Plane back flattening (odd question)

Re: Plane back flattening (odd question)

Rolf Schmid

>My idea is that the purpose to flatten the back is to create the ideal straigth line of the cutting edge. If the whole blade is not entirely flat, that seems for me not to be a problem, because it will be pressed flat on the planebed by the levercap or the wedge.

regards

Rolf

Re: Plane back flattening (odd question)

WoodburnBob

>I think if you study your irons you'll find they vary quite a bit in flatness and rigidity...quite apart from their surface finish. There's been much said on the topic of mistaking surface finish for flatness. The test on flatness would be to blue up a surface plate and see where the blade is bearing. Alternatively, you might sight with a good straight edge.

At one time I took some blades, old and new, thin and thick, and clamped them one at a time in a big metal vise. To the vise I had attached a dial test indicator via a magnetic base. I set each blade to project 3 inches above the vise jaws and then set the indicator at zero touching the cutting edge. Using a screwdriver, I then pushed on the iron at 0, 1 and 2 inches from the edge, with a force of about perhaps 5 or 10 pounds...fairly light pressure. I recorded the displacement, i.e. flexibility. The numbers I obtained seemed typical of a beam-loading-displacement function...as one would expect.

It was a simple little experiment that informed me that these irons, which we imagine are rigid, are in fact surprisingly flexible. Try it yourself. I theorized that when we clamp a blade assemble under the lever cap of a Bailey style plane, we're really pressing down on a three legged object...at best.

It leads me to think that when we lock down the lever cap, we're actually bending the blade into a state of tension onto the bed of the plane. If we're really interested in contact down the full length of the bed, we'd make the bed convex up and the blade convex down.

Of course, this wouldn't matter with arbitrarily thick irons. Maybe a 1/4" iron makes moot iron-bed mating. With a thick enough iron, maybe all our machinations about plane design, precision, fit and finish, and craftsmanship are nothing more than ad copy and delusion. Oops, for some reason I must be feeling a little cynical this morning. Forgive me.

Re: Plane back flattening (odd question)

joel

>On bevel down planes the back of the iron isn't in contact with anything except the cap iron at the tip.

On bevel down plane thats not the case.

I see tons of irons, an every one where someone tried to lap the back flat more than the mimimim amount I see a back that is convex on the edge left - right. Not good. it's just too easy to accidently do.

During the sharping process you will need to chase the burr and do work on the back, over time the back will get a touch thinner at the edge. Doesn't seem to matter.

although the bedding issue on bevel up planes could be why I personally have never seen a bevel up plane which worked as well as my bevel down planes (but that could just be me)

Forged chisels and irons were intentionally made slightly hollow top to bottom. with surface grinders that's not the case anymore with plane irons. don't know if it matter.

as you hone the iron and chase the burr the area of flattenss on the back will extend up so there is no reason to worry about this when you just get a new iron.

Re: Plane back flattening (odd question)

Jeff Spirer

>Actually, I don't really flatten the whole back, just give it a quick polish to clean up any rust.

But, was wondering about this.

Thanks for the answers.

I might add that in the 30+ years of woodworking, I've learned more about the great advantage of a sharp blade in only the past 5 or so years.

I guess I was pretty lazy.

Jeff