Negative rake scraper angles

[Trevor Mills]

I attempted to grind a negative rake scraper using a 30 degree angle for the top and about a 70 degree angle on the bottom but it doesn't cut very well at all. I think I remember those angles from a UTube video but I could easily be remembering it wrong.

I'm willing to experiment but wonder if anyone has any suggestions.
Trevor

[Thom Sturgill]

If I recall correctly, a scraper has to have an included angle less than 90 degrees. Yours IS 90 degrees. I would have made the top angle more like 15-20 degrees. According to Cindy Drozda she uses a 5 degree top bevel and about a 70 relief bevel. On the Stuart Batty scraper she uses a 25 deg top and 40 degree bottom to get a 65 degree included angle. She just uses the same 40 platform she grinds everything else on when she needs to resharpen.

The larger the angle the more work required to get it to cut and the longer the edge lasts.

I would regrind the bottom angle and try again.

[Nick Stagg]

Your negative rake scraper won't cut, it's only intent is take off very fine shavings for finishing work. The "burr" you create by sharpening does the work and it has to be on the top of the tool. A coarse wheel, 46 grit, raises a much better burr that lasts longer. Once the burr is gone, you're back to scraping so you need to renew it often; if you can't feel one, it's gone.

You can do the same thing with a skew by just sharpening one side at a time and reversing it for sharpening. However 10 V steel holds a burr much better than regular HSS and it's handy to have a dedicated tool for negative rake scraping,

[Thom Sturgill]

A scraper, any scraper, can make heavy or light cuts depending on the edge. You can hog wood off with one - ask Reed Grey AKA Robo Hippy or just go watch his videos.

A grinder burr is like using a tomato knife and breaks off leaving an even more jagged surface. Hone it off. Any harder steel can be rubbed against the edge to raise a clean burr. A light burr for finishing cuts can produce paper thin wisps of cuttings just like a cabinet scraper and a heavier burr can be used for rapid waste removal. It does take some learning not to 'roll' the burr completely over to where it will not cut.

By the way, I use a HONED skew occasionally as a scraper, works fine, no burr just a sharp edge and about 30 degree included angle.

[James Combs]

If I recall correctly, a scraper has to have an included angle less than 90 degrees. Yours IS 90 degrees. I would have made the top angle more like 15-20 degrees. According to Cindy Drozda she uses a 5 degree top bevel and about a 70 relief bevel. On the Stuart Batty scraper she uses a 25 deg top and 40 degree bottom to get a 65 degree included angle. She just uses the same 40 platform she grinds everything else on when she needs to resharpen.
The larger the angle the more work required to get it to cut and the longer the edge lasts.
I would regrind the bottom angle and try again.

Thom, correct me if I am wrong but doesn't the OP's original statement of angles at 30 and 70 actually give him an included angle of 100* instead of 90* which actually makes things worse for him. An obtuse angle(>90*) for practical purposes will not cut at all? It takes an acute angle(<90*) to give you any sort of cutting edge, not considering burrs. Just thinking out-loud here maybe I am missing something in yours and the OP's post.

Edit: Although I disagree with Thom's assessment of the OP's included angle I think everything else He says is right on the money.

[Reed Gray]

I was across from Stuart in the Vendor area in the Phoenix Symposium, and got to take in parts of his demos at his booth. He is probably THE negative rake expert. I don't know if he did a video on it with all the considered angles, included angles, and what not. I have only one negative rake scraper. It is ground 1/4 round, or kind of like a swept back scraper, and 45 degrees on both sides. Some times I want to use one side, some times the other, so I just flip it over and grind with the side I want to use on top. The burrs ground on a CBN wheel are much better than the ones from standard grinding wheels which are more of a wire burr that will break off easily. The CBN wheel has an almost burnished quality to it, and can stand up to a lot of heavy roughing on my bowl scrapers. They last a lot longer on negative rake scrapers as well. I have experimented with burnishing a burr on one, and it does work, but isn't any improvement on the CBN wheel burr. Yes, you can hand burnish a burr on HSS tools. With the negative rake scraper, as well as standard scrapers, the triangle burnishing tools work best, you need very little pressure, and you want to be maybe 10 degrees off the bevel angle, so if your bevel is 70 degrees, then you want the burnishing tool to be at 75 to 80 degrees. If you tip it out too far, you can make a burr that breaks over the top like an ocean wave, and they cut terribly if at all, or you have to hold the handle way up in the air. I have seen a lot of negative rake scrapers that are 60 to 70 degrees on the bottom, and 10 to 20 on the top. Mostly you hone off the burr, then kiss the face to a grinding wheel.

