This will make you wish for an adjustable mouth (ala block planes) but there just isn’t enough room.
I don’t have this step wired, I do it in three settings: rough out the ramp, reset to cut the front ramp and then reset again to finish the ramp. I think I should be able to remove one setting but maybe not [until I get a 4 axis CNC mill].
Cutting the ramp. The basic idea is to set the sole at the required angle (20º) and cut until the mouth emerges. If only it were that easy. I finally made a angle block (after using a protractor for a bunch of planes), it works much better. I sometimes use a stop (a rod on a stick) clamped in a T-slot to keep the block from sliding. Remove the block after clamping the sole so you can see and feel under the sole as it is machined.
Scribe layout lines the width of blade for the kickers. After most of the ramp has been machined, it is widened, leaving 1/8″ kickers that will center the blade on the ramp. The kickers should be positioned about 1/4″ after the ramp enters the sole.
It would be really convent to scribe the kickers and ramp but since the reference is the bottom of the sole, it would be a pain or sorta good enough, so I don’t. That has lead to some tense moments however.
The length of the ramp, along the ramp and through the sole, is about 5/8″. I use a 1/4” roughing endmill to rough things out because it edge cuts really well. But, be careful as it pushes out the bottom well before it should, so stop at about the kickers and switch to a regular endmill. SLOWLY work the bottom of the ramp until the endmill corners just starts to bulge the bottom of the sole (you can feel this way before you can see it). This is really critical! The mouth opening needs to be less than zero. So use your fingers, use a flashlight, don’t go too far.In fact, don’t go far enough and finish it later.
Stay within the kicker layout lines!
The front ramp. This is a pain. I do this solely to get room for the finish pass of the ramp. The 1/16″ endmill I use is too short to cut the front of the ramp so this is a relief cut. I use an 1/8” endmill for this operation because things are tight in there and my mill has so much backlash that a bigger endmill will catch (during the climb cut) and launch into the ramp (tested and verified). And, since the endmill is short, an R8 collet will hit the sole. So I use a collet chuck (with ER-16 collets) to buy some room (it is still tight as you can see from the dings on the collet nut). I aim for a fat 1/16″ front wall (at the mouth) as this will grow a bit during the ramp finish pass. Kiss the gutters.
I mount the sole at 45º (not critical).
The finish pass. Now that we have made some room at the mouth, we can finish machining it. Since the sole has been in and out of the vise a few times, it is unlikely to be at the original angle. And I can’t re-set the endmill to exactly at the right depth. So a skim pass is needed to create the true ramp and finish up the mouth.
Using a 1/16” endmill with a square edge, skim off as little as you can. Really, just a tiny bit (see the photo) or the mouth grows.
Finish the kickers and mill out to the gutters to give the blade room to rotate. Be sure and test to make sure the blade fits.
Now be super super careful when doing the final cut at the mouth. You want to just feel the bump on the sole and you don’t want dimples in the mouth front wall. Mill out to the gutters and be thankful that fat part of the endmill cuts (as it gets stuffed into the side wall). You don’t actually have to mill to the gutters as we’ll be filing the mouth but it is nice not to have to worry about keeping the mouth straight (when filing).