There are instructions for this type of jig here **LINK**

Bill … You may find it worth trawling through this previous thread.

It will be difficult to follow, because the original post and [I think] all of the OP's postings have been deleted … but you might still find it interesting.

MichaelG.

Bill

Perhaps the pictures here :- **LINK** will help you set up the Potts jig. There is also a two part article in Model Engineer around November / December 1963 by Duplex describing a built up jig essentially identical to the Potts. I have a PDF covering most of the first part which runs through the theory and implies that the Potts doesn't always give correct results so this version has been modified either dimensionally or in set-up details.

However according to Professor J Hugel the Potts jig, if properly set up, gives near perfect geometry for the range of sizes its made for. When rooting around the internet sometime back I found a PDF about an Excel spreadsheet he developed around 2009 for analysing swing around a pivot type drill grinding jigs. Although entitled Instructions for Using the Excel Tables <Drill Performance XLS> most of the text and figures are concerned with the geometrical analysis underpinning these jigs, the mathematical formulae arising from it and sample results for typical jigs. Unfortunately the spreadsheet formulae never seemed to have escaped and the instructions have now disappeared. A reasonably careful read through convinces me that Prof Hugel has done a good job of the analysis and his conclusions are trustworthy.

Having mentioned Prof Hugels work it might be useful to very basically run through whats going on.

As you know the main problem for all variants of the swing around a pivot type drill grinding jig is how to generate an essentially constant clearance angle running circumferentially around the web at all radii and for any size drill within the operational size range when the basic motion of pivoting the drill across the grinding wheel is along a radius. Correct geometry requires the depth of grind, as expressed relative to the end of a plain cylindrical blank, to be suitably increased as the swing moves from it starting point at cutting edge to finishing point at the end of the web. Obviously the actual grind depth at the end of the web needs to increase with drill size. An angular inclination of the pivot relative to the desired cutting edge angle naturally gives the desired increase but the form is more radial than circumferential so clearance decreases as you move round the web. Manipulation of the drill centre-line offset and point projection relative to the pivot adjusts this erroneous basic geometry to a sufficiently accurate approximation of correct form. The simpler jigs have a fixed offset of the drill point relative to the pivot which severely limits the range of drill diameters for which an reasonably accurate approximation to correct geometry is produced. In practice the range of drill sizes which will cut adequately is rather greater but its still unsatisfactory. The Potts has a built in drill size measurement capability permitting intrinsic adjustment of cutting edge offset relative to drill diameter which is geometrically much more satisfactory giving very correct results over a useful range.

Clive

Bill

I find that with this sort of thing the lightbulb moment tends to come rather faster if you can do some measurements.

I wonder if a dial gauge could be set up to measure the travel of a round rod moving in the Vee groove when the jig is swung against a stationary grinding wheel or equivalent mock up mimicking the movement used to sharpen a drill. I guess you'd need a lubricated shim or thin piece of metal sheet covering the grinding wheel to get a decent smooth slide. Its not unreasonably tricky to get a good estimate of the axial distance between the tip of a good drill and the trailing end of the web so the movement of the rod could be compared with the grind depth relative to the point needed on a drill.

Do that for a couple of three suitable sizes of drill and rod and you should be able to confirm if the setting gauge gives accurate results and, if not, make suitable empirical adjustments. Even if it does work as it should I'd be inclined to make measurements at deliberate miss-setting to get a feel for what happens when things go wrong.

Hafta say its probably over 30 years since I saw one of these jigs. I've never used or seen one used although the owner did tell me how it worked and why it was better than the Picador one I'd recently bought. Both of us could get good sharp drills so, at the time, I considered the point somewhat moot.

Clive

I thought I'd resurrect this thread as I seem to be having the same problem that peak4 has had.

Many years ago I bought one of these made by Picador:

It was left unused until I read Harold Hall's book on tool and cutter sharpening and after one or two modifications I managed to get some reasonable results.

Recently I bought a kit of parts, from a well known auction site, for a Potts drill grinding jig. I bought it because it would be an interesting project to build and also Tubal Cain in his book on drills taps and dies said it produces drill points "very near to perfection".

Some of the parts were already machined and as far as I can tell are in accordance with the drawing supplied (from Woking Precision Models).

The jig is now sufficiently complete to be able to give it a try, however the result is disappointing.

The drill on the left is an untouched 3/8 inch drill as supplied by Presto, the centre one is a 23/64 inch drill (Dormer) that has been sharpened on the modified Picador jig and the one on the right is a badly worn 10mm drill of unknown parentage that I'm trying to sharpen using the Potts. On this the relief angle is now much steeper than the first two and looking directly at the point the chisel angle is wrong also. This seems to indicate to me that the drill point is too close to the axis of the jig pivot, giving too small a radius on the ground surface of the drill. This can be confirmed by putting a straight edge on all three drill points and comparing the curvature.

When using the Picador the minimum distance of the drill point to the pivot axis is about one inch, any less and you end up grinding the tip of the jig off. On the Potts it is much less than this:

This is part of the drawing as supplied by Woking Precision Models. The scale is full size and appears to show the jig set up to grind a 1/4 inch drill. It's not too clear in the side view, but the pivot axis is only just (by about 1/4inch) behind the front of the jig which is where the drill tip should be.

Peak4, did you manage to sort your problems? I have the set up instructions if you're still looking for them. Can someone who has one of these tell me where I'm going wrong?

John

Thanks for that Colin, I'll check that out.

John.

Edited By Jss on 18/03/2016 18:49:33

… It may be worth skimming through these

MichaelG.

.

Edit: and this page.

Edited By Michael Gilligan on 20/03/2016 20:09:59

houstonceng

Sorry I don't know where Profesor J Hugel originally published his work. Certainly not Model Engineer, Model Engineers Workshop or Engineering in Miniature. I found the article by pure happenstance on the internet, around 2011 I think and grabbed it. A few months lather I decided to have a determined try at finding the spreadsheet and any other data and it had vanished along with some of his other writings on drill 4 facet drill sharpening which I'd also grabbed. Best policy with the internet is to grab it when you see it! I seem to recall a mention by Hemingway, possibly in one of their catalogues, that the geometry of the sharpener produced from their castings had been verified by Professor Hugel.

This thread **LINK** on bbs.homeshopmachinist implies that the original was written for the Society of Model and Experimental Engineers but the link is dead. Maybe someone ahs a paper copy of the work? That thread has what looks to be some interesting comments on setting up the Potts. Some mention in this one **LINK** in this one too.

Clive.

.

**LINK**

I have been sent a copy of the pdf, and am checking to see if we can put it on this website.

Neil