Sunday, August 31, 2014

Kanna help you, perhaps? (VII)

A series describing the set up of a new plane, from the ground up.

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One trick I forgot to mention in my previous post was a method to compensate for the tendency of a blade to damage the osae-mizo when it is tight down low in the mouth. If you're not careful when tapping the blade down in the fitting process, the edges of the ground corners, mimi, will tend to dig in and cut their own way:


As the osae-mizo ramp is undermined, the blade is forced to curve slightly as well. In the end the fit is spoiled and instead of solid purchase low down in the mouth at the blade sides you have a bit of loosey-goosey.

To preclude this possibility, all you need to do is dull the ura side of the mimi so it can't cut so readily. A few drags of the corner across a medium grit stone will do the trick, first one corner:


And then the other:


Equal amount each side:


Trying to catch the line of light that reflects off of a dulled corner with the camera was outside my range of luck today, but that's what you should have when done:


Another point made in the previous post which I would like to elaborate on a little further is that of the zones of pressure between the dai bedding surface and the surface of the main blade, as shown in this sketch:


There are a couple of refinements that could be made to this drawing. One is that the most important zones of contact, where the fit should be the tightest, are actually those zones directly under the osae-mizo:


The dark green areas in the grooves are those where the contact pressure should be greatest. In the vast portion of the field, including the tsutsumi (if present), the contact pressure should be 'soft'. In the area up at the top of the bedding surface, where the plane blade first enters, the contact pressure should be 'very soft'.

There's also a detail, shared with me by Kunimoto-san, about the width of the zones of high contact pressure under the fixing grooves. These contact zones, depicted in red below, should be narrow and not much wider than the surface of the groove ramps:


The key point here is that the high pressure zones serve to keep the main blade straight in both axes, along the grooves and across the opening. If you are not careful in how you fit the blade, having wider zones of contact which are canted slightly, say, then the blade can be bowed upward by being forced down into the opening, like this:


It can be easy to realize this result, particularly with blades which have a more pronounced hollowing on the contact surface, as you try and fit the blade, so please guard against it. If the blade is bowed upward, the edge will not be straight, which effectively means the blade corners will contact the wood first, digging in and making a mess.

Finally, you want even contact along the bottom of the mouth, but not too tight a contact as this can also cause the blade to bulge up a little in the middle and, conversely, cause the wood in that area to be bulged downward slightly which will throw off the registration of the dai to the wood you are planing.

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Onward and upward. Time to deal with fitting the sub-blade, osae-gane, to the main blade and the dai.

First off, check how the sub-blade fits in the opening of the dai. There should be ample lateral clearance for the sub blade:


If the opening is tight, the recourse is to pull the osae-bō out and pare the sides with a chisel until the sub-blade fits easily.

Now we check the fit of the osae-gane to the main blade, kanna-mi. Fit one atop the other, in the same position they would be in within the dai:


The sub blade sits on three places. At the front there is the line along the ura-suki of both blades, which, if the initial preparation of the ura on both blades was done correctly, and each ura is dead flat, should be a zone of continuous contact. The other two points are the thickened or folded-over ears (also called mimi) on the top of the chipbreaker. You want the chipbreaker to sit upon the main blade solidly, on all three points of contact at once. If it sat on only two points - say one ear was high or low in respect to the other - then when the two parts are pressed together in the plane by the osae-bō, the pressure will be unevenly applied and this will very slightly deform the main blade, press one side of the main blade more than the other, and tend to make the sub-blade tight fitting on one side and not the other, which can affect ease of adjustment.

So, better to have it fit evenly atop the main blade. We do this by tapping on one corner...


...and then the other:


If the fit is imperfect, you will detect a slight rocking or clacking between the two pieces of metal where there is a gap - 'gata-gata' is how the Japanese refer to this clacking fit.

Even if the fit of the sub-blade to main blade is dead on, it still does not mean everything is hunky-dory in regards to the total fit of the package (dai, kannami, osae-gane and osae-bō), as we shall see soon enough.

