|
| More Scrub Irons |
|
| Finest abrasives. | ||
| Microbevels front and back. | ||
| Use a jig. | ||
| Copyright (c) 2002-09, Brent Beach |
Introduction
This page includes a couple of other issues with cambered blades:
Bevel up cambered blades
In the fall of 2007, largely in response to an item by Derek Cohen on cambering bevel up planes, the general topic of camber on bevel up irons was discussed on a number of woodworking forums. In all cases the discussions continued on for hundreds of contributions. There was a lot of heat generated, and just a little light. The discussion on the UK Workshop Hand Tools Forum contained many good ideas. The discussion on the Fine Wood Working Knots Forum added a few details, but a lot of sparks. The following is the condensed version of what I learned from those discussion.Cambered blade preparation
When sharpening a blade with a particular camber you proceed in exactly the same way, independent of whether the blade will be used in a bevel up or bevel down plane. We will see that you need larger cambers with low angle planes for the same effect. However, once you decide on a camber, you grind and hone that camber without regard to the final use of the blade.Derek's use of a template clamped to the blade is an attempt to solve a problem that does not arise if you use an angled tool rest. Derek uses a round bar as a tool rest (as do the Tormek and Jet sharpeners and several others). If you do not have an angled tool rest then you must clamp something to the iron at a known extension from the edge to get a desired angle. Even then, it is only the starting angle. You can continue to grind as the angle increases to 90 degrees and the system will not stop you. If you use this type of setup, you must scribe the desired edge profile on the back of the blade, set the extension from that line, then grind down to the line. The solution is more complex than a simple tool rest at the desired angle.
From the point of view of blade preparation then, there is no reason to prefer bevel down to bevel up style planes.
Geometry matters
Geometry plays a large role in this discussion. The geometry describes how the blade moves along the bed as you set the depth of cut. With plane iron bedded at 45 degrees (a bevel down bench plane), for each 0.001" you advance the blade along the bed, the middle of the edge drops 0.00071" (sin(45)). For a plane iron bedded at 12 degrees (a low angle bevel up plane), for each 0.001" you advance the blade, the middle of the edge drops 0.00021" (sin(12)).This difference in geometry means that you have to advance the blade in a low angle bevel up plane 3.4 times as far along the bed as a blade in a standard bench plane bedded at 45 degrees. The factor for a bevel up blade bedded at 20 degrees is 2.1.
The need to advance the blade so much more for a given increase in set is one reason that bevel up planes usually have adjustable mouths. Increasing the depth of cut by 0.001" means advancing the blade by 0.0034". This would close the mouth on some infill planes.
Aside from the need for an adjustable mouth, are there any other implications?
Typical Camber - bevel up Jack
Consider the idea of a low angle bevel up jack plane. That is, a low angle bevel up plane that we want to perform the functions for which we would normally use a jack. If we normally use a jack plane with a 1/16" camber, to get the same effect our bevel up iron would need a 3.4/16" camber on your iron (0.21" instead of 0.006"). The corresponding radius of curvature would be 2.5" (rather than 8").Typical Camber - bevel up Scrub
How about a low angle bevel up scrub plane that worked about the same as a bevel down scrub plane. Using Leonard Lee's camber of 3/16 for a 45 degree bed, you would need 10.2/16" camber on your on your blade. This corresponds to a radius of curvature (assuming a 1.5" wide blade) of 0.76". That is, the edge almost forms a half circle! Notice that if the adjustable mouth was set just in front of the middle of the blade, the opening at the side would be almost 3/4". This would indeed look strange. I suspect you might have some trouble with this camber.A bevel up blade with extreme camber has an interesting additional feature -- near the sides it looks to the wood like a skewed blade. At the middle of the blade the shaving moves straight up the bevel, so there is no skewing. Near the side of this highly cambered blade, the edge is almost perpendicular to the direction of motion. The shaving does not travel up the bevel, it travels along the bevel, the equivalent of a skew angle of almost 90 degrees. Near the edges then, the blade appears have an effective cutting angle that nears the bedding angle. This might be quite interesting in figured woods (tearout in the middle, smooth at the sides).
Discussion
Most woodworkers meet with extreme cambers like this only with bevel down moulding planes. I doubt if many have used a bench plane with such extreme cambers. It is well known that with moulding planes the blade cannot cut more than a 60 degree arc. For bevel down blades cutting a larger arc, the action near the blade corner is a scraping action, not a cutting action. The result is a poor surface quality, rapid edge wear. So, extreme cambers are bad.This general caution may well not carry over to low angle bevel up planes used for scrubbing. First, these planes are not finish planes, so the finish quality problem does not arise. Second, the fact that the bevel faces up means that there is a skewing effect on the sides which eliminates the scraping action. So, experience with extreme camber and standard bench planes may not apply on low angle bevel up planes.
Having said that, I still don't think many people would actually use a low angle bevel up jack plane with such an extreme camber to duplicate the scrub plane effect. However, Derek has used moderately cambered blades and gotten good results.
Two additional factors may differ between the bevel down standard and the bevel up option. First, planing effort. Does the more extreme camber for the same effect result in more or less effort? Second, edge durability. Does the more extreme camber for the same effect result in better or worse edge durability. I have seen nothing that informs either point.
Planes with Cambered Soles
In all the examples above, the sharpener is free to select the shape of the edge (the amount of camber) before starting. The plane soles are all flat, the blade extends a variable distance from the sole across the blade.
There are many planes with cambered soles and cambered blades - moulding planes, in particular. Here the sharpener must produce an edge that matches the shape of the plane sole. No room for user choice here at all.
The first option in this case is to avoid honing the edge at all! If you flatten the back of the iron you are removing some of the roundness of the edge.
This is my standard drawing of side profile of a worn blade. The black outer lines are the profile after sharpening, before use. Use wears the blade on the upper and lower faces, with the worn blade having the profile shown by the red lines. You can expect more wear in the middle of a cambered blade than near the edges because the middle is removing larger shavings, is getting hotter because it is pushing against thicker stock, and is wearing faster. In spite of this, the blade front profile will still match the plane sole profile pretty well.
If you hone just on the back of the blade, with metal removal shown by the upper blue line, you improve the side profile, without changing the front profile. The blade is not really sharp, but it is not a dull either. You can also grind the front bevel on a belt sander, grinding the primary bevel almost up to the edge. The new lower bevel is shown by the lower blue line. If you do not grind through the dull edge, you retain the edge front profile while improving the blade sharpness.
If you have a plane you use a lot and want to put a really good edge on it, you can build a honing appliance like the FID discussed earlier. There is a bit of geometry involved in figuring out the shape FID based on your blade camber, and hence the shape of the blade you will have to use to create that FID. Send me an email if you have such a blade and would like to build the appropriate honing FID.
Navigation
- Introduction. The scope of the problem.
- How much camber do you want? It depends on the shaving thickness you intend to take.
- Shaping a Radiused Blade. How to create the radiused edge using a belt sander.
- Grind and Hone - belt sander.
- Grind and Hone - abrasives on glass.
- Hone - using a convex hone
- Bevel up cambered blades. More camber required!
- Planes with cambered soles. Much harder.
Return to the Nitty-Gritty page.
Return to the Sharpening home page.