Precision analysis of turning shaft-type workpieces

During the turning process of shaft-type workpieces, issues such as precision and surface roughness not meeting requirements may arise. An analysis is conducted based on the specific process during turning.

1. Reasons for not meeting dimensional accuracy requirements and their solutions

1. Due to the operator's carelessness, errors occur during measurement, or the scale is misread and used improperly.

Therefore, it is very important to measure carefully and correctly use the scale. For the scale of the carriage handle, the scales of various models of lathes are different. The value of each small scale can be calculated using the following formula:

Distance moved by the tool per scale turn = Distance of the carriage screw / Total number of scale lines on the scale (mm)

After knowing the value of each scale, it is also important to note that due to the gap between the screw and the nut, sometimes the scale may turn, but the tool may not move until the gap is taken up. Therefore, when using it, if the scale has turned too many divisions, it is not allowed to simply turn back a few divisions; instead, it should be turned back completely and then realigned to the scale.

2. The measuring tool itself has errors or is not positioned correctly during use.

Before using measuring tools, they must be carefully checked and adjusted, and they should be properly positioned during use.

3. Changes in temperature cause changes in workpiece dimensions.

During cutting, chips deform, and the molecules of the chips move relative to each other, generating a lot of heat due to friction during their movement. Additionally, friction between the chips and the front of the tool, as well as between the back of the tool and the surface of the workpiece, also generates heat, which directly affects the tool and the workpiece. Of course, the heat is mainly from the chips (about 75%), followed by the tool (about 20%) and the workpiece (about 4%, with 1% in the air). When the workpiece is heated, its diameter increases (about 0.01 to 0.05 mm, with cast iron changing more than steel), and it shrinks upon cooling, leading to scrap. Therefore, measurements should not be taken when the workpiece temperature is very high. If measurement is necessary, sufficient cutting fluid should be poured during turning to prevent the workpiece temperature from rising; secondly, rough and finish turning methods should be used.

4. Insufficient raw material allowance.

The raw material itself may be bent and not straightened, or the center hole may be misaligned.

2. Main reasons for not meeting geometric shape accuracy and mutual position accuracy requirements.

1. Causes of ovality.

a. The ovality of the spindle journal is directly reflected on the workpiece. If it is a sliding bearing, when the load size and direction remain unchanged, the spindle journal is pressed against a certain position on the bearing surface under load (due to the gap between the spindle and the bearing). When the spindle rotates 90°, the center position of the spindle changes, resulting in two center positions during one rotation, causing variations in the tool's back engagement and leading to ovality in the workpiece. The ovality of the bearing hole does not affect the workpiece.

b. Uneven raw material allowance, combined with the gap between the spindle and the bearing, causes changes in the back engagement during cutting.

c. Misalignment of the front and rear center holes (the two center holes form an angle with the workpiece center), causing the center hole to only contact one side, resulting in uneven wear and axial movement leading to ovality.

d. Front swing.

2. Causes of non-straightness (curvature, convexity, saddle shape) and taper.

a. Incorrect mutual positioning of the lathe guide rail and the spindle centerline, especially horizontally. For example, if the guide rail is bent, the workpiece may become convex or concave, and if the guide rail is not parallel to the spindle centerline, it may produce a taper.

b. The front and rear centerlines are not parallel to the bed guide rail, resulting in taper.

c. The increase in workpiece temperature can cause the shaft to bend. For example, when machining a long shaft on a heavy lathe, if the temperature rises to a certain point, the workpiece will elongate, but since the distances remain unchanged, the workpiece will bend because it cannot elongate in the length direction. Therefore, when turning long shafts, it is important to minimize temperature and frequently retract the tailstock.

d. The influence of internal stress in the workpiece. Internal stress often exists within the workpiece, and during the cutting process, due to plastic deformation of the surface layer, internal stress can also be generated. This internal stress maintains a balanced state within the workpiece, keeping it in a certain shape. However, when the workpiece is removed from the fixture or lathe, it will deform. To solve this problem, aging treatment methods are generally used.

3. Causes of radial runout.

Previous sharp pulsations; the center hole is not round or has chips and other debris; of course, the surface of the workpiece with ovality can also cause radial runout.

3. Main reasons for not meeting surface roughness requirements.

1. Insufficient rigidity of the lathe.

For example, if the carriage is loose or the drive is unbalanced, it can cause vibrations. Additionally, if the lathe is not installed securely, it can also cause vibrations, leading to a decrease in the surface roughness of the workpiece.

2. Insufficient rigidity of the cutting tool causing vibrations.

Therefore, it is advisable to use a rough tool holder to reduce the length of the tool extension; insufficient rigidity of the workpiece can also cause vibrations, so when turning slender shafts, a tailstock should be used, or a combination of a clamp and a support should replace two clamps.

3. Incorrect geometric parameters of the cutting part of the tool.

Select reasonable and suitable cutting angles based on the machinability characteristics of the workpiece material to reduce surface roughness.

4. The formation of built-up edge reduces the surface roughness of the workpiece.

During cutting, the presence of built-up edge can cause the workpiece surface to exhibit roughness or scratch marks. Efforts should be made to avoid its formation during turning. Based on the above analysis, it is important to be proactive in identifying and preventing issues during processing to eliminate problems at their source, improve workpiece accuracy, and meet design requirements.