That's a great picture, Derek. In hard woods (meaning woods that don't deflect much, not hardwoods in the usual sense) it's possible to get surfaces that are both tearout-free and chatoyant, as your picture shows. The advantage of low planing angles is in woods that are soft and deflect enough due to planing forces to be bruised a little.
I've used the same supersharp plane with a 47-1/2 degree cutting angle on Port Orford cedar (a soft wood) and achieved a great surface, and then on Douglas fir (which has extremely soft earlywood) and gotten a cloudy surface. The effect of planing angle on surface quality depends on the species being worked, and to some extent on the individual piece of wood.
Re: Adding to the physics - a touch of commonsense
TomD
"Knowing the resources that a company can and will apply to optimizing a mechanical device(Supersurfacer) I can believe that a 40 degree bed angle and 30 degree blade bevel must be optimum for dealing with a wide variety of hard and softwoods. On the other hand, if one sufficiently searches exceptions can likely be found. If the exception is some wood or wood figure that is rarely used in building stuff this discovery does not detract from the utility of the conclusion. It would seem that effort and electrons would be better devoted to exploring the utility of the cap iron effect in more relevant timbers. "
I'm not sure about that. Mark needs to jump in, but my understanding is that the cutterhead assembly on an SS is/can be designed to rotate in the plan view, and therefore a wide range of cutting angles can be tuned from the bedding angle to angles theoretically near zero bedding angle at zero width. Not sure what the actual working limits of offset are.
Also, I would ask whether your scientific bent is now willing to conclude that 40 degrees is the perfect angle, now that you have read it is so. Or whether it was arrived at out of convention where that is the standard softwood angle for Japanese planes. And some softer hardwoods.
Further, I would ask Mark, what that means, are SS designed mainly for surfacing softer range woods, and is this whole discussion off track as a result, since there are Japanese planes at much higher angles also.
Re: Adding to the physics - a touch of commonsense
Mark Hennebury
Hi, as i see it:
You are trying to cleanly sever the cell fibres with the least amount of stress damage possible.
Keeping in mind the structure of wood is cylindrical tubes bonded together, so think of cutting wood in terms of cutting tubes like a bunch of drinking straws.
For example lets just take one straw;
If you wanted to cut a 8" straw in half and make two 4" straws.
Which do you think would cut the easiest with the least amount crushing or distortion of the tube.
A straight razor with lets say a 5 degree bevel or a plane blade ground with a 60 bevel.
It would seem to me that the straight razor would cut with the least amount of effort and damage.
I think that the lowest bevel would cut the best for most cutting tools. Plane blades are a compromise; to make the edge last longer and be less fragile they are ground 25- 30 Degrees. Heavy duty mortise chisels are ground a steeper bevels than paring chisels.
In a single blade plane:
When planing wood with the grain: the force from the knife edge cutting the cells is pushing the wood fibres down and together. The force from a well sharpened knife edge severs the cells of the wood with little distortion or damage leaving a clean surface.
When planing against the grain: the angle of these tubular cells in relation to the knife cutting them, means the force from the knife edge cutting the cells is pushing the wood fibres up and apart. The force required to sever the cells is greater than the cell walls and the bond that holds them holds them together can counter, and they are crushed and torn and separate ahead of the knife edge and down into the wood creating “tearout” and a bad surface.
So the conclusion would appear that the lowest angle cuts with the least resistance and damage. Think straight razor.
In “against the grain” a low angle creates lift.
What to do:
1. Use an angle high enough to prevent lift.
A low creates lots of lift. A knife angle at 90 degrees doesn’t create any lift and would be considered scraping, and somewhere in the middle are angles for the best trade-offs of “cut and scrape” for various species of wood.
2. Use a low angle blade and a method to provide backup to the wood cells to support and hold them in place so the knife edge can cut them.
a. For this you have the Chip bender and/or the leading edge of the throat.
For supersurfacers:
Supersurfacer are made to cut all range of woods from softwoods to tropical hardwoods.
40 degrees blade bed angle is not a divine magic angle.
The actual blade bed angle is around 34 degrees with the blade micro bevel at 32 degrees, so a clearance of only about 2 degrees not 10 as in the Kato video.
The blade bed and blade grind angles are compromise angles due to the multi species of wood to be cut.
