Feb 15 2009
I know it seems like an odd name to call the process, but it is based in science, and what happens to wood while being worked isn’t really any different when you are going slow with hand tools.
Planing woods is a process that has a number of considerations, which require their needs met all at once, in order for the process to be successful. It isn’t just one thing happening at a time. There are causes, effects and recognizing which you have. The answer isn’t simple until we understand all the usual suspects involved.
If you haven’t already, read chapter 9 of Understanding Wood By R. Bruce Hoadley. It is a real good primer about how wood reacts to planing and machining to brush up on. Overall, the book will improve your skills as a woodworker. It is available in many places, and both Taunton Press as well as Amazon.com are good sources.
Wood reacts to machining in observably repeatable ways. If we understand these ways and learn to recognize the conditions, our own success in working wood is repeatable as well.
During planing operations, it is desirable for wood to be severed right at the cutting edge of the plane blade. It would seem obvious that this is happening. It is when things go well, but this does not always occur in practice.
When you plane, you make a shaving. Technically the shaving is called a chip. Shavings curl, but there is science behind the curling. The tightness of the shaving’s curl is significant. The tightness of the curl is a derivative of how frequently the chip is broken.
What breaks the chip is the blade bevel angle, just after it severs the wood fiber. The angular change from wood being at 0 degrees on the boards face, cut, then suddenly at the angle the final bevel angle the iron presents, causes the break in the chip. The wood being planed is running into a wall. The steeper the wall, the closer the breaks are, and the tighter the curl. The further the breaks are the looser the curl.
The chip (shaving) is referred to in types. There are 3 types of chip.
Type one is loose curl, and is the result of being planed at a low angle such as 45 degrees, which is known as “Common” pitch.
(Yes there are planes meant to plane at 38 degrees but this is generally meant for planing for end grain, which is a cross grain planing operation, and as such, is not generating a chip, per se. It is severing wood fibers crosswise, and as such is not considered applicable to this discussion, despite possible exceptions to this rule.)
Type two is a medium curl, and is the result of being planed at medium angle such as 50-55 degrees, which is known as “York” or “Middle” pitch.
Type three is a tight curl, and is the result of being planed at a high angle such as 62-63 degrees, which is known as “Half” pitch.
75 degrees is the next significant pitch angle for planing operations, and is considered scraper country.
Now there are a few important considerations going forward.
The lower the angle that the bevel angle the plane severs wood fibers at, the easier it is to push, and this is desirable. Planing is work! But the lower angle that makes work easy, isn’t always capable of getting the desired results in every wood.
Something that can occur when planing, even with a sharp blade, is tearout. Tearout is defined as when the angle of the blade lifts a wood fiber that is supposed to be being cut by the cutting edge, but the chip is not breaking like it is supposed to, because perhaps the wood fiber is too strong, or the planing angle is too shallow for the strength of the fiber being planed.
When the chip does not break at the cutting edge, and instead breaks after the fiber lifts, by riding up the bevel ahead of being cut, it also causes lifting and peeling ahead of the cutting edge. The blade is no longer in control of the depth, and usually the tearing that results from the lifting fibers is deeper than the intended surface desired.
This is at the heart of why planes are developed to plane at specific angles, such as described earlier. The steeper angle is meant to be an alternative available to generate a different chip type. The higher angle, forces the chip to break at a more frequent interval. Every time the chip breaks more frequently, that is a shorter length chip and the shorter length chip does not allow the fiber to ride up the blade. When the chip does ride up without being broken, tearout is likely happening.
The essence of stopping tearout is about understanding wood grain, and the planing bevel angle that will generate the chip type most compatible for the grain type being worked.
The rest of the process of tearout reduction falls to a couple factors. Tool and Human.
Tool wise, The Plane can also help reduce tearout by adjusting for a tight mouth if the plane can be adjusted. Stanley Bailey style planes use adjustable frogs and some steel planes which bed the iron bevel up offer an adjustable sole. I am not going to offer specific clearance numbers here because I want to avoid helping to develop a belief that specific mouth clearances are perfect. If you plane has variable adjustment, then it is best to experiment. You will be working in the .003-.010 range on high-end smoothers.
Another tool factor is blade sharpness. Sharp tools sever fibers. Dull cutting edges tend to blunt them an rip them… Remember, reducing tearout is the want here.
Finally, tool factors wise, Planing depth. Smoothing is a final dimensioning process. You are creeping or sneaking up on things here, so the thinnest shaving is going to offer the least resistance to being cut, and as such, leave the smoothest glossiest surface.
The human factors are about perception and action. Reading the grain is a biggie. If the grain is straight then planing with the grain is usually doable. Just remember against the grain is usually as effective as petting a porcupine, and so it goes. However if planing with the grain is problematic, sometime changing the angle a bit can help.
When changing planing angles, sometime a shallow angle off the grain direction will help reduce tear out, and there is no rule stating that going fully across the grain is taboo, as long as it will provide the finish quality you are looking for.
On flatsawn boards there is a place where tearout can hide, some people refer to it as “cathedral grain”. What cathedral grain is, is where the board was milled and the blade passed through the growth rings and a shallow skew angle. It looks reminiscent of the gothic arches seen on cathedrals. The concern with cathedral grain is that early and the late wood of the growth ring lay right there, and they come up all the way to the surface.
Something common that happens when planing over cathedral grain, is that when planing with the grain, the plane whisks over these areas, and the thin late wood of the growth ring just separates and pops loose. Seems the same as tearout, but it isn’t. It is the late growth ring’s (dark) inability to adhere to the next year’s early growth ring (light) at such a thin and shallow angle.
The trick to approaching these areas is to do it at angles the run closer to longwise with the “cathedral”, with a sharp iron set for a thin shaving and the plane body skewed to the direction of planing. Sometimes a light wipe with mineral spirits can soften wood fibers enough temporarily to assist in this. I have even seen where rays and chattoyance have given similar planing issues, and so again, be vigilant.
In curly grain, many of the same aforementioned strategies will play. Be willing to come at the grain at any angles that will work. However, please reach for half pitch planes in this situation, and don’t hesitate to simply stop and move to scraping planes, even card scrapers if that helps. Sometimes it is simply acceptable to resort to sanding.
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