Reply

How to calculate Thread depth and Hole depth clearance.

Hello All,

I need some help regarding Thread depth and hole depth clearance.

How do I calculate the thread and hole depth clearances if I know the screw/bolt length? What formula do we use for these clearances?

 

I got the information below from http://www.me.metu.edu.tr/courses/me114/Lectures/screw_threads.htm

Just want to make sure if this is correct and applies as a general rule.

Specifically, I want to know B and E from the image below.

Any help would be greatly appreciated.

4 REPLIES

Re: How to calculate Thread depth and Hole depth clearance.

didn't know there was a differance between "Hole Tapped" & "Cap Screw in tapped hole" We have tapes sizes and depthes set up for all of our threads. When we calcuate a bolt length it's no more than 2xD and no less than 1-1/2xD. we stay around 2X. we show the tap driil about a 1/8 longer the tread.

Re: How to calculate Thread depth and Hole depth clearance.

[ Edited ]

If anyone wonders "n" = pitch. Edit: threads per inch. For metric it would be 4*n instead of 4/n

 

@TDB,

It seems to me that you can't really use 1/8 as a constant. The lead-in chamfer to full thread of a tap will vary. You have to know the tap design and should also allow for overtravel and chip build-up in the bottom of the hole.

The machinist no doubt makes allowances but it could be a problem in "don't break thru" cases, etc.

Bruce Shand
ST9 MP8 - Insight - Win10 - K4200

Re: How to calculate Thread depth and Hole depth clearance.

I'll jot down all my rules-of-thumb for blind threads.

 

Thread Depth:

Thread depth (C & A) is 2x nominal diamter (d) for Aluminum and 1.5x nominal diameter for Steel.

 

Thread Clearance:

Thread Clearance (B) is at least 2x thread pitch.  This is because you assume that the last two threads of a tap will not result in a full thread of engagement, so you need to "overshoot" the tap in order to get the actual full-thread depth.  This wikipedia page has a good picture of a typical tap, and you can see the chamfer that will not create full threads.

https://en.wikipedia.org/wiki/List_of_drill_and_tap_sizes

Exampe: I'm using a .250-20UNC.  There are 20 threads per inch, or .05 inches per thread.

.05 x 2 = .1 inch drill depth minimum beyond your full-thread depth.

 

Drill Depth:

The drill depth (E) is the same as the clearance above.  In order to get the full-thread depth per the requirement, the chamfered tip of the tap must have room to extend past the threaded portion.


____________________________________
--Scott Wertel, P.E.

Re: How to calculate Thread depth and Hole depth clearance.

Here the tap drill depth dosn't matter to a point, it's up to the machinest on what depth to go. We call out a thread depth and they have to meet it. So if they want to drill through they can even knowing the drawing doesn't show it. If we don't want them to drill through we will note it "DON'T TAP DR THRU" this is common in our industry