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shear cut
- Thread starter squirrel
- Start date
Here's a brief summary
Squirrel,
A shear cut refers to the angle in which a blade's edge enters the wood. For example, assume you have a spindle mounted on the late between centers. If you were to draw a line from headstock to tailstock and then hold your tool so that the edge is parallel to that line, the edge would enter the wood at an angle that is 90 degrees (or square) to the direction of wood's rotation. (It would also be square to the wood's fibers.) If you were to cant you tool in one direction or the other, such that the edge of the tool intersects the line at a 45 degree angle (more or less), the edge of the tool would now enter the wood at skew angle to the direction of rotation and you would be making a skew cut.
In general, tools cut much more efficiently from a skew angle than from a square angle. Which is why people using a hand plane tend to run the plane at a slight angle. It allows the plane's blade to slice through the wood fibers more easily. Each fiber is sliced by running along some length of the blade rather than clipped off. It's the difference between pulling the edge of a pocket knife against the tip of your finger rather than merely pushing the edge (not the point) straight against the finger. Pulling it across your finger will slice pretty deeply without much effort. Pushing your finder against the edge may do nothing more than dent the skin.
Just about any turning tool can be held so it's either square to the wood or skew to it. Look closely at the way the tool is cutting. Is the edge intersecting that imaginary line at a fairly steep angle? You're making a skew cut (assuming you're riding the bevel and doing everything else right). Is the edge more or less parallel to that imaginary line? You're not making a skew cut.
Hope this helps.
Squirrel,
A shear cut refers to the angle in which a blade's edge enters the wood. For example, assume you have a spindle mounted on the late between centers. If you were to draw a line from headstock to tailstock and then hold your tool so that the edge is parallel to that line, the edge would enter the wood at an angle that is 90 degrees (or square) to the direction of wood's rotation. (It would also be square to the wood's fibers.) If you were to cant you tool in one direction or the other, such that the edge of the tool intersects the line at a 45 degree angle (more or less), the edge of the tool would now enter the wood at skew angle to the direction of rotation and you would be making a skew cut.
In general, tools cut much more efficiently from a skew angle than from a square angle. Which is why people using a hand plane tend to run the plane at a slight angle. It allows the plane's blade to slice through the wood fibers more easily. Each fiber is sliced by running along some length of the blade rather than clipped off. It's the difference between pulling the edge of a pocket knife against the tip of your finger rather than merely pushing the edge (not the point) straight against the finger. Pulling it across your finger will slice pretty deeply without much effort. Pushing your finder against the edge may do nothing more than dent the skin.
Just about any turning tool can be held so it's either square to the wood or skew to it. Look closely at the way the tool is cutting. Is the edge intersecting that imaginary line at a fairly steep angle? You're making a skew cut (assuming you're riding the bevel and doing everything else right). Is the edge more or less parallel to that imaginary line? You're not making a skew cut.
Hope this helps.
- Joined
- May 16, 2005
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The word shear means at an angle. For turners, it's often referred to as skew. The basics of cutting and machining terminology can be found here, if you have no respect for copyrights. http://woodtools.nov.ru/mag/understanding_wood/understanding_wood_id.htm Or with less of the original, but proper permission, here: http://homepages.sover.net/~nichael/nlc-wood/chapters/caop.html
If you follow the terminology and the theory, you'll understand why the business of "riding the bevel" as in toe to heel, or perpendicular to the edge is not desirable. You have a particular bevel angle on the tool, and you cannot adjust the sharpness angle to compensate for the wood direction and density properly unless you keep the bevel off the wood. A bit of clearance angle is always needed, so you should never have the entire bevel on the wood anyway.
The way you maintain control of the tool is to angle it to the work so that the edge is supported across the breadth of cut. The way you lower the sharpness angle is to shear or skew the edge to the direction of feed. A lathe tool is the same as any other edged tool, so all the basics work.
