Undersized Tapped Thread

[chartwar02]

I'm threading a high silicone cast aluminum using a Guhring 1/4-28 UNF STI 3B Modified Bottoming TiN coated tap about 7/8" deep. I'm using the same type of tap (5/16-24 and 10-32) with no issues. However, with the 1/4" tap, my GO gage will not go all of the way down. I've tried chasing it several times with a new tap but no success. I'm wondering if there's anything I can try in the machining process to help cut the thread to size.

FYI: I'm circular interpolating the minor diameter to the high end of tolerance. I was worried that the drill was wondering and creating a "bent" and/or out of round hole.

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[Jim Dawson]

Maybe use a 2B or 1B tap?

You might want to take a look at this article

The Relationship Between H-Limit and Class of Fit

[chartwar02]

Jim, that article was really helpful! The customer requires a 3B tolerance. I'll hunt down a larger "H" limit and see if that helps.

[Jim Dawson]

My pleasure to help out. I had this same problem about 45 years ago. All of my tapped holes were undersize, interestingly enough I was working in high silicon cast aluminum also, but back then it took a call to the tap manufacturer to sort it out. Ain't the Internet great