I’ve had a nagging question in my mind about the use of the term “shearing cut” in woodworking for a long time. Reading another article this morning where the author referred to changing the angle at which a gouge engaged the work in order to do a shearing cut to produce smoother cuts prompted me do a little quick research. It seems that according to Webster, shear means “to cut through something with or as if with a sharp instrument”.
http://www.meriam-webster.com/dictionary/shear
and from Wikipedia: http://en.wikipedia.org/wiki/Shear
“Shear stress in physics, refers to a stress state that will cause shearing (see verb) when it exceeds a material’s shear strength;
“Shearing (physics), the deformation of a material substance in which parallel internal surfaces slide past one another;”
http://en.wikipedia.org/wiki/Shear_Stress
“Shear stress is a stress state where the stress is parallel or tangential to a face of the material, as opposed to normal stress when the stress is perpendicular to the face.”
From the above it seems that any cut made in woodworking with any kind of knife-like instrument is a shearing cut. A chisel, or plane “skewed” to the direction of force is doing no more of a shearing cut than a chisel or plane held with the cutting edge perpendicular to the direction of the cut.
The impression that I get from the usage of “shearing cut” is that most people mean orienting the cutting edge, e.g., chisel, hand plane, at an angle to the direction of grain in a board — or to include end-grain cuts, etc., at an angle to the direction of motion of the cutting instrument. It is also used wrt power tools when people recommend angling a board as it passes over a jointer or through a planer. But, I also get the impression that they don’t understand why angling the cutting edge produces a better/easier/smoother cut.
Now, to start off the argument, er, discussion, I’ll state my hypothesis: “All of that is baloney!” — I don’t mean that it isn’t effective. What I mean is that, until someone shows me differently, I think the mechanics involved are considerably different from what people seem to think is causing the improvement in the cutting. If the process is examined from the physics perspective, the change in the orientation of the cutting tool doesn’t change how the cut is being made. In once sense, the smoothness of the cut is totally unaffected by the angle at which the cutting edge is pushed through the work. The only thing it changes is the effective angle of the cutting edge as it impacts the fibers to be cut. Think bevel angle. (The physics for rotary cutting heads in a jointer or planer are slightly different, so let’s stick w/ chisels, planes, etc.). Thus, it seems that anyone that is constantly having to skew their cutting edge wrt to the direction of the force really should consider changing the bevel at which they sharpen their tools.
For the most part, this topic is nothing more that a pet peeve of mine on the popular use of the term “shearing cut”. However, when people start giving advice on how to correct problems encountered by others, I think it would be helpful if the physics of the cutting action were more clearly understood. Does any one have any insights they can offer?
Thanks…..
Replies
A tempest in a teapot.
If you can slice a loaf of bread by pressing the knife directly down on the loaf, be my guest.
However, if you choose to move the knife forward as you press down, I submit you are engaging in a shearing cut.
Frosty
Frosty,
are you saying that you are not going to get involved in this thread because it is shear nonsense? :-)I do my bread on the band saw. I use a 3 TPI blade on Italian bread, but a higher tooth count on softer breads. I try to make sure that the back of the blade has been honed and that the weld mark has been honed as well. Good eating!
MelMeasure your output in smiles per board foot.
Mel,
Come on now. Everyone knows that's really the reason they made the schultmeiser! Just fit it with a 9" blade! Then you can over the shoulder slice your Italian bread.
For Gods sake don't use a splitting maul, at least not the blunt end.
Regards,
Bob @ Kidderville Acres
A Woodworkers mind should be the sharpest tool in the shop!
Edited 6/28/2007 9:36 am ET by KiddervilleAcres
Bob,
I know you'll never believe this, but I'll tell you anyway. I am the person who is responsible for the fad of putting soup in a hollowed-out loaf round Italian bread. All of the fashionable sandwitch/soup restaurants do that now. I invented the idea of using the schultermesser to hollow out the bread to put the soup in it. Well, I never got credit for that, but at least you know where the credit lies. It took a creative woodworker, using forgotten tools to revolutionize the food industry. (well, maybe that is putting it a little too strong.)
MelMeasure your output in smiles per board foot.
Mel,Good one, gotta like it!
Regards,Bob @ Kidderville Acres
A Woodworkers mind should be the sharpest tool in the shop!
Mel & Bob,
There's some shear nonsense getting talked in this thread.
Lataxe, all askew.
Lataxe; Your answer just has to be a cut above the others, with that I'll part with a scythe of relief.Work Safe, Count to 10 when your done for the day !!
Bruce S.
Mr. Froe,
"A little nonsense now and then is relished by the wisest men."
Wow, I'm a poet and didn't know it and my feet show it 'cause they're Long fellows.......................
Regards,
Bob @ Kidderville Acres
A Woodworkers mind should be the sharpest tool in the shop!
Edited 6/29/2007 8:27 am ET by KiddervilleAcres
THE BOY STOOD ON THE BURNING DECK
HIS FEET TOUCHED THE WATER...Longfellow.
