When pursuing a hobby such as miniature engineering or even DIY for that matter, it is not long before the necessity to sharpen a twist drill raises its head. Some prefer to soldier on with blunt drills because the prospect of the sharpening process fills them with dread. It is not long before the drill succumbs to this abuse and sharpening is no longer an issue as the drill now needs replacing.

While grinding a drill free hand is not easy, it is also not as difficult as some make out, like all things in engineering it improves with practice. Having said that I have seen skilled men fight shy of sharpening their own drills. As an apprentice I was taught to sharpen a drill freehand, while I never found it that difficult many of my fellow apprentices really struggled.

Because of the time involved to successfully sharpen a twist drill freehand, most drills in industry below a certain size are considered consumables, at least this was the case in the multi-national companies that I have worked for. As the size of the drill increases then so does the cost of these larger drills. Often these were then either sharpened in house using a Brierly or Christen drill grinder, or shipped out to a firm specialising in regrinding cutters of various forms. When it comes to hobby usage however every drill is of value, as the money saved from not purchasing new drills can be better spent on materials for the models, etc.

There has been on the market for a number of years a Drill Grinding Jig. The early ones were made by Picador, there was also a similar unit on sale in the USA at about the same time, which came first I do not know. I was given one of these Picador units many years ago as the previous owner could not “get on with the jig”, (more on this later). The newer versions it seems does not have the same geometry as the Picador unit even though at first glance they do look the same. Like my donated Picador unit these too seem to end up under the bench after a while as the owners cannot get a satisfactory result from them.

Over the years I had used my Picador unit but mainly to sharpen the neighbours or relations drills which seemed to come by the sackful. The jig while it produced a satisfactory ground edge it was to my mind fiddly to use, not helped by the drill location device. It was for this reason alone that I still sharpened my own drills freehand. It will be obvious that any jig that grinds a drill symmetrically, with the correct angle and with a satisfactory facet is going to be superior to any freehand ground drill, no matter how skilled the person that ground it may be.

Fig 1 The prototype clone jig still under development.

It was after reading about these jigs not producing very satisfactory results that I began to look into the matter. While the Picador jig was fiddly it did always turn in a good drill, therefore I could not understand where the problem lay. I was spurred on to a large extent by my friend who had a newer version and wanted to get a satisfactory result from his jig. In the end I obtained a newer jig from a car boot sale for a fraction of the new price and set about inspecting the differences. It is not until you get the two units side by side that the differences can be seen.


For a start on the Picador jig the name moulded on in several places and the rotation axis about which the attachment pivots in use is truly vertical, this produces a cylindrically shaped facet on the drill. However on my newer version this pivot was leaning towards the grinding wheel, the facets produced from these jigs are conical.

Here too I have found that there are variations as regards to how much lean there is. Some leaning as little as 6.5° while others lean about 12-13°. With this current jig leaning towards the wheel I began to wonder if the actual drill sharpening angle was correct, as by eye lining up a drill in the device with the door jamb it did not look right.


Fig 3 The Picador jig is clearly marked, Photo

courtesy of Baron.

Placing the grinding jig base on my surface plate and gripping a new drill in the drill vee location with the cutting edges vertical, the stock of an engineers square was placed on the surface plate and with the blade vertical to simulate the vertical face of the grinding wheel the two were brought together. My suspicions were confirmed in that I had to tilt the base a further 7.5° in order to get the edge of the drill to line up with the blade of the square.

This confirmed one account that I had read that “a drill ground in the jig had too much ground off the heel”. Which is what would happen if a correctly ground but blunt drill was sharpened in a jig similar to this one. The other problem with the above statement is that the poster did not make any reference to the included angle being correct after grinding. I suspect that had he or she checked this as well then this too would have been found to be wrong.

