Drill Sharpening Jig Geometry

[Burnsy]

I have been following a few of the drill sharpening jig builds with interest but one thing has me puzzled. I was always taught to bring the cutting edge of the drill into the grinding wheel face with the shaft pretty much parallel to the ground. This creates a centre chisel edge of 45 degrees. I notice with the jigs however (and I think the Tormek jig is the same) the cutting edge is introduced to the wheel with the shaft lowered some what. I like the accuracy that these jigs provide and wouldn't mind having a go at making one up but need to understand the geometry of the shaft angle before I make any plans.

Why the angle and what is the angle? Does this not affect the chisel angle adversely or is this a completely different sharpening geometry to the more traditional style (which I believe is what I was taught)?

I did a bit of reading on drill geometry and this site details pretty much what I do (but in far more detail when it comes to clearance angles - I tend to wing it when it comes to that).
http://www.smithy.com/machining-hand...apter-6/page/4

[th62]

Geez, everyone must be on holidays, I’m on holidays permanently so I’ll jump first: I would say holding the drill parallel to the ground is firstly to obtain a known base from which to work ( it’s easier to judge what is parallel that to judge angles) secondly you can see what you are doing – lower the item or angle it and either the wheel or work may obstruct.

Not sure I understand what the chisel angle of 45 degrees you refer to is in relation to; however, the chisel angle is generally supposed to be around 100 degrees to the cutting edge, but that is governed largely by the relief and cutting angles and generally increases if grinding on the front of the wheel (unless grinding multi facets) The cutting edge is generally set around 8 – 9 degrees and back rake at around 35 degrees.

Anything more than the 8 -9 degrees on the cutting edge will cause the drill to dig in savagely – mostly manageable on steel, but not so manageable on soft materials like wood. The back rake has a similar effect, but not any where near as bad. The point angle varies for the material being cut, but most are set around 118 degrees.

If you introduce an item to the face of the wheel at centrepoint and parallel to the ground, the ground face will be at 90 degrees to the work and the face slightly concave due the shape of the wheel. The further above or below centre the item is introduced, providing it is still parallel to the ground, the more acute the angle of the grind. Similarly, if you introduce an item at centre point again, but this time the item is not parallel to the ground, then the grind will emulate the angle at which the item was introduced.

So when sharpening a drill you have two methods of obtaining the primary and secondary angles: either keep the drill parallel to the ground, raising or lowering it’s height to achieve the appropriate angles; or, adjust the angle of the drill, keeping the drill point at much the same height in relation to the wheel. Most jigs rely on a combination of both height and angle adjustments to obtain the appropriate angles.

Of course this is only what my experience has taught. There are a number of ways of grinding drills and an even greater number of jig designs. I grind with the cutting edge parallel to the ground and so my methodology will differ from one who grinds with the edge at 90 degrees to the ground.

apologies if my nomenclature is not in line with the accepted norm.

[th62]

Perhaps I should also mention that the angles I mentioned I pulled of a web site; however, they are not correct, at least from my perspective. Conical bits have just the one angle and according to my three sets they are all angled the same at about 4 degrees. And the chisel point angle is 120 degrees.

I’ve ground my bits with a primary facet of around 5 or 6 degrees, any greater and I’ve found they dig in. The relief angle is somewhat difficult to measure: from the heel of the relief face to the rear of the primary edge is around 15 degrees, but because of the concave face on the relief, the angle is considerably sharper directly behind the rear edge of the primary facet - probably nearer to 30 degrees. This then gives a chisel angle of around 125 degrees.

I think the relief angle in particular should be considerably less which will decrease the chisel angle as well as the chisel length. The drills cut nicely with the primary facet as it is so I’m reluctant to change that. Next time I sharpen I may decrease the relief angle, thus shortening the chisel and reducing it’s angle – we shall see.

[Michael G]

If the drill axis is parallel to the ground then there is no relief on the back of the drill and it will have trouble cutting (as well as the drill rubbing). By tilting those few degrees you effectively relieve the cutting edge. If you look at a sharpened drill bit from the side, the cutting edge should be the highest point with the rest of the drill (that is, the bit to the left of the flute (normally)) should slope away.

Michael

[Burnsy]

OK I forgot to mention that my hand shapening method introduces the drill above the centre of the wheel (I rest my hand on the rest) which gives the cutting relief.

So I take it that those jigs that introduce the bit at an angle and not parallel to the ground are centering on the wheel so are getting their relief angle by dropping the shank of the drill in relation to the cutting tip? If this is correct, what is the accepted angle for this, 8-9 degrees as th62 has mentioned?