Shear scraping question

[germdoc]

I am honing my skills on end-grain bowl turning. I've gotten pretty good with the bowl gouge and now am able to get a pretty clean surface using this tool.

I'm curious what type of scraper you all use for final shear scraping. I was thinking about getting the Ellsworth 3/4" radius skew--not sure if Woodcraft still carries it. Or, possibly getting a 1" square scraper and grinding a skew edge on it.

[Jim Boyd]

I have a right-handed and a left-handed bowl scrapers. They are both about 1 1/4 wide and 3/8 thick.

[kwgeorge]

Sorby makes a couple of shear scrapers that work pretty well. I have purchased the small one and like it.

http://www.robert-sorby.co.uk/shear_scrapers.htm

Alternatively you can use a normal scraper by raising your tool rest above center line and then presenting the scraper tip tilted back to center line. Normally I only do this on the harder more dense woods and this can produce an almost polished finish. On the exterior of bowls and what not I like to use a gouge with a long grind in a shear scraping presentation. Also a piece of advice, do not have a bur on a scraper that you intend on using in a shear presentation or for a final cut. It will be too aggressive and defeat the purpose.

[germdoc]

Rockler sells that Sorby scraper and I might have to give it a try. Woodcraft no longer carries the Ellsworth skew scraper I was interested in, but I guess I could get a square end scraper and grind a new profile on it if I wanted to.

[Tom Miller]

Can anyone point me to a good photo or diagram showing the "angle of attack" of the scraper for shear scraping? (The little photo in the Sorby link is not at all clear to me.) I guess I'm still unclear of the concept.

Thanks,
Tom

[kwgeorge]

Well let’s take a look at shear scraping with a scraper since that is the discussion. Normal scraping on the inside of a bowl or such would mean you would present the tool to the work piece parallel with the lathe bed. You would have your tool rest at or just below center line with the lathe attempting to make the scraper tip cut at or just above center line as illustrated here;



This is a very aggressive cut especially if the scraper is sharp and has a small bur on the tip. This is good to remove quite a bit of wood in a single pass but will also lead to some potential catches and a surface that is a bit torn. To shear scrape we want to present the tip of the tool at center line but at a downward angle so that only the very cutting edge of the tool contacts the work piece. To accomplish this we move our tool rest at or above center line and present the tool at a more downward angle as illustrated here;



This will produce a much finer cut and will “shear” the wood fibers leaving a much better finish. It also has the advantage that if you do have a catch the tip will be simply pulled downward and cause less damage to the work piece. Then the question becomes’ how much angle? Well that depends, at some point the angle becomes too great and you are no longer cutting wood with the tool but rather just rubbing the tool against the wood. Also the increasing the angle will make the tool harder to control. My thoughts on this are to present the tool at a great enough angle to keep the bevel from touching. Now a word of caution, this type of scraping requires the removal of the bur. The bur is created by sharpening the scraper on the grinder. Normally this is desirable and it makes for a more aggressive cut but on a finish cut you need to knock this bur off the tool with a diamond hone or such.



Another issue while we are on scrapers is how much of the scraper is really touching the wood. Even with a large scraper only a portion of the cutting edge is in contact with the work piece. This can be on any part of the cutting edge.



If too much of the cutting edge comes in contact with the wood you will experience a poor cut and potentially a bad catch.



The hardwood scrapers available now commercially are based on the above principles the only real difference is they allow you to keep the tool rest pretty much where it is. Another type of scraper called “Shear Scrapers” commercially available have a flat ground edge on the tool tip and are made to present this tool tip at 45 degrees parallel to the surface of the piece being turned.

So, shear scraping is nothing more than presenting the cutting edge of a tool to the work piece at an angle other than 90 degrees. The more angle of the presentation the shearer the cut. Again though you do reach the point of diminishing returns by too much angle.

Hope this helps someone.

[Tom Miller]

Thanks, Ken. Your description of shear scraping is essentially what I had in mind, but somehow the description of that Sorby tool in the link threw me off. Sounds like one important aspect I was missing, though, is the removal of the burr for shear scraping. I'll have to give that a try.

Regards,
Tom

[gerti]

It is also possible to get a very fine slicing cut with the burr by rotating the tool to about a 45 degree angle and slightly downwards. It helps to round over the corners of the tool for this so it easier glides over the tool rest.

Gerd

[Tom Miller]

quote:Originally posted by gerti

It is also possible to get a very fine slicing cut with the burr by rotating the tool to about a 45 degree angle and slightly downwards.

So, the axis of the 45 degree rotation is the handle? I think that's what was confusing me.

I must say, I am getting some pretty wispy shavings with the burr, and the tool angled down. Now I'll try it with a twist.

Regards,
Tom

[kwgeorge]

Well, you have entered a world too dangerous for me. Rotating a flat tool and using it by supporting only its edge on the tool rest is just a bad idea. The area of the tool that comes in contact with the tool rest is severely altered and in my opinion completely unsafe. A good catch can you get you in a world of trouble and pull the tool flat to the tool rest potentially snapping the tool. There are ways to do this safely by taping a dowel to the underside of the scraper and using it as a pivot point. With this you at least end up with two contact points for the tool.

My opinion so do what you think is right.

[Tom Miller]

quote:Originally posted by kwgeorge

Well, you have entered a world too dangerous for me. Rotating a flat tool and using it by supporting only its edge on the tool rest is just a bad idea.

I agree. I should have mentioned I was using a homemade version of one of these:



Regards,
Tom

[kwgeorge]

Now your talking Tom!

[guycox]

I've got an Ellsworth 3/8" bowl gouge -- I use it in a shear scraping mode per the instructions that came with the tool .. It takes some practice and the rest has to be really close to the work -- but I can pull tissue paper thin shavings off the bowls and I can start sanding at about 150 for dry blank.