through hole clearance calculation

MrSalts

Joined Apr 2, 2020
2,767
For a machine screw 2 x 8 how would I calculate the hole clearance diameter for a PCB?
Just like for any other hole in any other material. I assume the hole is not plated. If plated, add a bit more clearance. Call your board manufacturer - they may not carry every drill size anyhow.
 
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Thread Starter

clangray

Joined Nov 4, 2018
261
Remember to include extra clearance to allow for mechanical tolerances on whatever to screws are going to be attached to.
So my screw is listed as 2 x 8mm. The "2" must be talking about screw width in mm. Now when I actually measure the screw diameter it comes out as 1.6 - 1.7mm. Which do I rely on? I cant determine the screw clearance hole if I don't know what the actual mm width of it is.
 

MaxHeadRoom

Joined Jul 18, 2013
28,519
So my screw is listed as 2 x 8mm. The "2" must be talking about screw width in mm. Now when I actually measure the screw diameter it comes out as 1.6 - 1.7mm. Which do I rely on? I cant determine the screw clearance hole if I don't know what the actual mm width of it is.
Generally hardware measure slightly different than the declared size due to machining.
 

WBahn

Joined Mar 31, 2012
29,871
So my screw is listed as 2 x 8mm. The "2" must be talking about screw width in mm. Now when I actually measure the screw diameter it comes out as 1.6 - 1.7mm. Which do I rely on? I cant determine the screw clearance hole if I don't know what the actual mm width of it is.
Metric screws normally have three numbers, starting with M.

Such as M2-0.4 x 8.

The measured diameter will be a bit less and will depend on how you measure it. If you use the sharp points of a typical caliper perpendicular to the threads, then the points will sit inside the threads and not be directly across from each other. Depending on how sharp the points are, you may or may not be getting down to the bottom of the thread cuts. If you put the screw between the flats of the calipers you'll be measuring the major diameter.
 

MaxHeadRoom

Joined Jul 18, 2013
28,519
So should I go on my measurements or the machine machine specs? I think you are saying go on your measured specs. Its the rea life values that we need.
Yes, measure the actual hardware you intend using.
As well as a digital caliper, I have my old mechanical Moore & Wright micrometer that has a larger dia anvil, so the measurement is a little more accurate for screw threads.
 

Thread Starter

clangray

Joined Nov 4, 2018
261
Yes, measure the actual hardware you intend using.
As well as a digital caliper, I have my old mechanical Moore & Wright micrometer that has a larger dia anvil, so the measurement is a little more accurate for screw threads.
As a rule of thumb how far away from the PCB can I put a through hole?
 

MaxHeadRoom

Joined Jul 18, 2013
28,519
If you are referring to hardware mounting hole for the board, I usually keep ~2mm or 3mm foil clearance from any hardware mounting, nut or screw maximum diameter.
When using nylon style mounting, concern is not as great.
 

Thread Starter

clangray

Joined Nov 4, 2018
261
If you are referring to hardware mounting hole for the board, I usually keep ~2mm or 3mm foil clearance from any hardware mounting, nut or screw maximum diameter.
When using nylon style mounting, concern is not as great.
I messed up: what I meant to ask -

As a rule of thumb how far away from the PCB edge can I put a through hole?
 
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