View Full Version : threading calculation

02-17-2005, 01:23 PM
I would like to cut a 24 TPI, 60 degree female thread to match a male thread which has an outside diameter of 1.365 inches. What should I bore the inside diameter of the female part to start threading? Could someone tell me how the calculations are made? Godbless, Ralph

Allan Waterfall
02-17-2005, 01:46 PM
The way I would do it would be the OD measurement less two thread depths. i.e. if your thread depth is .025" the bore needs to be 1.315" .

I don't mind if someone corrects me on this.


[This message has been edited by Allan Waterfall (edited 02-17-2005).]

02-17-2005, 02:11 PM
I was taught to throw in an extra .005 for clearance.

As to "how the calculations are made..", assuming you're talking about unified standard threads, the single depth of thread is calculated as .6495 X pitch (in your case .6495 X 1/24 ~= .027, so two thread depths = .054 + the .005 clearance = .059

[This message has been edited by lynnl (edited 02-17-2005).]

02-17-2005, 02:17 PM
Just like tapping a hole, you want the "tap drill diameter." You can figure out the theoretical hole diameter for any thread percentage using the following formula:
Hole Diameter = Bolt Diameter - 2 x K x Pitch x Percentage
K = .6000 for BA threads
K = .6403 for BSF and BSW threads
K = .6134 for ISO Metric threads
K = .6495 for NC and NF threads
For example, if you want to find what theoretical tapping hole diameter will yield an 80% thread with a 2BA tap, the numbers (in inches) are:
.1544 = Hole Diameter =.185 - 2 x .6 x .0319 x .80

In your case, it would be:
x = 1.365 - 2 x .6495 x (1/24) x .80

Actually, since you're not tapping the hole, you might use .90 (90%). I suspect you want to allow at least a little (not do 100% thread), just to be sure you have some clearance.

[This message has been edited by SGW (edited 02-17-2005).]

02-17-2005, 03:29 PM
The theoretical thread depth is half the pitch divided by the tangent of half the angle. In this case it would be 1/2 x 1/24 / (Tan (30)) or 0.03608"


02-17-2005, 04:12 PM
Bill, wouldn't that formula be for a sharp "V" thread?

02-17-2005, 05:30 PM
Simple rule of thumb is 1 divided by TPI subtracted from OD.

1/24=0.04166 1.365-0.0142=1.323

Close enough, especially if you have the male thread to test fit.

02-20-2005, 12:22 PM
Thanks to everyone for your assistance with my problem. I did follow the specifics posted by SGW with success. Godbless, Ralph