Hi Rik,

thanks for the tip. I have a similiar jig (grey though) lying in a drawer somewhere, my experience is not very different from yours. May be I should try again.

Thor

Edited By Thor on 22/04/2014 18:23:44

Thanks Michael for giving my website a mention. For an unbiased recommendation, see what Russell Eberhardt said here about the pages on drill sharpening **LINK**

Harold

Putting aside the fact that many of the cheap import versions are so badly made as to the completely geometrically unsound there is no great difficulty in getting good results from these jigs providing some effort is made to follow the instructions. It does help if you have some understanding of the underlying geometry involved.

These jigs put the drill centre line offset to one side of the pivot axis so a simple side to side sweep brings the drill in front of the pivot by an amount proportional to the sweep angle. This basic movement produces a clearance angle across the drill radius which won't cut unless the angle is so exaggerated as to approximate 4 facet form. To achieve the correct shape where the clearance angle runs around the drill circumference requires the depth of grind to be increased as the swing progresses.

There are two ways of achieving an adequate approximation to the desired geometry with these simple jigs.

One way is to set the pivot stem vertical and the drill cutting edges at an orientation best described with reference to a conventional clock face as 25 past 11. The original Picador jig uses this geometry. The projection is set by lightly butting the lip of the drill against the lip rest. Although it is in some ways advantageous to use the same projection for any given drill size at all point angles the grind accuracy does depend on how closely you manage to set the 25 past 11 cutting edge position. It doesn't help that there are no on jig references. The lip finger fit is usually pretty poor too which merely adds to the difficulty. Especially with small drills.

The alternative method is to set the pivot point at an angle, usually a little under 20°, as in the jig described by Howard Hall whose operation he completely misunderstands. I can only assume Howard did not have the benefit of a proper set of instructions. Here is a link http://www.generaltools.com/assets/i…825_Manual.pdf to one set. The geometric results of the angled pivot plane are essentially similar to the vertical pivot and 25 past 11 drill orientation of the Picador Jig but introduces a component dependant on drill projection distance. This means that you can, in principle, set the drill position by direct measurement where-upon contact between drill and lip rest will produce the right orientation of the cutting lip and heel. Naturally it is a complete pain in the butt to measure the projection of each and every drill prior to sharpening so the carrier trough offset and pivot angle in a correctly made jig are such that setting the cutting edge vertical with the drill in contact with the will work for the common 59° and 68° point angles. For other angles you need different projections.

Although simpler to set-up for normal angles, vertical is much easier to judge than 25 past 11, the angled pivot geometry is not quite as good an approximation as the vertical pivot one. A high heel needing a touch of hand grinding to remove is pretty endemic with larger drills. A slight twist of the drill will, I understand, fix this.

On the Picador jig the drill carrier trough is offset by about 3/16" from the pivot axis. The angled pivot versions I've seen are similar but there is an interaction between pivot angle and offset so there is room for variation.

These devices work much better using a properly dressed cup wheel rather than just rubbing across the side of an ordinary grinding wheel. It's also imperative to provide some means of guiding the base so it remains correctly orientated ay 90° to the grinding wheel face as you slide it back and forth to get the drill to just touch the wheel before grinding. It should be obvious that any twist will completely muller the motion geometry, most especially with the angled pivot version. The vertical pivot is a touch more tolerant but still not very. The simple slot and bolt idea is monumentally inadequate. Its important to judge just the right amount of downwards force on the back of the jig when swinging. The vertical pivot Picador is very sensitive to this. Simply letting things float in the crude vee and plate bearing will not produce consistent results.

The linked instructions from General suggest that the jig can be used on the periphery of the wheel. Something I find hard to believe unless the wheel is very large.

Clive

Edited By Clive Foster on 22/04/2014 23:51:39

I wrote about my experience with a similar jig to Rik's and provided my alternative solution in MEW 176.

The jig I described is not foolproof but the the results are at least repeatable. Having seen the instructions Clive posted I will have another look at the version of that design I have out of interest. Perhaps it will be useful after all?

Regards

John

I'd like more heel clearance than Rik's example.

I have a cheap drill jig which I have modified to remove slop (the pin was a loose fit in the casting). I find it tedious to set up.

I have a Plasplugs multi-function sharpener which sometimes does a good job, but is tricky to get right.

The best results I have had on large drills has been a hand ground 4-facet point.

I suspect that grinding a 4-facet point using my T&C grinder is little different from sharpening a slot drill. I'll have a go later.

Now I have some decent split-point and four facet drills. in due course I'll probably use them to guide me in making a simple sharpening jig. the advantage with these types is that if you use a chuck to hold the drill, you don't need to change the angles.

Neil

I have the silver /grey version of the above jig. It has the angled stem.

After initial fettling it now does a very acceptable job on drills from 3mm to about a max of 19mm. I found the following mods turned it into a very useful addition to the workshop.

I first made Harold Hall's 180 deg. drill holder modification and I can vouch for its effectiveness. The original sliding lip stop on the jig does not give repeatable results and causes only frustration.

Secondly I made a sliding base with a fine screw-in feed. A version of this was shown in MEW some time back. Well worth the effort!

The main stem is held in a v-slot by a piece of rubbery metal and two screws. Find a piece of flat spring in your scrap box to replace the original metal retainer. I also ran an end-mill down the vee and converted it into a half round and it gave the stem a much snugger fit. It does not wobble now.

I lengthened the square bar in the body of the jig by welding a similar sized square bar from a door lock to it. That greatly increase the capacity of the jig.

On this jig it is crucial to have the cutting lip of the drill vertical in the holder for sharpening. Once set, Harold's 180 deg. attachment takes care of the second lip.

As regards the length of drill protruding from the jig, the theory is heavy going, but I have got very good results by having the drill protrude the same amount as the diameter of the drill being sharpened, ie. 6mm for a 6mm drill or 12mm for a 12mm drill. (You could experiment with slightly less to get a smaller relief angle.)

Lastly, whenever I do get it wrong, it usually comes down to one of three user errors. 1. Drill lip not set vertical. 2. Drill not snugly seated in the V of the jig. 3. Drill protruding too much/little.

John

Edited By Falco on 23/04/2014 22:46:48