Parting Off – Part 5: Troubleshooting Common-Place Issues
Parting off is one of the most common lathe applications in a shop. In this series, we will discuss various challenges, tips and tricks to make your parting off applications more productive and trouble free. In previous articles we looked at the basics of parting off operations as well as suggested best practices to achieve successful and efficient parting off operations.
In this final section, we will look at the common problems that arise during parting off and suggested troubleshooting methods to fix the problems.
Large residual pip/burr
As discussed earlier in this series, burr or pip left at the center of the workpiece is one of the main challenges in parting off operations. Many measures can be taken to eliminate this issue, depending on the application and setup background. The following recommendations are as follows:
- Reduce the feed value by 50-75% at a diameter of 1.5 × s or above (s = cutting edge width), for the reasons explained earlier.
- Use a cutting insert with a lead/inclination angle also called as handed insert.
- Use a narrower insert. A narrow insert width naturally reduces the width of the burr, also reduces the feed forces that would otherwise push off the finished component prematurely. It also weakens the remaining burr in the face of feed force, and burr tends to come off easily.
- Use a smaller corner radius. A smaller corner radius means a ‘sharper’ insert. A sharper insert tends to have a clean shear cut rather than a blunt pushing force, leading to reduced leftover burr.
- Use a more positive geometry. A positive geometry also tends to have a cleaner shear cut leading to a reduced leftover burr.
- Check the center height. Reduced deviation from center has a huge positive effect in reducing the burr.
Poor surface finish is usually a function of feed, stability or insert geometry. It can be a combination of those factors, or all of them combined. To improve surface finish:
- Use a more stable tool. Wider seats of the insert and wider insert widths add more structural stability and reduce tool vibration tendency.
- Clamp the tool at a shorter length. This again leads to a more stable setup with reduced vibrations and improved surface finish.
- Check whether the insert seat is damaged. This is often an overlooked factor. A damaged insert pocket not only reduces tool life but also allows for a tendency of the insert to ‘wiggle’ and vibrate in the pocket, leading to a poor surface finish.
- Use a more positive geometry, which is again targeted towards reducing overall vibrations in the operation.
- Increase the feed. Higher feeds usually lead to better chip curling and thus lead to a corresponding improvement in surface finish.
Poor chip formation
All grooving applications, including parting off, are critically dependent on efficient chip formation and breakage. Recommendations for better chip formation:
- Reduce the cutting speed as explained in earlier section.
- Improve the coolant delivery. If possible, use precision guided thru coolant tools.
- Check the chip formation. Use correct chip formers, suitable to the materials being cut and recommended for the specific application.
- Increase the feed. Higher feed leads to the chips better conforming to the form of the chip breakers. This leads to better curling into clock spring shapes as well as folding into themselves axially, thus, avoiding possible scoring of the finished shoulder surfaces.
Poor face flatness
Poor face flatness can be attributed to two primary causes: tool deviation or ‘walking’ away from finished surface and tool alignment. All factors that reduce these two causes should be deployed. Here are some steps to take to improve face flatness:
- Use a cutting insert with neutral cutting edge. If a neutral cutting edge cannot be used for some reason (such as need to reduce burr), then select an insert with the smallest possible lead angle. This problem was further illustrated in the earlier section.
- Use a tool with the shortest possible cutting depth. This reduces the vibration of the tool, allowing a more stable cut and correspondingly improved surface finish.
- Reduce the feed for cutting inserts with a lead angle.
- Use an insert with smaller corner radius. Smaller corner radii reduce the overall cutting force, thus reducing overall vibrations and lead to a better surface flatness.
- Use a more positive geometry. With a more positive geometry again, the overall cutting forces are reduced, leading to reduced vibrations and a better surface flatness.
- Align the tool properly. Proper tool alignment also reduces the overall vibrations in the operation leading to a better surface flatness.
Damage caused by chips
Damaged cause to the finished shoulder surfaces is a direct result of poor chip formation and chip control. Use all the measures suggested under the section poor chip formation.
With this, we come to the end of the 5 article series about parting off. We hope this series armed you with the necessary knowledge to tackle the parting off challenges you face in your shop, and that you learned a few new tricks to help you trouble shoot your operations in the future.
For more information about parting off operations or tools, please visit Walter-tools.com.
Author: Sarang Garud, Product Manager, Walter USA LLC.