countersinking specs

[MerDeNomsX12]

Im looking to find out the specs for countersinking through thin material. Meaning, if i need to countersink for a number 8 screw, i cannot use a number 28 drill bit for my pilot hole because when i countersink the hole will be too large and the screw will fall through. So what size pilot hole do i use for each screw size?

 

[eromlignod]

Look up clearance holes for socket head capscrews in the Machinery's Handbook. These clearances are universal no matter what the head style.

Then take a look at the dimensions for flat-head screws. If you want the screw to be flush you have to countersink to the "theoretical sharp" value. Go by the outer diameter of the resulting c'sink and you can't ever "fall through".

If your material is thinner than the head height of the flat-head screw, then the head will penetrate past the bottom of the sheet. This will require that you also make a small countersink in the tapped part that you are attaching to. It also means that the resulting clearance hole in the sheet will be larger than the recommended one, but as long as you go by the c'sink OD, you should be OK.

Don
Kansas City

 

[mcgyvr]

You might be better off using "undercut" flat head screws for thin sheets. Then you are not required to c'sink the mating parts.. (obviously all depending on the size of the screw and how thin is thin.

 

[KENAT]

I'm having trouble following your question.

28 drill = .1405
#8 = .164

So a #8 screw wont fit through a .1405 hole.

You must be completely eliminating the hole with the size of your CSK, which makes sense as you call it a pilot, so changing pilot drill dia wont make a difference.

I think the above 2 posts should cover you though.

Machineries gives suggested 'clearance' holes, for #8 I believe this is .194 (10 drill) for a 'normal' clearance.

Size of CSK is driven by head sharp diameter assuming you want the resultant fit to be at or below flush.

Max sharp head dia for #8 hex flat head is I think .359 (phillips etc are slightly smaller). So you can do the math at 82° or 100° and work out at which point you need to CSK the mating part or use undercut screws.

KENAT, probably the least qualified checker you'll ever meet...

 

[mcgyvr]

Standard hole/c'sink size for standard #8 flat head screw
.177 thru hole x .335" x 82deg C'sink.

Minimum material thickness to not effect the .177 thru hole is .0892" material thickness. Thinner sheets and the thru hole diameter will just get larger (which is typically fine because the c'sink will center the screw)

Head height of the head for a #8 flat head is .100" (82 deg C'sink). So anything under .100" thick material will require a c'sink in the mating part.

Head height of undercut #8 flat head .072 (100 deg C'sink)

Get yourself a Machinery's Handbook.

Also discuss with the vendor on what they have for c'sink tooling for their fabricators.

Here is a list from one of our vendors of all their tooling
.079 X .157 X 90 DEGREES
.079 X .157 X 90 DEGREES
.105 X .148 X 100 DEGREES
.105 X .148 X 100 DEGREES
.106 X .197 X 90 DEGREES
.106 X .197 X 90 DEGREES
.106 X .217 X 90 DEGREES
.106 X .217 X 90 DEGREES
.120 X .190 X 90 DEGREES
.120 X .190 X 90 DEGREES
.125 X .218 X 82 DEGREES
.125 X .218 X 82 DEGREES
.125 X .230 X 82 DEGREES
.125 X .230 X 82 DEGREES
.126 X .236 X 90 DEGREES
.126 X .236 X 90 DEGREES
.129 X .230 X 82 DEGREES
.129 X .230 X 82 DEGREES
.132 X .250 X 120 DEGREES
.132 X .250 X 120 DEGREES
.134 X .236 X 90 DEGREES
.134 X .236 X 90 DEGREES
.136 X .230 X 82 DEGREES
.136 X .230 X 82 DEGREES
.136 X .240 X 100 DEGREES
.136 X .240 X 100 DEGREES
.138 X .236 X 90 DEGREES
.138 X .236 X 90 DEGREES
.140 X .230 X 100 DEGREES
.140 X .230 X 100 DEGREES
.140 X .235 X 82 DEGREES
.140 X .235 X 82 DEGREES
.147 X .234 X 82 DEGREES
.147 X .234 X 82 DEGREES
.147 X .234 X 82 DEGREES
.149 X .234 X 100 DEGREES
.149 X .234 X 100 DEGREES
.156 X .260 X 90 DEGREES
.156 X .260 X 90 DEGREES
.156 X .356 X 82 DEGREES
.156 X .356 X 82 DEGREES
.156 X .360 X 82 DEGREES
.156 X .360 X 82 DEGREES
.157 X .236 X 90 DEGREES
.157 X .236 X 90 DEGREES
.162 X .280 X 90 DEGREES
.162 X .280 X 90 DEGREES
.169 X .289 X 82 DEGREES
.169 X .289 X 82 DEGREES
.173 X .250 X 82 DEGREES
.173 X .250 X 82 DEGREES
.186 X .289 X 82 DEGREES
.186 X .289 X 82 DEGREES
.188 X .332 X 82 DEGREES
.188 X .332 X 82 DEGREES
.201 X .338 X 82 DEGREES
.201 X .338 X 82 DEGREES
.204 X .303 X 82 DEGREES
.204 X .303 X 82 DEGREES
.213 X .299 X 90 DEGREES
.213 X .299 X 90 DEGREES
.213 X .413 X 100 DEGREES
.213 X .413 X 100 DEGREES
.213 X .433 X 100 DEGREES
.213 X .433 X 100 DEGREES
.218 X .395 X 82 DEGREES
.218 X .395 X 82 DEGREES
.218 X .402 X 82 DEGREES
.218 X .402 X 82 DEGREES
.221 X .395 X 82 DEGREES
.221 X .395 X 82 DEGREES
.221 X .402 X 82 DEGREES
.221 X .402 X 82 DEGREES
.235 X .275 X 100 DEGREES
.235 X .275 X 100 DEGREES
.236 X .276 X 100 DEGREES
.236 X .276 X 100 DEGREES