What causes the deformation of workpiece in mechanical processing,? How should we avoid it?

The problem of workpiece deformation in machining is a difficult problem to solve. First, we must analyze the causes of the deformation and then take measures to deal with it.

Our common reasons are: shaft parts in turning are easy to produce ellipse or edge circle, taper, bending, especially the processing of slender shaft is easy to produce bending deformation.

The reasons for the error are:

(1) Misreading drawings, wrong sizes and scales.

(2) No trial cutting and careless trial.

(3) In the trial, the measurement is not allowed.

(4) The size of the workpiece changes due to the influence of cutting heat.

(5) Blunt edge causes minimum cutting thickness change.

The measures to eliminate errors are:

(1)To read the drawings carefully, read the drawings repeatedly several times, master the way to use the feed dial and see the scale grid.

(2) The cutting depth is calculated according to the machining allowance, and then the shallow cutting is carried out, and then the cutting depth is corrected. We should be careful when cutting, and measure 2mm to 3mm in order to prevent scrap.

(3) Before measuring, the parts must be carefully inspected and adjusted, and the measuring tools should be used correctly. Especially when finishing the car, vernier calipers and micrometers must be used in conjunction, in order to avoid measuring more or less than one circle.

(4) It can not be measured at a higher temperature of the workpiece. If it is a precision part, rough turning first and then high temperature aging, semi-finish turning after low temperature aging, and then finish measuring.

(5) Choose the chamfering edge of the knife point, the knife point with small arc radius, fine grinding edge, improve the rigidity of the knife.

Fault phenomena: shaft parts in turning are easy to produce ellipse or edge circle, taper, bending, especially the processing of slender shaft is easy to produce bending deformation.

The causes of roundness error are:

(1) The clearance of spindle is too large.

(2) The allowance is uneven, and the cutting depth changes during the cutting process.

(3) When the top is clamped, the top contact with the center hole is not good.

(4) Unbalance of fixture rotation.

The measures to eliminate the error are:

(1) Check the spindle clearance before turning and adjust it properly. Generally speaking, only the front bearing can be adjusted. Only when the required rotary precision can not be reached after adjusting the bearing, the rear bearing can adjust the axial clearance and accuracy of the spindle.

(2) There must be 2 processes of rough machining and finishing.

(3) The workpiece should be properly clamped to check the accuracy of the center rotation and repair or replace in time.

(4) Match the balance block with careful adjustment.

The causes of taper error are as follows:

(1) When the workpiece is clamped with one clip, the tailstock tip deviates from the axis of the main shaft.

(2) After clamping with chucks, the workpiece will be affected by the radial cutting force when the overhang is too long, so that the front end will produce elastic deformation and taper.

(3) When the small scooter is out of circle, the position of the small sliding board is not correct.

(4) The lathe guide rail is not balanced with the spindle axis.

(5) The tool wear is too fast, and the cutting depth of both ends of the workpiece is different.

The causes of bending deformation are:

(1) The weight of the blank and its own bending.

(2) The workpiece is not rigid enough or the rear top is too tight.

(3) The internal stress of the workpiece is large.

(4) Improper selection of geometric parameters and cutting parameters resulted in excessive cutting force.

(5) Hot deformation occurs during cutting.

(6) Because of the weight of the shaft and the cutting force, the slender shaft is easy to appear the waist drum shape and the bamboo shape.

The measures to eliminate errors are:

(1) Workpiece blanks should be straightened and heat treated.

The slender shaft should be straightened before turning. When roughing, the workpiece blank should be fed into the turning circle once. Otherwise, the use of the heel tool holder will be affected and the bending degree will be checked before finishing. Otherwise, the turning difficulty will be increased.

(2) The workpiece should be properly clamped.

When machining the long shaft, properly relaxing the top force or using the elastic movable top, using the elastic movable top, can effectively compensate the thermal deformation and elongation of the workpiece, workpiece is not easy to bend, and then with the use of auxiliary support, slender shaft should be used three-jaw tool holder and elastic shrinkage rotary top and reverse turning. The axial tension of the workpiece can eliminate vibration and improve machining quality. The size of the contact pressure between the top and the workpiece depends on the center's rotation with the workpiece and a little bit of force. Too much pressure is easy to bend the workpiece, too small, it is easy to cause vibration when starting to eat.

(3) Appropriate elimination of stress.

Prevent pre-stress during clamping and cause deformation of the workpiece. Manual aging is best after roughing, before finishing. For shaft parts with high precision, it is necessary to remove internal stress annealing for many times during rough and semi-finish machining.

(4) Reasonable selection of tool geometry parameters and sharp edge can reduce feed times, properly increase the rake angle, reduce cutting force and reduce the vibration caused by turning.

(5) Pay attention to heat dissipation and cooling.

Sufficient coolant should be supplied during the machining process, which can not only reduce the thermal deformation and elongation of the workpiece caused by the temperature rise, but also prevent the workpiece from being supported by the heel holder, and improve the tool life and workpiece processing quality.

(6) In order to prevent the occurrence of "waist drum shape", the two supporting claws should be adjusted at any time during processing, so that the center of the two circular surfaces of the supporting claws coincides with the rotation axis of the lathe spindle. In order to increase the contact area and reduce the wear and tear, grind the claw into an arc slightly larger than the radius of the workpiece before using the claw. The main deflection angle should be increased to reduce the radial force during turning. When the slight "bamboo shape" appears in the turning process, the upper supporting block's compression force can be adjusted, the middle slide handle can also be adjusted, the back feed quantity can be changed, and the gap between the lathe saddle and the middle slide plate can be reduced, so as to keep good contact with the workpiece.