Am I doing this calculation right? I have a 1.485 hole threaded 28 tpi. I wand to screw a cap into that hole. I think I should turn the cap down to 1.512 and then thread it 28 tpi to fit with 75% threads. That's if I'm doing the math right/using right formula to figure it out.

OD thread - ((.01299 * % thread)/TPI) = Hole size
OD thread is what I'm solving for so:

X - (.01299 * 75)/28 = 1.485
X - .0348 = 1.485
X = 1.485 + .0348
X = 1.512

It hurts my brain because it just doesn't look right. If I have 1.512 OD & 28 tpi threads are about .024 deep I have 1.152 peaks & 1.464 valleys on the male part. If I have 1.485 ID & 28 tpi threads I have 1.485 peaks & 1.533 valleys on the female part. Should there be a .021 difference? I know they can't be the same or they'd never go together with no clearance but .021 seems like a lot.

Might help if I did the simple math right.......

OD thread - ((.01299 * % thread)/TPI) = Hole size
OD thread is what I'm solving for so:

X - (.01299 * 75)/28 = 1.485
X - .0348 = 1.485
X = 1.485 + .0348 X=1.520

It hurts my brain because it just doesn't look right. If I have 1.520 OD & 28 tpi threads are about .024 deep I have 1.520 peaks & 1.472 valleys on the male part. If I have 1.485 ID & 28 tpi threads I have 1.485 peaks & 1.533 valleys on the female part. That's a .013 difference. Does that sound right?

Do you have some scrap on hand to use for a test? If so, and you get it right, you'll end up with internal and external gauges on hand to check your work when you cut the real thing.

Stan1 wrote:Do you have some scrap on hand to use for a test? If so, and you get it right, you'll end up with internal and external gauges on hand to check your work when you cut the real thing.

This is what I do.

Try this it works every time.
28 tpi crest to root = .024
.024x2-.005+ID=OD of plug with a thread depth of .024