Edited 6/30/2007 6:18 pm ET by Steinmetz
I love this forum because it brings light into my life with responses such as yours.Perhaps we should sit down with Bob of Kidderville Acres and have a "bowl" of soup, cut, in a shearing motion, from a loaf of bread.In all seriousness, I think John White has hit on the heart of this story.Frosty
Yo Frosty and Mel,
It might not be the season for you folks but I'm prepared to make up a fresh pot of venison stew along with a proper beverage and a fresh baked strawberry rhubarb pie with perhaps some aged cheese to go along with it.
Let's meet at my summer cottage just north of the Canadian border.
There's some really nice Maple syrup available at a good price up there also.
Regards,
Bob @ Kidderville Acres
A Woodworkers mind should be the sharpest tool in the shop!
Edited 6/29/2007 8:26 am ET by KiddervilleAcres
BobYou make my mouth water - which results in slippery keys on my computer.I'll have to pass for now but hope that a raincheck is available.Frosty
Mel,
It helps to lubricate the blade by holding a stick of butter against it while it is running. I like to rub a clove of garlic over the top of the table too from time to time.
Ray
Marie Antoinette chose the guillotine over hanging just out of shear curiosity. She wanted to prove to the world that French women did not shave their hair, but, preferred to shear it with angular strokes. (To avoid 'nicks' and 'Beard Rash"Fortunately, the Thong was not invented 'til much later. Nor was they're
any need for a 'Bikini line'trim.
Edited 6/30/2007 6:01 pm ET by Steinmetz
Frosty,
I tried using a shearing motion with the drawknife last night instead of just pulling it straight through the wood. Made a much cleaner cut and was a lot easier to control too.
Am I doing something right for a change?
Regards,Bob @ Kidderville Acres
A Woodworkers mind should be the sharpest tool in the shop!
Frosty,>A tempest in a teapot.Sure!......but, I'd still like to understand the physics a bit better & perhaps, either become more enlightened, myself, or perhaps get a few people to change their minds on what is occuring.Now, to pick an argument with you. No, I don't slice my bread pushing straight down, but just sloping the knife doesn't make it much easier. And, that analogy doesn't accurately related to the shearing cuts most people talk about. When you move your knife forward, it becomes a sawing motion. If you look at them, most dedicated bread slicers from the major knife companies have teeth -- for sawing. Sawing is a totally different operation. I've never seen any one use a sawing motion while at the same time pushing a plane down the edge of a board. I think John White's answer may be the key to why just changing sharpening bevels is not the bottomline answer in all cases, but I've still got to digest what Chris is trying to tell me.Thanks for the response, anyway.....
Edited 6/28/2007 11:29 am ET by bd
If we consider face planing, the effect of the shearing cut is to reduce the width of the plane blade from perhaps 2" to 1.5". That reduction obviously reduces the effort required.
Bill
Agreed. That works if you're working on something wider than your cutting tool, but not for the case of edge planing a board.Still chewing on this.......
bd,
Seems to me there are actually two things being discussed here:
1) the presentation of the blade edge to the wood- askew vs. square- which has an effect on the cutting angle.
2) the "shearing" motion, you call it sawing, I think of it as slicing, wherein the entire length of the edge is used, in progression, as it is drawn, or pushed, across the wood. Think of the example of using the entire length of an 8" drawknife to cut a shaving 1/2" wide off a cabriole leg, or a knife slicing bread or a tomato.
Using the slicing motion makes the cutting easier than simply pulling, even when the blade is held square to the wood being cut, but a little experience soon convinces one that when one handle is advanced, skewing the blade, that less effort yet is required.
I'll leave it you physics students to analyse the why, utilising bigger words than I can digest.
Ray
That seems to be where it's gone, but that's not what my problem statement was, or what I intended........but sometimes hard to be both concise & clear in brief text messages. My contention was that many people, you see it in the mags all the time, use the term "shearing cut" to mean skewing the blade to the grain orientation. It doesn't matter if they're talking about hand planes, chisels, Shelix type cutter heads in power planers & jointers, lathe tools or the spiral knives that Powermatic uses on some of it's machines. It seems that anytime anyone refers to the benefits of angling the cutting edge to the direction of movement, the justification given is that the angling produces more of a "shearing cut". My contention is that terminology/explanation is not correct. All cuts where a knife edge is moved along the work (without a sawing motion) are "shearing cuts". It doesn't matter how the edge is oriented wrt the grain -- that's a fact that can't be argued.I don't know if my impression is correct, but it appears to me that 'most' of those people that give some explanations think the benefits derived from skewing the blade are a direct result of cutting the grain at a different direction, where, in fact, the benefits that are derived are only the result of effectively changing the bevel of the cutting edge. The effective bevel at the point the cutting edge meets the wood is a function of only the angle between the cutting & the direction of motion, and, of course, the original bevel put on the cutting edge by sharpening. My contention is, & I think I can show it analytically, that there are virtually no improvements relating to any "perceived" change in grain orientation. Provided the skewed cutting edge is moved along the work in the same direction that the unskewed cutting edge would be, the only changes wrt to grain orientation are in the woodworker's head.My analytical geometry & trig skills are more than a bit rusty, but I'll post a diagram or two when I get a chance to complete them. I guess there are two sides to engineering. One is knowledge about the laws of physics ruling the world & something of a desire to see processes accurately defined/described. OTOH, most engineers are not scientists, &, thus, see no need to demand absolute accuracy when "close enough" will allow the problem to be solved. So, what the heck?? The understanding of many woodworkers on why it works may be wrong, but does it matter at all if the action it leads to solves the problem?
