Harold Hall
The advantages
The advantages of the system are, the drill will start from a smaller centre punch mark, or even without one where positional accuracy is not that important. An even greater advantage is that the change in the chisel to a point makes the drill require less pressure when drilling, especially useful on light weight machines. However, when drilling some materials, some grades of brass for example, that grab the drill, this can be a disadvantage.
The disadvantage
This being that the sharpening process is much more critical but once conquered should not be that difficult using a good jig for the process and with a grinding rest able to guide the jig. It must be understood that the drill is not sharpened using the jig described earlier.
The procedure explained simply
1 Grind the first facet at 10°
2 Rotate the drill 180° and grind the second edge at the same setting.
3 Leave the drill in the jig at the same position but set the grinding angle to 25° and grind the secondary facet. Do this stopping just short of the centre of the chisel. Note that the primary facet has parallel edges so the drill must not be rotated.
4 Now rotate the drill 180° and grind the fourth facet at the same setting.
5 Inspect the drill and if the process has left too much of the original chisel place on a very little more depth . Then regrind the secondary facet on both edges.
6 If though you have ground too much away you will need to set the angle back to 5° and skim just a very little off the primary facets.
If all this sounds a long process, it is initially, with practice though it will become second nature and you will like the method, of course it depends on having the appropriate equipment for the task.
To achieve the above, a special fixture will be required to hold the drill and set the 180 degrees rotation between edges. Also required, a grinding rest to guide the fixture to the wheel and including fine feed to enable the depth of cut to be accurately set. A typical jig can be seen in Photograph 12 and being used with one of my grinding rests in Photograph 13. Photograph 14 shows the first facet having been ground and Photograph 15 the fully ground drill.
Again, my book Tool and Cutter Sharpening gives more details regarding the process and also the drawings for making the jig. See other pages on the site for my Advanced and Basic grinding rests.
12
13
14
15
Drawings
Metalworking
Workshop Processes
Error. There is an omission on the drawings in the Tool and Cutter Sharpening book for the four facet jig. The overall length of the Base, item 6, should be 140mm.