Hi,
I have posted some thoughts and calculations about different ways to sharpen a plane blade in a bevel down plane and the affect on cutting edge support. I'm interested to know what people think. I am planning on analysing bevel up planes and back bevels in bevel down and bevel up planes. Any bright ideas could be incorporated easier now than after.
Thanks, Peter
man that some serious brain work (G) wonder how much it matters in real life? you forgot laminated blades though (G) hollow grinding though is slower then flat grinding. even if I went to this method it is not practical to use a grinder they are slow and heat up the steel too much. I do all my grinding on my belt sanders. the speed is so much faster it's not even worth thinking of grinding wheels anymore. I like the back bevel idea myself I have not played around too much with it yet. I recommend it to people who can't afford more then one plane. it's cheaper to get two irons for the different angles.
Wow! That's pretty complicated. I do hollow grind & then finish them off on a flat stone - no microbevel, usually. I'll keep touching them for quite a while & eventually they get to a point where I put them on the grinder again, maybe a year or two. (I'm a hobbyist, so I don't use them for hours a day).
I will put a microbevel on one occasionally. Seems to work better on some woods some times, usually wild grain. Not sure I always do it as well as I should, though. I tend to resharpen for the single bevel soon after.
I'm not an expert, though. I've been using hand planes for quite a few years & have always gotten a lot of satisfaction out of them. There's something about a long, thin curl & a glass smooth finish that is very satisfying.
Jim
Peter,
Here are some random thoughts. Sorry if they seem a bit blunt or sharp... My sole intention is to improve your analysis.
It seems to me that your analysis needs some more analysis. You state:
"I wanted to investigate how different practical sharpening methods affected the edge and blade support in bevel down planes. The cutting edge is supported by both the blade metal behind the cutting edge (edge support) and where the blade contacts the plane bed (blade support). I don't know which means of support is more important. I suspect the closer to the edge the greater the importance."
Since you do not know which means of support is more important and can not show why it would be important, it is impossible to make any sort of conclusion.
A simple free body diagram would show the blade subject to a force at the cutting edge which is downward and backward (from the point of view of the person planing). If we analyze the forces around the point where the blade contacts the frog it is true that the closer to the bottom of the frog we get the moment around that point would be less. This moment is counteracted by whatever force is applied by the cap iron or wedge or whatever is holding the blade down. So if the "problem" with plane blades was that they tend to rotate too much about this point, then hollow grinding might be a solution. However, if you look at the geometry of the situation, simply making the blade thinner has a much greater reduction.
It would seem to me that other factors will play a larger part in plane performance than the shape of the back side of the blade. I have no real analysis to back this up, and I've got neither the time or desire to do so, but it seems to me that it is possible to conduct this sort of analysis - though it requires more than just geometry to do so. Geometry is necessary for the analysis, but it is not enough to really understand the situation. In my opinion contouring the bevel side of the blade is most useful in reducing the time necessary to put a new edge on the blade (as there is less material to remove) than it is useful in increasing the structural integrity of the plane iron. Remember that the blade is most flexible where it is thinnest (at the cutting edge) and is thousands (if not millions) of times less flexible by the time you get to the point where it contacts the frog. Stiffness is proportional to the cube of the thickness.
So, how about the case where you have a flat ground bevel, but the clearance angle is minimized? What would you define as the minimum clearance angle? Why?
-j
Steve,
Maybe it doesn't matter much. I did want to debunk the idea that hollows are theoretically inferiour to other methods in all cases. In this case it would be the best with the proper equipment. Also it points out that using a 25 degree primary bevel and a 30 degree microbevel is probably a bit wasteful in terms of edge support. I was just curious what would be the best geometry for a given blade thickeness and then I thought some other people might be interested in what I was thinking about but not able to do the calculations.
this method it is not practical to use a grinder they are slow and heat up the steel too much. I do all my grinding on my belt sanders. the speed is so much faster it's not even worth thinking of grinding wheels anymore.
This is great feedback! I had an idea about this last night. A way to hollow grind with a belt sander is to use the curved part. Maybe make a belt sander with 6" diameter wheels to rotate the belt and then set a tool rest in front of one of those. If you use the middle of this curved surface I do not think that "dubbing the tip" would be a problem because the belt is tensioned from both sides as it continues to move past the blade and along the wheel. You could even go up to the finest abrasives that come in belt form. I don't know if that is fine enough. Also the wheel diameter would never shink like grinding wheels do.
Thanks Steve.
Peter
Hi J,
You have brought up some interesting points. Yes a deeper analysis could be done. But I'm sure that my analysis has debunked the myth that hollow shapes are necessarily worse. Also I'm satisfied that primary angles should be close to the microbevel angle (the 25/30 degree double bevel combo is probably wasteful in terms of support.)