The next problem with the jig was that the moulded “Vee” that is supposed to locate the adjustable drill angle portion of the jig onto the moulded scale. The Vee was so far away from the ‘Ident’ on the moulded scale when it was fully clamped up that it allowed an additional plus or minus 2.5° change in the angle. Gripping the moveable part of the jig in the bench vice with just the Vee part of the casting above the jaws I tapped it very gently with a brass drift and a hammer towards the joint face which finally cured this problem.

Fig 4 Moulded scale on the clone jig, note the 88 position relative to the edge, Photo courtesy of Baron.


Fig 5 Compare the 88 position on the Picador jig, Photo courtesy of Baron.

Because of the construction of the jig there is an offset of the rotation centre line of the jig with respect to the actual centre line of the drill vee location. On my donor jig this was of the order of 5mm, but this too does vary across the different suppliers and eras. This offset or eccentricity is necessary in order for the drill to be ground with some relief on the cutting edge, or facet. If this offset was not present then the drill would not cut as the facet would have no relief angle.

The main reason why this eccentricity is so large at 5mm is due to the jig being asked to grind various included angles. It will soon be realised that a drill ground to a 90° included angle is going to require a lot more relief than a drill ground to a 118° included angle. If the 90° drill was ground with the same relief as the 118° drill then the heel of the drill would rub due to the smaller included angle. This therefore is an inherent design problem for this jig, in order to grind a drill correctly at a 90° included angle we must accept that the drill ground at 118° is going to have more relief than really required. This again would also cover the earlier statement concerning “too much being ground off the heel of the drill”.

Another aspect which may be why these jigs put up such a poor performance and end up “under the bench”  is the instructions supplied with the jig. Some instructions say the drill should be ½ a diameter, (D), beyond the front edge of the drill vee location, while others say 1D. Some instructions say the drill cutting edges should be vertical while others say they should be at a slight angle, but the instructions do not quantify what that angle should be. While most adverts for these jigs show the jig mounted to the right of a bench grinder such that the drill is being ground by the side of the right grinding wheel, I have seen one jig advertised mounted directly in front of the grinding wheel. Under this latter set-up the facet will be ground with a concave face, the radius equalling ½ the grinding wheel diameter. While this may not be such a problem on a 3mm or ⅛” drill, it will certainly do nothing for the performance on a larger drill. It may come as a surprise to the reader that the direction of the grinding marks on the drill facet with regard to the cutting edge have a profound bearing on the longevity of the cutting edge. Ideally the grinding marks should be at 90° to the cutting edge.  The last example above gives the worst case with regard to the grinding marks. Grinding marks parallel to the cutting edge severely weaken the edge allowing bits of the edge to break off.

I therefore came to the conclusion that the basic idea of the Picador has been copied and modified over the years to provide a conically ground facet. This modification although an advantage had been carried out by someone that did not truly appreciated the finer points of drill grinding. Plus during the alterations to the mould tool to move the pivot point away from the vertical, the drill scale also needed moving. This scale however ended up being moved the wrong way in the mould tool and this is borne out by the wrong drill angle found on the donor jig.  Finally the instructions provided are intentionally vague, possibly due to lack of understanding or due to the knowledge of the above faults being present in the jig, but we will never know the real reason.

Fig 6 Checking the Picador drill angle, Photo courtesy of Baron
Fig 7 The same check on the clone jig, Photo courtesy of Baron.

On the face of it then these jigs would appear not to be worth messing with, but I feel given a little technical insight and some TLC they can grind drills to an equivalent standard as those supplied from the factory. Given that these jigs can be picked up for a few pounds they represent a very valuable asset for such a small outlay. I also need to point out that those donor jigs with the wrong moulded angle are not scrap, the addition of a wedge piece under the base will rectify this problem.

To this end I have carried out a number of modifications to the jig that has transformed this Ugly Duckling into a very desirable piece of equipment. We look at these next time.

PART TWO HERE

 

TWIST DRILL GRINDING MADE EASY

By Graham Meek. Part one.