Edited 6/29/2007 1:27 pm ET by bd
Now that we have considered cutting bread, let's consider the surgeon cutting skin. I can testify that the scalpel works much better when drawn across he skin. I'm not sure what that proves.
Tom (M.D.)
Yes, if you made the bevel angle smaller on chisel or hand plane you could achieve the same result cutting with the blade square to the direction of the cut as you would with a higher angle blade being skewed. The problem is that you can't reduce the bevel angle down to 15 or 20 degrees without creating an edge that would be too fragile to use.
John White, Shop Manager, Fine Woodworking Magazine
Look at a pair of scissors, also known as shears, but think of one blade as a force, not an object. One blade equates to the gouge and the other is the force, provided by the rotation of the piece being carved. I think they just mean that the cutting edge and the line of the tool need to be aligned with the outer diameter of the material. That would be shearing the material instead of scraping.
BD,
your hypothesis holds for isotropic materials.
But since wood is highly anisotropic you have three shear angels (one of them does not matter since you plane in a plane). Due to the anisotropy the strength strength of wood is different in different directions and different shear angles lead to qualitative changes in the chip formation. Chip formation (and in general crack introduction, formation and propagation) is a highly complex phenomenon that appears to be little understood even for very simple materials.
In short, I believe you are right that science can not fully explain how skewing a plane works.
Hope this was useful.
Chris
Chris,Thanks for the response. You hit on something I had not thought of. Now that you mention it, (I think) I understand chip production/propagation has a lot to do with the smoothness of the cut produced by a plane or chisel. In their last Technical Bullentin, Lee Valley had an article on plane blade geometry and included some microphotos of chip production by different planes. It was all excerpted from Leonard Lee's "The Complete Sharpening Guide". I've never seen a copy of that. I think I may have to order one. I still have some questions, but it's going to take some time to formulate them & generate some diagrams. Will have to get back to this later.Thanks, again.......
bd, this would be a really good time for you to get a copy of "Understanding Wood" by R. Bruce Hoadley, and study chapter 8, his description is about the clearest that I have seen.
<Thus, it seems that anyone that is constantly having to skew their cutting edge wrt to the direction of the force really should consider changing the bevel at which they sharpen their tools.>
If you have a tool that is sharpened to 25º because that is the sharpest angel that will hold an edge without dulling rapidly, and cut with the blade askew, you can effectively present a sharper angle by sending the fiber up the blade at a longer path, but if you tried to sharpen the blade to a 12.5º angle, it would be too fragile to hold an edge.
Keith,Thanks for the response & reference. I've been wanting a copy of Bruce's book for many years. That's just one more reason to keep it on the list. Maybe one of these days........On the issue of the bevel, I think we're basically in agreement. While I had overlooked the extreme case of very low bevel angles that you & John White have pointed out, it appears that we are in agreement on the actual effects of what changing the angle a which a cutting edge is moved across a piece of wood accomplishes. Also see my response to JOINERSWORK above.
bd. Sometimes when I am trying to think through things as you seem to be doing with this, I try to imagine these changes such as skewing the blade way beyond the point that is normal and think of the results of the extreme. One of those things is the difficulty of cutting with the blade square across the end-grain is the hardest, to planing against the grain, then with the grain, finally with the blade aligned with the grain, slipping between the cells, and the blade moving across the grain being the easiest. Part of the benefit of skewing is that the cells being severed are traveling a longer path up the blade, but they are also able to bend aside in a way that is easier than straight up the blade. As for your reference to articles that you have read that you take exception to, I probably didn't see them, but I will assure you that there is a lot more information out there that most woodworkers never know about which has been compiled from many years of experience running machines, and keeping good records of all manner of cutting geometry, feed rates, knife cuts per inch, etc. So it is not just hypothetical.
It appears to me as if the surfacing process is more complicated than a a shear failure. For example the chip is significantly compressed during the cutting process. Various types of chips have been identified, all of them indicate that wood surfacing is much more complicated than simple shear failure. I believe some of the instructional sharpening books in the popular literature have some nice photographs of various chip types.
Here are a few results from a group at the Dresden Institute of Technology (no affiliation). In this work they focus on empirical observations, I am not aware of any attempts to explain the observations. As far as I know this is the only somewhat scientific published work on linear cutting (as opposed to rotary cutting as used in power tools).
The work was done with a super surfacer, with the following geometry:
Projection of cutting edge above sole: ü = 0.05 mm (0.00197 in), mouth width: b = 0.5 mm (0.01968 in), bevel down blade with bevel angle: β = 30.8°, bed angel: δ = 36.0°, distance tip of blade to tip of chip breaker: k = 0.25 mm (0.009842 inches), angle of chip breaker : ζ = 35.3°, cutting speed: vc = 50 m/min (2.7 ft/s).
The shear angel was varied between 0 degrees and 60 degrees relative to the direction of planing in steps of 10 degrees. Measurements were done for fir (picea abies), pine (pinus silvestris), beach (fagus silvatica), oak (quercus sp.), willow (salix alba) and red lauan (shorea negrosensis).
