I am a nerd, I confess it. I like to understand the "why". I bow to our more experienced turners who have the steady hands and years of practice who can grind their gouges by hand and eyeball, but I'm like most of us who use jigs. I have been using the Vari-grind jig for years, and for several years in combination with Don Geiger's adjustable height Pro Sharp (I think that is the name, his new device is the Pro Sharp Supreme).
I like making my own grinds and experimenting, but also like to have a base to start from. I have a list of my grinds on a clip board hung by my grinder. I might set the leg of the VG to the 4th notch and the four position (in/out) peg to "red" to set the height of the V notch so that the nose bevel is 60 dgs. (the test bar Don provides). Then I might grind at a different leg setting on the VG, and a different peg on the V notch.
Sounds complicated, but it really isn't, and it has served me well as I can duplicate any grind I've come up with with a brief pass, saves me a lot of steel. But it has always bothered me when I see different set ups, particularly the extension of the gouge tip from the VG - and I've wondered about the effect. I got back onto this question a couple of days ago when I saw Don's new device, and his advocacy of Doug Thompson's grinding chart using a 1 3/4" extension - my older Geiger Pro Sharp has a built in socket for a 2" extension (no criticism Don, just investigating).
Let me explain one matter of jig geometry. The angle of the leg does not define the angle of the gouge as you roll it, the pivot point on the gouge is not the tip. The Ellsworth jig has a fixed leg, the VG a variable leg, but each of them have a different distance from the tip, even with the same extension, due to the location of the pivot. This affects the steepness of the side grind as you roll the tool - and that is the shape I like to play with.
I am attaching a diagram, and I hope it will come through readable. You will have to tilt your head to read it, sorry about that but I've tried to reorient the scan of my drawing. I was amazed, as I'm sure you will be, at the very small difference in effective grind between the 1 3/4" extension and the 2". My numbers aren't exact, nor is my drawing, as I was not using drafting tools (and my compass doesn't go to 6 1/2").
I'm going to work with this drawing and see if I can come up with a true geometry for the rolling of a gouge with a jig. My guess is that a 45 dg. angle from tip of the gouge to the foot of the leg would give an equal bevel from tip to sides. It is not an easy concept, the bevel reverses in a three dimensional way.
Your comments encouraged, your complaints accepted.
Best, Jon
I like making my own grinds and experimenting, but also like to have a base to start from. I have a list of my grinds on a clip board hung by my grinder. I might set the leg of the VG to the 4th notch and the four position (in/out) peg to "red" to set the height of the V notch so that the nose bevel is 60 dgs. (the test bar Don provides). Then I might grind at a different leg setting on the VG, and a different peg on the V notch.
Sounds complicated, but it really isn't, and it has served me well as I can duplicate any grind I've come up with with a brief pass, saves me a lot of steel. But it has always bothered me when I see different set ups, particularly the extension of the gouge tip from the VG - and I've wondered about the effect. I got back onto this question a couple of days ago when I saw Don's new device, and his advocacy of Doug Thompson's grinding chart using a 1 3/4" extension - my older Geiger Pro Sharp has a built in socket for a 2" extension (no criticism Don, just investigating).
Let me explain one matter of jig geometry. The angle of the leg does not define the angle of the gouge as you roll it, the pivot point on the gouge is not the tip. The Ellsworth jig has a fixed leg, the VG a variable leg, but each of them have a different distance from the tip, even with the same extension, due to the location of the pivot. This affects the steepness of the side grind as you roll the tool - and that is the shape I like to play with.
I am attaching a diagram, and I hope it will come through readable. You will have to tilt your head to read it, sorry about that but I've tried to reorient the scan of my drawing. I was amazed, as I'm sure you will be, at the very small difference in effective grind between the 1 3/4" extension and the 2". My numbers aren't exact, nor is my drawing, as I was not using drafting tools (and my compass doesn't go to 6 1/2").
I'm going to work with this drawing and see if I can come up with a true geometry for the rolling of a gouge with a jig. My guess is that a 45 dg. angle from tip of the gouge to the foot of the leg would give an equal bevel from tip to sides. It is not an easy concept, the bevel reverses in a three dimensional way.
Your comments encouraged, your complaints accepted.
Best, Jon