Planing Difficult Woods

Excerpts from The Message Boards

Rob Lee: It's not about the planes - it's about the wood, and how the wood fails. A plane is a carrier for a blade used to induce controlled wood failure. Much of the confusion over which plane is best, or which angle is best really comes down to which wood are you using... tougher, more "failure resistant" woods can be well worked with a low angle plane.... Y'all have have a bunch of tough, failure resistent woods down there [Australia] - which may run contrary to the experiences N Americans and Europeans have with their common domestics. Wood failure generally falls into two types - Type 1 and Type 2 chip formation (creative naming, eh?). Type 1 is typical at lower bevel angles (angle between the bevel and the wood), and involves having the wood "splinter" ahead of the blade...usually evidenced by tear-out... For a really tough wood - this may not happen!

Type 2 chip formation is where the wood fails right at the cutting edge - essentially, the wood fibres are severed by the blade before they fracture. Type II chip formation (or behavior) is what we strive for, for a clean surface..

Now, there will be some exception woods to all of this...

Really soft/fragile woods can be difficult to get Type 2 failure .... so now we have to discuss Type 3. This is where the blade actually pushes the wood fibres ahead of the blade, inducing a compression failure - often leaving a fuzzy or furry surface. It looks a lot like the way a snow plow pushing sticky snow does....you can picture that, eh? :D (couldn't resist!)

This is why softer pines don't scrape well.... there's compression failure....

So - now we come to plane geometry...

Standard angle planes have a 45 degrees effective cutting angle, and are generally bevel down - a generic "best" angle for NA and European domestic woods...Keep in mind too, that planes were developed a century ago, when the quality of wood used was far better (more plentiful, old growth woods, and lots of mahogany) - today we work generally more "demanding" woods....

Low angle planes are generally below 45 degrees, and are typically bevel up...

High angle planes are generally 45 degrees plus, and bevel down...

So why bevel up/bevel down? Well - there are engineering constraints imposed by each method of construction... If you want an adjustable mouth - then there's a limit to how small an included bed angle you can have. Using a frog - it's larger. Using an adjustable sliding plate ahead the blade - it's smaller. With a low bed angle - a bevel up configuration gives a cut angle of "bed angle + bevel angle" - with modern blade steels - this can effectively be as low as 12+20 , or as high as 12+ 78... (a 58 degree range)

A higher bed angle - with a bevel down blade - is fixed at 45 degrees (or whatever the bed angle is). In order to increase the effective cut angle - we have to introduce the concept of a "back-bevel".... Using back-bevels - the effective cutting angles can range from " bed angle" to 90 degrees ... (a 45 degree range for standard planes). Additionally - using a back bevel has the advantage of strengthening the edge on the blade - as the included angle on the blade tip is greater.

So for bed angles - there are also performance differences. Lower bed angles make the plane sole more susceptible to distortion - as tightening the lever cap can exert enough force to cause sole deflection. This is commonly observed in LA shoulder, rabbet (rebate) block planes etc., and is a technique often used purposefully to "adjust" blade projection.

Low bed angles do have the advantage that the blade is held in an orientation more in-line with the force applied - with should resist chatter more effectively than a higher bed angle plane made to the same tolerances.

A list of "truisms" (not really rules) I'd put forth would be:

1. A back bevel works at least as well as a change in bed angle - and possibly better if the blade is not perfectly bedded, as a blade more in-line with the applied force can resist chatter better. (note - an adjustable mouth is usually necessary if using back bevels)
2. A bevel up plane will work at least as well as a bevel down plane with the same effective cut angle - same reason as above...
3. A low bed angle (bevel up) plane gives you the widest range of cut angle choices (rapidly changeable, if you have extra blades!)
4. A narrow mouth with a light blade feed may allow a plane to "emulate" type 2 chip formation by reducing the possibility of the wood tearing-out (the sole ahead of the blade reduces the magnitude of, or stops the type 1 chip)
5. How the wood you're using fails is really the most important factor in determining which cut angle is best...

