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Hi
I’m trying to understand thread but I’m not having a lot of luck.
We are trying to cut a 4-1/2-12 UN -2A on a brass part and we are having trouble getting
The O.D. to size, it is cutting it under the min. by .006. We are using a iscar thread mill that is Ø.625 with a 12 pitch.
I have looked in the Machinery’s handbook and tried to figure how I can check to see if the pitch line is
In the right location on the thread mill, and I’m having no luck. Can you tell me how muck clearance the pitch
Dia. gets from the int. to the ext. thread, and what determines how much clearance it gets.

Since you are cutting external threads, have you considered that your stock is undersized?

If this is not the case they you may need to adjust your program to compensate for the error.

Have you contacted your ISCAR supplier or ISCAR directly for technical assistance?

As for measuring the threads to confirm the dimensions the best way to do it is to use go/no go gauges.

Alternately you can measure the threads over wires.

For more information on measuring threads you need to look at the ASME/ANSI standards

ASME/ANSI B1.2-1983 Gages And Gaging For Unified Inch Screw Threads
ASME/ANSI B1.3-2007 Screw Thread Gaging Systems for Acceptability: Inch and Metric Screw Threads (UN, UNR, UNJ, M, and MJ)

These standards will provide you with more information than the summary contained in Machinery's

Niel Leon
engineering.com