Perfectly ground Twist Drills every time.

On my drill sharpener, those two pozi screws (in your last picture) can be slackened off and then the guide can be moved up or down to get the cutting edge vertical.

As I read the instructions that lip stop can only be up against the helix of the drill behind the cutting edge so there would only be one position for any given drill size.

May be the order of setting the offset and the lip stop is important - I'll go and play.

Andy

Ok I have played and it would seem that if I set the drill to protrude by half it's diameter from the end of the trough and then position the lip stop the angle looks a lot better (that is, the cutting edge of the drill when viewed end on is closer to vertical), I could get it closer to vertical by eye but believed the amount by which the drill protruded from the jig was important for the geometry of the thing. 

Edited By AndyP on 15/05/2014 21:35:37

While some aspects of its operation need care and careful setting, I think you don't need to get too hung up on the angles being off a couple of degrees as in your fourth photo. Far more important that it's ground the same on both sides.

Andy,

I found the same problem on my jig. It was endlessly fiddly trying to set the lip stop while keeping the cutting edge vertical. I tried lots of times on different sizes of drills and every time the setting had to be changed. I gave up in the end, removed the stop and now I eyeball it to vertical on the first edge. I use Harold Hall's 180deg. attachment to get the two lips the same. Results are generally very acceptable.

One other problem contributes to poor results with these jigs. It fooled me for quite awhile! I found that the lock-down mechanism on the cradle could lock the drill slightly to the side and not exactly in the V of the cradle. This naturally led to poor results. This is not always obvious without actually looking down the length of the V before grinding. Graham's new V insert and the two clamps should eliminate this problem.

John

Due to the grossly inadequate instructions and dubious setting abilities of the lip stop which are invariable features of these jigs its best to have a decent qualitative understanding of the operating geometry before attempting to put into reliable service. There is a considerable range of drill projections, offsets and pivot tilt angles which can produce adequate results if correctly combined. If your jig gives the correct, or close to correct, point angle its probably worth doing some mock sharpenings of new drills with a flat, blued surface set up to mimic the grinding wheel. With any luck you will find that a little refinement and addition to the instructions will enable satisfactory results from a range of drill sizes. Note that the jig must be operated with light downwards pressure on the back which will alter the point presentation andgle by a degree or so. Its also best to remove the drill and replace it after setting the depth of grind using the screw. This ensures that the set-up is the same for both sides. The base must be at right angles to the wheel. If it isn't the pivot tilt messes everything up.

If the point angle is wrong then go with Grahams modifications as they produce the purest and easiest to understand geometry but it does require measurement for individual set-up of each size of drill to give of its best. Which, as Graham has shown, can be very good indeed.

A good way to visualise whats required for a factory drill grind is to consider half of a conical "dunces cap" lying on the point with its brim following the edge of the ground portion. Clearly the point of the cap is both offset and skewed in relation to the drill axis. To sharpen the drill the grinding wheel face needs to lie on a line down the surface of the cap and the drill moved about an axis passing through the cap point. Fairly obviously a simple swing jig is only going to give an approximation to the factory grind but if its a reasonably close approximation over an adequate range of sizes the device will be useful.

Contemplation of the common conical hole and blade pencil shaperner is an adequate starting point for getting a handle on how the swing jig works. Imagine the sharpener with the blade vertical and the pencil pushed up into it. The geometry is effectively that of the swing jig with the Vee trough directly in line with the pivot. Putting the pencil in the Vee and spinning the pencil will sharpen it. Substituting a steel rod and grinding wheel for the pencil and sharpener allows a conical point to be similarily produced by rotating the rod. Clearly if the rod is fixed in the trough with the correct projection in front of the pivot point an arc of the same cone can be produced by swinging the jig. Too much projection and the cone arc radius will be larger, too little and it will be smaller.

The pure conical point produced when the Vee and pivot are directly in line causes grinding to start directly on a line across the diameter. Useless for a drill as it has neither rake nor clearance and cannot cut. Offsetting the trough puts the start of the grind behind the drill introducing rake so the drill can cut. Easily seen if you draw a circle and tangents on the diameter and on a parallel, but offset line. This offset also causes the Vee trough to move slightly in front of the intitial pivot axis as the jig swings so increasing the depth of grind relative to the drill body helping to provide the necessary heel clearance. The forwards tilt of the pivot axis also acts to generate heel clearance.

Ultimately these jigs always fail for drills outside the a certain size range by producing insufficient heel clearance. However there is a good deal of scope for manipulation of projection, offset and tilt angle to produce satisfactory results over a reasonable size range. Increasing projection makes the basic grind cone radius larger, rapidly producing negative clearance, which can be countered by increasing offset to deepen the grind as the jig swings. Increasing tilt also increases the amount of heel clerance ground. If the front end of the Vee trough is forward of the pivot axis setting the drill projection in relation to the Vee rather than the pivot introduces a constant factor governed by the height of the drill centre line which varies the geometry as the drill size changes increasing heel clearance on larger drills. Twisting the drill lip slightly forwards of vertical, as in Andys last picture, can also be exploited to increase the effective heel grind on larger drills but its important that the lip tip always has some offset relative to the pivot so that a positive rake is ground. Generally jigs that exploit this twist will have longer projection, larger offsets and a Vee trough front edge further forwards from the pivot axis than those requiring a vertical lip for all sizes. Theoretically the twist allows a wider range of good grind sizes but it makes setting up much harder as it relies on the inadequate lip finger to simultaneously get both projection and twist right.

