Author: huboboe
Date: 2011-03-31 00:32
AHA! Cooper, you are hoist on your own petard.
What you describe is not a difference in gougers but a difference in blade profile (perhaps as a difference in the manufacturer's preference).
Let me throw out a couple of terms here for simplicity. Radius is the size of the circular shape of the grind. The 'angle' is angle behind the cutting edge that the radius is imposed on the flat blade. 'Mounting angle' is the slope on the carriage to which you mount the blade. 'Back relief' is a term from metal cutting tools referring to the steepness of the angle behind the cutting edge that allows the tool to cut into the metal without the heel rubbing on the cut surface.
On every gouger I have looked at (a lot of them) the mounting angle has been 45º, so we can call that constant. For argument's sake, let's assume the radius remains constant as well...
If the blade is ground at 45º and mounted at 45º the cane will see a circular cutting edge at whatever radius you choose to grind and the heel of the blade will rub on the cane just like the guides. (Think of a rod parallel to the bed and angled at 45º on it's front face.
In practice, that's what the RDG gouger is, although the angle is more like 43º to lift the heel of the blade slightly off the cane, presenting only the cutting edge to contact the cane. (That 2º is the relief angle).
BUT, if you change the angle at which you grind the blade, then the effects you spoke of come into play. As you decrease the angle of the grind (grind the radius 'flatter' on the blade) the contour of the cutting edge becomes more elliptical; the nose elongates, and although the radius has not changed, it takes longer along the length of the blade to reach the edges.
If you decrease the angle of the grind (grind the radius 'steeper' on the blade) the profile becomes less elliptical, fatter and squatter and ultimately square across.
So your egg description is accurate, but how you get from one end of the egg to the other is simple geometry: grind angle.
The hard part is figuring out what profile you want on the interior of the cane to optimize your reed architecture.
Kind of like most other facets of reed making, you either take what comes with the gouger or you experiment with the blade until you find what works best. At that point, measure the angle and the radius and you can repeat it forever. If your profile is not a pure ellipse, that's another story; you can't grind it on my setup but could make a template controlled setup (sorta like a cane profiler) that would give you repeatability. If you grind by hand, I defy you to make two of anything quite the same. (But maybe you've got micrometer fingertips.) ; - )
Attached are photos of my blade grinding setup. You can see that the blade is mounted at an angle to the wheel: the angle of the grind is produced by the angle on the end of the shaft upon which the blade is mounted. (The blade shown was roughed out before heat treat but not yet ground...) The Radius depends on how close the grind is to the centerline of the shaft; the further away, the larger the radius.
For a single radius gouger, size on size is probably the best approach. For a double radius machine a smaller radius or a shallower angle is probably a better choice, though for what radius and/or angle I defer to your judgment, Cooper.
This is a long winded version of what I was trying to say in my first post. Hope it's clearer...
Robert Hubbard
WestwindDoubleReed.com
1-888-579-6020
bob@westwinddoublereed.com
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