Learning Curves

Plane blades that are sharpened straight and square are essential in all shoulder and rabbet planes, and they have other applications such as when shooting an edge with a bench plane. However, I have a very strong preference for using cambered edges in most of my bench planes, most of the time.

There are two powerful arguments for using a slightly curved blade. The first has to do with perfecting the square edges of your work. Let us suppose that you are preparing the edge joints for a tabletop and your powered jointer’s fence was a few degrees off square and that you wish to correct the errors of squareness in the edge of your timber.

I have no magic built-in spirit level, which would allow me to plane a perfectly square edge with a straight blade in my plane, and I have no idea how this could be done. The curved blade is a sophisticated device, which allows us to take three different kinds of shavings without having to adjust the lateral-adjustment lever at all.

1. If the plane is centered over the edge of the wood, then an even-thickness shaving will be removed and no change will be made to the angle of the edge.

2. If the center of the plane is moved so that it is over the left-hand side of the edge, a tapered shaving will be removed and the left-hand side of the edge then will be lowered.

3. If the center of the plane is moved so that it is over the right-hand side of the edge, a tapered shaving will be removed and the right-hand side of the edge then will be lowered.

In all these positions the plane is kept completely flat against the edge by firm pressure from the ball of the thumb on the plane body casting. I do not use the front knob at all. The thumb is always positioned over the center of the edge, regardless of where the center of the plane is. I use the surface of my fingernails as a fence to keep the plane from wandering about during the length of the stroke. The grip on the back handle is gentle, so that we do not twist the plane sole out of contact with the edge. (Editor’s note: You can find more details of this technique in Charlesworth’s second book and second DVD; see the Supplies box for details.)

Finally, by carefully allowing the plane to drift from side to side towards the high points during the cut you can also correct a twisted edge. This surely has to be the most cunning plan of all time. Of course the curved blade needs to be well centered in the plane for all of these techniques and I will be describing my method later in this article.

The second powerful argument for a curved blade is that you will not leave whiskery tracks (sometimes called "steps") from the corner of the blade when you plane across a wide surface. The surfaces you leave will be minutely scalloped in their width, but the depth of these depressions is small indeed and they can be removed easily by sanding if desired. I will explain my method for taking a set of shavings from a surface in a future article.

Blade Angles
I like to grind my bench plane blades at 23°, which is a little lower than tradition suggests. All sharpening and shaping is then done at 33° on a coarse waterstone, such as an #800- or #1,000-grit stone. This is 3° steeper than usual. I then polish the extreme tip of the narrow bevel on an #8,000-grit waterstone at 35°.

Because there is a significant difference between the grinding angle and the angle formed on the #800-grit stone, very little metal needs to be removed for either sharpening or shaping a blade. Shaping a curve does require more metal removal than normal sharpening, but much less than if a single bevel were used.

Forming a Curve
I take a freshly ground blade and set it at 33° in an Eclipse-type side-clamping honing guide (see the Supplies box for information on purchasing one). The information for blade projection cast on the side of the guide will not mention 33° so you will have to experiment to find the necessary distance. I draw the angles I want on white card or melamine-faced scrap. The angle is checked by sighting across the blade with the card propped behind. Once I have found the correct projection for a blade I measure the distance the cutting edge projects from the honing guide and scribe this measurement directly onto the top of the blade so that it’s always available. This saves time in the future. It is also worth noting that different thickness blades will require slightly different projections to achieve the same honing angle.

I prefer the Eclipse-type guide because it has a narrow roller. Jigs with wide rollers tend to dictate to the user. We need some lateral tilting to form a curve, and the narrow roller is easier to tilt than a wide one. I have an old honing guide with a barrel shaped roller, indicating that creating curved blades is not a new idea.

Expert sharpeners produce a curve on a flat stone by subtly applying more pressure to the outer corners of the blade while honing to and fro. I have broken this process down into simpler stages so that the beginner can succeed from the start. I noticed that many students were struggling to start a curve with my old method, which depended on point pressure only. The new technique was developed during a short course a couple of years ago.

The idea is that a 5/8"-wide strip of thin plastic is laid along one long edge of the #800-grit stone. This tilts the blade slightly; and as pull strokes are made, metal will be removed from the opposite corner of the blade. Finger pressure is placed over the area of blade that is touching the stone. Some woodworkers have told me that the plastic sleeve that comes with a cheap 6" ruler is ideal for this job. I use strips of plastic from the covers of cheap ring files; we originally used cardboard, but it did not last long. I think precise thickness is not important, but something around .012" to .015" will do.