If you made a machine for only cutting Balsa you could maybe grind the knife to a very low angle of say 10 or 15 degrees, whereas that wouldn’t last long cutting very hard woods. If your machine was only for cutting very hard woods you would probably have a much higher knife grind angle of maybe 35 degrees, but the would cut well on Balsa. So knife grinding angles are a compromise based on making a knife that will hold up to the stresses it will have to face. A straight razor cuts very well and has an extremely low angle.
Supersurfacer knives are mounted in turntables.
This turntable can rotate from 0 to 60 Degrees.
At zero degrees all the tool geometry is as stated above.
As you rotate the turntable all that geometry is modified.
For instance when a 34 degree blade angle is skewed to 60 degrees the effective blade angle is reduced to 18.6 degrees!
Plus you have added the skew factor into the equation.
Don’t dismisses the skew factor, the skew effect is the slicing effect of an acute angle forcing the individual wood fibres to slide slightly along the blade surface exposing the fibres to more of the edge of the blade.
Supersurfacers also have an adjustable throat plate.
This throat plate is adjustable in two planes or axis.
It is adjustable in one axis as to open and close the distance / gap between the knife edge and throat plate.
It is adjustable in the vertical axis to increase or decrease a pressure line at the edge directly ahead of the cutting edge of the knife. This loading or pressure on the wood cells ahead of the knife edge provides the support for the wood cells in against the grain cutting to prevent cell fracturing ahead of the cut.
Supersurfacers also have the variables of down force, and feedspeed.
Supersurfacers are designed to be very finely tuned to produce the best cutting for a wide variety of woods.
Re: Adding to the physics - a touch of commonsense
Derek Cohen (in Perth, Australia)
.. so a clearance of only about 2 degrees not 10 as in the Kato video.
Hi Mark
That is interesting.
We have had much debate on this forum about clearance angles. These range from "7 degrees is sufficient" to "12 degrees is not enough". And you come along with 2 degrees!
Can you say something about this, for example, issues of spring back in the different hardnesses of woods.
Re: Adding to the physics - a touch of commonsense
Mark Hennebury
Hi Derek,
The thing I find most important to learning is to keep an open mind, which means being willing to challenge our own logic, which is tough for all of us because we see evidence, we form conclusions and from then on, seek confirmation.
For example: many years ago a customer came in to my shop to get me to make him some night tables.
He explained that he was very interested in woodworking and well read on the subject and had even take n classes. So with delight I gave him the "peers" tour, proudly showing him all of my prized machinery and what a great job my supersurface and mortiser did and explaining how I could produce the most flawless woodwork fast and efficeintly.
At the end of the tour he stated that he had passed up the machine course in favor of the hand tool course, "as machines did all the work and didn't require any knowledge or skill"
I was gutted, I realised that he literally hadn't seen a thing that I showed him or heard a word that I had said. He came in with a picture in his mind of what he had decided constituted good craftsmanship and what he saw did match that picture, so was dismissed, Totally.
I was blown away! but this was pivotal in my understanding of how we dont see what is in front of us.
I had my idea that craftsmanship was "the best work done in the most efficient manner" and blindly assumed that he would see what i was showing him.
In fact selling anything is very simple, like all of the old cliche's that everyone tried to tell me. Find out what keeps the customer happy and give them that. Thats it, thats all.
Another wise businessman put it this way; he said Mark "I like red trucks, but i sell blue trucks."
So back to your specific question about relief angles; Assume nothing, maybe a 10 degree angle works great, but dont lock that in and look no further, maybe an 5 degree relief angle wont work, but maybe a 5 degree relief angle with a 2 degree micro bevel may work better.
One tooling example: Relief angles are very important on rotating tools such as jointers; as the wood progresses forward the heal of the knife will contact the wood and push the wood out of the knife, so we put enough of a bevel on planer knives to have clearance at normal feed speeds. I tried jointing my planer knives with a water stone while the jointer is running and the cutterhead is spinning. I just lightly touch the knife tips, to create a micro bevel that has no clearance from the cutting circle, and bringing all knives precisely into the cutting circle. This does an incredible job and produces a very fine finish, as long as the micro bevel is very very small. It is also very dangerous, so I am not recomending that anyone do it, I am just saying that experimenting reveals things that you may have ruled out or not considered because of your logical analysis, or previously dismissed because you didn't see, realize or factor all of the variables
The variables are not always easy to see, so our logical analysis is often based on incomplete knowledge and reaches the wrong conclusions.
If you try various releif angles, you may find that a very small relief angle works,
Or a very small relief angle with a lot of downforce works better.