The results are:
Specific cutting force kc as function of shear angle λ. As expected the cutting force reduces with increase in shear angle. Cutting force is measured in direction of planing, specific cutting force is defined as cutting force Fc in the direction of the cut divided by chip width and thickness, the unit of kc is N/mm2, 145.03 N/mm2 = 1 lb/in2
Average chip thickness hm as function of shear angle λ. There was no change in average chip thickness hm with change in shear angle λ, 25.4 mm = 1 in.
Average surface roughness Rz as function of shear angle λ. There is a slight tendency towards rougher surfaces for for beach and oak with increase in shear angle λ, and a slight decrease in surface roughness with increase in λ for fir and . But the results are somewhat inconclusive. 25,400 µm = 1 in
Again, those are all empirical data, no model that would attempt to explain the measurements was given and could be the basis of your one work.
Hope this help.
Chris
Galoot-Tools
Atlanta, GA
Chris,You obviously have a lot of knowledge in this area. Thanks again for the responses & links. I have couple of questions related to that data. If you can provide some additional clarification, I would certainly appreciate it. I've milled tens of thousands of board feet of lumber on a bandsaw sawmill. Quarter sawing and flat sawing. The odds of a board coming out of a log with the grain completely parallel to the milled face is infintesimal, at best, I would think. It would seem that the only way to obtain boards with the grains aligned to the faces would be to use splitting techniques. Given that, it seems most likely that the boards run thru the Super Surfacer would have grain orientations that were skewed (in 3 dimensions) with the surfaces being planed. In order to be meaningful, wouldn't the shear angle in the tests have to be adjusted for those offsets instead of being measured wrt the direction of travel of the board? If that was not done, the testing procedures would seem to require a large number samples to be run to get some meaningful averages. To your knowledge, was either of those done in that testing? Or, in other research that you might be aware of?Well, that's only one question, but my back's been bugging me lately & I've got to get away from the computer a while.Thanks again for the info....as Ahhhnold said, "I'll be back".......
Chris,In attempting to combine your comments & the info in the links you provided w/ the information that Lee Valley published, it still seems to me that my observation may still be valid. That is esp. true in the case of the article that prompted me to start this threat, oops, thread. It seems obvious, at least to me, that if skewing the cutting edge on the lathe while doing a turning provides better results, the only generalization that can be made, because of the widely varying grain directions encountered in a complete revolution, is that the improved cutting action is due primarily, but maybe not completely, to the change in the effective bevel. Other cases may need some "caveats", but no need to get into that here.I would like to pick your brain further on some related topics. Maybe later, if you don't mind?
I agree, it is always good to question the validity of measurements.
The data they present is based on 126 separate measurements. In addition they mention 156 supporting measurements to identify friction coefficients (this is essential to calculate the actual cutting force, a rather tricky measurement). There are also 120 more measurements on other cutting parameters.
The authors have done relatively simple statistical analysis on the measurements, but nothing close to a measurement capability analysis. Nevertheless, one would expect that systematic variations in the measurements would average out.
Various other measurements are given in the paper but it appears as if they could not vary the bedding angle; so your question (i.e. is changing the angle of attack equivalent to changing the skew angle) remains unanswered at least from the experimental side. In my biased opinion conclusions based on theoretical considerations without experimental justification are of questionable merit.
There is one other group in Japan that has performed studies on the effect of the chip breaker on chip formation. As far as I know none of the groups conducts any research in the filed today. I believe some work on chip formation has been published in refereed journals as well as work on dulling of cutting tools.
I am not aware of any other studies on linear cutting action in wood (i.e. super surfacers and hand planes). The work I cited contains well executed measurements but fails to give explanations of the measured results (objective was to improve the total throughput of super surfacers while maintaining surface quality; super surfacers are too slow for today's production shops).
I'll be happy to help wherever I can either through this forum or ping me offline.Chris
Galoot-Tools.com
bd, ya know fella, when I have a gnarly grained board that needs to be flat, I skew my very sharp smoothing plane and 95% of the time it works. If it dosen't as it may be really gnarly I get my scraper plane out and it usually solves the problem. All of the rest of any discussion on this topic is too much stress skewing my tolerance level on academic waste for no good purpose. Paddy
I hear you!.......but, interest, like beauty, must be in the eye of the beholder. It's interesting to me, even if too skewed for you to enjoy & Lataxe, & many others, think it is shear maddness. Actually, your response raised another couple of questions in my mind, but since I seem to be on the verge of being burned alive for witchcraft, heresy & dealing in the black arts, I'll hold on to them for another time....
bd, many thanks. BTW the eye of this beholder is the eye of an old Industrial Engineer who is a Knot head for the joy or the "glow" that fww provides. The LAST thing I need, even recognizing the satisfaction it yields, is another esoteric analysis. All the best, enjoy your quest.
Paddy, the over educated wood butcher.
Hi All,
Maybe a shearing or skewed cut is a sawing motion on a microscopic level.
Paul
Paul,
I've been thinking the same thing. Remembering my days of yore when in barbers school I saw pictures of microscopic views of a straight razor blade and there were small teeth running diagonally across the blade, much like a saw only obviously much much smaller.