I use gouges forged or shaped in some of the traditional forms for shaving, applying them to the work as shown here, if you have broadband.
http://s108.photobucket.com/albums/n28/MichaelMouse/?action=view¤t=35mmGougeRounding.flv
Still. http://i35.photobucket.com/albums/d160/GoodOnesGone/Frame-From-MVI_1017.jpg
For most of the clip I'm letting the gouge geometry do the work, cutting on the wing. As it is fairly straight, it works fine on a mild downhill. As I get toward the end I move to the more rounded portion and swing the handle left to put some extra skew or shear in the cut. Same as going to a low angle block plane for end grain, and you can see that the shavings are broader, though the feed rate is pretty much the same.
http://s108.photobucket.com/albums/n28/MichaelMouse/?action=view¤t=CherryPeelIn.flv
Shows hows the shear or skew angle increases as I cut inside. Note the wire shavings to begin and the wide shavings at the bottom.
A still, if you don't have broadband. http://i35.photobucket.com/albums/d160/GoodOnesGone/12_1024.jpg
If you follow the terminology and the theory, you'll understand why the business of "riding the bevel" as in toe to heel, or perpendicular to the edge is not desirable. You have a particular bevel angle on the tool, and you cannot adjust the sharpness angle to compensate for the wood direction and density properly unless you keep the bevel off the wood. A bit of clearance angle is always needed, so you should never have the entire bevel on the wood anyway.
The way you maintain control of the tool is to angle it to the work so that the edge is supported across the breadth of cut. The way you lower the sharpness angle is to shear or skew the edge to the direction of feed. A lathe tool is the same as any other edged tool, so all the basics work.
I use gouges forged or shaped in some of the traditional forms for shaving, applying them to the work as shown here, if you have broadband.
http://s108.photobucket.com/albums/n28/MichaelMouse/?action=view¤t=35mmGougeRounding.flv
Still. http://i35.photobucket.com/albums/d160/GoodOnesGone/Frame-From-MVI_1017.jpg
For most of the clip I'm letting the gouge geometry do the work, cutting on the wing. As it is fairly straight, it works fine on a mild downhill. As I get toward the end I move to the more rounded portion and swing the handle left to put some extra skew or shear in the cut. Same as going to a low angle block plane for end grain, and you can see that the shavings are broader, though the feed rate is pretty much the same.
http://s108.photobucket.com/albums/n28/MichaelMouse/?action=view¤t=CherryPeelIn.flv
Shows hows the shear or skew angle increases as I cut inside. Note the wire shavings to begin and the wide shavings at the bottom.
A still, if you don't have broadband. http://i35.photobucket.com/albums/d160/GoodOnesGone/12_1024.jpg
Steve Worcester
Admin Emeritus
I use a couple different forms of a shear cut.
A shear scraper, which is noting more than a regular flat sectioned scraper with a 90 degree angle, 45 on each side. It is held flat on the toolrest and used to clean up gouge marks and refine shape. The nature of it is that when sharpened has a very fine burr on the top side and it doesn't last long, but cuts real well. Especially hard woods and burls when using a very light touch.
The other is using a gouge (or skew or shear scraper like John Jordan sells). With a fingernail gouge, you hold the top of an inside wing to the wood at a 45-60 degree angle. Sort of hard to describe, but best shown in the Ellsworth videos.
With Johns tool, you can see how it is use on his page, under tools, then click om shear scraper.
A shear scraper, which is noting more than a regular flat sectioned scraper with a 90 degree angle, 45 on each side. It is held flat on the toolrest and used to clean up gouge marks and refine shape. The nature of it is that when sharpened has a very fine burr on the top side and it doesn't last long, but cuts real well. Especially hard woods and burls when using a very light touch.
The other is using a gouge (or skew or shear scraper like John Jordan sells). With a fingernail gouge, you hold the top of an inside wing to the wood at a 45-60 degree angle. Sort of hard to describe, but best shown in the Ellsworth videos.
With Johns tool, you can see how it is use on his page, under tools, then click om shear scraper.