When you look at the Kato video, you wont see much of the springback that we have been told the relief angle is neccesary for.
"For instance when a 34 degree blade angle is skewed to 60 degrees the effective blade angle is reduced to 18.6 degrees! "
I thought I had the mechanics of this situation figured out but the more I thought the more complicated I made the situation, correctly or not. I guess the issue is how the forces of the blade distribute between shearing across the fibers vs splitting them apart. What has become clear is that with the blade skewed some fraction of the force to cut or shear the fibers has been translated to a force perpendicular to the direction of the of plane travel(and wood fibers). A force in this direction doesn't lever the fibers apart to make a crack ahead of the blade tip. No doubt this has all been studied and reported somewhere with equations I wouldn't understand.
I expect an experienced planer takes advantage of subtle feedback of these forces, consciously or not, to adjust the plane to optimize these forces to reduce tear-out.
The Kato-Kawai pictures, as well as Steve Elliot's pictures, show us that the clearance angle at the blade tip becomes zero very quickly as a result of the bottom of the blade wearing as it rubs across the wood. Hence, much of the time we are planing with a zero microbevel. This wear surface eventually grows sufficiently large to creat a lifting force on the blade. I don't think I am up to trying to figure out the compensating forces of a lower blade angle and this lifting force from this wear surface. However, I once made a low angle Krenov style plane with very little clearance angle and it worked fine.
Spin the cutter head by pulling the drive belt with the stone engaged. Because only a bit of metal is to be removed this slower means of removal works and it is less stressful.
Warning to those who have not done it. It is easy to do too much and then the machine will pound the lumber.
Many years ago I read a Karate book written by Mas Oyama. At the end of the book was a section on self defence against a knife attack. With nothing to use as a sheild, Mas Oyama suggested that you can grasp and hold the knife blade quite tightly without getting cut, as long as you move your hand with the direction that the attacker is moving the knife. It is only when you move in opposition and create a slicing action that you will get get.
This I found quite fascinating.
It is quite true that to cut something with a sharp knife by pushing requires quite a lot of force on the tip, slicing requires very little. Same knife, same object being cut, slightly different approach, whole different outcome.
I do use feedback to skew the plane in use to get better results with the board that I'm planing, but it's really not much more than, "Oh, that pass wasn't too great. I'll turn the plane a little and see if that makes things better."
Ron Hock's blog has a short table of how skew angle and bed angle interact to give you lower effective cutting angles.
Ron Hock's formula is incorrect as I pointed out in his comments section. Here is what I posted on this forum five years ago. I think engineer Wiley Horne posted the same results I had on this forum sometime around 2009.
See, that's my thought, these things are really different. One thing that has not been discussed here are the super low angle smoothers. These also have their place. And you mention some low angles for these machines.
Another thing we are not quantifying is planing speed. What are the feed rates? I was reading this thing on the difference between Japanese experts and western ones. It mentioned that in Japan, the expert is fast. Not the slowest most deliberate worker. It reminded me that when I took the plane building course, the only positive comment I elicited was when I entered the planing contest and planed very quickly. That was "good", though I actually lost out to a more deliberate pace.
Anyway, another detail, cutting power is probably most easily affected by pace since energy at the edge will go up to the square of pace. Of all the things one can do while planing, moving it twice as fast is actually pretty easy to do, and you get 4 times the cutting power. But this would be something a machine could easily do. And another thing that is probably not dreamt of in n Horatio Tilden's philosophy.
One argument I like to make, with numbers and examples, is that the extra pace required to get a Ti hammer to hit as hard as a steel one, is not natural. It is actually a massive increment. But I don't think that is as true of planing. Maybe doubling speed is unrealistic, but a major increase in speed seems possible, because it is not something, unlike hammering, where people are already maxing out their speed.
BTW thanks for the accurate formula. The lower effective angle would increase the levering force on the fibers leading to more tear-out, if that was all that was going on. Hence, the favorable action of skewing the blade must have more to do with how force is distributed to the bundle of wood fibers ahead of the blade tip.
There are levering forces, as Kato and Kawai call them, that split the fibers apart ahead of the blade tip. Wood is not isotropic, which is to say that its properties to resist splitting vary by the direction the force is applied. It is desirable to minimize these forces overall, and/or direct them in a direction where the wood is stronger, which is across the grain.
There are shearing forces that sever the wood fibers and again the resistance of the wood structure to shear(slice) is direction dependent. For ease of pushing the plane these forces should be minimized.
