Saturday, April 19, 2014

Skew mitre plane: the mortise

Not much time for planemaking or blogging lately, but I did make some progress on the mortise.
Chopping the mortise was not really more difficult than with an unskewed plane--just more time-consuming.
As usual, the first step was to make a guide block for chiseling. I just used a piece of southern yellow pine scrap. I laid out the lines, cut it with a handsaw, and then did lots of careful planing, checking frequently with my two bevel gauges (set at 38° and 20°) and a straightedge, until the angles were perfect and the surface flat.

Here's a completely phony, posed shot, taken after the fact, that shows how the guide is used.

I generally chop freehand until I get within about 1/4" of my front and rear layout lines, then use the guide to finish off. Here's the mortise roughed out:

Then I use a flush cut saw to widen the bed and define the abutments. The bed cuts are simple--i just lay the saw on the bed and cut 1/4" deep. For the abutment cuts, it helps to make a spacer block. This was the first point in the project where the geometry got a little tricky. My first attempt was a failure, but it helped me figure out the correct the angles. The main angle is 10°--a standard wedge angle. But the skew is not 20°, because I'm referencing off the 38° bed, rather than the sole of the plane, and this decreases the skew angle. I suppose I could have figured this out with trig, but in practice it was easier to just do a little trial and error. The final skew angle turned out be 15°. The sides of the spacer block are angled at around 10° (relative to the bottom of the block) to match the sides of the mortise. Anyway, here's the spacer block:

I shot some Spray 77 adhesive on the back of the block to temporarily hold it in place for the saw cuts. After I was done, the block was easily removed with a few hammer taps.
Finally, I chiseled away the waste between the saw cuts. Here's a shot of the more-or-less finished mortise:

I've got some stupid cosmetic scars on the bed (from chiseling the abutments), but the cutting iron won't care. Overall, I think it's looking pretty good. The next thing is to deal with the iron.

1 comment:

  1. Steve,

    I just discovered your blog while browsing for advice on making a skewed iron shooting board plane! I have been messing around making laminated planes, and recently purchased an old iron and chip breaker from a skew plane. I don't have the plane it came from, so I don't actually know the original pitch and skew of the blade. It is ground at an angle rather than 90 degrees. Given that angle, and that I'd like the pitch to be 42 degrees, how do I find the skew angle that keeps the blade sides vertical in the mortise? I can do basic trigonometry, but I can't wrap my head around this problem. I'm thinking I'll just physical hold/rest the iron at 43 degrees and measure the skew I need. I don't want to have to grind the chip breaker or iron if I don't have to, so I'd like to be within a couple degrees of correct.

    I hope that all makes sense.