Threaded Cup Difficult to Remove
Threaded Cup Difficult to Remove
(OP)
Hello,
I have two cups from two different suppliers. The design of the cups call for an external thread M85X1.50 with a 6g tolerance. The hardness value of both cups were checked around 30-36 HRC. There are a few differences in the way the cups were made:
Supplier A Cup:
Made from a casting
Hardness value towards lower end of spec
Supplier B Cup:
Made from a billet
Hardness values slightly higher than cast piece
Threads cut using single point cutting
The female component has the same thread with 6H tolerance. The component is made from a casting, and the material of the female component has a hardness value of around 23-32 HRC.
After both cups were used in the mating component, they were disassembled for inspection. We noticed that after testing, the billet material cup will un-thread fine about half way, then it gets stuck and needs some force to remove. Threads themselves look fine, and do not appear to be damaged. The cast cup has no issues, and did not get stuck during removal.
Can anyone explain why the billet cup gets stuck halfway during removal?
Would opening the tolerance on the threads help in any way?
Does the cast cup in cast mating component have a benefit?
Any thoughts would be appreciated.
Thank you,
mjgarrin
I have two cups from two different suppliers. The design of the cups call for an external thread M85X1.50 with a 6g tolerance. The hardness value of both cups were checked around 30-36 HRC. There are a few differences in the way the cups were made:
Supplier A Cup:
Made from a casting
Hardness value towards lower end of spec
Supplier B Cup:
Made from a billet
Hardness values slightly higher than cast piece
Threads cut using single point cutting
The female component has the same thread with 6H tolerance. The component is made from a casting, and the material of the female component has a hardness value of around 23-32 HRC.
After both cups were used in the mating component, they were disassembled for inspection. We noticed that after testing, the billet material cup will un-thread fine about half way, then it gets stuck and needs some force to remove. Threads themselves look fine, and do not appear to be damaged. The cast cup has no issues, and did not get stuck during removal.
Can anyone explain why the billet cup gets stuck halfway during removal?
Would opening the tolerance on the threads help in any way?
Does the cast cup in cast mating component have a benefit?
Any thoughts would be appreciated.
Thank you,
mjgarrin





RE: Threaded Cup Difficult to Remove
RE: Threaded Cup Difficult to Remove
RE: Threaded Cup Difficult to Remove
RE: Threaded Cup Difficult to Remove
The actual hardness of the material should not present a problem, but the heat treat process used might not provide sufficient dimensional stability in the finished parts.
Also, what is the length of the thread engagement? If it is more than about 5p, accumulation of pitch spacing errors can be a problem.
RE: Threaded Cup Difficult to Remove
Anything else might be surface finish related.
RE: Threaded Cup Difficult to Remove
Thanks again,
RE: Threaded Cup Difficult to Remove
Making the internal thread tolerance class 4H likely will make your problem worse since you still have zero deviation associated with the H tolerance position but the 4 tolerance grade has a smaller range than the 6 tolerance grade.
RE: Threaded Cup Difficult to Remove
Heat treatment makes each distortion in each thread from the original mold, then the shrunk part, then the heat treatment movement and re-contraction even worse.
RE: Threaded Cup Difficult to Remove
I'd say that's your most likely problem. An engaged thread length of 18p is way too many threads. That many engaged threads is probably creating substantial interference. Try reducing your engaged thread length to something closer to 6p.
RE: Threaded Cup Difficult to Remove
I suspect most threads are checked over small distances, so that increased engagement allows lack of straightness of the pitch diameter to result in interference.
It is still odd that one becomes more difficult to remove as the engagement increases, hence my thought about taper.
RE: Threaded Cup Difficult to Remove
http://www.boltscience.com/pages/screw8.htm covering accumulation in mismatch in thread pitch.
RE: Threaded Cup Difficult to Remove