Compared with general parts processing, deep hole parts processing has the following process characteristics:

(1) Deep hole parts have high quality requirements. The dimensional accuracy is within the range of IT6~IT12, the surface roughness R. is within the range of 25~0.2um, and the form and location accuracy, work hardening layer, residual stress and surface physical and mechanical properties are required to be high.

(2) The machining of the inner surface of deep hole parts is carried out in a semi closed state. The operator cannot directly observe the cutting condition of the tool. In addition, the chip space is small, the cutting heat is not easy to spread, and it is difficult to remove chips, cool and lubricate.

(3) The rigidity of the process system is weak (especially the rigidity of the tool system), and the error duplication is prominent. The processing stability is low, and it is easy to produce vibration and deformation. The processing accuracy and surface roughness of the hole are not easy to ensure. Therefore, the tool guidance and the rigidity of the process system are very important.

(4) Long cutting path, difficult chip discharge, uneven cutting edge load of the tool, high cutting temperature, easy tool wear, crushing and chipping.

Selection of datum for deep hole machining

It is an important problem to choose the locating datum for deep hole machining. The axis line and hole center line of the machined parts are the main datum for deep hole machining. When the axis line and hole center line are used as the measurement datum, positioning datum and correction datum for deep hole processing, it depends on the specific processing situation. The axis line and hole center line of the parts to be processed are the most important benchmarks for deep hole processing from beginning to end. Whether the selection is correct is the key to ensure the quality of deep hole processing. The axis line and hole center line datum are respectively embodied by the outer cylinder surface and the inner hole surface. Therefore, the datum for deep hole processing is generally selected according to the following principles.

Blank is solid

Obviously, the datum is the axis line of the blank. When installing the workpiece, the workpiece is aligned and clamped according to the outer circle, so the axis line is the positioning datum during processing. When the runout of the workpiece is checked (that is, the curvature of the workpiece. Because the workpiece is long, the operator should make such a check according to the convention to judge whether the allowance of the workpiece is sufficient), the axis line of the workpiece is the measurement benchmark. If the bending degree of the workpiece exceeds a certain range, the blank shall be corrected according to the measured bending degree and orientation, and the correction reference is also the axis line of the blank.

(2) Datum selection for boring (except blank tube material): there are two cases for selecting the datum for boring.

1) If the hole centerline of the previous procedure deviates greatly from the axis centerline of the outer circle, continue boring according to the hole centerline of the previous procedure.

It will cause the danger of insufficient excircle turning allowance. At this time, the workpiece should be turned around and the axis line of the excircle of the workpiece should be selected as the benchmark for alignment and installation, that is, boring with a certain eccentricity (commonly known as "borrowing").

2) If the deviation of the hole centerline of the previous procedure is small relative to the axis centerline of the outer circle, and the inner margin is also small relative to the outer circle, the boring can be continued according to the benchmark of the previous procedure. At this time, the boring margin is relatively uniform.

(3) Datum selection for deep hole honing: the main task of deep hole honing is to complete the semi finishing or final finishing of the aperture size, in order to reduce the surface roughness value of the hole and make its surface smooth and smooth. The grinding shall keep the axis line of the original hole unchanged, that is, the datum is the axis line of the hole.

(4) Datum selection for turning the outer circle: Generally, when the length diameter ratio L/d ≤ 15, because the hole is relatively straight, the outer diameter allowance is usually sufficient, and the outer circle can be directly turned against the inner hole based on the hole. The length diameter ratio of most deep holes is relatively large. Due to the poor rigidity of the deep hole processing system, the stress generated in drilling, boring and honing causes the workpiece to deform, that is, the hole centerline of the deep hole has been bent, or even bent in several directions. When turning the outer circle, the bending size and direction of the outer circle, that is, the axis centerline of the outer circle, should be consistent with the hole centerline, In most cases, it is realized by measuring the wall thickness of the workpiece, that is to say, the wall thickness shall be uniform before turning.

(5) Selection of workpiece calibration datum: after the workpiece is processed by drilling, sewing, honing, turning the excircle and other processes, the workpiece is bent in most cases and must be calibrated. The calibration datum is the axis line of the excircle (that is, the external cylindrical surface of the workpiece).

(6) Benchmark selection of step holes: the benchmark selection of deep hole steps, especially the benchmark selection of multi step holes, is very particular, because there is a certain coaxiality requirement between step holes. Generally, a datum hole is machined first, and another hole is machined by positioning the inner hole surface of the datum hole (that is, taking the hole centerline as the reference). The datum of the datum hole can also be transferred to the corresponding outer circle, that is, the outer circle datum of the corresponding length is turned out with the hole as the reference, and then the workpiece installation is aligned and adjusted according to its outer circle datum. The guide hole of another hole is added, and then the deep hole processing of another hole is carried out.