Screw hole size

[nichzimmerman]

There are plenty of places on these aircraft that use machine screws. I'm embarassed to ask such an elementary question, but I couldn't find a definitive answer through Google.

What drill bit size is considered "best practice" when drilling for these screws (#4's, #6's, and #8's)? I can find the actual diameter of the screw on Google (and using a caliper) but I imagine the hole should be slightly larger and I'm sure there is a table out there somewhere that specifies this. Is this in 43.13B? Thanks,

-Nic
#1217

 

[S Lathrop]

The information about clearance hole sizes can be found in the Machinery's Handbook under clearance hole sizes for machine screws.

 

[Zzz]

The # sizes correspond to number drill sizes, Those are the through hole sizes for that size screw. So for a #10 screw, you use a #10 drill for the clearance hole.

Do you use a #8 drill to drill for a 8-32 screw?

 

[nichzimmerman]

Thanks for the response. My Google skills are challenged, but the drill charts I can find show that #6 and #4 drill bits are larger than 0.2" while the screw sizes are much smaller, like 1/8" or less. Larger #d drill sizes represent smaller sizes, and vise versa for screws. What am I missing?

 

[JBC]

Google "clearance hole for machine screws". There are a number of charts out there to show the "tight fit" and "free fit" hole size for each screw size.

John Ciolino
Patrol

 

[S Lathrop]

The number size is the major diameter of the screw. That is the diameter if the sides of the threads were extended to a point. Because the threaded proton of a screws will not equal to the major diameter of the of the screw, the major diameter will give plenty of clearance for a screw.

And you can go tighter but you need to look that diameter up or calculate it based on the pitch diameter of the screw thread you are working with.

 

[Flygirl1]

Nic, I found this one: http://eberhardt.bz/GME_Wood_Land/GM...rill_table.pdf. I know it says "wood" in the link but the title on the chart says machine screws.

 

[Bdflies]

Nothing. I think you got it just about right...

Bill

 

[H. Ivan Haecker]

#6 screw, use a #28 drill bit.
#8 screw, use a #19 drill bit.
#10 screw, use a #12 drill bit
I can't remember what the clearance hole size is for a #4 screw, but if you need it, I can post that later

 

[S Lathrop]

#4 uses a #32 drill for close fit and a #30 for free fitt.

I apologize for posting wrong information. I got the above numbers information from a copy of the Machinery's Handbook. And I should have looked in the correct place the first time.