If there is some gata-gata in the fit, the adjustment is by way of the sub-blade ears. One ear is high and one is low. Ears can be adjusted, if they are of the folded type, by hammering the ear up or down upon an anvil, like this fellow is doing, a process termed mimi-age (耳曲げ), which translates directly as 'ear-bending':


(photo from mandara-ya)

Notice he uses a bronze hammer so as not to mar the sub-blade's upper surface, and in the above example he is  hammering down the ear which was not contacting the main blade. This is generally the way to go. The anvil surface can be marked out beforehand with a 45˚ line, using a sharpie or similar, to help align the sub-blade and fold of the ear. The edge of the anvil is rounded so as not to leave marks on the underside of the sub-blade from hammering.

Some types of osae-gane, often the more expensive ones, do not have a folded corner but a gradually thickened corner. Tasai's planes have a unique treatment:


 (photo from: http://blogs.yahoo.co.jp/y8077jp/60638509.html)

These sort of mimi treatments don't lend themselves quite so well to adjustment by hammer as shown above. It can be done by hammer in some cases, however you will likely approach the issue by filing the under surface of the corner which holds the sub-blade high in order to get both ears in contact with the main blade at the same time.

Once both ears are touching, you can make a final check to see that the interface between the ura-suki of both blades along the front is light-tight:


You do not squeeze the blades together hard for this step, just a light pressure pinch. Unless you messed up the basic flattening of the back of both blades there should be no gaps present along the interface. The above photo also shows, by the way, one type of sub-blade which has gradually thickened corners instead of the folded corners.

Stepping back a moment, now that there is an overview of the process for adjusting the ears of the sub-blade: we had a choice, once we found there was some gata-gata, to bring an ear up or down. It is worthwhile placing the blade assembly back in the plane to assess the relative tightness of the fit of the sub-blade when pressed down by the osae-bō:


The sub-blade really only needs to be pressed down enough by the fixing pin so that when the shaving presses against its edge it does not move, and such that it is held tightly enough that it can be readily adjusted.  If the sub-blade is held is too tightly, not only does adjustment become more of a struggle, but the added down pressure from the sub-blade presses the wood under the main blade down even more, which causes it to bulge more than necessary. A tight sub-blade also will be marred more by scraping harder against the underside of the fixing pin, and deforms the fixing pin so that it is not bearing nice and cleanly against the middle of the sub-blade, as shown several posts back.

A sub-blade should be able to seat well with relatively light taps of the end of the hammer handle:


If you can push it all the way down into end position with your finger it is too loose a fit.

So, before adjusting a wayward sub-blade ear, first scope out what sort of fit you are obtaining under the osae-bō. If the fit of the sub-blade seems a hair tight, then it makes sense to file or hammer the mimi which is in contact with the main blade so as to bring the entire mimi down closer to the main blade. This moves towards a more relaxed fit.

If, after bringing an ear down with the aim of relaxing the fit a bit under the fixing pin, the fit is found to still be too tight, then the recourse is to remove the fixing pin, plug the pin holes, and re-drill so as to place the pin slightly further out. One could also lightly file the underside of the pin to obtain the perfect fit - however, one must judge the situation carefully - if you have to file too much material off of the pin, you will weaken it too much and it would be better to remount the pin.

On the other hand, if the fit seems a bit on the loose side under the fixing pin, then one would look to bend one (or both) mimi so as to effectively raise the entire sub-blade slightly up away from the main blade, which will slightly tighten the fit.

If, after bringing the ears up - or having a sub-blade with the  gradually thickened ears that cannot be so readily hammered downward - the fit remains a bit on the loose side, there are a couple of options. One would be to remove the pin, plug the holes, and remount the pin a bit closer to the main blade. The other path out of the woods, which is generally to be preferred, will be taken up in the next post.

Thanks for coming by and taking a look. Hope to see you next time.