|
| Grinding and honing angles |
|
| Finest abrasives. | ||
| Microbevels front and back. | ||
| Use a jig. | ||
| Copyright (c) 2002-15, Brent Beach |
I use the same final included angle is almost all my tests so that the results are comparable. It does not mean that you should use the same angle for all planing situations.
My standard angles, using a 2" jig and slips that are 0.08 and 0.13" thick:
| Front | Back | |
|---|---|---|
| Primary | 25 | |
| First microbevel | 29 | 1.9 |
| Second microbevel | 30 | 3.1 |
| Third microbevel | 30.6 | 3.9 |
A general goal of sharpening is to use the smallest durable included angle. You can experiment with smaller angles than this, especially if you use more durable steels like M2 and D2. Elsewhere in the FAQ there is a discussion of steel composition and properties.
The primary bevel angle should be at least 4 degrees less than the first microbevel - otherwise the first microbevel gets too wide too soon, requiring more grinding. As the primary angle gets smaller, the primary bevel gets wider and takes longer to grind. The primary at 20 degrees is almost 25% wider than the primary at 25 degrees. The edge is also 25 percent thinner and hence less able to dissipate the heat generated during grinding. The longer you grind the more chance of burning the edge. As well, a smaller angle means a weaker tool. In my experience blade edge durability drops rapidly as the final included angle gets smaller.
A final back bevel angle of 10 degrees, or even 15 degrees with certain woods, will reduce or eliminate tearout. You can do this easily with my jig, but you will need a jig with a thicker back jaw. Use the extension calculator to find a suitable jig size.
For example, if you build a 2" jig with back jaw about 1/2" thick, you will have a final 9.5 degree back bevel. A 3/4" back jaw will produce a final back bevel angle of 13.1 degrees.
The total included angle will larger - 40.1 and 43.7 degrees in these two cases - so the plane will be duller. This could be a problem for tough or fibrous woods.
You can compensate for the large back bevel by using a smaller front bevel angle. For example, a jig with a 2" front jaw and a 1" back jaw with a blade extension of 5 6/8" would have a final front angle of 21.2 degrees and back angle if 11.1 degrees, assuming normal slips. The jig is illustrated in this measured sketchup model.
To get this geometry, you would need to begin with a front primary bevel angle of around 15 degrees. Grinding a 15 degree primary is more difficult that grinding a 25 degree primary. For one thing, the 15 degree bevel is over 60% wider than the 25 degree bevel. This means the iron will get hotter during grinding and will take 60% longer to grind.
Unlike the following method of getting a high planing angle using a back bevel, I would use only microbevels on the back of the iron - no grinding there, just honing.
To make it clear that no one living can lay claim to first discovering the back bevel as a solution to the problem of difficult woods, here is a drawing from a paper in Transactions of the Society of Arts, 1825, vol. xliii., p. 85. A bit more than the back micro bevel that I use - Mr. Williamson has full bevels on both sides of the iron. Williamson got 10 guineas from the society for his invention, which, with inflation, is worth about 686 UK Pounds today, or about $1,080 CDN. You may find it strange that a Society dedicated to Arts was awarding a prize for a woodworking tool. Back in those days, Arts included: agriculture, chemistry, polite arts (book making and printing, ...), mechanics (our guy), and economics and trade.
The geometry in this drawing: the bedding angle is 50 degrees, the lower bevel is angled at 30 degrees, the upper at 60 degrees, for an included angle of 30 degrees. So, the lower bevel is 20 degrees, the upper 10 degrees.
This invention is mentioned in Volume 2 of Holtzapffel - Turning and Mechanical Manipulation, which contains an extended section on plane operation. This diagram is taken from this reference, rather than the 1825 reference. Holtzapffel has two objections to the design. First, "it is obviously much more difficult to produce a true right-lined edge, by the meeting of two planes, each subject to error in sharpening, than when one exists permanently flat as in the broad surface of the blade." Second, you can get the same effect with a blade having a 30 degree bevel and a 60 degree bedding angle (or bevel up and a 30 degree bedding angle).
While Holtzapffel is generally pretty good, I dispute his claim that it is much more difficult to produce a "true right-lined edge. by the meeting of two planes". Of course, he thought you would need to grind bevels on both faces. In fact, you need only grind one face, and hone only microbevels on the other face.
Holtzapffel recommends making a new plane with a higher bedding angle. I recommend making a new jig with a larger back jaw. The jig option may be easier for many.
When planing end grain the key consideration is the cutting angle - the angle between the upper face of the blade at the edge and the wood surface. Here lower cutting angle reduces effort and improves surface quality.
With a 12 degree block plane you must use almost no back bevel. To get a 30 degree included angle, you will need to put all of that on the front. The resulting cutting angle will be around 42 degrees. When honing a blade with no back bevel, you must hone the front bevel using your coarsest abrasive until you have removed all the back wear, then continue as usual with the next two front microbevels. The additional front honing with the coarse grit will widen the first microbevel quickly. You will have to grind the front bevel almost as often as you hone to reduce honing time on the first microbevel.
