Personally I would grind a 5mm drill to the correct angle and stone the flutes to give zero rake. That way if the depth varies a little you will still be on diameter. You don't say what will be filling the holes but usually screws have something of a thickness to the lip of the countersink, they are rarely dead sharp so using a drill will allow for that last little bit to be sunk bringing the heat dead flush with the surface. It's a much prettier way of doing things than countersink bits where any extra depth varies the width of the sink and to get the screws flush you have to go too deep in any case which leaves an annulus round the head. Set your depth stop on the drill press/mill and everything will be identical.

regards Martin

Edited By Martin Kyte on 14/03/2021 11:07:11

Make yourself a “clickspring” one they are extremely good on brass, one cutting edge and zero rake. Made myself 3 in different sizes and only use them on brass MAGIC.

My understanding is that 'general purpose' drills for brass and bronze are ground with a chisel angle of 118°.

Select a drill of the same diameter as the head of the screw being used in the countersink (180° – 118° = 82° QED) and use the drill to give the required profile and include a depth of cut such that the head is flush, as described by Martin Kyte, above. Better still, I think, would be to drill the 'countersink' first, then follow up with the through hole to reduce the possibility of drill wander.

John

UNC & UNF are 82deg

I googled 82 degree countersink and got lots of hits. Five flute countersinks were easy to get hold of and would be my choice. Practice on a spare piece of brass to get the optimum speed which should be slow to minimise chatter.

If you make your own out of old drill bits, it would be best to make them as short as possible to increase their stiffness.

Using a larger 2 flute drill bit to centre on the hole does not always work because it sometimes digs in more on one side than the other (both flutes have to be ground exactly the same to cut equally), producing an offset chamfer. Even worse with a long slender drill that can flex. Using a rigid cutter, held on centre with the hole is a better option. Or a cutter with 3 or more flutes if held by hand.

This type on the left works well, as it has a large flat area for locating on centre with the hole. For larger holes I use the type with a hole thru the side. (these are easy to sharpen by grinding the cone end)

Edited By Paul Lousick on 15/03/2021 03:06:31

It's best to modify twist drills to give zero rake to the cutting edge for working on brass, even for small sizes. It gives a much better finish. The modification process is very simple, see here:

Clickspring video

Russell

I have a nice selection of countersinks with integral guide pin, they all are made to suit standard clearance holes.

In use I use a slow speed with no chatter and of course they can be set to depth on the drill. so can make the c/sunk flush with the surface or lower.