Pilot Hole for Threading

And since it is just a ball detent that will never have a significant load on it you could even go a bit bigger than the "Q" IF it is still tight, you could try a "R" / .339 or even an 11/32" / .343.

If you have a good tap the "Q" should be fine.

It is rare that you will find a good tap at a hardware or big box store. Craftsman never had good taps.
 
CJ5Dave said:
Thanks. I have more but they are import, probably as bad.
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Quality spiral flute taps are absolutely amazing. They cut through your material so much easier than conventional HSS taps.
Less chance of breaking one too.
I bought a few common size, YMW or Yamawa taps. They are a Japanese Mfg.
Not cheap, but I've used a 1/4 -20 tap for going on five years Still going strong.

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I second the use of Spiral Flute Taps. and the larger hole for the use to which it will be put.
 
CJ5Dave said:
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?
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while 5/16 is the common recommendation, it's got a very high engagement at 75 ish percent, which means a nearly full chip coming out. 21/64 is closer to 60 ish percent, which is very recommendable for standard use on general steels, for fasteners seeing "real" loads. Given your application, you could (probably) do this with with a 2-56 screw if you wanted to close the hole back up after the ball was in.... I'd have no problem going to a Q drill (50 ish percent) or even (slightly) more. I'm talking about your detent here, not a structural bolt....... That detail is important.....

I'll also (kind of) second @Janderso in that machine taps do WAY better than hand taps in hard materials. I say "kind of", because my go-to is gun taps. In a three flute variety they are the strongest taps you can buy. The angled shearing cut (gun taps OR spiral flute) makes hand cutting far easier. I prefer the stronger gun taps because I usually either hand tap, or tap in a loose, floppy, hard to control drill press, which means inevitibly finishing the operation by hand..... In a rigid machine, spiral flute does fine. Hand tapping (including finishing what you started with a floppy crooked drill press....) the spiral flutes dont' like to back up and start again. They prefer to be driven all the way home, and backed out all the way if they're backed out at all. Personal preference probably, but either my gun taps (spiral POINT taps) or Janderso's spiral FLUTE taps are hands down winner over a hand tap any day of the week, and twice on any day of the week that you're working on gnarly mystery steel, which probably has it's own flavor of tap with it's own special coating but you don't know what it is to even consider if it's worth buying the right stuff......

There's a lot of ways to skin this cat. This is just my two cents. Take it for what it's worth.
 
Maybe we should revise the title of this post to TAP DRILL SIZE, and not PILOT DRILL SIZE. a pilot drill suggests making a hole that would be later enlarged.
 
Well, we are hobby machinists. A one off part, to be given away, use the steel and taps I have hopefully. Found a better quality tap in my stash. Hate to buy 20 ft of steel to make a 3 inch long part and give it away.
 
mmcmdl said:
I have a bunch of this unknownium and unobtanium laying around the garage . :)
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Can’t be unobtanium if a) it is also unknownium and 2) you have it!

Also why I mark stock when I “obtain “ it. Which reminds me, I need to find a chart of the colors they use to mark rod stock.
 
The tap drill size charts are a convenient simplification and usually work fine. But when things are getting a bit more difficult and you want to play with thread percentages (as ChassC & Jake pointed out), I use a web calculator like http://theoreticalmachinist.com/TapDrillSizeCalculators
Guhring points out that thread strength is not directly proportional to percentafe and that 60-70% greatly increase tool life and lower required torque.

BTW, some of the older Craftsman taps (Kromedge) are among the better tap sets in my opinion -- but I learned to stay away from sets and buy quality taps in the size I need. I came across the reasonably priced YG-1 spiral flute combo taps for machine use (specifically T5C series for metric and T2C series for inch) and quite like them. Otherwise Guhring is my personal go-to. YMMV.
 
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