Hey, I'm on the west coast... Just commencing coffee infusion.... Plus I am a night owl. I was up in Salem last night to take in Ashley Harwood at the demo up there. I need to practice my 'dainty' skills....
Anyway, on bowls, and side grain turning, a shear angle cut will cut more clean surface than a scraping cut. On end grain, it is a different story where you are able to get comparable surfaces with either cut. So, scraper flat on the tool rest will leave more tear out. Negative rake scrapers are a whole different story, in part because, just raising the handle of a standard scraper way up in the air does not let it cut the same way that a negative rake scraper does. Most bowl turners want a big heavy scraper for sweeping across the bottom and slightly up the transition areas on the inside of a bowl. This is the most difficult part of the bowl to turn. Part is due to not being able to see the shape and surface as you cut like you can do on the outside. The other part is because we generally don't follow good body movement on that transition area (I have one video clip up about smoothing the inside curves). I have not used a negative rake scraper much. I have found that for the inside curves on bowls, it does work better on harder woods than it does on softer woods. On softer woods, it is about the same as a regular scraper (depending on sharpness and burr). A shear scrape is better. However, what exactly a shear scrape is differs a lot from turner to turner. I remember seeing several turners, Bill Grumbine was one, who did a 'shear scrape' on the inside walls of a bowl with a swept back grind gouge, with the gouge horizontal, and rolled over on the side. That to me is a scraping cut because the cutting edge is 90 degrees to the spin of the wood. Shear to me means at an angle to the spin, 45 degrees or more. The higher the shear angle, the more gently the cutting edge is at getting under the wood fiber and lifting it off. The lower the shear angle is, the more 'pull' there is on the fibers, so most of the time you will get more tear out. A scraping cut also will tear out more as you go down through the fiber, like down the side wall of a bowl. It tears less if you are sweeping back and forth across the bottom of the bowl, but some times some wood will just tear more. If it saves you time sanding, then use it, but keep working on gouge skills, and shear scraping skills.
I don't use the swept back gouges. I do use a 45/45 grind, and more standard nose shapes, and for finish cuts only. No practical use for my style of turning. I do all of my roughing with my scrapers. There is a thread about this over in the Newbie section about favorite roughing tool, and a link to my clip on turning a bowl with just scrapers. The wings on the swept back gouges are pretty good for shear scraping the outside of a bowl, but impossible to use properly on the inside of a bowl because you can't drop the handle to get the shear angle. I use scrapers for all of my shear scraping. Swept back, aka inside bowl scraper on the outside, and a ) nosed one on the inside. They are easy to roll up on the edges, once you round over the back side corners, to 70 or more degree angles. Also, you don't have to roll them over so far as you do the gouge flutes to get the cut. This improves visibility. Other than that, the burr you use makes a bid difference. I have been experimenting with 80 grit CBN, 180 grit CBN, honed, hand burnished, and now a 600 grit CBN burr. 80 grit CBN burr great for hogging out lots of material very fast, keeping a good edge for a long time, and gives a pretty good shear cut. 600 grit CBN burr, not so good for roughing, but excellent for shear scraping. Not a whole lot of difference with honed or burnished burrs, but both need some more experimenting... I hope to have a shear scraping video out later this year.
Jimmy Clewes commented at a demo that he does not like a bevel rubbed surface because it actually burnishes the wood surface, and when you start to sand, first thing you have to do is cut through the burnishing. That does have a good degree of fact, but depends in part on the turner. 'The bevel should rub the wood, but the wood should not know it' is a huge lesson we need to learn. Watching Ashley turn an ebony spindle that was about as thick as the bottom E string on my guitar with a bevel rubbing cut, and no support on the back side of the spindle really said 'Hippy, you got to practice your dainty skills'.
To date, to the best of my knowledge, no one has done a really comprehensive video or tutorial on negative rake scrapers. Best use I have found for them is for end grain box lids and bottoms. There is so much more that can be done...
I think the coffee has kicked in, off to Thai Chi class soon....
robo hippy
