|Ask the Expert: Negative-Rake|
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Q: How to make and use a negative-rake scraper?
I hear a lot about negative-rake scrapers and it gets confusing how to make one and when to use it. ~John Berger, California
A:Using Negative-Rake Scrapers
Being such a proponent of negative-rake scrapers, this is a question I am asked quite often. I started using negative-rake scrapers when I first began turning after meeting Stuart Batty and seeing the work he was doing on his wing bowls and deep vases. I do a lot of thin wing type pieces and platters and found they worked superbly and safely. I feel it is a very worthwhile change to make on a scraper, but first there are some things you should be aware of regarding negative-rake scrapers.
Negative-rake scrapers were designed primarily for and work best on hard, dense, dry woods. Some turners say that they do not work on softer woods or green wood. I personally have tried them on all types of woods and I have often been surprised how well they worked. Regarding green wood, negative-rake scrapers most often will “fuzz up” the surface, but if you do not get a smooth finish with your bowl gouge, I believe it is easier to sand out fuzz on an even surface than tool marks or ridges on an uneven surface.
It is essential that a burr can be felt on the cutting edge for it to work. Once the burr is worn away, the scraper will not cut efficiently, which causes you to apply more pressure and this will begin tearing grain and causing thin walled pieces to flex. Neither is good.
Negative-rake scrapers should be used in a level, horizontal position on the centerline and not tilted downward in a trailing position or on edge at an angle in a shear scraping position. Used correctly, I find them to be virtually catch-free. So if you are currently using your scrapers to hollow end grain boxes and such, you should probably preserve those as they are and purchase others to be ground as negative-rake scrapers.
The recommended included angle (as measured across both bevels) is generally between 45° to 75°. A negative-rake scraper with a 45°angle will cut a bit quicker, but the burr will not last as long as one at 60°. I have always ground mine with equilateral angles on both sides and have settled on the included angle measuring about 55° (it’s really not that critical). Some people refer to these angles as being 30°/30°. That would be 30° down from both sides g
iving an included angle of 60°. By grinding the scraper in this manner it can be used in either a right hand or left hand position on different curves of a bowl or on either side of a wing or platter by simply flipping it over and regrinding the scraper to raise the burr on the top side.
There are many different methods of setting the platform angle to the grinding wheel. I am of the opinion of keeping it simple. Having said that, rather than try to explain how to set the angles, I have come up with a jig that makes it quick, simple and easily repeatable.
9/4/2016 » 12/29/2016
Turning 30 - AAW Annual Member Exhibition - St. Paul, MN