It's true that my conclusions are not based on an analysis of the stresses in the metal and resulting flex. This is only a concern when comparing the double bevel to the hollow grind/flat hone. And this ended up being the important comparison. For really thin blades (0.080") the two geometries are within thousandths of an inch and I don't think there would be a substantial performance difference. But a properly sized grinding wheel could match any primary bevel angle. For thicker blades you can use a larger wheel to make the hollow and so that method wins again. So over all the hollow grind/flat hone method can always win in a geometry comparison against the double bevel. But the "other factors" affected my conclusions. I think I'll reword my conclusion.
My idea was to look at which way to sharpen a blade of given thickness. Thicker blades seem to give better performance and manufactures of high quality planes use thicker blades than the 0.080" Stanley blades. So I presume that the increased contact height of a thicker blade is not a serious penalty to pay for the increased stiffness of the thicker blade. The thickness of the blade is so important that Steve Knight's
1/4" blades apparently give incredible performance even without a cap iron to pretension the blade and with a larger contact height as he uses single or double bevels. When comparing different blade thicknesses it is not the flex of the bevel that counts, it is the flex along the length of the blade.This stuff is complicated. In fact modeling it will always be insufficient. Experimental evidence would be the only way to ensure all things are accounted for.
In theory the minimum clearance angle is the bed angle. That is like using a chisle to chop a mortise. But if you did this then after the first few shavings are made and the blade edge crumbles and dulls then the cutting edge will be above the bevel surface and you will not produce another shaving because you will not be able to push down the wide bevel to compress the wood enough to get the cutting edge into the wood.
Because the plane blade extends below the sole, it is slowly worn down just like in honing but here the wood is the abrasive. Brent Beach suspects that when this worn area gets large enough the blade feels dull as you drag it across the wood with great pressure. So a large clearance angle would reduce the frequency of sharpening. I guess it is a trade off between edge toughness due to included angle, supportive geometry, sharpening frequency and desired wood surface finish. For a particular wood and task (face or eng grain) there will be an optimum clearance angle for your assessment of the correct balance of the tradeoffs. There probably isn't one optimal clearance angle for all woods/tasks. I think I will add something about this to my page. Thanks. I did the analysis at 15 degrees clearance because that seems to be about what people like to use.
Leonard Lee writes "Anything between 30 and 35 degrees is quite acceptable. If you go much lower than 30 degrees you encourage blade chatter; if you go much higher than 35 degrees any wear dulls the blade much faster, but, more significantly, you reduce the relief angle unacceptably, particularly on planes with a 45 degree bed. Bevel-down planes as a group represent an exception to the rule that blades should be sharpened at the lowest angle consistent with edge retention, because the bevel angle has no bearing on the cutting angle. A sturdy edge is wanted, and a basic grind angle between 30 to 35 degrees will give you good blade stability and the least chatter."
It is too bad Lee doesn't say what he thinks the relief angle is for. He has implied it is involved with more than how quickly the blade dulls as Beach has suggested.
Thanks for your thoughts!
Peter
Knife makers do that. but I don't think you can get enough accuracy to go all the way with belts. myself when I make irons I grind the bevel at 30 degrees before heat treat. then I use the makita freehand with the zirconia paper at 120 grit to flatten the back and get the bevel accurate. then I free hand on my shapton stones. from the makita to sharpened takes me at most two minutes and usually less.
So you are using a single bevel?
yes since I can make planes at any angle. but if someone can only afford one I recommend a back bevel since I grind to 30 degrees a double bevel would be too much. plus it's easier to free hand sharpen when you do the whole bevel. if you use good fast sharpening tools like shapton stones the speed is not really a issue.
wrote
Although not specifically related to planing, there is a discussion of the properties of hollow ground blades on my web site. Please look at 'Sharpening Notes' - 'Hollow Grinding, Good or Bad'.
Perhaps Peter means 'secondary bevel'? A microbevel is a third, very narrow, highly-polished bevel.
wrote
If we take J's very valid point about blades being most flexible near the cutting edge, it could be argued that in view of the hollow-ground geometry, hollow-ground blades caan be theoretically inferior. For notes about the strength of hollow ground blades, please see my web site - Sharpening Notes - Hollow Ground, Good or Bad?
It may be of interest to note that the tapered, often laminated older irons fitted to traditional bench planes were given a primary bevel ground at about 12-1/2deg, taking the point of support even further higher up the frog.
A very common belier, but in what regard is this true? Much is made in woodworking literature of chatter whereas the most common problem with a properly set-up plane is skitter. Please check with my web site - Planing Notes - Skitter and Chatter.