With a 20 degree block plane you can reduce the cutting angle, while retaining a large enough included angle, by relying on the back bevel. By large enough included angle, I mean an included angle of around 32 degrees.
A final front microbevel angle of 25 degrees with a 7 degree back bevel (total 32 degrees) gives you a cutting angle of 45 degrees: 25 honed angle plus 20 degree bedding angle.
This model achieves as 42 degree planing angle, using a 22 degree final honing angle with a 10 degree back bevel. The model shows honing the first microbevel, no slips. This iron begins with a 16.5 degree primary bevel. Low primary bevel angles require care during grinding. The long bevel will overheat quickly. The thin metal at the edge cannot carry away much heat. Be very careful grinding such low angles, dipping in water every few seconds.
In certain situations different angles might be appropriate. Lee Valley sells an edge-trimming block plane - a plane with a built in 90 degree angle fence. The plane is used mostly for squaring up wood - planing the edges - before glue up. The blade that comes with this plane has a 20 degree bevel. The blade works quite well on clear softwoods sharpened at this angle. The edge is immediately destroyed planing end grain white oak.
Paring chisels - pushed by hand and used to remove fine shavings - can have very small included angles. I have paring chisels with the final included angle of less than 20 degrees. Such a chisel cuts very well but will dull quickly in hard or abrasive wood.
Mortise chisels require much larger angles. Not only are the used with a mallet, they are also often levered against the shoulder to remove chips deep in a mortise. If you intend to work quickly with a heavy mallet in hard woods a final include angle of 35 degrees may be best.
The standard solution is to regrind the blade back past the pitting, then go through the usual steps of grinding the primary followed by honing.
You can often remove all the pitting in a minute or less if you have the hand grinding setup I use. The page on hand grinding includes a discussion of such a repair. The blade used in a grinding demonstration had a badly pitted back. The pits were removed in a few moments using a coarse Silicon Carbide stone with a standard back bevel setup.
I do this myself almost all the time when face planing, even if I am not getting tearout.
Skewing the plane has several effects, most of which would seem to have no reason the reduce tearout.
First, because the blade edge is at an angle to the direction of motion, the blade takes a narrower shaving. If the blade is set for a full width shaving, then the width of the shaving taken by the skewed plane is decreased by a factor of COS(S). [The cosine of skew angle S. If you do not know trigonometry, but are interested, a cosine is a number between 1 and 0, near 1 for small angles, near 0 for angles near 90 degrees.] Taking a narrower shaving means less work for each shaving. It also means more passes along the board.
Second, because the plane itself is skewed, it behaves like a wider and longer plane. The plane in the model on the right is skewed 15 degrees. The plane acts as if it is only slightly longer - 9 5/16" rather than 9". This greater effective length means the plane will produce a slightly flatter board, lengthwise. The effect on its effective width is much greater - 4 3/4" rather than 2 1/2". This greater effective width means the plane will produce a flatter board, width wise.
Third, skewing the blade changes the way the shaving moves up the front of the blade. When we think of tearout, we think of an unbroken shaving moving up the front of the blade and causing the wood fibre to rip below the surface. When we skew the blade, the same length of wood shaving does not rise as high before the edge reaches the base of the fibre and cuts it.
This final effect is so significant that many planes are built with the blade skewed to the body. Skewed blades are found on a variety of specialty planes - both wooden and metal. Stanley made a number of metal dado and plough planes with skewed blades, as well as the #95 Edge-trimming block plane and the #140 Rabbet and Block plane. Many manufacturers of wooden planes put skewed blades into Rabbet, Dado, and Fillister planes as well as bench planes. You cannot skew dado plane or an edge trimming block plane to get the skew effect - you must skew the blade in the body of the plane.
How much does skewing the blade to the direction of travel change the effective planing angle? I will present two derivations of a formula for the effective planing angle as a function of the original planing angle N and the skew angle S. [The original planing angle N is the bedding angle of the blade plus the angle of the upper surface near the edge. It includes any back bevel angle on a bevel down plane iron.]
Derivation 1. Thinking in terms of how far the shaving moves up the face of the blade.
|
Prompted by a visitor, Tony Yeates, I corrected the above derivation and looked around the net for other derivations. I found another and compared its result with my corrected result and found a difference. To explain the difference, I have added the sketchup model on the right.
The second expression for the effective angle in terms of the original angle and the skew angle is
| ATAN(TAN(N) * COS(S)) | From the website Investigations |
These formulas have similar appearance, but produce slightly different results. Why is that?
This small model shows the geometry. It is a model of a plane iron, with planing angle 45 degrees, resting on a flecked grey surface. The triangle on the left with the 45 degree angle at the corner shows the line of motion for skew of 0 degrees. The second triangle shows the line of motion for a skew of 60 degrees.
