JonL
Elder Statesman
WWSME (Wiltshire)
Posts: 2,990
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Post by JonL on Jan 20, 2019 21:24:09 GMT
I'm pretty sure I used to be able to do this, so why has this skill deserted me!
I tried to sharpen a drill today, 7.5 mm Dormer. I'm aware of the need for the correct angles required, but for the life of me I couldn't get a sensible balanced tip to it?! Every time I tried I seemed to get a very long intersection point between the bevels. I tried various angles but had to stop as I just seemed to be making it worse.
Has anyone got any top tips for drill sharpening please?
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timb
Statesman
Posts: 512 
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Post by timb on Jan 21, 2019 18:56:13 GMT
How are you performing this task, by hand or jig etc?
Tim
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JonL
Elder Statesman
WWSME (Wiltshire)
Posts: 2,990
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Post by JonL on Jan 21, 2019 21:43:44 GMT
By hand. I'm sure I had the knack once...
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Post by 92220 on Jan 21, 2019 23:10:08 GMT
It is possible it is the type of drill you are trying to sharpen. In the 'old days' drills used to be made in Britain. Nowadays they are generally made in the far east. I have noticed when sharpening drills, that the central 'stalk' of the drill is thicker than it used to be in the British made drills. Like you, I used to be able to sharpen drills freehand and they would drill perfectly afterwards. Nowadays I can't get the drills to drill as they used to. Now, if O am drilling a large diameter hole (anything above 1/4" dia) I use a series of larger and larger drills until I get the size right. We all buy cheap far east drills. If we bought British made drills, I think you would find we could sharpen them just as we used to, but our purses would be a lot lighter! I did hear from an engineering [professor that far eastern drills and milling cutters are are made to a slightly different shape to British made drills and cutters, in that the flutes are all a bit shallower and at a slightly different angle. This would give a thicker central 'stalk'.
Bob.
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Post by gwr14xx on Jan 22, 2019 8:29:42 GMT
As Bob has said, these days the centre web of twist drills tends to be thicker than it used to be. As well as making sharpening of the drill a little more difficult, it necessitates a larger centre punch mark to get the drill started without wandering. The centre web also gets thicker as the drill gets shorter. During our apprenticeships, we were taught to hand grind twist drills - the normal point angle being 118 degrees, but if you have the correct relief on the point, the angle between the cutting edge and the line across the web should be 120 degrees - that gives the correct point shape. If the web is now too thick - using the corner of the grinding wheel, chamfer the back of the flute (the non cutting edge) (grinding slightly into the web). This will reduce the web thickness, making the drill much easier to locate in the punch mark and a lot freer cutting.
Regards,
Eddie.
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timb
Statesman
Posts: 512
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Post by timb on Jan 22, 2019 8:50:02 GMT
Looks like the drills have been covered as above, the only other things to think about are technique and grinding wheels. In my limited experience, if the wheel is not true then you wont get a good grind, the slightest axial curve in the periphery of the wheel will prevent a straight grinding face and cause issues. Incidentally the thicker webs mentioned above might lend themselves to four facet grinding, I made a jig some time ago and I can sharpen drills from 3/32" up to 1/4" with accurate four facets in no time at all. Of course it took a long weekend to make the jig!
Good Luck
Tim
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Post by 92220 on Feb 1, 2019 19:12:50 GMT
Sounds handy! Let's have some details Tim.
Bob.
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timb
Statesman
Posts: 512
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Post by timb on Feb 6, 2019 11:35:56 GMT
Sounds handy! Let's have some details Tim. Bob. Hi Bob,
If you pm me I will send you some details.
Tim
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Post by runner42 on Feb 11, 2019 6:58:45 GMT
Looks like the drills have been covered as above, the only other things to think about are technique and grinding wheels. In my limited experience, if the wheel is not true then you wont get a good grind, the slightest axial curve in the periphery of the wheel will prevent a straight grinding face and cause issues. Incidentally the thicker webs mentioned above might lend themselves to four facet grinding, I made a jig some time ago and I can sharpen drills from 3/32" up to 1/4" with accurate four facets in no time at all. Of course it took a long weekend to make the jig!
