This just might work

Problem: How to cut the mouth for a bevel up plane with splitting the sole? For some reason, I have a mental block against splitting the sole, cutting the blade ramp, putting the sole back together and then cutting the dovetails (the traditional method). So, how to do it? The narrowest slot I can cut is 1/16” (how many ways can you say “watch that end mill break”?). There is not enough room to get tools in there after the sides are attached. A frog would be cantilevered a looong ways (ie a diving board). I can’t figure out how to make movable mouth (like block planes have) fit the aesthetic I’m going for. Arg!
Read more: Bill Carter on Cutting the Bevel for the Bottom of the Bed and The Tongue and Groove Joint (scroll down a ways).

Here is what I came up with. First, some math to see what kind of dimensions I’m dealing with. Well, I tried the math (several times) but it is a bit messy so I decided “proof by picture” would work better. What you see is a CAD drawing of the sole, blade and a slot. The leading edge of the slot is at the point where the blade is level with the bottom of the sole (ie a zero width mouth opening). Then I measured the perpendicular distance from the ramp to the top of the sole. This distance tells me how far into the sole to cut the ramp.

That looks sorta OK, even though I don’t like that much lip at the bottom [back] of the ramp. But how do you measure that distance? Ideally, I’d just stick it on my CNC mill and press the GO button. If I had a CNC mill. Got no DRO on the Z axis either and the depth reading is really crude (still not sure it would work anyway). I could index along the X axis if I had measured the mouth block (and had a DRO). Ouch my aching head! So I just used feeler gauges. Tedious (cut a tiny bit, move the end mill out of the way, measure, repeat) but worked pretty good. The photo shows the set up: the sole (with the mouth block brazed on) mounted in the mill vise at 25º, a 1/8” end mill and feeler gauges at the ready.

After the ramp was cut, I used a 1/16” (center cutting) end mill to cut the slot, aligned with the top front edge of the ramp cut. Surprisingly enough, the measurements were spot on and a blade (in this case, a chisel) on the ramp hits the front of the mouth just before it protrudes. Now I have to be careful and not remove too much when I flatten the sole. If I do manage to screw that up, my backup plan is to cut a “dado” across the sole and braze in a patch to close things up.

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