Reply To: Single Point Threading

[Bob Tascione]

Hey David,
I think William may be referring to the theroretally perfect thread where a flat (or radius) is cut into the valley of the thread. In normal home type machining practice William this is usually considered unnecessary and a sharp 60 degree pointed V shape cutter is used. Main reason is that the dimension of the flat will change with each thread pitch and diameter. I think it’s like 1/8 the pitch of the thread (not sure so someone please correct me on that one). That’s a lot like the helix angle which also changes with pitch and diameter. The flat or radius is important when the thread must be exact but in most home and basic machining applications it’s often considered unnecessary and overlooked and as David suggests just truncating the threads a tiny bit will take care of any friction using the sharp tip would cause. I’ve got a good link somewhere to a site with an online calculator for calculating the helix angle and flat if you would like it William. It’s on my other laptop so I’ll fire it up in a bit (after breakfast!) and will post it up here for you.
On a side note it’s not a bad idea to grind a tiny radius at the end of the tool with a stone to help keep the cutter from dulling or breaking and to relieve stresses that the sharp angle will leave at the root. This can help keep the part from cracking at these sharp edges – especially if heat treating the part.
I’ll be back with that link (if I can find it) in just a bit.
Bob