Pilot Hole for Threading

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CJ5Dave

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I am making a part in the lathe, which then needs a detent ball socket installed to hold the part inside tubing. The material I am using for the part is unknown steel, but apparently harder than I realized. The detent socket is 3/8 x 16 TPI. My Fastenal chart on the wall says use 5/16 bit for the pilot hole. Really tight, would thread 1/2 turn, lock up, thread back out and clean, etc. Finally broke the tap off. I had threaded some mild steel with the same tap and it seemed tight as well with a 5/16 hole. It is a quality tap I think, an older Craftsman. Is 5/15 the size other charts show for a 3/8 v 16 hole?
 
5/!6 is the proper size for 3/8-16, it is likely that your tap was simply dull, hence the tight fit. Likely it was a carbon steel tap, not HSS and it would not tolerate harder materials.
 
Thanks. I have more but they are import, probably as bad.
 
Dave,

You are using a good cutting oil when running the tap in? If you are dry tapping and the material is hard or sticky, it will give you the results you are getting. The next thing I would suggest is to figure out the material you are tapping. Not knowing can lead to a lot of guessing.
 
Since the material seems hard and it's only for a ball detent, it would be entirely acceptable to drill a slightly larger hole.
 
5/16” is correct for Aluminum (75% engagement), but “Q” / 0.332” is recommended for steel (50% engagement).
 
WITH A GOOD SHARP TAP, THERE SHOULD BE NO TROUBLE IN TAPPING RELATIVELY SOFT STEEL WITH THE COMMONLY USED 5/16" DRILL, IF THE DRILL HAS BEEN HAND SHARPENED, IT IS LIKELY TO CUT OVERSIZE ANYWAY.
Please excuse the capital letters, I did not note that the caps lock was on, and did not want to re type the message ---
 
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