Hope this helps.

Clive

Thanks, Clive

That's a very helpful description

Now; if one of our 3D CAD modellers would be so kind as to render it, the World would be a much better place.

MichaelG.

Wallace,

It's all a devious plot by Presto, Dormer, et al ...

... to make sure that we keep buying new drills.

MichaelG.

Couple of points here.

I have one of these jigs, generic die cast one, probably Taiwanese because of it's age, absolutely no markings on it.

I a conversation with Dennis, a guy down the road who has literally one of everything but never uses anything - he collects, but he's a great bloke.

Anyway he has one [ surprised he hasn't got 10 ] so I asked to borrow it to compare to mine. So Wednesday he drops it off and it's identical to mine, that identical I cannot see any differences at all. Grabbed two 12mm cobalt drills of the rack, both identical drill and bought at the same time so gotta be same batch and ground these up, one on my jig, one on Dennis's then teates them.

For a test I drilled, on the lathe, into the end of a brass bar with no pilot or centre. Only went 5mm or so deep.

One drill cut cleanly and didn't require a lot of force, second drill took more pressure and when I looked at the end you could see brass rubbing marks on the heel, that was the reason I did this 'test'

So one jig works OK, one doesn't but I can't see the reason. Problem now is got to do the test again to find out which jig it was as I never marked them plus don't know who's is who's.

Conclusion ?

With that many variations of jig out there and subtle differences in the design and manufacture is it worth playing with one or building one of the aforementioned from an article that is known to work. In my case It would take some serious study to find out why one works and one doesn't.

Dilemma now is which jig do I give back to Dennis ? Knowing he's 87, never going to use this do I give him the crap one or do I give him the good one ? Second question, which one was his to start with ?

Answers on the back of a box containing a decent drill grinding jig please.

Edited By Neil Wyatt on 16/05/2014 12:48:58

Michael,

"Now; if one of our 3D CAD modellers would be so kind as to render it, the World would be a much better place."

That is not so easy a task, at the least it would be necessary to know what the grinding wheel form was along with the diameter so you could replicate the flute form - then the easy bit would be the end grinding to whatever design you wanted.

Mark

John,

Give your friend both of them I'm sure as a collector he will love you for that, either that or let him choose

then create the best drill grinding jig ever!. .

Dull

I mentioned my oddly ground 6.5mm drill in an earlier entry in this thread, just for interest I've just drilled thru 1/2" black steel with it, no problems,didn't even center -pop. Don't understand it at all. The thing is half-round on the end, one flute has no back clearance for half the length, the other flute has a chip out near the flute, faint remains of the chisel point. This was done in a floor mounted drilling m/c at slow speed. Back on topic- I have an old silver coloured jig which I got to work well after about 5 years. I removed the bit of tin that is supposed to be a stop against the flute and just set up by eye. Biggest difference is instead of vertical I offset about 5deg. to the right (looking on the drill end ) Seems to work well.

Reflecting on last nights epistle (thanks for the kind comments Michael) it occurs to me that a pivoting rod spring loaded away from vertical towards the jig could be an effective indicator as to the fidelity of the jig ground point in comparison to factory original. Let the rod be vertical with its pivot arranged so as to mover perpendicularly to the nominal plane of the grinding wheel face. Consider a brand new test drill inserted into a "perfect" jig capable of accurately repilcating the factory grind. When the test rod is vertical the drill lip will be in perfect contact over its full length and both perfect contact and vertical alignment of the rod will be maintained as the jig is swung through its sharpening arc.

An imperfect jig unable to match the factory heel clearance grind will cause the rod to swing back and forth over an angle porportionate to the error. Swinging away from the jig indicates excessive heel clearance will be ground, which probably doesn't matter, but swing towards it indicates insufficient heel grind which will soon eat up the clearance stopping the drill cutting. Probably easier to see via some sort of scale or an indicator in contact with the rod. I suppose a dial gauge fitted with an elephants foot would work directly but it seems a bit delicate to me. If the jig point angle is a degree or three out simply shifting the nominal vertical position should stiil give an adequate comparison.

I guess that the base and vertical rod of a surface indicator could be pressed in to service given sufficient low cunning. However in practice a square rod (key stock?) or narrow bar would be much easier to set up than a round rod which needs to be set so the cutting edge contacts at the full diameter when the jig swing is set to grind the edge. Probably a better mimic to the actual conditions too as grinding contact is with the wide wheel face not a line as simulated by a round rod.

Despite certain cynicsm in many engineering quarters these jigs, if correctly made, work and work well. Heck even Which managed to get decent results from a Picador version ages back and they are a dozy crew at the best of times. Back in the day I used use a Picador jig to sharpen drills for my father who was a semi skilled sheet metal worker for an air conditions system builder. Co-workers who borrowed his drills reckoned they were sharper and cut faster than new factory ones.

