The sub-blade is illustrated in white in the drawing:
I'll throw out a few Japanese plane terms here and there as I go in this series of posts, and you'll see I have referenced all of the terms to be used on the above drawing. Of the two Japanese terms for chipbreaker, uraba (back blade) and osae-gane (pressing-metal), I prefer the latter term osae-gane, and intend to use that term, or or 'sub-blade', fairly frequently in this series of posts. Either is, after all, shorter to write than 'chipbreaker'. And by the way, the term '~gane' is pronounced 'gah-neh', not like 'gain'.
The sub-blade does not cut anything - the main blade does all the cutting. The osae-gane does manipulate the shaving produced by the cutting action of the main blade. And it does a few other minor things besides:
- pre-tensions the main blade's cutting edge so that it deflects less under loading
- adds a slight heat sink effect which prolongs the edge retention of the main blade
- helps compensate for a loose fitting blade at certain times of the year
The heat sink effect is rather minor, so I tend to think it is best ignored. The mass is in the main blade, so most of the heat sink potential is in the main blade. The sub-blade is fitted onto the main blade under tension, the load imposed on top of the osae-gane by the metal pin known as the osae-bo, (lit.: 'pressing stick'). As far as the fit-compensating effect of the osae-gane on top of the main blade, that effect should not be too large so long as the main blade is properly fitted. If the main blade is properly fitted, the dai should be fully supporting the blade nice and low down in the pocket. Having a crappy fit there and expecting the sub-blade to make everything all right is not going to be a recipe for success in most cases.
The pre-tensioning effect is this: the main blade's bevel, as one moves out to the cutting edge, becomes progressively less supported and therefore more prone to deflection under load. A certain amount of that deflection associates to the inherent elasticity of the laminated blade and also to the angle of bevel on the main blade. The osae-gane pushes down onto the very end of the main blade and some of that elasticity is taken up. This pre-tensioning effect should therefore lead to a reduction in any propensity of the blade to chatter while cutting.
If the fit of the blade is not the best, or becomes sloppier at a certain time of the year, then the sub-blade can help to take out some of the sloppiness however the pre-tensioning potential will be reduced. One can bend the ears of the sub-blade a bit more so as to increase the pressure on the edge of the main blade, or shim under the main blade with paper, however a better solution, ultimately, is to have a separate plane block for use at those times of the year when the fit of the main blade is unacceptably loose. Winter planes and summer planes. If you live somewhere without substantial swings in humidity, then you will probably be able to get by with the same plane year round, so long as the fitting of the main blade is done properly from the get-go.
The sub-blade should not be considered a means of compensating for a poorly fitted main blade! If the fit of the main blade is irretrievably poor, cut another dai.
Let's assume that the back of the main blade has been properly prepared and there is a sufficient landing at the cutting edge for the sub-blade to press against. Let's assume the sub-blade sits on the main blade without clattering, and that the fit of the sub blade edge is perfectly tight against the main blade. What does the sub blade really do - and does it offer any advantages in comparison to a single blade plane? Is it, as Odate suggested, a means by which less-skilled carpenters during wartime could accomplish good planing results, or more than that?
In this series I am going to look closely at the true functioning of the sub-blade, at least as far as I understand it, and will argue that it does more than overcome inept carpentry technique. The sub-blade, as we shall see, allows a plane with a lower blade bevel angle to successfully work difficult materials - to punch above its weight, to borrow a boxing analogy. There is a place for both single blade planes and planes with sub-blades in the tool set, and neither type should be considered inferior to the other. This post series will also feature, for the first time on the Carpentry Way, a guest post by an experienced West Coast craftsman. I hope you'll stay tuned.
On to post II ←← link