It must turn out that the increase in splitting resistance as the blade's force is apportioned between the favorable and unfavorable grain orientations gets better as the blade is skewed even though the overall levering force becomes unfavorable. Once again practice likely preceded explanation by centuries.
I am imagining that some mechanical engineer has proposed some equation relating all these forces and solved it to determine the minimum levering force in the unfavorable with-grain orientation.
You wouldn't chop vegetables with a 60 bevel on your kitchen knife, because it would crush not cut. Wood is no different.
The different application is when cutting against the grain, the the force required to sever the fibers is greater than the the wood structure and it fails, by tearing and separating the fibers, even from a sharp blade.
So if the blade is a sharp as you can get, and the throat and chipbender accurately set, this should give the best finish with the least amount of effort.
High angles require more force because its harder to cut that way, that higher force is from the tip pressure against the wood, it creates more stress on the wood structure and the blade and is felt in the pressure required to push the handplane.
Low bed angle, low bevel angle, minimum clearance angle, with throat opening and chipbender closely set would make the most sense to me.
Then look at Skew effect other than just change of effective angle.
Feed speed and down force.
Supersurfacer feed speeds are from 60 -100 meters a minute!
Feedspeed may improve the effective downforce of the chipbender, by delay the lifting long enough for the cutting action. Most of the action is on the micron scale, so it is not easy to get definitive answers, without an indepth scientific study of all the variables.
Steve may be working for he has not yet responded on this topic. He would be the best source of information on the effect of velocity. Steve got a thesis on studying the mechanics of planing. Steve told me that the study found no large effect of tip velocity on the shaving formation.
That could be good news since it would indicate you get similar shaving formation while the powers that are making the cut are clearly multipliable. It would be a bad deal if the energy forming the shaving made it more likely to cause tear-out. Surface speed is normally a key variable in any cutting process.
I now have the two doctoral dissertations referred to in Appendix I of Leonard Lee's book on sharpening. Norman Franz addresses the question of whether cutting velocity has a significant influence on the type of shaving ("chip") that is formed or on the surface produced on the wood and concludes that there is not a significant effect. To quote him, "Chip formation is independent of cutting velocity, except at near-static conditions. This conclusion is diametrically opposed to current opinion in woodworking practice."
William McKenzie explores the question of how the cutting velocity affects cutting forces, and concludes that various factors (strain rate, friction and heat) may tend to cancel each other. He also acknowledges conflicting evidence in the existing literature and suggests a need for further investigation.
The speeds in question went way beyond the range possible when hand planing, since the high speeds referred to rotary cutter heads of the type used in jointers and planers.
If I could see a difference in the wood surface or shaving that result when I move a hand plane at different speeds I'd trust that more than the scientific speculation, but I haven't noticed any practical differences when I use a plane myself.
I was wondering if the speed /chipbreaker combination may have some effect on tearout in against the grain work. The thought that maybe the forward speed and the down force on the chip caused by the chipbreaker at higher speeds may negate some of the lift caused by the blade.
Dont know if it would make any difference, but an interesting thought.
Do the other studies that you mention factor in the chipbreaker effect or not?
Neither dissertation considers the effect of a cap iron. With all the variables involved in shaving formation I think it will be difficult to say for sure that velocity is never a factor, but so far I haven't seen much evidence that it is.
The fact that Japanese woodworkers (or others working in a craft passed down through tradition) believe something carries some weight with me and I hesitate to say they are wrong. At the same time I believe it's possible to pass down misinformation, especially on issues that are difficult to test. In this case I want to keep an open mind in case better evidence comes along.
Wood fibers are quite rigid in their long axis. It may be the time scale for transmission of the blade force along the grain axis is fast relative to the blade speed. In this case the force propagating the crack won't lag the force severing the fibers, ie. there is no shock "absorber effect".
This stuff is too interesting to have been overlooked by some mechanical engineer at a university with a wood science department. I'll bet trolling through the publications out of VA tech or Wisconsin will uncover a study or two.
However, a conversation with an experienced planer could reveal the answer quicker.
Re: depends on mechanical prop. of the wood fibers
Mark Hennebury
If you look at the way they test Samurai swords, by slicing a bunch of straws tied together, you will agree that speed is very important to the task. I think that it is ovbious that there is a lag in the response time of the straws reaction which facilitates the cutting, rather than just pushing the straws.