The diagonal placement of the teeth were a result of holding the blade at a 45° angle which was the preferred angle when honing on the stone.
Also stropping was done only to remove any particles left behind from the sharpening stone; it offered no additional sharpening capability, at least as it applied to straight razors.
Regards,Bob @ Kidderville Acres
A Woodworkers mind should be the sharpest tool in the shop!
Bob,The references to scissors or shears in your response may be part of the key to why so many people seem to be getting confused over this topic or the point of my discussion. The cutting action of scissors, tin snips or metal shears, etc. is, of course, related & pertinent, but since we can't reach agreement on what the most basic definition is, let's leave them out of the discussion for now. The most basic definition of shear is "to cut through something with or as if with a sharp instrument" (per Webster), so let's stick with the most basic of sharp instruments & wood cutting operations that aren't a sawing action. To try to explain my point a little further, the following is copied from a web page ad for Byrd Shelix heads for planers/jointers:"Woodworkers know that a shear cut is far better than a straight cut." ---- (yeah, but we can't agree on what that term means!)"SHELIX¯ - shear, helical, staggered, insert tool"From a recent WJ article on turning bowls "Use the cutting edge on the side of the gouge for a shear-scraping final cut". This was in reference to finishing the outside of the bowl. In one revolution, the cutting edge would "see" a wide variation of grain orientations, esp. in figured woods.In the Q&A section of a relatively recent online "e-zine", when one reader asked how to get better results planing a curly grained board, the respondent suggested trying to find someone with a Powermatic planer with the spiral (not carbide insert) head because it produces more of a "shear cut". (IMHO, that's no more of a shear cut than one gets with a regular planer head. It DOES have it's benefits though & that was some of what we have already discussed.)In the Q&A sections of magazines when people ask how to improve their results planing or jointing curly grain, the most frequent answer given is to skew the tool to get more of a shearing action. (Again, IMHO, telling someone that skewing the cutting tool to the work improves the cut because it increases the "shearing action" is wrong. What it does do is effectively decrease the bevel of the cutting edge, thus, presenting a sharper edge to the wood fibers being cut. That's all that can really be said because all of the responses I've seen along these lines ignore the direction of the grain.)The term is used a LOT in the press, but never explained -- & that's why I opened the thread.......
Keith,I'm sure that those that are involved in tool design, whether it be hand or power tools, and many of those that have amassed significant amounts of experience from either hands-on work or education, are aware of the details & have signifcantly more knowledge and insight than I do. And, yes, it has been some articles either in the popular press or online sources that I caused my interest/concern.Actually, it was an article about turning bowls on the lathe that got this started. Especially in that case, there certainly appears to be no valid generalizations that can be made wrt improving the cut by changing the aspect of the cutting edge to the grain. In the case of turnings, the grain varies widely over one revolution.On the point that you make about the deformation of the fibers, Chris' revelations about the importance of chip formation probably negate the need to get into that for any along grain work. Maybe even for the end-grain case, but let's assume not, for the moment. For the end grain case you mentioned, I don't think it can be said that any particular orientation of the cutting edge is going to provide a benefit by permitting the grain fibers to bend aside. There is always some deformation at the microscopic scale before the fibers are sheared. Consider the case of having, say, an end-grain cross-section wheel cut from something like a 20 in log. No matter where you push your cutting edge thru that end-grain, each fiber is pushing against it's neighbor to resist the force of the cutting edge until it is actually sheared. But, still in those cases, you can feel the benefit of skewing the blade to the path-of-travel. I think the only thing that can be generalized from that action is that effective change in cutting bevel improves lessens the force required to push the cutting edge through the work.
Edited 7/1/2007 5:54 am ET by bd
In regard to the skew cut, I have always regarded the mechanics as similar to gearing down, in that one is extending the area over which the work is carried out and the time taken whilst expending the same force. The cutting edge might be ground at a less steep angle, but the length of the bevel needed then would be clumsy.
yup, you're absolutely right there -- the total force that would need to be applied would be less because the width of the shaving would be less. I was thinking of edge planing where the cutting tool is usually wider than the work. There the width of the shaving stays constant -- but, just thought of this, the actual width of the cut is the length of the diagonal the cutting edge makes.......hmmmm, wonder what that does to the total force required to push a tool???
I tried to post a reply a while ago but it somehow got lost in my computer. Oh well, I'll try again. Speaking of baloney, there might be a little in you post as well. I think most of us understand the difference between shear cutting and scraping. Language being as imprecise as it is, maybe other terms might be better. How about cutting, slicing or paring? We all know that cutting with the grain produces the best results. Take for instance, the task of cutting a branch with an ax. Chopping at an angle of 90 degrees to the branch is very difficult and produces poor results while taking an angled cut makes the chips fly. I think we're getting hung up on communication instead of mechanics. Along this line, I have seen another term tossed around in recent times. The term 'shear scraping' is one that I have not seen until the last couple of years. The term is contradictory within itself. I and I believe most turners in years past, have used this technique many times in a situation where a paring cut might induce a catch. I believe it to be a simple scraping cut using the bottom edge of a gouge rolled on its side. Sometimes terms are coined in an attempt to communicate and are taken to be something new when in fact, they are not.
