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Post by Roger on Feb 19, 2019 0:16:27 GMT
I've been reading about grinding drills differently for brass for years but still can't picture what people mean by it. Does anyone have a photo or diagram of what a drill ground for brass looks like? Not off hand, but if you think of it in Lathe tool terms it might help. For plastics, you want the top rake to slope at a steep angle. That's the same as a very severe helix on a twist drill. The cutting edge is at the same steep angle so it digs in. If you picture a lathe tool for Brass, the top could be completely flat, ie no slope at all front to back. That forces the chips to crumple up on the edge rather than have a shallow wedge try to pull itself into the work. This is equivalent to a twist drill that has no helix, just straight flutes. That's the sort of drill you find for drilling the hardest of materials. Some even have the slope the other way, all for the same reasons. So back to your question... if you have a normal twist drill, you get that steeply sloping face leading away from the cutting edge because there's a helix that you don't want for Brass, or at least it should be much more shallow. To overcome that problem you just cheat by grinding a small flat on the cutting edge which makes the first short section appear to have no helix. That's all.
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Post by David on Feb 19, 2019 1:59:12 GMT
So this is nothing to do with the clearance behind the cutting edge. The small flat is axial and means a keen edge won't dig into soft material and drag the drill down into it. It might be more like a scraping action.
On the subject of lathe tools given the work is held solidly in the chuck and the cross-slide should be locked against movement how is a problem caused and what is it's effect? I accept there is a problem as people have been working around it for probably over 100 years, I just can't see how the tool can get dragged into the work. Or is it more a problem of the quality of cut and finish on a lathe?
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uuu
Elder Statesman
your message here...
Posts: 2,800
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Post by uuu on Feb 19, 2019 6:25:53 GMT
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Post by Roger on Feb 19, 2019 9:17:11 GMT
So this is nothing to do with the clearance behind the cutting edge. The small flat is axial and means a keen edge won't dig into soft material and drag the drill down into it. It might be more like a scraping action. On the subject of lathe tools given the work is held solidly in the chuck and the cross-slide should be locked against movement how is a problem caused and what is it's effect? I accept there is a problem as people have been working around it for probably over 100 years, I just can't see how the tool can get dragged into the work. Or is it more a problem of the quality of cut and finish on a lathe? Exactly, this is only to do with the angle of the wedge presented to the workpiece. Yes, it's more like a scraping action, and that suits brittle materials. A smooth flow of softer materials is better controlled by a wedge entering the work. There's no such thing as 'solidly' in Engineering, everything is elastic. The more rigid the machine/tool/workpiece combination is held, the less elastic it will be and the less it will grab or chatter. This includes the stiffness of the leadscrew/slide combination which includes backlash. This is why bigger lathes with more metal in them are much more capable than lighter ones, regardless of whether they're cheap Chinese ones or not. Chatter and a tendency to grab are inevitable consequences as you move to ever lighter built machines. Industrial machines are massively rigid in every department for these reasons. A lot of hobby lathes have plain bearings, that that doesn't help with chatter or grabbing either. If you put a fine clock on a precision part held in the chuck of a lathe with plain bearings and turn it by hand, you'll see the spindle rise up on the film of oil as you start to turn it, then settle down again when you stop. Roller bearings are preferable in my opinion, particularly for super precision work because of this.
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Post by RGR 60130 on Feb 19, 2019 9:23:21 GMT
A picture paints a thousand words! Well done Wilf.
Personally I have virtually given up sharpening drills unless I need a Special or chip the cutting edge of one and don't have anything else I can use instead. I buy the Dormer drills from Greenwood tools and every year at the Doncaster show I replace the ones that are getting a bit dull. Usually that means about 5 new drills. The old ones are either saved for specials or passed on to someone else.
Reg
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timb
Statesman
Posts: 511
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Post by timb on Feb 20, 2019 10:02:51 GMT
Perfect illustration Wilf, thank you.
Tim
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