If you coat the grinding bevel with a permanent marker before starting, you will see that an elongated triangle of metal has been removed. It is difficult to specify how many pull strokes will be needed. I press fairly hard on coarse stones and would expect to achieve a result after about a dozen strokes.

The plastic is then placed on the opposite long edge of the stone and the process is repeated on the opposite corner of the blade. Try to produce a symmetrical result. We now have a blade with three straight facets, whose shape is an approximation of a curve.

To judge the shape, I offer the edge up to a piece of flat plastic. I would expect to see gaps of about .012" at either outer edge. This is approximately the thickness of a quality business card.

I now use point pressure about halfway from the center to the corner of the blade by stacking my forefingers on top of each other. The two positions for this pressure are indicated as positions "4" and "5" on the diagram below. This operation is done without the plastic strip. It’s important that weight is kept off the jig itself and only applied to the blade, near the edge. The idea is to create new, smaller flats at the meeting points of the three previous flats. Fewer strokes will be required at these points, possibly six. A certain amount of balance and judgment will be required here. However we do get good feedback from the surface of the stones, and it is usually possible to see a track on the surface where the blade is touching.

The blade edge and shape can now be examined again against the flat plastic. If you have succeeded it will appear as a gentle symmetrical curve. If it is not, just take more strokes, with the finger pressure in the appropriate position. You will see that I have indicated five possible finger positions for a 2-3/8" plane blade.

When satisfied, clean the blade and the roller of the guide to avoid contaminating the superfine #8,000 grit stone, which is used next. This stone is prepared by spraying the surface with a little water from a plant mister. A Nagura is then rubbed over the surface to produce a little muddy slurry or paste. The blade projection is shortened slightly in the jig, to raise the blade angle to 35°. Four gentle strokes are then made with the finger pressure in each of the five positions shown in the illustration. This polishes the tip of the coarse stone bevel, and is all that is required for a razor edge.

Sometimes, mostly for fun, I take a couple of extra strokes on the polishing stone. During a pull stroke, I start with finger pressure on the right, and try to transfer it steadily to the left. My forefingers are on the two outer edges of the blade. If done well, this will draw an elongated x on the surface of the stone. This is just a fun way of trying to smooth those five facets into an even smoother curve. It is not necessary, but a good exercise in controlling finger pressure. (It seems to go better when accompanied by a sound effect, such as a rising or diminishing hum!)

The wire edge created on the unbeveled face of the blade is then polished off in the usual way, using the ruler trick on the #8,000-grit stone. (For more on this procedure, see "The Ruler Trick" in the November 2004 issue.) I always dry the blade and coat it with a thin smear of camellia oil, as this protects against rust. The whole process is much quicker to do than it is to describe.

Different Curves For Different Work
When planing relatively narrow edges, say about 1/2" to 5/8" thick, I find I need a pronounced curve on the blade.

When planing a wide surface such as a tabletop, I use less curve. The gaps seen at the edges of the blade, when it is offered up to a flat surface might be around .006", roughly half as much as you would employ for edge planing. If you have a very shallow curve in your blade, the plane would need moving a very long way to the right or left, to have any squaring effect at all. When the plane is moved this far off center, it can be quite difficult to keep it balanced flat on the narrow edge.

Resharpening A Curved Blade
When the blade dulls, it is likely to be worst in the center. I set it in the jig at 33° and go to the #800-grit stone. Using point finger pressure in the center, I find how many strokes it takes to produce a minute wire edge. The small wire edge is a signal that enough metal has been removed to get past the wear on the blade. The same number of pull strokes are then performed with the stacked finger pressure in all the remaining four positions i.e. halfway to each edge, and just inside each edge of the blade. This will maintain the existing curved shape. If you wish to change the shape, more strokes are used in the appropriate positions.

After cleaning the tool and roller of the honing guide, adjust the projection to give 35°, and do four gentle polishing strokes on the superfine stone in all the finger positions. Polish the wire edge off using the ruler trick.

With each sharpening, it will take more strokes to produce a wire edge. This is because the #800-grit bevel gets wider with each sharpening, and you are honing a larger area of steel. After about seven sharpenings, I regrind the blade and start the cycle again.

When grinding, I never go right to the edge of the blade. A small sliver of the #800-grit bevel is left at the tip, as this contains the shape that we have worked so hard to produce. Grinding right to the edge shortens the life of a plane blade, and it’s not necessary unless you have a large chip in the cutting edge. Sharpening a recently ground blade takes me about four minutes.