>I think most of us understand the difference between shear cutting and scraping.I think I know the difference between cutting & scraping. None of the references that I have seen used used the terms in a confusing manner. However, given my apparent lack of understanding of the real definition of shearing, I might just agree w/ those that use the term "shear scraping". It seems perfectly logical to me, IF I understand what most people mean by shear cutting. If someone would just post a working definition of what "making a shearing cut" actually means that most would agree with, instead of pointing out my obvious madness, it might clear up some confusion. I would make a stab at it, but I'm not sure that I can come up with a consensus opinion. We could leave the chip formation issues that Chris brought up out of it to start with.......Anybody willing to stick a definition out here???
bd,
I would make a stab at it, but I'm not sure that I can come up with a consensus opinion.
Why not post what you have and then perhaps our collective minds might be able to develop a definition as a consensous.
Regards,Bob @ Kidderville Acres
A Woodworkers mind should be the sharpest tool in the shop!
>Why not post what you have and then perhaps our collective minds might be able to develop a definition as a consensous.Well, I thought I did that earlier, but nobody offered any corrections -- just disagreements. This is based on my understanding of how people use the terms, not how I would define them or how I use the terms involved. One other caveat/constraint is that it should apply to all cases, not just planing with the grain along the long edge of the board. Remember the usages that have been mentioned are bowl turning on a lathe, planing gnarly or curly grained wood, etc.The easiest & simplest definition ignores the difference between the direction of travel of the tool and the direction of the grain. It also seems to me to be the most common usage. "A shearing cut is made anytime the angle of the cutting tool is skewed, i.e., not perpendicular to, the path of travel of the cutting tool across the work."or if you wish "work across the cutting tool."
bd,
I keep thinking about my days as a barber when I go thru this discussion as I've looked at it many times. Cutting hair with scissors creates an action whereby the blades force the material to slide along the cutting surfaces. The microscopic teeth on the blades grip the hairs slightly so they can be sheared off. Does that make sense in terms of the topic of the discussion?
If you/we are confining the topic of the discussion discussion to wood, then I would think that the process of carving would most likely apply as the blade is drawn across the material in a shearing fashion, yes?
To that end I would think that any cutting process that causes either the material or the cutting blade slide across the material would in fact be shearing.
Regards,Bob @ Kidderville Acres
A Woodworkers mind should be the sharpest tool in the shop!
bd,
what is all this talk of shearing cuts? Why use blades such as planes when you can use a sanding thickness planer. Forget about sharpening or shearing. Just put the board through the sander and it comes out perfect. Them little pieces of sand just shear their way through, but there are so many of them that it looks like it was done with blades.
Have fun.
MelMeasure your output in smiles per board foot.
Mel,"what is all this talk of shearing cuts? Why use blades such as planes when you can use a sanding thickness planer"Why?...I think I tried to answer that some in my response to Bob, above. I see it used all the time & I was just curious to what people really thought it meant. It seems that there are a wide range of interpretations. That's odd for a term that's used in machine ads & in the press so often. Basically, I am just curious as to what the common understanding of the term is. Not sure that I have discovered that yet.WRT your comments on the sanding thickness planer. Interesting you should bring that up. I've been looking around, trying to find someone with the means to plane some boards I have. I've got about 2000 sq ft of hickory, most boards with some good tight knots, that I cannot plane. Even with a brand new blades in the planer, angling the boards as much as possible & wetting them down before running them thru, I get a lot of tearout around the knots & other grain reversals. I wanted to use it for flooring (with the knots), but I could buy prefinished hardwood cheaper than having someone with a commercial shop abrasively plane them. No one around here, that I can find, has a spiral/Shelix/carbide insert cutter head. Even if I did find one, their labor charges would probably make buying prefinished flooring the preferred option. Thus, I'll probably be joining the "shearing fraternity" by buying a Shelix head.....as soon as the finances allow.
Edited 7/13/2007 3:32 pm ET by bd
BD,
as you guessed, I was just having fun making a comment about using a thickness sander.About shearing cuts. I do a bit of carving. Carvers use shearing cuts all the time. You quickly find out that by turning your gouge, and slicing your cut, rather than just pushing it through, does a much better job.
MelMeasure your output in smiles per board foot.
Mel, "I was just having fun making a comment about using a thickness sander."...........something like that from any one on this forum that does a lot hand work has to be suspect. It's totally out of character.When we first built our house, I edge-jointed a bunch of pine boards to build doors with a Record jointer plane. Couldn't afford anything else. When I finished, I greased it back up, put it back in the box. It hasn't been out since. That was a little over 30 yrs ago.More recently, I managed to flatten a board that was too wide for my (power) jointer with a hand plane. Swore the next time I had to do something like that, I'd buy a big dish bit for the router & use some router rails......have never felt the satisfaction of experience that many of you refer to when using hand tools. Beginning to think that that falls into the same category as a "runner's high" --- either completely fictious or totally beyond my capabilities.
bd,
When I finished, I greased it back up, put it back in the box. It hasn't been out since. That was a little over 30 yrs ago.
To solve your shear madness perhaps it's time to resurrect that plane. By using it you may find your answer.
Regards,Bob @ Kidderville Acres
A Woodworkers mind should be the sharpest tool in the shop!
