Veritas skew block plane

[TheTiddles]

Tiddles, I've just fused my brain trying to do the trig. in my head to confirm that, but I cannot see how the apparent edge angle can change across a skewed blade! You'll have to explain it to me with a diagram or something.
I've made a few skewed planes, so I know the angle of the bed is the same at both sides (you use the same template to set out both sides) so presumably any parallel slice at any point between, is also at the same angle. The blade edge is skewed, and the same angle is applied across the edge in grinding & sharpening. It seems to my untutored mind that the edge meets the work at the same apparent angle from one side to t'other. What am I missing..??

Cheers,

Imagine zig-zagging up a hill instead of going straight up, that’s literally it.

 

[D_W]

Imagine zig-zagging up a hill instead of going straight up, that’s literally it.

That's actually a pretty good way to describe it. if you had to drive a 30 foot diagonal to go up the same elevation as a 15 foot straight on incline, you know the effective change in elevation is half.

 

[D_W]

just to make things interesting, it's about 59% as steep to drive twice the distance. If the adjacent and opposite sides of the triangle are equal straight on (45 degrees), increasing the distance driven to double (half height same distance driven equates to an angle just over 26.5 degrees.

I would guess the gut reaction to driving double the distance is that the angle would halve, but that would be the case only if the distance of a different leg of the triangle was doubled (laterally), which results in more than double the distance driven.

At any rate, the angle change from a skew is relatively small and not nearly as valuable as the fact that it's slicing the tops of straws rather than pushing through them. It's even more drastic with a moving filister in rebates where a regular rabbet plane is pushing the fibers and scraping many off rather than cutting.

the fact that holtzapffel shows no use for a low angle bevel up plane, but does for one with a high effective pitch (miter plane to shoot thin edges and avoid tearout - set at about 60 degrees effective) should be a clue.

It's also one of the reasons that these little block planes only seem useful if one never comes across what they're supposed to replace.

 

[IWW]

Imagine zig-zagging up a hill instead of going straight up, that’s literally it.

Sorry to labour the point Tiddles, but while that's a simple explanation, I don't think it is in any way analagous to the situation with a skewed plane blade.

Think of it like this. The angle of the blade bevel at the leading edge is x (haven't got a 'theta' sign, so trigonometry wonks will just have to put up with the 'wrong' symbol). The angle of the blade (& therefore the bevel, is exactly the same at the trailing side measured in the axial direction (as I said, I have laid out numerous skewed beds so I can vouch for this!). any axial slice of the blade, at any point between will also have the same angle of x, so unless Euclidian geometry is different in the northern hemisphere, the cutting angle does not change across the blade. We're talking about a block plane with a bevel-up blade, but the same would apply to a BD blade.

I much prefer the alternative explanation, that the the skewed edge is simply making a slicing action, even if the plane is pushed in line with its axis - it's really just the same as slewing the whole plane with an un-skewed blade....

Cheers,
Ian

 

[WideningGyre]

Not a formal geometric proof, but interesting nonetheless:

https://grantcrawley.uk/what-really-happens-when-you-skew-your-plane/

 

[D_W]

https://grantcrawley.uk/what-really-happens-when-you-skew-your-plane/

>> Conventional wisdom when tear-out occurs is to plane in the opposite direction, reduce the mouth opening (if possible), sharpen your blade, increase the cutting angle of the blade or skew the plane.<<

giggle...Or, you can do what works best - use the cap iron.

the objective of the skew is to prevent stripping fibers off laterally like a float. It allows a plane cutting across the grain (where you can literally just push straws off by accident) to cu as if it's cutting diagonally.

Someone new to planing can get a really good feel for this easily. cut a 1.5" wide stick directly across the grain. Put it in a vise. Plane it directly across the flat surface and notice the finish and the feel and the look of fibers being pushed off instead of severed.

Then, plane the same piece with the plane held askew about 20 degrees.

This guy, interestingly, gets the effect on pitch backwards. the shaving travels a slightly longer distance if the plane is held askew but pushed straight. The effective angle decreases, If there is a reduction in tearout it's because the orientation of the grain is "downgrain" relative to the skewing of the plane.

Before cap irons were marketed, it wasn't uncommon to have a small plane to clean things up planing small areas. It's a tiny fraction as effective as using the cap iron.

Where is a skew common on older planes? badger planes, not to reduce long grain tearout - but to plane across end grain. Moving fillisters. Same - the skew isn't for cutting long grain rebates, it's because there will be a time that you need to cut end grain rebates. Same with rebate planes being skewed for cleaning up - some cuts will be directly across the grain and planing directly across the grain always results in a poor surface.

The reason I mentioned a panel raiser being a bit of a novelty is that the bulk of the work fielding a panel is far better done with a regular bench plane held slightly askew on end grain - a jack for really heavy work. The clean up work is then supposedly left to this small block plane? It's not really up to the type of work that's involved - there's not enough plane to get a hold of. the reason these planes live on is because they're not used much and because the features look like they'd be useful.

 

[WideningGyre]

Functional cutting angles of a skewed plane appear in this article, though again I don't know if any of this withstands mathematical scrutiny.

https://www.finewoodworking.com/project-guides/handplanes/handplane-blade-angles

 

[IWW]

Now you're confusing two separate issues.

Skewing the plane itself does change the geometry of the blade relative to the direction the plane is moving in. I need a few props.

1. This is a straight 45 degree bed:



Skew it relative to the board axis & measure the 'bed angle' along the board & the angle of attack of the blade has decreased:
I'm not debating that.

What I'm saying is that the cutting angle of a skewed blade is the same from one side to the other, measured on the axial axis. Here's a 45* bed, skewed at 20*:

The angle of attack at the right side:

....is exactly the same as the left side (or any point in between):

relative to the plane's axis. The cutting angle is not altered when the blade is skewed in the standard manner. The only way it could change is if the bed is canted at an angle so that the left & right sides are different angles, which as far as I know is never the case.

Or if you skew the plane....

Cheers,

 

[raffo]

I'm sorry Ian, I thought that's what we were discussing, the angle of attack of the cutter in the direction of the axis of the plane.

The only other plane with an extra rotation is the badger plane. I jus tried to measure the angle of attack of mine and the angle seems to be the same left and right.

I'm not sure what distinction you're making between the angle of attack and the cutting angle. What do you mean by cutting angle?

 

[IWW]

....I'm not sure what distinction you're making between the angle of attack and the cutting angle. What do you mean by cutting angle?

Yeah raff, the debate has gotten thoroughly confused and too may red herrings were tossed in the mix (like riding bicycles uphill!??).

Cutting angle = angle of attack, some folks prefer one or t'other but I'm happy with either. For a bevel-down plane, they are also the same as the bed angle, and I've probably used them all interchangeably, to add a bit more confusion - apologies for that.
BU planes differ, the cutting angle /angle of attack is the bed angle plus the sharpening bevel angle.

My original post was to counter an assertion that skewing the blade of a block plane alters the cutting angle. I I hope I've now demonstrated that it doesn't. That was all I was trying to establish.

Skewing the plane and pushing it along the axis of a board will alter the cutting angle, as I tried to show above. You can easily do the experiment yourselves with a bevel square and a block of wood if still in doubt....

I would expect that your badger plane will have the same cutting angle/bed angle across the full width of the blade. I've not had a genuine badger (plane) in my hands, so I can't say for sure that it is so, but I read Konrad Sauer's description of making reproductions of a Norris pair & he set out the bed as I would expect.

Cheers,
ian