Good Luck
Tim
Hi Tim, I assume that the size of the grinding wheel is also a factor, an 8" dia would be better than a 6" dia? Brian |
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timb
Statesman
Posts: 512
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Post by timb on Feb 11, 2019 17:30:10 GMT
Absolutely Brian, the bigger the wheel the 'flatter' the grinding arc, the trick is to get the ground edge along the axis of the wheel.
Tim
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Post by Roger on Feb 14, 2019 9:31:58 GMT
I use the side of the wheel rather than the diameter for drill sharpening. If you look at the action those drill sharpening jigs use, you'll see the way you need to pivot the drill to get the required angle. It does take a bit of practice though.
The jigs are hopeless in my opinion, they don't hold the tool well enough.
I did have a big session on the Tool & Cutter Grinder, setting them up for 4-facet grinding, but that geometry is nowhere near as good as the normal type. 4-facet drills tend to grab much more so I won't be doing that again. I ought to make a decent fixture to grind them the proper way really, I probably will one day.
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Post by David on Feb 14, 2019 10:38:09 GMT
I did have a big session on the Tool & Cutter Grinder, setting them up for 4-facet grinding, but that geometry is nowhere near as good as the normal type. 4-facet drills tend to grab much more so I won't be doing that again. I ought to make a decent fixture to grind them the proper way really, I probably will one day. So you ground a bunch of drills with a 4-facet point and didn't like it? That's the first time I've heard anyone say anything bad about them! When you say "proper way" do you mean with the chisel point and the relief rounded away behind the cutting lips? |
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Post by Roger on Feb 14, 2019 13:29:23 GMT
I did have a big session on the Tool & Cutter Grinder, setting them up for 4-facet grinding, but that geometry is nowhere near as good as the normal type. 4-facet drills tend to grab much more so I won't be doing that again. I ought to make a decent fixture to grind them the proper way really, I probably will one day. So you ground a bunch of drills with a 4-facet point and didn't like it? That's the first time I've heard anyone say anything bad about them! When you say "proper way" do you mean with the chisel point and the relief rounded away behind the cutting lips? Hi David, Yes, I must have sharpened twenty or more drills with the four facet method, they looked spot on. The trouble is that they dig in much more readily than the usual style, to the point where they would be practically unusable on something like Brass. There's a very good reason why drills are not commercially ground this way, if it was better, that's how they would be supplied. |
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Post by coniston on Feb 14, 2019 23:23:11 GMT
So you ground a bunch of drills with a 4-facet point and didn't like it? That's the first time I've heard anyone say anything bad about them! When you say "proper way" do you mean with the chisel point and the relief rounded away behind the cutting lips? Hi David, Yes, I must have sharpened twenty or more drills with the four facet method, they looked spot on. The trouble is that they dig in much more readily than the usual style, to the point where they would be practically unusable on something like Brass. There's a very good reason why drills are not commercially ground this way, if it was better, that's how they would be supplied. HI Roger, I'm sure your own experiences with 4 facet ground drills is exactly as you say, however it IS one of the most common methods of grinding industrial drills. Along with 6 facet and common single point methods. Yes I agree most of the drills we see from model engineering suppliers generally of low price are traditionally ground with a chisel point, but go buy a Dormer , Presto or other industrial manufactured drill you will get many options for drill points. What is important for different materials is the rake angle at the cutting edge, by the nature of manufacture of a twist drill the rake angle changes from centre to edge, commonly we used to 'blunt' or 'Dub' the cutting edge to provide a constant rake angle when drilling brass/copper etc. Have a look at this article which I found quite interesting. www.newmantools.com/machines/drillpoint.html Chris D. |
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Post by Roger on Feb 16, 2019 12:09:00 GMT
Hi Chris,
That's odd, I've never seen anything other than traditionally ground commercial drills. When I was with General Motors, we only used that style. I can see why you might want to change the helix angle and perhaps alter the tip geometry for some materials though, and it makes sense to optimise them for mass production. The link doesn't appear to return a viable page.
