Most frequently used in the production of threaded rods, bolts, screws, and other metal fasteners, thread rolling is a cold metal forming process that creates threads using precision thread rolling dies that are the mirror image of the thread being produced. Though a reliable and flexible practice, there are a number of techniques that can improve your output. Extensive experience in thread rolling makes CJWinter’s team of well-versed experts the go-to source for anticipating and preventing common thread rolling problems.
Slivers/Flakes in Threads
Overfilled, mismatched, or incorrectly measured rolls can result in slivers or flaking in the threads. If encountering this problem when using rolls of the correct diameter, make sure that the centerline between the rolls is parallel with the centerline of the work.
A rough finish on a blank or a slow feed rate can also cause flaking. Regrinding the tooling to improve finish on the blank diameter, as well as increasing the rate of penetration helps solve these issues.
After checking the above factors, if slivers or flakes still appear in the threads, consider a change in material. For best outcome, make sure that the utilized material can be cold-worked.
To ensure an accurate pitch, use a comparator to measure as many fill threads as possible. Divide this measurement by the total number of threads measured in order to find the exact pitch. If the theoretical value and the pitch do not match, using modified lead (ML) rolls may work well to resolve the issue.
When experiencing problems with pitch, you may want to consider sending rolled parts samples to the manufacturer, who can quickly identify the problem design properly gauged parts to correct the error.
Mismatched Helix Angle
"Screw jacking", a common issue, arises if a thread roll stays in contact with the blank for too long. In turn, mismatched helix angles can occur when attachments or rolls get pulled into the collet as a result of being kept on the part for too long.
Solve this issue by reducing roll time. Avoid prolonged contact with the part by increasing penetration rate of the rolls in and out of the blank. Allow enough time for the thread to form properly, but be sure to come off the part as quickly as possible to reduce the length of contact.
Different Rolling Conditions
In rolling conditions for forgings, the grain structure of a material may also cause potential problems. If the grain structure does not remain consistent within a certain section of material, then grain flow becomes an issue.
Additionally, a piece can occasionally harden before the thread rolling process is complete. Caused by the tool becoming dull, avoid running into these work hardening issues by simply increasing the rate of penetration when rolling the thread.
Hollow Work, Closed Hole, or Out of Round Conditions
When encountering these thread rolling issues, checking three important factors may help. First, be sure that the mandrel is supported. If so, check to see if the wall thickness is sufficient. A lack of thickness may warrant a decision to drill later in the cycle. If the feed rate is too high, slowing the penetration rate may help alleviate any issues. Use the minimum wall thickness requirement chart below for guidance.
Minimum Wall Thickness Requirement Chart
Thread Filled Out in Center But Not End
A few issues may cause the thread to fill out in the center but not at the ends, such as:
- The blank doesn't maintain diameter throughout
- The roll centerline and work centerline are not parallel
- The thread is too short or too long
Short threads will measure at approximately two to three threads and can be resolved by generating a concave blank measuring 0.0002–0.0003. Long threads will measure at 15 threads or higher, and can be resolved by generating a convex blank measuring 0.0002–0.0003.
Even if the rest of the process runs smoothly, a poor finish ruins the final product. First, check to be sure the chosen material can be cold-worked and that the rolls are not being overfilled. If they are, the blank diameter may need to be reduced.
Finish may also be impacted if any material has begun to accumulate in the threads. If the threads cannot be adequately cleaned, replacement of the rolls should occur. But if the threads are absent of any material buildup, nearby jobs may be the culprit for any part contamination. Eliminate this problem with a steady stream of clean oil to flush out the work zone adequately.
Poor Thread Form
Poor thread form results from a number of possible issues. During rolling, if the work bends, alter the setup to properly support the part. Resynchronizing the rolls may prove necessary to guarantee their timing matches or adjust the attachment/roll orientation to confirm that their centerlines are parallel.
As with many other thread rolling problems, inadequate rolling speed may be causing the problem. If experiencing too many work revolutions, simply increase the rate of penetration.
Crests Not Filled Out
A slightly concave crest may develop if blanks are too small, or the thread is too deep. Many users do not consider crests a serious problem, and allow thread forms to be produced with crests not completely filled out with a slight concavity. This can help avoid roll overload and prolong roll life.
Scuffed crests happen as a consequence of a number of complications, including:
- Material buildup in threads
- Feed rate too high
- Attachment not retracting quickly enough
- Rolling misalignment
- Insufficient wall thickness
- Rolls and gear train binding
- Improper supporting mandrel
Solutions may involve increasing speed roll retraction, checking oil jet flowing on rolling position, checking the gear train for foreign matter, or slowing down the penetration rate.
CJWinter Thread Rolling Solutions
Whatever the thread rolling issue, CJWinter is here to help. Our troubleshooting page provides further guidance for solving all your thread rolling concerns, from scuffed crests to split threads.