robo hippy

[Pat Scott]

According to Stuarts 2014 catalog, his Negative Rake Scrapers had an included angle of 50 degrees (so each side was 25 degrees). I just noticed that a later 2014 catalog says that the Negative Rake scrapers came with a 40 degree included angle. His conventional scrapers had a 70 degree angle, and his skews came with a 40 degree included angle.

Here's a paragraph from that later catalog:
It is essential that the blade have a secondary ground angle on the top of the blade to be negative rake. Lifting or tilting a
scraper does not make it negative rake, only the secondary top angle does. The included angle of the ground bevels must
be 70˚ or less to produce a satisfactory burr that Negative Rake Scrapers require. If there is no burr this technique becomes
ineffective. It is the negative bevel combined with a burr that makes this technique so effective; hence, carbide cannot be
used because it cannot create a burr.

Glenn Lucas uses an included angle of 66 degrees (33 on each side), for no other reason than his angle gauge has a 66 degree reading. This is the angle that I use too, and I don't hone. I think I read that honing is only beneficial for certain woods? I figure the burr doesn't last very long anyway, so I just use the tool as is straight off the grinder.

It's sure a lot easier to have the same angle on both sides when grinding. Do one side, flip it over and do the other. Otherwise if you have 30 degrees on one side and 70 on the other, you're constantly changing the tool rest.

[Thom Sturgill]

Thom, correct me if I am wrong but doesn't the OP's original statement of angles at 30 and 70 actually give him an included angle of 100* instead of 90* which actually makes things worse for him. An obtuse angle(>90*) for practical purposes will not cut at all? ...

Quite right, my numbers were off and as you said that must makes it even worse.

[Thom Sturgill]

I've never tried a burr from my CBN wheel, but now that I have a 350 grit wheel I think I will give it a try even though I have been happy with burnished burrs.

[David Delo]

I've never tried a burr from my CBN wheel, but now that I have a 350 grit wheel I think I will give it a try even though I have been happy with burnished burrs.

Thom, Not sure (perhaps it will get posted further on down the line) but don't you get a better burr for a scraper with 80 grit CBN versus the higher grit???

[Thom Sturgill]

Thom, Not sure (perhaps it will get posted further on down the line) but don't you get a better burr for a scraper with 80 grit CBN versus the higher grit???

My take on that is if I wanted 80 grit scratches in my work, I'd just use a grinder.

Any grooves or bumps in a cutting edge will telegraph onto the cut. Ask any flat worker who has planed or jointed a board with a nick in the blade. The scale may be finer, but the effect is still there, and needs to be sanded out. 80 grit would probably work for those little scrapers used on boring bars, not for larger scrapers used on bowls.

[Pat Scott]

My previous post was done while I was at work, and trying to do other things. Now that my work day is over I have time to read over Stuart Battys catalog a little more. I read the rest of the page on Negative Rake Scrapers from Stuart's catalog, and thought I would share some more information for what it's worth:


What is Negative Rake Scraping?
This technique has been used since the 1500s for turning ivory and dense exotics. Stuart Batty defined the rules on how, when and where to use this technique, naming it as Negative Rake Scraping to distinguish it from Conventional Scraping over a decade ago.

