Countersinking

[Theo Markettos]

I have some 1.3ish mm steel that I need to countersink some machine screw holes into. The metal currently has some 4.6mm holes which I also need to enlarge to take M5 screws. I don't know the size of the head of the screw (I won't get them for 2 weeks so can't measure them), but they're 6mm long:

is fairly critical - clearance is tight so the screw head cannot be too proud of the surface, but the screw has to take a lot of weight so I don't want to weaken the metal by overdrilling the countersinking. The diameter and length of the screws are also fixed (and M5x6mm is a slightly difficult size).

Ordinarily I'd bodge it by drilling a 5.5mm hole and then (say) an 8mm hole part way through. But I wonder if using a proper countersink bit would help. Do they have a means of stopping them going too deep? In which case, what size should I get? When bits are listed with a shank size, is that the size of the shank of the screw, or the shank that goes into the drill? A random M5 countersunk screw that I have lying around has a 9.1mm head (and a shaft of 4.9mm) - does that mean I want a 9.1mm-ish countersink?

Thanks Theo

[NT]

g:

> Geometry is fairly critical - clearance is tight so the screw head cannot= be

it would give you the correct angle, so yes

countersinks dont come in different sizes, just stop drilling when you get the right hole width.

drill bit shank size

no. Even if you wanted the screwhead perfectly flush, the hole diameter would be greater than 9.1mm. Looking at the screw head shape will show why.

NT

[Bob Minchin]

long:

>>> Geometry is fairly critical - clearance is tight so the screw head cannot be

Standard head size for circular headed screw is twice the nominal thread size ie 10mm, Countersinking that deep in 1.3mm sheet steel will leave part of the head hanging through the hole so the component below will have to be countersunk too otherwise the screw will not be able to be tightened.

Basically M5 is too big for such thin material.

Bob

[Martin Brown]

length that is in stock and use a couple of M5 nuts to cut them to the required length. The nuts have to be on the bolt before you cut it so that after tidying it up with a file you can unwind the nuts and clean off any burrs. Assumes you have a couple of decent files to hand.

No they are basically cone shaped cutters. But into steel it will put up enough resistance to cutting that you will probably be glad when it is deep enough. BTW why are you using brass screws for this if the thing has to carry a lot of weight - much better to use steel bolts.

Don't do a thing until you have the screws in hand and are sure that they are up to the job. I am sure one of the generic local hardware stockist should have something along these lines.

Countersink bits are not that expensive.

Regards, Martin Brown

[Dave Baker]

It's very simple. You can't do it. The screw head is far too big for the thickness of the metal.

head thickness of a csk screw is half the thread diameter i.e. 2.5mm in your case so half of that is going to be sticking out unless you countersink into the material behind the case which will enlarge the 5mm hole in the case and lead to a poorly located and weak fixing. To get the head of the screw flush with the surface the screw will only just be catching the case on its very outermost edge and will pull through if any great load is applied. So basically don't do it.

As to the countersink bit, all that needs to be is bigger than the screw head which is twice the thread diameter in a csk screw. How much bigger is irrelevant. Unless you can countersink on a drill press or mill with a depth stop it's a matter of trial and error trying to make the hole the right size every time.

You'd be better off with panhead screws and find some other way of creating clearance for the heads to stick out.

[The Natural Philosopher]

Classic way is a countersink bit on a pillar drill set up with an end stop.

Some hand drills have adjustable depth gauges..

[Rick Hughes]

yep .. that is a bodge ... drill tip angle is not same as angle for counter sink head. Buy a proper csk bit ...

I use a drill press so can set the deph, otherwise do it by eye.

Best to use some lube oil, csk bits often chatter otherwise.

[Jon Fairbairn]

long:

>>> Geometry is fairly critical - clearance is tight so the screw head cannot be

Yes they do:

Since screw heads do not have a rounded rim (rather than being a sharp cone) the ideal countersink (for metal work) has a straight part to accomodate that curve. The above Farnell search gives an indication of which countersink to use for which size of screw. The Ruko ones seem pretty good. (In fact the Ruko deburring cutters make excellent countersinks for wood, though that?s off topic for this thread)

[docholliday]

I remember being advised to take the bulk of the metal out using a suitable size drill, then use a countersink bit to finish to the correct angle and depth. This was on the basis that drills are easy to sharpen, countersink bits bloody difficult, so make the drill do most of the work. I can't remember if this was in BR Loco works, Derby, or a small engineering shop where I spent the following summer...

[Theo Markettos]

It's to attach a drawer slide to the drawer (but industrial racking not kitchen cabinet). The drawer has a special fitting that is only available with an M5 or an M6 thread. The screw head has to go inside the slide - so is limited in height and width as otherwise the slide can't move over it. Standard M5 pan or cheese head screws are too high and too wide. The slide is sized for M4 (or similar Imperial, not sure) screws, but they won't fit into the drawer. The drawer's fitting is a specially shaped captive nut about 3mm deep and won't accept any thread protruding from the back because they'll foul. The special nut is the only way to anchor to the (non-flat) framework of the drawer. Essentially the geometry is constrained from all sides.

I tried bodged countersinking (making an angle with an 8mm drill in a 6.5mm hole) and it was sufficient to enable the slide to get past the screw, so I really just want to do this in a slightly less bodgy way (there are quite a few of these to drill) with minimum reduction of structural integrity.

Hmmm... that's what I was worried about.

I'm using a pillar drill, so might be able to set up a depth stop. I'll give it a go.

Short of filing down the heads of panhead screws, there isn't any clearance to do much else. Hex head might be slightly better, but it's the height that's more of a problem than the width.

Theo

[Terry Casey]

Is 10.5mm too long?

should solve your head problem but you might still have to cut them shorter ...

[Jon Fairbairn]

Damn. that should of course have read ?Since screw heads have a rounded rim (rather than being a sharp cone)??

[Theo Markettos]

Those look handy, but are they not woodscrews rather than bolts? The thread looks very coarse.

Theo

[Terry Casey]

Ah yes, I think you're right! Its just that I remembered seeing them recently and the M6 bit of the description was what stuck in my mind.

I should have looked at the picture more carefully!