Skew Cutting Angle

**Dan** · 17th August 2004, 01:29 PM

As a follow on from a recent thread about ramped shooting boards and skew cuts with hand planes, found here .
I decided to try and work out how much affect skewing the plane has on the cutting angle. By using (re-learning) a bit of trigonometry and a few diagrams I came up with this,

tanA = sinB / (cosB / cosC)

A=New cutting angle
B=Original cutting angle (HNT Gordon 60deg, Stanley 45deg)
C=Angle of skew

Because we know B and C, you can get an answer for the bit to the right of the equal sign, then use the reverse Tan function on the calculator to get the new angle.

By using the formula, a stanley plane skewed at 30deg will have a cutting angle of just under 41deg. A HNT Gordon plane skewed at 45deg will cut at 50deg. The answers look about right but if there's any geometry boffins who would like to check and see if I've cocked it up somehow, I would appreciate it.

Not sure if knowing the new cutting angle is really all that helpful but I was curious about how much skewing will affect cutting angle.

**Dan** · 19th August 2004, 10:49 PM

For anyone that's interrested, I found a page that has a formula for working out effective pitch (and plenty of other good info) when skewing a plane. The only trouble is it's different to the one I ended up with. The results from both methods come out close but there is a 5deg difference when using a standard pitch plane skewed at 45deg. I get 35deg, website method gets 30deg (20deg skew gives 43 and 41.6 degrees respectively). I guess I should assume I was wrong, but close.

http://www.sover.net/~nichael/nlc-wo...ters/skew.html

**Dan** · 24th August 2004, 07:52 PM

Ok, not the most popular thread in history, but I have found another page that gives a different result to the formula obtained from the website above. More importantly the new sites results seem to agree with mine.

This new page has also been right under my nose for quite a while but I still missed it.

http://www.amgron.clara.net/planingp.../slewindex.htm

It didn't show up in searches because he calls it slew and I was searching for skew.

I'll stop now

**cyclops** · 24th August 2004, 11:42 PM

Hi Dan

I came up with:
tanA = tanB*cosC
which I later realized is precisely what you had:
tanA = sinB / (cosB / cosC) = (sinB*cosC)/cosB = (sinB/cosB)*cosC = tanB*cosC

Anyway, lets think we're right and John Whelan is wrong

What struck me most is how small the effect is! (see figure)
In order to make a standard 45 degree plane work like a 37 degree plane you need to skew it about 40 degrees!

Jasper

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