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Shear scrape speed (Read 1,354 times)
 
Rick Caron
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Shear scrape speed
May 8th, 2016 at 6:28pm
 
To remove my tool marks and minor tear out i use a negative rake scraper.  Never been able to use a shear scrape, Leaves more marks.  Maybe to scared to turn above 1200 rpm.  Once the outside is shaped  should i run lathe faster?   6 to 8 inch bowls, regular and natural edge.   Roll Eyes
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Don Stephan
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Re: Shear scrape speed
Reply #1 - May 8th, 2016 at 6:34pm
 
What is the shape of the end of your scraper?  If rounded to a smaller radius it will only be contacting the wood in a narrow band, even narrower if rotated to shear scrape.  That could make it more likely to leave parallel grooves.  Don't want to take a shaving wider than 1/4" or so in my mind to avoid heavy drag, but still can have a larger radius of curvature at the end of the scraper to reduce grooving.
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John Grace
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Re: Shear scrape speed
Reply #2 - May 8th, 2016 at 6:53pm
 
Rick, check out robots hippy's videos on shear scraping, good information.  Also see his use of Doug Thompson's fluteless gouge for that exact task.  Excellent clean up tool and technique.
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robo_hippy
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Re: Shear scrape speed
Reply #3 - May 8th, 2016 at 11:44pm
 
I do need to do one video on just shear scraping.... probably negative rake scrapers, but they are still new to me and I have lots of experimenting to do... I have been playing around with some wet myrtle, finishing cuts, shear scraping with swept back scrapers (gouges just don't do it for me), negative rake scrapers, and different grits for sharpening, 80, 180, 600, and 1000. Main conclusion thus far is that dry myrtle gets a much better finish off the tool than wet myrtle....

First, speed, I keep it at the same speed I use for turning, which for me is high, up to 2200 rpm, or for normal people, 1000 is fine for that size of bowl.

On the outside, I prefer a swept back scraper which has a nice long edge on it, maybe 2 inches or so. Any longer, and the cutting edge digs into the tool rest. I am moving the tool down as I cut so I am keeping a fresh edge on the wood, which means I am cutting closer to the nose to start, and down at the bottom of the wing when I get to the end. Since this cut is so light, you can push or pull, or go with or against the grain. I do get some small lines, but they are not dig in marks or bruising, and for me, on green wood, I can start sanding most of the time at 120. On dry wood, much finer than that. Two key points here, one is to move with your body, and keep the tool anchored on your body. Move forward and back by moving and shifting your balance, not your arms. This is much more steady. The other point is to have a rest with the hardened drill rod on top. Mostly this is because there is less friction so the tool slides across it much more easily, and it doesn't dent or ding so you don't get the divots in the wood because of the divots in the tool rest.

On the inside, I use a scraper with a ) shaped nose, not half round, but that will work. Again, move with your body, and hardened drill rod tool rest. With that nose profile, handle down working/cutting with the lower half of the cutting edge so you can not catch. Most of the time I am pulling only, from bottom up to rim, and since the cut is so light, you have minimal tear out. It is pretty much impossible to shear scrape with a gouge on the inside of a bowl. Part of this is because the rim of the bowl, the banjo, and the lathe bed all seem to get in the way. You can hold the gouges pretty much level, but then it is a scrape, and has no shear angle.

With both inside and outside, you are not rubbing the bevel, and you are only cleaning up, not removing stock.

Now, negative rake scrapers. I find these easier for cleaning up the inside of the bowl. However, they don't seem to work well on wet myrtle, and leave a fair amount of tear out. On dry myrtle, they work great. On madrone, they work beautifully, but anything cuts well on madrone. On softer woods, they don't cut well, and a shear scrape seems to do better. On harder woods, they seem to do about the same. Also, just like when turning bowls, the outside is a lot easier to do than the inside, and I get equal results on the outside, but better on the inside with the NRS, than a shear scrape.

Now, how sharp?? There does seem to be a big difference between dry and wet woods. For sure, the dry woods clean up better with a very fine edge, honed or ground to 600 or 1000 grit. On wet wood, it doesn't seem to make as much difference, with the 180 grit CBN wheel giving at least as good of a cutting edge, and maybe even better than the finer grits. Still playing around with that one. It doesn't seem to matter what I do with green wood, I pretty much always have to start sanding at 100  or 120. On dry wood, I can start at 180 or 220.

robo hippy
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Louie Powell
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Re: Shear scrape speed
Reply #4 - May 9th, 2016 at 11:10am
 
This may be a dumb question - but what is the difference between a negative rake scraper, and a standard scraper held at a downward angle?
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Louie
 
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robo_hippy
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Re: Shear scrape speed
Reply #5 - May 9th, 2016 at 11:37am
 
Louie,
That is a question I have been trying to find an answer for. You would think that if you just raise the handle enough that they would cut the same, but they don't. It is supposed to have some thing to do with the 'included' angles, which means the top angle and the bottom one combined. First NRS were skew chisels, which if sharpened the same on both sides at 30 degree bevel, leaves you with a 60 degree included angle. Supposedly the included angles should not be more than 90 degrees. Standard scraper at 70 degree bevel, and top flat is to me a 70 degree included angle. I have some that are 70/20 that I use inside boxes which would be 90 degree included angles, and they cut fine. I have tried a number of different grit wheel burrs, and no matter what I do the NRS and standard scrapers, they do not cut the same way...

robo hippy
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Rick Caron
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Re: Shear scrape speed
Reply #6 - May 9th, 2016 at 5:34pm
 
My scraper is a 1inch wide, curved nose,  75 degree single angle.  Maybe i need a straight end scraper so it will cover a little  more wood than a rounded nose?
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robo_hippy
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Re: Shear scrape speed
Reply #7 - May 9th, 2016 at 8:15pm
 
Rick, the round nose is good for being able to go to the right or the left. With the longer sweep to my scrapers, I can drop the handle more for a steeper shear angle than I can if the edge is shorter. If the edge is at a 45 degree shear angle, that is fine, but you may need to touch up the edge slightly more. Just like with gouges, I like a fresh edge for finish cuts.

robo hippy
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Lee Watermann
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Re: Shear scrape speed
Reply #8 - May 14th, 2016 at 2:51pm
 
Robo, is there any difference to you for a push cut or pull cut and if so where?
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Don Stephan
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Re: Shear scrape speed
Reply #9 - May 14th, 2016 at 6:40pm
 
Rick

Richard Raffan's dvd on bowls shows extensively how he uses scrapers both outside and inside.  He uses a spear point and three with different radii sweeping down the left edge slightly.
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robo_hippy
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Re: Shear scrape speed
Reply #10 - May 14th, 2016 at 8:04pm
 
Lee, for a shear scrape, there is no difference between push or pull. I know that Mike Mahoney and Stuart Batty have a very good demo of 2 ways to turn a bowl. Stuart does his push cut, and Mike does a pull cut. To me, the biggest difference is that Stuart holds his tools more level cutting with the nose, and Mike drops his handle cutting more with the wing. Mike is on the headstock side of the bowl. Some say that the difference is that with a pull cut, the handle leads and the cutting edge follows, but most of the time they seem to line up with the direction of the spin. That cut can be done as a push or pull, from either side of the headstock. Both are methods for adapting to turning on a long bed lathe so you don't need to bend over to turn the outside of a bowl. When they turn the inside, they are pretty much the same. More than anything, I think it is which feels better for you.

robo hippy
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