You want a different rake for drilling brass and bronze, which can be achieved by stoning the cutting edge with a small stone. So not actually blunt, but with the opposite rake to what you would use for steel. On a drill the rake for steel is built in by the spiral angle but it is enough just to stone the cutting edge back so that you can see a small flat on the spiral side, not on the end.

If you think of it like lathe tools, for steel the top of the tool slopes up towards the cutting edge. This peels off those nice spirals. On brass, such a tool will cause a large force dragging the tool deeper into the job, so by instead sloping down towards the cutting edge we reduce the dragging in force. The brass will sheer off in small chips and everything will behave itself.

The rake does not need to be very large, maybe 5 to 10 degrees. Ideally you would have a separate set of drills for brass, and I believe drills have been made with a different spiral angle for brass, but modified normal drills are fine for most amateur work.

John

Edited By John Olsen on 14/12/2021 02:30:21

Another vote for stoning the cutting edges. I do mine with an extra fine diamond slip (credit card size) with zero rake. Essentially straight along the length of the drill. However D A G Brown, famous for drill sharpening demonstrations at shows with the Quorn maintains that a correctly and very sharp 4 facet drill can be used on brass with no problems. My take on this is with a sharp drill less pressure is required in order to start a cut and so the depth of cut is smaller. In consequence the first facet is able to control the cut. I suspect that when the self infeed force gets bigger the controlling effect of the first facet has reached it's limit and is overcome and the drill is pulled into the work.

The self feed effect is much more pronounced on breakthrough when drilling brass and sometimes aluminium so care should be taken. For holes in sheet where I am not too fussed about getting the position of the hole to any high degree of precision I generally drill a small pilot hole and then follow up to final size by drilling half throgh the sheet, turning over and through drilling from the other side. This avoids the breakthrough 'grab' and also any tendency to create burrs on the exit side creating a good sharp edged hole. This is especially usefull in aluminium and in dural no deburring at all is required. For brass the point is more to avoid grab even with a stoned drill.

Brass drills used to be produced having straight flutes which when you think about it is exactly what you produde when you stone the cutting lips.

regards Martin

Stoning the cutting edges has never really worked for me, and ruins the drill for anything else. I prefer to use slow helix drills specifically designed for brass. I have a small collection of the sizes I use most often. Brass isn't too bad for snatching, bronze, and especially gunmetal, are much worse.

Andrew

For very small holes I use spade drills. On a course at BHI years ago we were shown how to make them from sewing needles. Over the years I have collected quite a few straight fluted drills

Roy

Totally agree

Tony

+1 – both in the lathe and mill.

Andrew

Hi, I my early days of work I had to drill a row of holes in a 2" x1" brass block and had the same problem, then a machinist told me to just touch the edges on the grindstone and that cured the problem and all the holes were drilled without any other issue. Getting any other kind of drill them days was not an option, especially for a one off job that needed to be done.

Regards Nick.

Edited By Nicholas Farr on 14/12/2021 12:49:51

I agree that minimising the pilot hole size or avoiding one altogether is good. Sometimes however, if it's necessary to enlarge an existing hole, this can best be done safely by stepping up the drill sizes in very small increments rather than in one go.

As usual lots of good advice but sadly mainly of the "stone" the edge variety to "blunt" the drill

I have seen reference to Rake and suggestions that you can alter this easily. Rake is the angle of the spiral relative to the cutting edge. For brass this can be an important factor. Slow helix/spiral drills have the rake required for drilling brass. Modifying the point by stoning or honing simply alters cutting edge clearance which is not the same as rake on a spiral drill.

If you intend to drill brass, of any type, I would suggest an investment in some "Brass Drills" (not made from brass obviously. LOL) and these are readily available at no great expense from our friends at Tracy Tools. These will have the helix, point and clearance required in a single drill bit. HTH.

You have clearly not understood the process, or so it seems. The material is removed from the face of the drill (inside the flute) which alters the rake. As I said I use a diamond credit card slip to achieve this and obtain a sharp drill with the same rake as a straight flute brass drill. I do agree with you that blunting the drill doesn't really get you where you want to go. I also agree that brass drills and slow spiral drills designed to cut these materials are the ideal. For my clockmaking though which is generally dealing with holes below 6mm I have a set of drills modified for brass as I have described and as they work well for me is cheaper than buying a complete set of specials.

regards Martin

At the Government Training Centre they taught me to grind a zero-rake 'platform' about 0,5 – 1mm wide to defeat the rake at the cutting lip. It's much like JasonB's recommendation above, but – for me at least – quicker and easier to achieve. It resolves the snatching problem completely. Time spent learning to get the bench grinder to do what you want is never wasted.