Bob, "To solve your shear madness perhaps it's time to resurrect that plane. By using it you may find your answer."Methinks that would drive me over the edge -- into the region from which no man has ever returned. I'm slow enough as it is. Being forced to use hand tools would be like being stuck behind a wreck on the DC beltway forever......
bd I think i'ts time to bring in the AUSSIES. They have more experience of shearing than anyone ,if you dont believe me ask the sheep.
I had occasion to be in a lawcourt once .The case was taken by somebody who had beeh injured by a wheel that had broken off a truck, An expert who was called said the studs holding the wheel on had sheared off. When asked how this had happened he said that the studs had been over torqued in his opinon. I dont no if there are two kinds of shearing but I've always thought that apart from sheep that only metal could shear off. Rgds.Boysie Slan Leat.I'm never always right but i'm always never wrong. Boysie
I've sheared plenty of bolts off myself, including a handfull of wheel lugs --- maybe that's a double handfull so I know what you're talking about there. From a materials science standpoint, I think a (perhaps overly) simplified definition would be when a material fails when subjected to forces going in opposite directions. I can't remember what the restrictions on the material might be (if any), but it's a fairly widely applied definition. Shear forces can be present in fluids as well. While I wasn't after a legal or totally technically correct definition of the term, your response is interesting. It appears that you don't see it used much, if at all, in the woodworking circles down under. Is that correct? If so, it may just be another Americanism.....
bd, The bit about the sheep shearing Aussies was my attempt at a joke.I've never been to that fine nation although I have many reletives there.
Whilst I dont have much experience in woodworking or metalwork I have never seen a wood shearing machine advertised anywhere. In the metal buisness they have what I believe are shearing machines which work on the principal of the scissors which someone else mentioned in an earlier post. Meaning that the blade comes down on one edge and cuts across the sheet, ie from left to right.
I hope this helps to settle this debate because,THIS IS THE WORD OF THE BOYSIE. Regds. Boysie Slan Leat.I'm never always right but i'm always never wrong. Boysie
bd,
Damn you.
Why did you come up with this infectious discussion! My mind has been going in circles since I first saw it! In truth it intrigues me; I just don't have the time to give it that much attention.
In my humble opinion any bladed tool causes shearing of the wood fibres, even sandpaper as well as an axe. If one could see a particle of sawdust magnified I would suspect that it would have similar characteristics to a shaving.
Teeth on a handsaw are shaped in a half diamond fashion. Why? To shear the wood fibres. Angling the blade has nothing to do with it.
Maybe we should redefine cutting wood with bladed tools as shearing wood?
Regards,
P.S. It is a great topic and I thank you for provoking my mind. It needs it now and again.
Bob @ Kidderville Acres
A Woodworkers mind should be the sharpest tool in the shop!
Bob, " My mind has been going in circles since I first saw it! "You get an "attaboy" for that!.......Actually, I don't think we're going to resolve anything here. Much less, change how people use the term. As Chris has pointed out, the real answer is much more complicated. However, it does bother me some, esp. when the term is used to give novice woodworkers an explanation of why a particular technique/tool is recommended. It sort of seems to be a self-propagating point-of-confusion --- well, confusion is not the right wood .....oops, word, but guess it will suffice here.When I opened the thread, I thought we'd be able to get thru the discussion on the basic cuts long before now & on to the use of the term in machining operations.
You know, I just might have thought of the reason for all this discussion. The extreme heat has driven us from our shops into a cool, airconditioned house. Having read all of our magazines and books, this forum is the next best thing to being in our shops. At least this is true in my case. Can't take this heat in my old age. You all have a good day.
"extreme heat has driven us from our shops into a cool, airconditioned house"
Ah. Someone with a real shop. Those of us who only have room for a shop in the basement find themselves looking for any excuse to stay IN the shop since it's the coolest place in the house. I actually found myself cleaning (!) my shop the other day, just to avoid leaving the place. ;-)
Mike HennessyPittsburgh, PA
This weather usually finds me in the shop by 7:00 AM and usually go for the AC around 11:00. I spent this morning wrestling with a chunk of spalted maple on the lathe. Roughed out the outside today. Maybe pick it up early tomorrow when its cooler.
What kind of heat are you experiencing in your old age. At age 74, I had a very comfortable day at 80°. Just about right, in fact. The basement of my shop has not warmed up enough yet to be comfortable.
Edited 7/11/2007 11:13 pm ET by tinkerer2
I'll be 70 in Sept. and have problems when it gets above 90 degrees. I think it's due to medication. You learn to live with what you are. By the way, we have very high humidity which doesn't help.
"seventy in September" Well time rolls on, doesn't it. This morning, when I went out to the shop it was 67°. That is kind of coolish for me. I put a sweatshirt on till I warmed up a bit. Humidity makes all kinds of difference. When I was out west (Wyoming) some time ago it was 90° F. It felt comfortable, but the humidity tends to be very low out there. You could just feel the sweat evaporating off your arms.
BD,
Here is a stab at a definition: 'A shear cut is any cut where the force applied to the cutting edge is not perpendicular to the cutting edge'. The further from 90 degrees, the more of a shear cut you have.
