Plane Physics

[outofstepper]

Good morning all. I got to thinking about planing in general and there's a little

something I can't figure out on my own. Standard caveat: I'm new to all this!

Why does a power jointer have a stepped surface and a hand jointer need a flat sole?

That is, on a power jointer, the outfeed table is higher than the infeed.

But on a hand jointer the "outfeed" and "infeed" are on the same plane.

But they both make flat straight faces.

If a power jointer had a flat top (and the blade stuck out), you wouldn't get a

flat face. How come you do with a hand jointer?

Again, just curious.

[jab73180]

i am just guessing here, but i think it has to do with knife placement. when you adjust a hand plane, you extend the blade past the sole. when you adjust a jointer you adjust the knife to the outfeed table height and then adjust the infeed table for the depth of cut. now, hand tools arent my thing but come of them do have adjustable soles, i think. hope that helps.

[Bombarde16]

Off the top of my head, because both tools will get the wood acceptably flat enough to cut joinery. Bear in mind that tolerances in woodworking stop at the level of what the human eye can see. Scientists and engineers would be appalled at how "sloppy" we are.

Safety is another matter, though. A jointer is designed so that you can use it for extremely aggressive cuts or even bevel the fence. When you have blades spinning at warp 8 threatening to make your fingers into hamburger, you want the workpiece supported on both sides of the cut. The hand plane, however, doesn't have that problem. So the fraction of a fraction that the blade projects from the sole of the plane isn't a safety concern and the tool will still get your stock flat enough for work.

[Roger T]

Good question and one I've never thought about. On a machine, if the tables are in one plane, you would never be able to get a flat surface as the board would pivot over the cutter of the jointer. Hence the outfeed table set almost at the apex of the cutterhead. You will have support on both sides of the head.

On a hand plane, you can get absolutely straight edges with nothing more than a small flat spot directly in front of the mouth of the plane, and a small flat spot on the rear end of the plane. The middle of the plane body, in theory, does absolutely nothing other than tie all the parts together. The thickness of a finishing shaving on a jointer plane is likely .010 or less in thickness. As long as you are bearing at the front of the mouth and the heel of the plane, it does not matter what happens in the middle. Your plane would be at a .010 incline, for the duration of the planing stroke, until the area in front of the mouth falls off the end of the board. If you keep the mouth of the plane closed up tightly, the few thousanths of an inch it travels will have no effect on the planed edge. As long as the toe of the plane, (front edge) is not lower than the area directly in front of the mouth, and effectively taking that area out of play, you should be able, given correct planing technique, to get very nice shavings and have little to no tearout. Without the area directly in front of the mouth contacting the wood, and slightly compressing it during the plane stroke, it allows the wood to fracture to far ahead of the plane blade edge, causing tearout and rough surfaces. There are different shaving types, classified as Type 1, Type 2 and so on.

I believe it was Brent Beach who did a rather extensive study on the sharpening of edge tools, and the different types of chips caused by them. Also, Wilbur Pan, a member here, did a bang up job of showing how to set up a Japenese Dai doing what I basically stated in the above paragraph.

Myself, I have never understood why some of the handtool users, send their planes off to have the soles ground completely flat. I can understand perfect, 90* sides to the soles, for use in a shooting situation, but the sole thing leaves me scratching my head.

I hope I have answered your questions, if not, reply back and one of us will try and explain better.

Roger