Proper method to countersink fasteners

[angelfj1]

I think this is a very basic question, so I apologize in advance if this seems too elementary! I am assembling a project and I need to mount a number of heavy sub-assemblies to a 1/2'' aluminum plate. I believe the alloy is 6061. The heaviest item weighs 20 lbs and will be attached by two screws. I am using 1/4-20, SS, socket-head screws and the head must not protrude above the bottom surface of the plate. The "items" to be mounted on the top surface of the plate are drilled and tapped to accept the 1/4-20 screws. The heads of these screws measures 0.222 in height, which would require a counter bore of at least that amount. This depth would represent approx. 44 percent of the material thickness. When the project is completed, the vertical orientation of the plate will cause the screws to be in shear.

My question: does this leave adequate material for torquing down on these screws. Is there a generally recognized method (rule of thumb) to establish the maximum depth of a counter bore, before risking tear out of the material?

Thanks in advance.

Frank

 

[Tom Griffin]

Frank,

I don't know about a rule of thumb, but a 1/4" flat head screw in a 1/2" plate holding a ten pound load is WAY on the safe side, especially since the screw is in shear. I used to do a lot of aircraft sheet metal work and we would dimple sheet metal for countersunk fasteners, so essentially you would have the screw flush in a .040" or .050" sheet and it would have adequate pull out strength. Sounds like you are good to go.

Tom

 

[DMS]

Your risk of pullout is going to be on the threads, not the head. If the threads hold up, and the fasteners don't shear, you are good. 10lb per fastener is well within the capabilities of a 1/4-20. Just make sure you have at least 4 full threads engaged (more is better).

 

[angelfj1]

Thank you all very much!!!)

 

[DMS]

One thing that just occurred to me; you may want to put use some blue loctite on the fasteners for final assembly. It is removable, but will keep things from vibrating loose.