Another question on sharpening angles

Re: Another question on sharpening angles

Reed Gray

I noticed that about machinists a while ago, they measure differently than we do.

The one I wonder about is with negative rake scrapers... If included angles mean both top and bottom bevel angles, if I have a skew type that is 30/30, then included angles are 60. If I have a standard scraper with a 70 degree bevel, then is it a 70/0 and therefor, included angles are still less than 90 degrees to fit into the ideal range that Stuart and others prefer????

robo hippy

Re: Another question on sharpening angles

John C Lucas

Thats the way i understand it Reed. I actually played with both today. I needed to make a regular scraper for a.photo i was doing. After the photo i compared ot to my negatice rake scraper.that has the same 70 degree included angle. Couldnt tell a h #ifference in cut quality but the neg rake was much more forgiving and less grabby.

Re: Another question on sharpening angles

John K Jordan

If writing something like an article this subject begs a good diagram!

How I think of it: In geometry an included angle is the angle between two sides of a triangle. We can consider all grinds with flat bevels identical, varying only in the number of degrees in the included angle and how much the triangle is "tilted" relative to the axis of the tool shaft.

We measure from the "horizontal", the top edge of the tool shaft (or the bottom of the flute for a gouge). The angle of a conventional scraper would be 90-deg if ground straight, 80-deg if the ground face is angled back 10-deg. The cutting angle of a scraper, NRS, bedan, skew, or gouge is the included angle.

The skew and standard parting tool normally have equal length bevels so the angle from the horizontal of one bevel is 1/2 the included angle. A skew with one side of the triangle aligned with the axis of the tool shaft is the same as a bedan (or we might call it a scraper depending on the angle). I use the angle from the horizontal to the upper bevel. A cross section through the tip of a gouge is also equivalent to a bedan. A side edge of a gouge is harder to measure since one side of the "triangle" is curved. The NRS needs another number to describe how much the triangle is "tilted" - I think it should be defined by the included angle and the angle of the top bevel from the horizontal.

That's the way I think of these angles.

Picture of measuring a gouge bevel angle.

https://22293-presscdn-pagely.netdna-ssl.com/wp-content/uploads/12355_5F00_trad21.jpg

JKJ

Re: Another question on sharpening angles

Chuck Engstrom

That little General protractor is just the right thing for measuring angles on our tools, and they're available on ebay for about $10-12, even less for "pre-owned," in several variations -- search on "General protractor." I carry one in my tool chest when teaching, and there's another parked permanently next to the grinder.

Re: Another question on sharpening angles

john lucas

That's what I use but I have at least 5 other ways to measure a flute angle. I don't really worry about sharpeness angles other than to answer questions for other turners or in my investigation on what angles produce better cuts. On a bowl gouge anywhere from 40 to 60 degrees will work. With 60 you get into a deeper bowl and still rub the bevel. With 40, not so deep but you get a cleaner cut because a more acute edge simply cuts cleaner.

Totally agree

Re: Another question on sharpening angles

Reed Gray

I played around for a bit, after watching Nick Stagg from Salem demo. He used a standard scraper with the burr honed off. It works great for hard woods, but not so good for softer woods, and I was using sugar maple. I won't put a standard scraper any where near the edge/rim of a bowl that has been hollowed out unless I am doing a shear scrape. though I have seen some use this method. I am also playing around with an 80/25 NRS like the 'hardwood bowl NRS' that Sorby makes. You use it for a peeling type cut on bowls, so you are rubbing the bevel. Makes my sphincters pucker to try it out, but it does a really nice job. Anything to reduce sanding time....

robo hippy

Re: My personal take on this.......

Jesper, Denmark

With a single bevel tool, always measure the angle between the c/l of the tool and the bevel - because this what you CAN measure directly. You cannot measure the angle between the perpendicular to the bevel directly.

With double bevel tools, you can measure the included angle directly - and that is what you are interested in.