Thread relief groove dimension

[patdttr225]

Hello Group,

I spec'd. out a 7/8-20UNEF male thread on an aluminum part and called out for a Ø.800 ±.005 thread relief groove. The shop doing the work said the groove wasn't small enough for the tool not to clear. Machinery's Handbook says minor dia. is .8155. I am .015 smaller than nominal, where did I go wrong and how does everyone else calculated thread relief grooves?

Thanks in advance,
Pat

[Konrad]

I don't calculate, Im a machinist and it's given to me..
If I was doing the work for you I wouldn't complain, the tool should clear,...if the tool might have a super sharp V point, then it might just scratch the groove?
The tool should always have a slight radius on the end then it should stay clear.
Konrad

[patdttr225]

Hello Konrad and thanks for the reply.

They said the tool would just scratch the surface and they used an CNC lathe with a standard threading insert, so I assumed it would clear also. I may back the diameter off by another .005 or .010 just so there are no questions in the future.

Hey, my wife is Canadian and from Edmonton. Her dad/family left there in 1963 and came to So. California to work in the oilfield machine shop business.

Thanks again,
Pat

[Konrad]

Pat, sure, another .010 or .015" wouldn't hurt, just make sure you have a Rad. U undercut.

Interesting...Edmonton now is a great place doing business in the oilfield, especially for the tar sand in Fort McMurray.
My daughter has a job there in chemical engineering.

Konrad