The angle between these lines of motion, as seen on the work surface, is 60 degrees. If we project these surface lines up to the upper blade surface, the angle is seen to be 50.8 degrees.
How can this be? If we compare the triangle formed by the skew lines on the upper blade surface to the triangle on the work surface, the side opposite the corner is the same length, but the side on the skew=0 line is longer. It is longer by a factor of 1/COS(45). So, the angle between the skew lines on the upper blade surface is ATAN(TAN(60)*COS(45)).
This second sketchup model adds a line, in the upper blade surface, that is 60 degrees from the 0 skew line. This is the line the shaving should follow if the original formula is correct.
Should we replace the angle S in the original formula with this adjusted skew angle? Should we use the second formula, which uses TAN rather than SIN?
What this comes down to is the question: Does the wood fibre follow the nominal skew line across the upper blade surface (original formula correct), or does it follow the projection of the line on the work (second formula correct).
If the latter, then the shaving climbs higher up the blade than we expect. Is this a plausible outcome? Given that the blade surface is pushing the shaving off to the side, would we expect the shaving, having been cut by the edge, to rise rather than fall?
There are three possibilities. Having been severed from the work, the shaving
Anyone prepared to test what happens to the shaving? What path does it follow, as it moves up the blade?
Finding the answer won't change how skewing affects tearout! It will advance the sum total of knowledge we have about skewing. It might resolve the question about which formula is correct. It might suggest a new formula. It might even suggest a reason why skewing reduces tearout.
Taking pictures of shavings as they climb the blade for various skew angles is now on my to do list, unless someone else does it first. This may justify my purchase of a chariot plane (or at least the making of one).
So, what does this mean in practice? With a standard bench plane where the planing angle is 45 degrees (more or less), here are the effective angles for various skew angles. (This table uses the first formula.)
| Skew | Effective angle |
|---|---|
| 10 | 44.1 |
| 15 | 43.1 |
| 20 | 41.6 |
| 25 | 39.9 |
| 30 | 37.8 |
| 35 | 35.4 |
| 40 | 32.8 |
| 45 | 30.0 |
| 50 | 27.0 |
| 55 | 23.9 |
| 60 | 20.7 |
In fact, as the blade moves forward through a wood fibre, it also moves sideways. That is, you get both a push cut motion and a slicing motion. The push tends to crush the fibre without necessarily cutting the fibre's outer layer. The slicing motion does cut the fibre's outer layer. Once the the fibre's outer layer is cut, the fibre can crumble. Once the fibre crumbles, the ability to transmit the pushing force down the fibre below the surface is lost. Tearout cannot occur.
Of course, tearout still does occur. That is presumably because not all fibres have their outer layer sliced through. In some cases, for one reason or another, the outer layer is not cut. The pushing force builds without the fibre breaking. The fibre is lifted out of the wood below the surface and tearout has occurred.
A ramped shooting board, one in which the ramp on which the plane slides is angled with respect to the surface on which the board rests, gives the illusion of skew.
The position and slope of the ramp is determined by the largest piece of wood you expect to shoot. Measure the distance of the blade edge right corner to the plane right side. The ramp should be a little more than this down from the working surface when it first hits the work - for the widest piece of wood you plan to use on the shooting board. At the fence, the top of the blade should be just above the top of the work. Measure the distance from the blade edge left corner to the plane right side. The ramp should be just a little less than this down from the top of the work at the fence.
If you are working 3/4 stock that is at most 5" wide, then the ramp would then drop 1 1/4" over the width of the 5" board, assuming a 2" blade. This means a 14 degree slope. This sketchup model shows a wooden plane with a 1 7/8" blade on a ramp designed to handle 5" wide, 3/4" thick stock. That is, if you were shooting such a board you would use the full width of the blade. Notice though that you do not make equal use of the edge - the outer bits still don't get a lot of use compared to the middle. Still, it is better than having all the wear in the middle of the blade.
You can do skewed shooting but you need to add a wedge to the plane body. For example, if you wanted to use a skew angle of 30 degrees, you would make a 30 degree wedge the length of your plane and adhere it to the right hand side of your plane (assuming the plane would run on its right side when shooting), sharp end to the back of the plane. If you slide the plane on the ramp, the blade edge is now skewed to the direction of motion by the wedge angle. You would have to drop the ramp by the thickness of the wedge at the blade. You could still angle the ramp to use more of the blade, again setting the ramp depth based on widest expected stock.
Previous page of the FAQ - The geometry of Honing
Next page of the FAQ - Back bevels and microbevels
Return to the Sharpening page.
Return to the Sharpening and testing home page.
Lost? Try looking around the site map. You can also reach the site map from the little map at the top of each page.
Questions? Comments?
You can email me here.
Back to the top.