I must admit that I have a preference for the "Reliance" as it has a more engineered
feel to it. One particular aspect that I liked was that after grinding one side the
upper portion of the jig could be lifted off to enable the drill to be easily inspected
and then rotated for grinding the second side. That having been said, the modern
unit does have the advantage of being able to be set at angles other than the standard
for twist drills of 118°. These are 82°, 98°, 136° and 176° just what some of these
angles are for I do not know. The 82° is though for sharpening countersinks but unfortunately
the present day standard for metric screws is now 90° and not available.
Fortunately, for accuracy, but unfortunately for flexibility, the unit cannot easily
be set at intermediate angles. Despite my liking for the "Reliance", I must say that
the modern unit works well. This, once one has mastered its use, as was of course
also the case with the "Reliance".
As mentioned above, the instructions for the modern unit often leave a lot to be
desired and so I think it is essential for the user to understand the basic principles
behind the process before putting the jig through its paces.
Photograph 3 shows the jig set up for sharpening the drill and obviously when using
the jig setting it up will be the first requirement. However, I am going to start
at the drill’s requirements when being sharpened. Having got to the stage in the
photograph, the only movement available is to rotate the whole jig about the pin
located in a V in the base casting, Photograph 4.
The pin on which the device rotates is not quite at 90° to the base but the angles
set by the jig compensate for this so it has limited effect on the sharpened drill.
Because of this, for simplicity, my explanation ignore this.
However, if you would like a little more detail, see Sk.10
Sk. 1 shows that the result of rotating the jig will be to grind onto the end of
the drill a surface that is part of a cylinder. If then, all other things being equal,
the drill projection from the jig is extended, the result will be similar but the
diameter of the cylinder will have been increased Sk. 2. The result will therefore
be a drill having a flatter surface giving less clearance behind the cutting edge.
Whilst there is an ideal clearance, Photograph 5 shows two that have gone wrong.
That on the left having too much clearance due to to too little projection and, on
the right, too little clearance having been sharpened with too much projection. This
understanding is vital in knowing how to use the jig.
What then is the correct projection and the correct clearance angle? Well, the answer
is the one that gives the correct value for a feature not yet mentioned, that is
the chisel size and angle. Again, rather than to just say this is what we want it
is better to explain the why.