It is essential that the blade have a secondary ground angle on the top of the blade to be negative rake. Lifting or tilting a scraper does not make it negative rake, only the secondary top angle does. The included angle of the ground bevels must be 70˚ or less to produce a satisfactory burr that Negative Rake Scrapers require. If there is no burr this technique becomes ineffective. It is the negative bevel combined with a burr that makes this technique so effective; hence, carbide cannot be used because it cannot create a burr.

Simply grinding the lower bevel pushes some of the metal up on to the top bevel creating a burr. It should be produced with a coarse wheel or belt, not with a burnisher. A sturdy grinding platform is strongly recommended so that you can create a burr at the same angle each time to get the longest life from your blade. Our Universal Grinding System combined with our angle gauges ensures accurate repeatable burrs, making this one of the most effective techniques a woodturner can use.

You should always be able to feel the burr on the top surface with your finger. If you can’t feel the burr it needs grinding again. The burr has a short life and will last approximately 90 seconds for each 1/2” width of blade in contact when using CPM 10V®, M2 only lasts about 20 seconds. Though the burr life is short, it can be resharpened in less than 10 seconds and will dramatically improve surface shape and finish, allowing the turner to start with much finer grades of sand paper.

When to use Negative Rake Scraping
Negative Rake Scraping is a neutral technique. It neither draws the wood in like Conventional Scraping does, nor pushes it away as would a gouge. This makes it an exceptional technique for working very thin walled pieces or broken surfaces, like square bowls or natural edge.

Negative Rake Scraping works best on medium density temperate woods and the densest exotic woods. It is also the very best tool on acrylics, plastics and acrylic impregnated woods because it will not grab at them. It can produce an unmatched finish on end grain or around the mixed grain surface of a side grain bowl.

A Negative Rake Scraper can be used on either side; therefore when cutting at the very center of a box or bowl use caution not to cut past center.

Negative Rake Scraping is never aggressive. It is slower at shaping than conventional scrapers, but it’s a far more effective finishing technique because it removes the risk of grabbing at the wood. Negative Rake Scraping can easily smooth out long cuts by removing unwanted tool marks from gouges or torn grain from carbide scrapers.

Negative Rake Scraping, when done correctly, will improve your shapes and dramatically reduce your sanding time. Don’t forget: the edge of the blade must have a burr present to be effective.

And here are some other excerpts:
I recommend creating a burr on a bench grinder. A coarse wheel creates a bigger burr than a fine wheel and, in most cases, this is an advantage. A bigger burr will last longer. I don’t recommend creating a burr with a burnisher. Burnishers don’t actually create a burr in the same way as a grinding wheel does; they tend to curl the edge over. This can make the scraper too aggressive.

Negative rake also requires a burr to be effective; once it has worn off the tool must be reground to create the burr again. A 40° included angle will create a bigger burr and last longer than a 70° included angle. A 90° or greater included angle cannot create a burr and is therefore not a Negative Rake Scraper.

[David Delo]

My take on that is if I wanted 80 grit scratches in my work, I'd just use a grinder. Any grooves or bumps in a cutting edge will telegraph onto the cut. Ask any flat worker who has planed or jointed a board with a nick in the blade. The scale may be finer, but the effect is still there, and needs to be sanded out. 80 grit would probably work for those little scrapers used on boring bars, not for larger scrapers used on bowls.

I've got a few extra scrapers that I'm going to have to try this NRS idea on. Don't have the 350 grit so I'll do it on the 180.

[Reed Gray]

A number of the free hand sharpeners use 60 grit wheels because they say it cuts cleaner. This is for gouges. At a shear angle, it does not leave 80 grit scratches..... I think I heard Chris Stott make that same claim about 80 grit scratches.

robo hippy

[Steve Trauthwein]

Just ground one yesterday from v10 straight scraper, a lot of grinding. Go to rudolphlopez.com and he has a hand out on grinding the Stuart Batty angles.

Regards, Steve