The principal value of thick blades lies in their inherent rigidy, necessary when bedded with wedge systems that are less effective than the lever systems employed by Bailey, Norris and similar designers.
It can be argued that the thinner Stanley blade is, because of its flexibility, more efficiently bedded by the lever cap system pressing it against a machined metal frog (that ,incidentally, is more stable than a wooden plane's frog).
However much as we might enjoy theorising, I reckon that the principal purpose of the cap iron is to deflect shavings. Please recall that many older bench planes did not use cap irons, and moulding planes still lack this feature. I supect that Mr Knight's and Krenov's planes have escapement geometries that allow easy passage of the shavings and hence do not need cap irons to serve as deflectors.
Jeff,
Thanks for your thougths.
I read the article on your site a while ago and also about hollow grinding in Leonard Lee's book. Both are about chisle sharpening and have created the myth that hollow shapes are less supportive in all cases. The conclusions cannot be easily translated to bevel down planes.
Two methods that require work on the entire bevel to sharpen are a single bevel and hollow _honing_ which is like the Tormek system. For a given included angle the hollow honed shape wins hands down. It has a lower contact height on the frog.
Two methods that require work on less metal when re sharpening are the double bevel (primary/microbevel) and hollow grinding followed by flat honing (as described by Krenov). They have the same geometry at the tip. They can both have the same support angle because you can choose the primary bevel angle or the grinding wheel diameter. But the hollow ground/flat honed method leaves more metal behind the blade and has a lower contact height. So the hollow shape wins this comparison too!
But both comparisons are based on geometry and after that available equipment and the time necessary to make a quality cutting edge must be factored in.
How well does a blade with 12 1/2 degree primary perform in difficult woods?
I wasn't concerned with skitter in my statement but your article is interesting and I'll have to think more about the chatter issue. If the chatter is occuring in the very tip then it doesn't matter how thick the blade is. If the chatter is due to the length of the blade flexing and your plane is holding the blade the best it can then it does matter how thick the blade is. You measured the frequency of the resulting sound which is interesting. What we really need to know is the wavelength in the blade to determine where the chatter occurs. We could ask for some experience...
Has anyone ever remedied chatter by buying a thicker blade? But the two blades were probably different metals with different rigidities.
Thanks again. Your comments will help me word my thoughts more carefully.
Peter
Steve,
Which shapton stones do you use? Professional series? What grit do you start on after the 120x belt? How far do you go? 8000x, 15000x, 30000x?
Thanks, Peter
I do mean a secondary bevel. In general the vocabulary here seems to be a bit mixed up. My secondary bevel is very narrow and highly polished. So it is a microbevel. I'm not using a third bevel.
I use 120 grit zirconia on the makita then 1000 5000 and 12000. I have the hippo stones and they are the normal shapton's not the pro's. even with my use I show very little wear on them. I can go pretty far between flattening's with those big stones. but I would prefer the 8000 over the 12000 but they don't make that grit in a shapton hippo stone.
Steve,
I've been reading the old posts and it seems like you have tried a lot of sharpening systems and have sharpened a lot of blades. Sorry to be picking your brain but probably you are one of the most worthwhile people to ask about this stuff. If I have it right you use the following sharpening system
120 grit ziconia PSA disk on a horizontal flat lap Makita 9820-2. and follow that by 1000x, 5000x, 12000x Shapton Hippo stones.
stones. It seems like you do not and would not use diamond stones. Do you flatten the 1000x on something (???) and then the 5000x on the
1000x and then the 12000x on the 5000x? Do you flatten them all on the same thing? Or do you rub three stones together?Flattening waterstones is really the only reason I have stayed away from them even though I have some (low quality ones). I know that the Shapton's require less maintenance.
It is interesting to see what your system has evolved into.
Thanks, Peter
yes I have played a lot for sure (G) wore out so many stones it's not funny (G)
yes they go if you push it maybe 10 sharpening's between flattening's if you don't have to rework the edge. I get maybe 20 out of a hippo stone.
There are some pretty sophisticated models out there so I wouldn't be so certain that it is insufficient. The real question is whether it is worth modeling or analyzing. Either flat or hollow grind works well enough that I would not bother with trying to determine which is "better". I'm not about to buy a grinding machine to put a hollow grind on any of my tools. That said, my shovel DOES get a hollow grind when I touch it up with the grinder...
-j
J,
You are right, there are some sophisticated models. But you have to include everything. Like how much slip is there between the cap iron and the blade as the blade flexes. Maybe there is a little slip there. Including all the little things would be tough. Probably easier to build experimental equipment and just test the systems. Probably even easier just to stop worrying and sharpen up a blade and do some woodworking. I was just curious.
Peter
Steve,
Is how you flatten the stones a trade secret? :)
Peter
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