All of these factors (and there are more - like skewing a plane to reduce the effective cut angle) can make for a real witches brew when it comes down to interpreting why one configuration works, and another doesn't...but it's really about the wood...

Lyn Mangiameli: Rob's discussion largely matches my empirical findings (and apparently, his own experimental findings).

I'll expand on his discussion slightly to add some of my own thoughts and findings.

First, only with a bevel up plane is the blade fully bedded right up to the cuttng edge minus the actual extension beyond the sole for cutting. With a bevel down plane, one looses bedding at the point where the bevel begins. The thicker the blade, and the shallower the bevel, the longer the unsupported distance will be for a given blade extension. (Though carefully note Rob's comments about how a back bevel changes the included angle, and thus-I would add --depending on its width, can change the amount of unsupported bevel,) Back bevels to the side, the above suggests that thick "chipbreakers" may be a better way to create a more massive blade assembly, than just thickening the blade itself on bevel down planes. It also suggests that chip breakers set close to the edge of the blade my help stabilize that unsupported--by the bed--blade edge in the area of the bevel, which of course says that such chip breakers are indeed best set close to the blade edge. The bevel up plane avoids all this compensation and usually also offers a superior damping surface--i.e., cast iron--closest to the cutting edge.

Second, and this builds on the above but is more speculative on my part, I don't think the influence of effective cutting angle can be divorced from blade width. Higher effective cutting angles result in greater resistance to cutting, and increase the perceived force required to push the plane through the wood for a given width. The muscular forces required to drive the plane through the wood, obviously increase as a wider cut is made. Wider blades (say those over 2 inches) are more difficult to push than planes with blades that are narrower (2 inches or less), this was clearly discernable in my last planing study. But I think another factor related to blade width comes into play. To throw away another finding from my last study, certain plane types actually DECREASED in performance at very highest effectively cutting angles, namely those with 2.25 inch and wider blades (namely both infill and cast iron bevel down planes). I think this is because their larger unsupported blade edge begins to flex slightly along their width under the greater resistive forces encountered in planing hard dense woods with a very high effective cutting angle.

Rob and others have noted that a plane is not just a blade carrier, but also can function to control wood failure ahead of the blade edge (the Type 1 chip). No where does the latter become more apparent than in a chisel plane that offers outstanding blade bedding (take the L-N chisel plane with its thick bevel up blade fully bedded to the edge). Yet as anyone who has used a chisel plane is all but too aware, Type 1 failure is totally uncontrolled, and in certain woods cut with the grain can result in wood failure far ahead of and much deeper than the blade edge.

In a plane with mouth that can be set close to the blade edge, the plane sole not only locates the blade with respect to the wood, but it also controls against forward failure. It does this by providing compression on the wood fibers. So I want to emphasize that a plane is a wood compression device as well as a blade carrier. Now I am going to go on to speculate that compression is significant not only for Type I failure woods, but also Type III failure woods. That is, the plane sole before the mouth not only can provide pressure to prevent cleavage well ahead of the blade, but it can also "pre-compress" softer fibers (i.e. less dense woods) and diminish the effect of high angle blades "pushing a raised wall of wood cells" ahead of the blade resulting in more ragged intermittent shearing as the compressed wall fails. The overall density, cellular structure, and dryness of the wood involved, will influence how much downward force is required to achieve functional compression. Functional compression will be achieve by a combination of downward pressure and how closely the compression occurs to the cutting edge --- not always will the tightest, closest mouth result in the optimal cleavage of wood fibers.

Anyway, I don't want to get too far afield from the original questions, so I will stop here, but to say that there are many individual variables that influence the quality of surface finish one will achieve by planing. Overall there is much greater range of variance in wood than in the tools we use. Many woods will be quite insensitive to the type of plane, or its adjustment. Others will be excruciatingly demanding. What the bevel up, movable toepiece, low bedding angle, plane style offers is a great combination of good inherent design and easy adjustability (including blade substitution) that allows it to adapt to a wide range of wood characteristics.