For our purposes though, standard drills ground in the usual manner can be used to drill any of the materials we come across. Yes, you can alter the cutting edge, but that's not necessary if you control the infeed so that it can't pull the drill into the work by lightly clamping the quill or using power feed.
The general point still remains, the 4-facet method on the same helix drill ground to the typical angles recommended in the articles describing this method do grab where traditional geometry doesn't. If a general purpose drill was better sharpened with the 4-facet method, I'm sure that's what they would supply.
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timb
Statesman
Posts: 512
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Post by timb on Feb 18, 2019 19:04:22 GMT
The trouble is that they dig in much more readily than the usual style, to the point where they would be practically unusable on something like Brass. There's a very good reason why drills are not commercially ground this way, if it was better, that's how they would be supplied. I have to agree that the cut with a four facet ground drill is more positive and therefore deeper and may 'dig in' under certain circumstances. I have found that the pressure required to drill is no where near that of a conventional drill bit and have adjusted my 'technique' accordingly. The cut effect is down to the angle of the cutting edge and as we know this will vary depending on the material to be worked. Steel with its propensity to provide a nice curl of swarf is more suited to the four facet drills which will provide a more accurately dimensioned hole than a relief ground bit. Brass however is a different kettle of fish requiring a zero rake angle, a drill ground for steel would be more likely to 'dig in' unless the cutting edge is relieved (much the same as a lathe tool). If you try drilling brass with a drill that has the cutting edge 'flattened' (not the best description but I think you know what I mean) you will find it cuts much more freely and accurately.
The 'standard' sharpened drills are for general use and are therefore ground in a maner that will suit most common materials, however if you were to purchase a drill for a specific material, say 304 Stainless, then the drill will almost certainly have a facet ground geometry along with its different alloy makeup. Personally I have found that the four facet sharpening method is easier to achieve consistently and produces an accurately sized hole for the smaller sizes of drills (say up to 6mm - 1/4"). Above this size I have found the cheaper drills will chip their edges on a heavy cut, which tends to agree with your point regarding 'digging in'. I have used six facet grinding techniques on some of my larger 2mt drills with great success.
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timb
Statesman
Posts: 512
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Post by timb on Feb 18, 2019 19:21:01 GMT
On a safety note, I had it beaten into me that grinding wheels are designed for purpose, ie the flat (or straight) wheel should be used for grinding on the periphery of the wheel only. Using the side can be dangerous as it can put the wheel out of balance and cause catastrophic failure. Conversely cylinder, cup and dish wheels should be used for lateral edge grinding only, never grind on the angled part.
Tim
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Post by 92220 on Feb 18, 2019 20:12:18 GMT
I remember when working in the machine shop during my draughtsman apprenticeship, when we wanted to drill thin sheet brass (we actually did this for all thin materials), to stop the drill 'grabbing', we used to rip a small piece of old cotton bedsheet into a ribbon about 1" wide and about 4" long, and fold it end over end to get 4 thicknesses. The drill was centred and the component clamped down. The cotton pad was then trapped between the drill point to the job. The machine was started and the hole drilled through the cotton pad and the sheet metal. A perfect hole with very little bur was made, and the drill NEVER grabbed, no matter what size, or what metal. The sheet metal was always supported on a thick piece of hardwood to stop it being bent during drilling. Another trick we used, to drill brass sheet was to take the drill to the grinder and take off the rake by grinding the flute square to the centreline of the drill. There was no way a drill ground this way, would grab in brass or aluminium.
Bob.
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Post by David on Feb 18, 2019 22:48:10 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?
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Post by simplyloco on Feb 18, 2019 23:17:10 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? I take my slim diamond file to the cutting edge of the drill point and just 'blunt' it by filing along the axis of the drill. Very little needs to be taken off. Works a treat and the drill is still usable elsewhere! John |
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