robo_hippy
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Lots of misconceptions about scraping, gouges, and shear scrapes. I have been wanting to do this video clip and it has been in the planning stages for a year or so. Both scraping and bevel rub gouge use is 'cutting' the wood. Tear out mostly comes from trying to cut the wood so fast that it tears before it gets a chance to cut. Dull tools does the same thing, pretty much with out any relation to speed. The idea of 'shear' is almost as old as woodworking itself with hand planes that have angled blades, or even the card scraper that you move across the wood at an angle to the grain. I compare it to going over speed bumps in the parking lot. Hit it square on and you get a fairly good bump. This would be scraping with the scraper flat on the tool rest. Hit the speed bump at a 45 degree angle, and you get much less of a sudden bump, but the bump is still there. Going at a 75 degree angle and the bump almost goes away. So, the shear angle is better at getting under the wood fiber more gently to lift it away. A bevel rubbing cut with a gouge is at a shear angle, most of the time in the 45 degree range. Shear scraping can be done with both gouges and scrapers. The shear scrapes are for clean up work, not for heavy stock removal. So, for bowl turning, first you get a rough shape, then you refine it with a pass or 5 with a bowl gouge. Then you use a shear scrape to clean up all the little tool marks and bumps and humps that maybe you could sand out, but it is easier, and most of the time faster to shear scrape. To get these marks out, you don't do it in one pass, but take several. Also, since this cut is so light, you can generally go with or against the grain. The higher the shear angle, the more gentle the lifting of the shavings is. I would say that the biggest difference between a shear scrape and a bevel rubbing cut is that with the shear scrape, the tool rest is your fulcrum. With a bevel rubbing cut, the tool rest supports the tool, and your bevel rub is the fulcrum. The bevel rub may help hold down the fiber as you cut it. It also does leave a some what burnished surface. Jimmy Clewes made the comment that he always shear scrapes because when you start to sand, the first thing you have to do is sand off the burnishing. I hadn't thought of it before I heard him make that comment, but it seems to be true. With the bevel rubbing cut, remember that 'the bevel should rub the wood, but the wood should not know it', so keep the pressure very light. I also like the shear scrape if I want a 'perfectly' round bowl. Most bowls will have a run out of maybe 1/16 inch, which means plus or minus 1/32 of an inch. Over about 1/8 of an inch thick, and no one will notice. Under that, you need to even it up in ways that you can't do with a bevel rubbing cut because when you rub, you get a tiny bounce from going uphill (against the grain), down hill, up hill, and down hill for each revolution. The shear scrape, since you are not rubbing the bevel does not seem to do this and you can get almost perfectly round, after reversing the bowl. I have a bunch of bowl turning clips up here and on You Tube, type in or search robo hippy
robo hippy
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