Internal thread is "self-blocking" - won't attach to threaded post.

l_p669

I'm trying to create a replacement tip to thread onto this very course threaded bit (see attached).

I have something that seems appropriate to start fit-testing, but the threads seem to "self block" at the exit of the hole. I've tried using Threadlab, and threadcreator. I thought maybe the issue is the # of starts, or a chamfer at the beginning, but can't seem to figure this out.

What am I doing wrong/what do I need to do?

PXL_20250423_230853273.jpg

Doc: https://cad.onshape.com/documents/e03cbd6ba70198bb46abf961/w/a4a68c9a345d72401b5f4a79/e/c33440971d524f09098b448d

l_p669

Update: Resolving this by modeling using helix+profiles+loft/sweep tools rather than thread featurescripts, to create rising/lowering threads at beginning/end. Slow going, but seems promising.

l_p669

Success. Had to split shaft into three sections to create 3 helix for rising, flat, lowering portion of threads (draft needed to be able to not get confused about direction.) Annoying but worked.

scott_willis165

Not sure what "self block" means?

I generally create the thread, then rotate a wedge shape (matching the slope of the thread profile) about the center axis in Remove mode to cut an entry chamfer and ease bolt insertion.

Is this for 3D printing, machining, or what fabrication method?

nick_papageorge_dayjob

Yeah, way too much work to create the threads manually with a helix. That's 1999 CAD methodology.

If the entry and exit of the thread is not right, use a simple revolve for clearance as mentioned above.