Setting up the Plane
I use a No. 5-1/2 bench plane for the majority of my work because I like the weight and the length. It is tuned up as a super smoother, with the sole lapped flat and the mouth set very fine, about .004" wide. (I recommend 1/32" for beginners.) This plane is used to perfect the accuracy and finish of the surfaces that come from my machine planer. I do not take heavy shavings, so the chipbreaker is set very close to the edge of the blade. I prepare the front edge of my chipbreaker, with a slight camber as well, which allows me to set it as close as 1/64′ to the blade edge. The connecting screw needs to be tightened very firmly.

When placing the blade and chipbreaker assembly into the plane, I hold the plane in my left hand with the frog’s surface horizontal, having brushed away any loose shavings or dust. This prevents the blade from sliding down the frog and colliding with the front of the throat. This would blunt the blade before we even get started. It is not a bad idea to retract the blade adjuster wheel by a couple of turns, too.

It is easy to see that when the blade-adjustment dog is engaging the slot in the chipbreaker, but care is needed to ensure that the lateral adjusting disc is engaged in the plane’s blade slot. I wiggle the lateral lever a few times to see that it is so, and try to set the lever in approximately the right position. The lever cap is now installed, and everything held firm with my left-hand thumb, while the lever cam is closed. I now advance the blade with the wheel, watching from the top, to see that the blade edge is not crooked enough to bang into the front edge of the throat.

Setting the Plane for a Fine Finishing Shaving
The plane heel is now placed on a sheet of well-lit white paper on the bench. Lighting the blade is not helpful. I hold the front knob with my left hand and sight down the sole. The right hand is available to advance the blade and adjust the lateral lever. Start with enough blade projection so that you can see it clearly, and adjust the lateral lever for a "balanced shaving." The projecting blade will appear as a black shape against the white background. I now retract the blade, just until nothing shows.

A Setting Trick
The setting can be confirmed with a small piece of thin wood. I use a close-grained hardwood, about 1/16" thick, by 1" wide, by 1-1/2" long. The long edge is held firmly down to the front sole and moved backwards over the throat, as if trying to take a shaving off its whole length.

I now advance the blade, very slowly, until the shim is just shaved by the center of the curved blade. You will encounter a phenomenon called backlash when you change from retracting the blade to advancing it. The wheel will turn freely for a while before you feel resistance. The blade will not advance at all while this happens. Once resistance is felt, the blade will begin to move. This "dead spot" is caused by play in the adjustment mechanism. It can be as much as two whole turns on an old worn plane. Less backlash signifies a high quality mechanism. All mechanical systems have some, and as long as you always set the blade while advancing the blade’s projection, it will not trouble you. Conversely, if you set the proper projection while retracting the blade, you are likely to lose all blade projection as the blade works its way back into the body.

When winding the blade out, I turn the wheel as little as "three minutes on the clock," — that’s an old-fashioned analog clock! — at a time. We only want a few thousandths of an inch projection for fine work, and it is easy to go too far, and then you have to start the whole process again.

This is where the setting shim is so useful. You can judge shaving thickness by feel. You can also confirm that the blade projection is well balanced.

To do this, rub the long edge of the shim to and fro, as if taking full-length shavings from its edge. Start at the outer edge of the throat and progress towards the center of the plane, and you will be able to hear, feel and see exactly where the curved blade edge first protrudes. With luck a small shaving will stay wrapped round the blade. Now repeat from the other edge of the throat. This will confirm whether the curved blade is well centered in the plane. Some woodworkers perfect the final lateral adjustments by tapping the top of the blade with a small hammer. This may be easier than trying to make minuscule adjustments to the lateral lever.

I don’t worry if the balancing is not perfect, as long as I know where the blade is! My final move is to take test shavings off the edge of a practice board. These can be measured with dial calipers to assess their thickness.

Suitable Shaving Thickness
For final finishing of difficult hardwood I take a .001"-thick shaving. For general cleaning of a machined board, I take a .002" shaving. In hardwood it is difficult to push a plane that is cutting much more than .004" to .006".

I do hope that this article helps you to get the most out of your bench planes. They are one of the most wonderful, versatile and accurate tools in your kit. PW

Supplies
Side-clamp honing guides are available from most woodworking catalogs, including:

Woodcraft
800-225-1153 or woodcraft.com

Lee Valley Tools
800-871-8158 or leevalley.com

Tools for Working Wood
800-426-4613 or toolsforworkingwood.com

David Charlesworth’s three DVDs on sharpening, planing and shooting boards are available for $25 each or the set for $70. His two books, "Furniture-Making Techniques" Vol. I and II are $18 each.

Lie-Nielsen Toolworks
800-327-2520 or lie-nielsen.com

Prices correct at time of publication.

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