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How Thick Does a Steel Plate Need to be for Countersinking?

Minimum thickness requirements for steel plates in countersinking applications in both 90, and 45 degree applications.

I have a 10mm diameter CSK screw that I would like to countersink into a 6mm thick steel plate (though it is closer to 6.4mm). The Lhead is 6.5mm which is the full thickness of the plate. Are there any issues with countersinking the entire screw head into the plate?
There would therefore be no bearing of the shank on the plate but rather bearing of the head.

 

We generally suggest a minimum thickness of 2mm for the remaining steel on the underside of the countersunk head. The reason for this is twofold: the screw shank will still bear the steel in shear, and to prevent the situation where a very thin steel thickness would act like a knife against the shank of the screw . While the outer diameter of the screw is hardened, we are unsure of the effects of the latter situation. Little, if any, guidance/code, or testing data exists to provide a glimpse into what behaviour would be exhibited in this case.

For a 90 degree screw, the 3mm socket height ensures that the application of force is perpendicular to the shank of the screw. If the socket were not there, the shearing force applied to the sloped head of the countersunk screw within the steel plate would create a separate force lifting the screw up in tension.

 

Fastener head type version diameter Bit confirmed Φ head  d1 d2 t
mm mm mm mm
Ecofast CSK 4 6 RW 40 12 6.5 16.1 4.6353
Ecofast CSK 4 8 RW 40 15 9 18.7 4.6836
Ecofast CSK 4 10 RW 40 18.5 11 22.6 5.6009
VG CSK CSK 4 8 RW 40 15 9 18.7 4.6836
VG CSK CSK 4 10 RW 50 20 11 25 6.7598
VG CSK CSK 4 12 RW 50 22.5 13 27 6.7598

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