There has been considerable interest in using the Varigrind jig to duplicate (or approximate) the 40/40 grind produced by manual platform grinding. Several approaches have been proposed that either do not quite do the job or that require large values of gouge protrusion from the jig. I decided return to basics and see if I could get a better solution.
Some background first: There is a fairly well known iterative jig setting process that can be used to match the jig settings to any gouge that has been sharpened previously on a Varigrind jig. The process is as follows: Set the gouge protrusion to any reasonable value. (I use 1.75 inches, which is near the minimum that will provide clearance between the jig and the grinding wheel during the swing. Larger values work equally well but reduce the useable lifetime of the gouge.) Starting with the Varigrind pivot arm set to somewhere near mid range, repeat the following steps in sequence several times until a good match occurs at both the tip and the wings:
1. Adjust the sliding V-arm until the outer part of the gouge wing matches the grinding wheel. Lock the V-arm.
2. Adjust the jig pivot arm until the nose of the gouge matches the grinding wheel. Lock the pivot arm.
This process will converge to the desired setting in just a few iterations unless there comes a point where you reach the end of the adjustment range of one of the settings. The final settings can be confirmed by blackening the gouge tip with a marker pen and rotating the grinding wheel by hand while swinging the gouge. The bright spots produced will indicate what corrections are needed.
I decided to apply this technique to try to match my Varigrind jig settings to a gouge ground to a 40/40 grind. Specifically I would match the nose angle and the outer wing angle of the gouge. I cannot (yet) prove that the grind will exactly match the 40/40 grind between these limits, but the result will be a smooth transition from nose to wing.
In carrying out the above iteration, I almost immediately found that the pivot arm of the jig ran out of adjustment range. Specifically, the arm could not be closed enough to achieve the needed value. However, inspection showed that the circular slot along which the clamping bolt moves could be lengthened about a quarter inch or so, allowing the arm angle to close by another 10 degrees or so. This turned out to slightly more than enough to obtain the desired 40 degree angle at both the end of the wing and the nose. Lengthening the slot took only a few minutes using a round file.
The following pictures show the result that I obtained using my modified Varigrind jig to sharpen a half-inch parabolic-flute bowl gouge.
Some background first: There is a fairly well known iterative jig setting process that can be used to match the jig settings to any gouge that has been sharpened previously on a Varigrind jig. The process is as follows: Set the gouge protrusion to any reasonable value. (I use 1.75 inches, which is near the minimum that will provide clearance between the jig and the grinding wheel during the swing. Larger values work equally well but reduce the useable lifetime of the gouge.) Starting with the Varigrind pivot arm set to somewhere near mid range, repeat the following steps in sequence several times until a good match occurs at both the tip and the wings:
1. Adjust the sliding V-arm until the outer part of the gouge wing matches the grinding wheel. Lock the V-arm.
2. Adjust the jig pivot arm until the nose of the gouge matches the grinding wheel. Lock the pivot arm.
This process will converge to the desired setting in just a few iterations unless there comes a point where you reach the end of the adjustment range of one of the settings. The final settings can be confirmed by blackening the gouge tip with a marker pen and rotating the grinding wheel by hand while swinging the gouge. The bright spots produced will indicate what corrections are needed.
I decided to apply this technique to try to match my Varigrind jig settings to a gouge ground to a 40/40 grind. Specifically I would match the nose angle and the outer wing angle of the gouge. I cannot (yet) prove that the grind will exactly match the 40/40 grind between these limits, but the result will be a smooth transition from nose to wing.
In carrying out the above iteration, I almost immediately found that the pivot arm of the jig ran out of adjustment range. Specifically, the arm could not be closed enough to achieve the needed value. However, inspection showed that the circular slot along which the clamping bolt moves could be lengthened about a quarter inch or so, allowing the arm angle to close by another 10 degrees or so. This turned out to slightly more than enough to obtain the desired 40 degree angle at both the end of the wing and the nose. Lengthening the slot took only a few minutes using a round file.
The following pictures show the result that I obtained using my modified Varigrind jig to sharpen a half-inch parabolic-flute bowl gouge.