that the tool is firm.
        5) For workpieces with very high accuracy requirements, the tool must be dynamically balanced before processing.
        6) Adjust cutting parameters and tool angles, etc., to control the force, heat and wear of the tool during              processing.
         7) Use anti-vibration and anti-vibration special tools.

Fixture aspect
1) Improve the clamping method to prevent the installation point of the compression screw and the rear support point of the pressure plate from being one on the fixture and the other on the machine tool to prevent the fixture from being pulled up by the compression screw and causing serious deformation of the fixture.

2) The fixture must have sufficient rigidity.

3) When the fixture adopts the mandrel for positioning, and the mandrel is provided with a compression thread, the thread must be ground to ensure the perpendicularity of the thread and the positioning end surface of the mandrel.

4) Perform dynamic balance verification on precision turning fixtures to control the centrifugal deformation of the fixture when it rotates.

The machine tool inspects and confirms the machine tool in accordance with the machine tool acceptance accuracy standards, such as checking the verticality of the machine tool spindle and the worktable, and the movement of the spindle.

Concluding remarks
1) When the workpiece, especially the new product, is out of tolerance in geometric dimensions, first determine what kind of deformation is causing it, and then find a suitable solution according to the corresponding deformation solution.
2) Explore the use of computer-aided finite element analysis technology to make an analysis of the deformation of the process system in advance, which is conducive to the continuous optimization of the process plan.
3) In daily work, we must be good at summarizing solutions and draw inferences from one another, so as to comprehensively improve product quality.