robo_hippy
|
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
|