Skewing and Slicing

[Joe]

I've just read an article by Mike Riley in the latest Good Woodworking which ends with some interesting information about the effect of plane skew on effective pitch angles. He also expresses doubts about the theory that skewing causes a slicing cut, which set me thinking (though I should probably find something more useful to do with my time).

Mike's point is that no matter how the blade is skewed, if the plane is always pushed in a direction parallel to the edges of the timber then no slicing action is produced. I don't have a deep understanding of the structure of wood, but is it not the case that the wood fibres will have a tendency to slide a minute amount towards the trailing corner of the skewed blade, thus producing the slicing effect? Just a thought...

Joel

 

[Jeff Gorman]

Joe wrote:

I've just read an article by Mike Riley in the latest Good Woodworking which ends with some interesting information about the effect of plane skew on effective pitch angles. He also expresses doubts about the theory that skewing causes a slicing cut, which set me thinking (though I should probably find something more useful to do with my time).

Mike's point is that no matter how the blade is skewed, if the plane is always pushed in a direction parallel to the edges of the timber then no slicing action is produced. I don't have a deep understanding of the structure of wood, but is it not the case that the wood fibres will have a tendency to slide a minute amount towards the trailing corner of the skewed blade, thus producing the slicing effect? Just a thought...

A spill plane has a blade with an extremely large skew that causes it to produce spiral shavings from the side of the plane. I suggest that this reinforces Joe's point.

 

[mr]

Doesn't the shaving from a spill plane curl in the way that it does because one side of the shaving is necessarily longer than the other as a result of the blade / mouth configuration?

Slicing involves using a greater length of cutting edge while skewing lowers the pitch of the cutting edge making the angle more acute but keeping the same length of cutting edge, which is not to say that one can't slice and skew at the same time. JK recommends doing just that on occasion, he mentions smooth planing in a circular motion which would have the effect of slicing and skewing.

Cheers Mike

 

[bugbear]

When you think you understand the geometry of all, you might like to consider what happens in a shooting board with a sloping running surface.

BugBear (feeling mischevous)

 

[mr]

When you think you understand the geometry of all, you might like to consider what happens in a shooting board with a sloping running surface.

BugBear (feeling mischevous)

I believe we have been here once or twice before

Cheers Mike

 

[jonbikebod]

Food for though certainly.
The ‘chip’ or shaving breaks along and usually just in front of the cutting edge (Hoadly/lee/Franz). It seems to me these lines of fracture are diagonal to the shaving as it is with a spill plane the shaving, the shaving will curl diagonally forming a spill.
I need to buy a copy of good woodworking for the argument on pitch angles.
Jon.

 

[Chris Knight]

Definitely Déja vue all over again!

 

[Joe]

Slicing involves using a greater length of cutting edge while skewing lowers the pitch of the cutting edge making the angle more acute but keeping the same length of cutting edge, which is not to say that one can't slice and skew at the same time.

I'm wondering if skewing doesn't in fact increase the amount of cutting edge used. Imagine pushing a horizontal plane blade straight into a strip of paper held vertically - the paper would move straight back in the direction of the applied force. Now imagine skewing the blade and pushing it - wouldn't the paper would move backwards AND sideways, sliding along the blade towards the side with the trailing corner? This lateral movement would result in more of the blade being used and therefore a slicing action. Is it not possible that wood behaves in such a manner, though obviously only moving an imperceptible amount?

Sorry if I'm going back over well trodden ground!

Joel

 

[mr]

Why the simultaneous sideways movement? If the blade isn't skewed there is no sideways movement so why would it be introduced simply because there is now a trailing edge. Skewing the blade makes the bevel angle more acute but I can't see how it affects the direction in which force is applied. In the real world the driver may also inadvertently or otherwise introduce some slice but in theory I don't see that it exists unless purposely introduced. Imagine the paper was some imaginary inflexible material - what then ?

Cheers Mike

 

[pam niedermayer]

I'm wondering if skewing doesn't in fact increase the amount of cutting edge used....

Of course it does, simple geometry. For a given blade width, a skewed blade's edge is the hypotenuse, which is longer than either edge.

Pam

 

[Digit]

I don't get that. If the edge is 5 cm long skewing it is not going to lengthen it,
swing the base of the triangle to the position of the hypotenuse and it will still be 5 cm.
In fact the width of the cut will be narrower.

Roy.

 

[Joe]

Mike, there is a good chance that I'm talking utter rubbish - I am not a physicist. I do know though that when you push against something, anything, it pushes back. The stiffer the material, the harder it pushes. So my thinking is that as the wood pushes back against the skewed blade it is going to follow the path of least resistance and move a little "down the slope" (just before it fractures). A bit like trying to push with your shoulder against a shiny steel wall moving towards you at an angle - you will tend to slide along the surface.

If this is true, given that the wood fractures in front of the cutting edge perhaps it is still inaccurate to call it "slicing" though. Maybe it's more of a shearing action if, as I am wondering, the wood twists a little against the advancing skewed blade.

As I said though - quite likely complete nonsense! Incidentally, I enjoy your articles Mike. I find there is always something there that I can relate to.

Regards

Joel

 

[Digit]

That's correct Joel the Moment of Forces has two components. But are we talking about pushing the plane forward with the iron skewed or are we talking about moving the plane forward and at an angle at the the same time? These are two different things.

Roy.

 

[Joe]

Roy, I'm thinking about moving the plane straight forwards with the blade skewed.

Joel

 

[mr]

Thanks Joe for your comments. My feeling is that skewing doesn't lengthen the cutting edge which, regardless of geometry, can only be as long as the thing being cut. There is a difference I think between a greater length of blade moving in one direction and an even greater length of blade used diagonally to the fibres being cut. One is skewed and the other is slicing. The slicing is only introduced if the iron is moved sideways (as well as forwards). Think of a carving knife in beef, there is little downward pressure applied the work is done by the backwards / forwards motion. Wheras a plane iron moves forwards in one direction and relies on the quality of the finish on the blade edge to do the work.

cheers Mike

 

[Digit]

That way there is two forces in operation Joel but I wonder if the timber wood respond differently if we simply planed with a straight push in the direction that the iron was previously skewed, if you follow me?

Roy.

 

[mr]

You would then be running diagonal to the grain if I understand you correctly in which case I would expect to see a different effect.
Cheers Mike

 

[Joe]

My point, I guess, is that something of this "planing diagonally" effect is achieved by planing straight with a skewed blade. I'm imagining that the shaving is twisted a little as it is pulled up (because it is deflected slightly sideways). The plane/blade acts like a wedge - if it goes in at a skew isn't the shaving going to be pulled up and a little to the side (as it deflects across the surface of the skewed blade/cap iron).

Joel

 

[Jeff Gorman]

mr wrote:

> ......... Skewing the blade makes the bevel angle more acute but I can't see how it affects the direction in which force is applied.<

It does not necessarily follow that the direction of the resultant of the forces the system actually applies is the same as the direction of movement.

> In the real world the driver may also inadvertently or otherwise introduce some slice<

Probably because the two hands are so aligned that it is more difficult to control the plane.

Actually I suspect that the real benefit from skewing the plane is the reduction of the tendency of the plane to skitter.

 

[Joe]

Ok, I'm going to stop droning on about this now before everyone loses the will to live, if they haven't already. My only other thought about why a skewed plane might be easier to push is that the skew might concentrate your pushing force on a smaller area of the blade edge, towards the leading corner, and therefore create greater pressure at that point.

Now I'm off to celebrate backing the winner of the Grand National...

Joel