As soon as the force is not perpendicular, I would think there is an increased tendency for whatever is being cut to slide along the cutting edge. Too much shear and the thing your trying to cut simply slides along the blade and not into the blade(particularly if not well supported). Too little shear to the cut and you lose your slicing action.
I believe this effect would be different from the effect of lowering the effective cutting angle by skewing a blade. A lot of times when carving, while pushing a gouge forward, you rotate the handle. This doesn't change the effective cutting angle but does make the cutting easier and cleaner. Here, you've added a slicing action while maintaining the effective cutting angle.
Matt
"Here is a stab at a definition: 'A shear cut is any cut where the force applied to the cutting edge is not perpendicular to the cutting edge'. The further from 90 degrees, the more of a shear cut you have."I don't think there's any significant difference in that & what I posted. From strictly a mechanics standpoint, the object must move in the direction the force is applied. But, I have to raise the question again, what do you mean by "shear". If we use Webster's definition, any cut made using a knife edged instrument is a shear cut. If you mean letting the cutting edge rub along the fiber to be cut, I would be inclined to call that a sawing operation and not a shearing operation. The teeth on the saw may be microscopic, but they're there. I do understand what you're saying & do agree that with the cutting edge skewed to the direction of travel, or direction of the applied force, the resistance of the fibers would tend to cause some variation in the path of travel of the cutting edge. I don't think that's as significant as the effective change of bevel, esp. when power tools, e.g., a Super Surfacer, is included in the discussion.I can't speak to carving at all, except maybe in putting a new point on a pencil. With a carpenter's pencil & a dull knife, yes, it does help to slant the blade & let it slide, but I think that's outside of the discussion, here.
1800 rpm motors are used more often with an 8 inch wheel. A large wheel has less rounding at the grind point. An 8 inch wheel turns quite fast on the outside as compared to a smaller 6 inch wheel on the same motor, thus less speed is used. The larger diameter motors on many common shop grinders along with the small wheel means that in some cases a tool such as a knife or possibly some scrapers can not be placed at a large angle for sharpening. The motor physically interferes with the handle.
Thanks for the replies. DW & I have been sharing a virus so haven't been keeping up with the postings. I'll start trying to catch up.
Paul,I understand what you're saying, but it has nothing directly to do with the issues that I'm trying to discuss. I'll re-post & rephrase everything try to reach a wider understanding of what I'm trying to discuss. I initially opened the thread because I had the feeling that a lot of those that write online & jounal articles did not really have the same understanding of what "shearing cut" means. I have seen the term used frequently, but never defined, properly or improperly. I think the (serious) responses I'm getting support that hypothesis.
"the change in the orientation of the cutting tool doesn't change how the cut is being made."
The change of orientation changes the angle of the wedge you are moving through the wood. The change of that angle changes the character of the chip.
At the lower angle the chip flowing over the cutting edge is simply bent. (At the higher angle the was suffering compression fractures.) The same can be accomplished by making thinner cuts, but ...
GHR,"The change of orientation changes the angle of the wedge you are moving through the wood. The change of that angle changes the character of the chip.".........we're in violent agreement.
bd
Methinks that most times the catalogs and such confuse the term shear with slice. Slice is defined as "cutting with or as if with a knife". Shear by definition cuts through. Think of the action of sissors or tin snips or a large metal shear. Those are shearing actions. I believe a chisel, plane, jointer and such would be a slicing cut. Maybe thats why the cutters are called a jointer or planer knife. With shears the blades are called a shear. A paper cutter is a good example of a shear.
Slicing does not need to go all the way through the piece to fit the definition while I think shearing does. Skewing a board through a planer will give more of a slicing cut. I think a hand plane is a perfect example of a slicing cut. I think of a slicing cut as producing a shaving while a shearing cut seperates the peice in two.
JMHO
Rich
The Professional Termite
Edited 7/15/2007 9:11 pm ET by trialnut
I believe you are absolutely correct. When you really drill down you will most certainly figure out that the literal definitions of shear are not very useful and you might want to use a technical (i.e. mathematical) definition of shear.
What wikipedia really says is that shear (stress, etc.) is the off-diagonal elements of the (stress, etc.) tensor. The wikipedia statement about 'parallel and normal to the face' [as measured by the surface normal] is just a way to visualize the off-diagonal terms.
Thinking about it, I don't think shear is ever used by itself in the technical literature; I believe shear is always a specifier to indicate a certain direction relative to a surface normal, direction of motion and a conveniently chosen frame of reference. Examples are 'shear stress', 'shear force', 'shear plane', 'shear wave', etc.
I know, I added absolutely no value to the discussion at hand.
hmmmmm.......maybe we should go to a {V, RxV, Rx(RxV)} coordinate system like they used in astrodynamics......just joking, of course. Still, even to invoke the use of the stress tensor, we need to know something about the directions of the principal axes, don't we? Wouldn't that be a function of the actual board being processed due to the variation in grains??? ......it does get complicated doesn't it? Sounds like a good problem for a Master's thesis, uh? While I'm interested in the actual mechanics of real cutting actions to apply to my own efforts, I was hoping that there would be a fairly easily arrived at consensus on what most people meant when using the term -- whether technically correct or not. There doesn't seem to be.
This forum post is now archived. Commenting has been disabled