|Robin Graham||14/12/2021 01:53:46|
|945 forum posts|
I'm sure this has come up before in general, but here is my particular problem.
I work with brass (CZ121) a bit and haven't had any problem drilling small holes (below 6mm) on either the lathe (drill in tailstock chuck), mill, or pillar drill using normal jobber bits. But last night I tried to drill a 9.7mm hole through a 10mm thick brass disc on the lathe in preparation for reaming to 10mm and it was a bit of a nightmare. A 6mm pilot went through OK, but the following 8mm just took off on its own and dragged the chuck from the tailstock. It felt a bit like climb milling when the tool takes charge over the feedscrew.
I was going at 330 rpm.
I ended up digging out a old set of Screwfix drills (useless for drilling steel) and further blunting the cutting edges on the 9.5mm by more or less random swipes with a diamond slip. It actually worked well - beginners luck maybe! And the hole reamed from 9.5 to 10 mm without problems.
I got away with it, but I'd like to know how to do it properly, so any advice would be welcome,
Edited By Robin Graham on 14/12/2021 01:58:10
|John Olsen||14/12/2021 02:29:17|
|1240 forum posts|
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.
Edited By John Olsen on 14/12/2021 02:30:21
|Thor 🇳🇴||14/12/2021 06:08:16|
1598 forum posts
You have discovered that when drilling brass the drill tends to grab the work with more or less disastrous result. As John says you must stone your brass drills, these stoned drills work well on other materials like gunmetal or copper. Have a look at these links:
|Martin Kyte||14/12/2021 08:50:25|
2721 forum posts
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.
|Martin Connelly||14/12/2021 08:55:05|
2123 forum posts
I have had a screwed shank Ø6 end mill unscrew from a Vertex Posi Lock chuck when working with brass. I only figured what was happening when I spotted the end mill suddenly stop rotating with the chuck just for a moment as it dug in and was screwed back in. I had to resort to pre-loading the end mill into the chuck, lots of discussion in threads in this forum about how these chucks should be used and pre-loading is not usually recommended. For large holes in plate I find broaching cutters are good but a hole saw may work if you can clean up the hole afterwards.
When I was at work there was a lot of copper bus-bar drilling. We had to use drills specifically designed for use on copper (Dormer I think) because of the grabbing of standard drills. I don't think you could justify the cost unless you did a lot of same sized holes in copper or brass.
|Andrew Johnston||14/12/2021 09:02:54|
6574 forum posts
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.
|David George 1||14/12/2021 09:14:09|
1808 forum posts
Robin I have seen many jobs ruined over the years by people not knowing about drills cutting angles and front rake grinding especially an apprentice drilling a brass plate on a pillar drill hand held. On brass drilling the drill needs to have a very small or negative front rake angle and you can buy such drills but you can just grind the angle on a standard drill. I have seen some people just stone a small flat on to the cutting edge but that can be overcome if you drill heavily and if you regring the drill to sharpen it the flat is removed. I always grind a substantial flat on the cutting edge and keep that drill for that use only. Here is a picture of some of my brass drills.
The one second from the left is a as bought brass drill and the others are modified by me brass drills. I would always for safety recommend a substantial flat on the cutting edge.
|roy entwistle||14/12/2021 09:16:44|
|1504 forum posts|
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
|2487 forum posts|
From experience, I think your mistake was an overlarge pilot making it easier for the larger drill to grab.
Again, the problem is likely to be less on a more massive machine.
|Tony Pratt 1||14/12/2021 11:08:40|
|1926 forum posts|
22560 forum posts
Like Andrew I don't stone my drills. I'll either nip up the tailstock quill lock to offer some resistance or if drilling on the mill use the head to feed rather than quill. If I do feel there is a risk then I'll get a 2 or 3-flute milling cutter out and use that.
|Andrew Johnston||14/12/2021 11:43:49|
6574 forum posts
+1 - both in the lathe and mill.
|Jon Lawes||14/12/2021 12:11:35|
872 forum posts
Again I'm learning. I assumed the bigger the pilot hole the better, but what is said here makes perfect sense when subjected to scruitiny.
|Nicholas Farr||14/12/2021 12:49:31|
3310 forum posts
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.
Edited By Nicholas Farr on 14/12/2021 12:49:51
|Andrew Johnston||14/12/2021 12:51:04|
6574 forum posts
If using an ordinary drill it's best to drill without a pilot hole and use plenty of pressure to keep the feedrate up.
|Clive Brown 1||14/12/2021 13:16:21|
|807 forum posts|
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.
|Pete Rimmer||14/12/2021 13:18:00|
|1219 forum posts|
Pilot holes should never be more than 1/4-1/3 the next drill size IMHO. Especially true for drilling a grabby material like brass.
|232 forum posts|
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.
|not done it yet||14/12/2021 13:20:48|
|6719 forum posts|
I generally use milling cutters if I need to enlarge a hole, with good precision.
|Martin Kyte||14/12/2021 13:59:18|
2721 forum posts
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.
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