Author: oboedrew
Date: 2011-05-04 14:59
Attachment: figure1.jpg (19k)
Attachment: figure2.jpg (18k)
Attachment: figure3.jpg (19k)
Attachment: figure4.jpg (19k)
huboboe wrote:
> So - if, instead of a burr, we have a sharp cutting
> edge angled at the same aspect that the micro-
> beveled burr would be, then it should 1) cut (scrape)
> the same as a burr would and 2) last for a long time,
> since it doesn't break down the same way a burr will.
>
> That's been my approach to sharpening my knife,
> and, as I have mentioned before, I make several
> reeds between sharpenings rather than several
> sharpenings per reed.
RobinDesHautbois wrote:
> Bob: so well put!
>
> And with the micro bevel, I don't have to trade
> frequency of sharpening for ease of scraping: as
> long as the metal is hard, I get them both.
Unfortunately, guys, we can't have our cake and eat it too. A bevel on the trailing side of a knife alters the relief angle, but not the rake angle. I've attached some sketches.
Figure 1 shows a sabre-ground knife (a common type, though seldom used by reedmakers). The scrape moves from right to left. The cane surface is the horizontal axis, and the red line is the vertical axis. The rake angle is the angle between the knife's leading side (or front face) and the vertical axis. The relief angle is the angle between the knife's trailing side (or back face) and the horizontal axis.
Because the leading side of the knife's cutting edge leans left (that is, toward the direction of the scrape), the rake angle is negative.
Figure 2 shows a beveled reed knife. Because the leading side of the knife's cutting edge runs parallel to the vertical axis, the rake angle is neutral (or zero).
Figure 3 shows a hollow-ground reed knife with a microbevel on the trailing side. The bevel affects the relief angle, but not the rake angle. The rake angle is negative, and negative rake angles require more cutting force. Moreover, you'll worsen the rake angle for scraping if you inadvertently create a second microbevel (on the leading side) when removing the burr.
Figure 4 shows a hollow-ground reed knife with a scraping burr. Because the leading side of the cutting edge (which, in this case, is the burr) leans right (that is, opposite the direction of the scrape), the rake angle is positive. Positive rake angles require less cutting force.
Earlier in this thread, I said that each knife stroke begins with the blade approximately perpendicular to the cane. But the starting point varies slightly from one reedmaker to another. My stroke typically begins with the knife's edge slightly behind its spine. The spine rests against my left thumb and acts as a fulcrum. I rotate my right wrist, and the burr glides forward across the cane, eventually lifting upward. Figures 5, 6, and 7 depict the beginning, middle, and end of a typical stroke. The rake angle remains positive throughout.
Figure 8 is similar to figure 5, but I've replace the burr with a bevel. The rake angle is negative and will remain so for much of the stroke. This requires more cutting force.
To begin with a positive rake angle, you'd have to hold the knife as shown in figure 9. But then rotating the wrist would merely lift the edge upward, removing no cane.
So that's it. That's why we use scraping burrs. At least, that's one of the more important reasons. They're just better for the job. Sure, they break down quickly; but, with a bit of practice, they can be re-established in mere seconds. I can't imagine why anyone would sacrifice control to save so little time.
Now, I've said that the once-a-week, burr-free sharpening method is "wrong," and that seems to have ruffled some feathers. So, I'll happily concede that it's not wrong in the same sense that genocide is wrong. I don't think anyone is going to hell for it. :-) To keep the peace, I'll say "less effective" or "less efficient" instead. But the bottom line is: It doesn't work as well. If you're satisfied with it, carry on. Heck, you can make reeds with a chainsaw if that's your cup of tea. But reedmakers who want to scrape as precisely as possible, and oboists who want to play as easily as possible, should learn to craft a good burr.
Cheers,
Drew
www.oboedrew.com
Post Edited (2011-05-04 16:37)
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