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16th February 2024, 11:41 PM #1Most Valued Member
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How does one calc the internal and external dimensions for custom threads.
I recently cut a 30mm diam 24TPI external and matching internal thread - thread data shows the thread depth for a 24TPI thread is 0.68mm, for the external thread the stock was turned down to 30mm and then cut the thread advancing the tool a total of 0.68mm, once at that depth I take 3 or 4 cuts or more until I see no more swarfe being cut.
For the internal thread I reasoned that with a thread depth of 0.68mm I would need to bore a hole 30-(2x0.68)mm or 28.64mm, I actually bored the hole 28.7mm to give a little clearance, however once the thread was cut the external threaded part would not even start - going a bit deeper with the internal thread didn't make any difference. No choice to but to either start again or try to fix by boring slightly larger, I opted to bore slightly larger, this was no fun as I had to remove the thread cutting tool switch to a boring bar and then try to pickup an internal 24TPI thread, I actually did this twice, as the first time I was really flying blind not knowing how much to increase the hole size by, the first time the thread would start but bind after 1/2 a turn so the second time I knew I was almost there, I ended up boring the hole larger by a total of approx 0.22mm on the radius.
The threads are now done - a nice close fit, but it would be nice to have some guidelines on the sizes to make the job easier.
Read the full thread at metalworkforums.com...
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