Clive

Edited By Clive Foster on 16/05/2014 11:35:11

Gordon

Sounds like your silver jig is of the Picador type normally seen with a vertical pivot. Standard drill position with those is cutting lips at 5 to 5 on the clock, ie 10° off vertical. They do work well, once you twig the relationship between projection, drill size and heel clearance, but understanding whats going on is a bear.

I suspect some of the ones with a small tilt on the pivot axis are supposed to be a variant or improvement on the Picador version with the important bit about the cutting lip being at an angle rather than vertical missing from the instructions. I think the Picador version could well work better with a small tilt permitting less rotation on the drill for any given heel clearance grind. As standard the amount of rotation needed to get the full heel clearance on larger drills puts the tip of the cutting edge so far over that it almost exceeds the offset so rake becomes very small. I've inadvertently ground a 1/2" drill with too much rotation and the thing ran out of rake before the full diameter was reached. Wonderful countersink but it just stopped cutting at a 1/16 or so undersize.

Clive

I don't think the cutting edge is meant to be vertical.

My somewhat different Plasplugs jig sets the drill so that the end of the cutting edge is vertically in line with the heel of the other facet.

Neil

Thank you for an excellent description of the geometry of the jig Clive, I may have finally got my head round most of it. Just to be clear when talking about "offsetting the trough" do we mean that measurement that would be affected by placing a washer in the adjustment position on the jig - the one calibrated as 49 deg 59 deg etc

As it stands my jig is going to cut a huge amount off the heel of a new, presumably correctly shaped, drill before getting to the cutting edge so I think I need to experiment with greater projections or look at making Gray's eccentric pivot and render the offset adjustable - it is 7mm at present.

Andy

My silver jig has no makers name, the spindle slopes towards the wheel about 5 deg. The thin tin that holds the spindle in place became worn and slack, replaced this with a thicker bit, about 3mm., big improvement. Never understood how the stop against the flute was supposed to work, it must depend on the spiral, and not all drills are the same. I bought a new jig of the same type but this does not work nearly as well, but just needs a bit of tuning, I hope. It looks as if an older jig was just used as a pattern for sand casting the new one!

Update. Since my spacer / washer mod. I've had chance to have a go at some more different sized drills with this Draper jig. Sadly my mod appears to work only on that size of drill. Inconsistent results seem to be the order of the day for this jig and my patience with it has come to an end so into the bottom draw it goes.

I will carry on sharpening drills by hand which I can manage OK down to 4mm. The larger ones I find much easier and the ones with a morse taper are easier still using this:

Rik

Sounds like Gordons jig is indeed Picador derived with the gentle slope on the pivot helping increase heel clearance with less drill rotation.

I have a Picador vertical pivot jig and haven't looked at the General style, tilted pivot, type for several years. That said I believe its correct to say that the Picador version has more offset and more drill projection than the General (et al) tilted pivot variety. My Picador has around 5/16" (4mm) of offset and operates with about 1" (25 mm) of drill point projection from the pivot axis. The instructions call for minimal drill point projection from the carrier Vee trough, 1/16" is suggested but "for drills larger than 1/4" the projection may be increased a little". Bought around 1975 so its almost certainly imperial by design. The instructions suggest that the lip guide simultaneously sets projection and angle. Theoretically it should but the practice sucks so, like Gordon, I generally do things by eye.

Andys 7 mm offset sounds way too much however far the drill projects. The offset is indeed that distance altered by placing a washer between the two parts of the body but that will increase the offset making things worse. Time to set-up some sort of measurement system to try and figure out whats happening. Or bite the bullet and write to Draper asking them how the darn thing works! Bet they don't know.

If Neils Plasplugs jig is anything like my orignal version the geometry is closely related to the Picador system so the lip should be at an angle, not vertical. Love the invertible drill carrier which ensures that both lips are ground at the same setting.

Be nice if someone could come up with an interactive computer generated animation of the actual grind shape produced at the base (full diameter) part of the ground portion. If we could actually put various values for offset, projection and axis tilt in to see the point shape appear in real time against a simulated hole it would be a lot easier to understand things.

Clive

Edited By Clive Foster on 16/05/2014 20:12:27

I have a Spiralux Drill Sharpener made by Holland & Blair Ltd of London which appears to follow the same designs as those shown in the photos. Using AndyP's method of measuring the angles using a digital indicator, the square bar on mine is set at 31.8° from the horizontal (58.2° from the vertical) and the pivot pin is set at 83.1° from the horizontal ( 6.9° from the vertical).

I've not done any more tests, but it does seem to me that certainly the grinding angle is more or less correct for the nominal 59° and therefore this particular drill sharpener doesn't need Gray's additional angle piece. I'm not sure how to interpret the other angle.

Looking at some of the larger drills I have sharpened and comparing with new ones of the same size, the sharpened ones do seem very similar - apart from one which has uneven sides, which is obviously my bad sharpening.

I'm beginning to think I may have got one of the better designs. I think I might try skimming all my older drills and then do a visual comparison against the new ones.

Peter G. Shaw