Plane Articles

Design and tuning considerations for bench planes
by Bill Clark & Larry Williams

Grain orientation and shape are critical design considerations in wooden planes. The best example of this is probably the coffin bodied smoother. Simplistically it seems shaped to fit the hand and there are a number of myths about the reason for its grain orientation.

We feel that the shape and grain orientation of a traditional coffin bodied smoothing plane evolved primarily for two basic reasons. The iron must be firmly forced against the bed by the wedge and the toe section of the sole must remain in the same plane with the heel section of the sole through the usual seasonal humidity changes.

It must be understood that wood is a relatively plastic material. Moisture moves through wood at a slower rate than through the air. This means that the surface of a piece of wood will generally have a different moisture content than its interior. Wood also expands and contracts according to its moisture content. The wood is usually under some stress because of this and the surface is actually deformed very slightly by this stress. This isn't a problem as long as the differential moisture content isn't too severe. We're all aware of the need to season green wood for this very reason. The thickness of the wood is an issue here...the thicker it is, the longer it takes for moisture to move through it.

We also know that water migrates through wood fastest through the length of the wood's cells. Most cells are oriented with the height of a tree and that's why we coat the end grain with an impermeable substance during seasoning. This slows moisture transfer with the air from the ends of these cells and limits the stress caused by uneven drying.

There are other cells that most woodworkers, incorrectly, don't consider involved in this moisture exchange. The cells of rays are aligned radially in length. In beech, the traditional plane material, as much as forty percent of the surface of flat sawn-wood (exposing the tangential surface) can be made up of rays. These cells also effectively move moisture.

The other property of wood involved here is dimensional stability. I'm sure most everyone likely to read this is aware that, in beech, quarter-sawn wood (exposing the radial surface) will move about one half that of flat sawn wood with humidity change. The most stable dimension is along the wood's length but strength properties limit the available uses of this.

Because of this difference and the fact that we want that wedge aligned with the most stable dimension available we want the sides quarter-sawn. This greatly limits the relative angle changes of the bed and abutments when ambient humidity changes. This helps keep the iron solidly bedded. This is also the main reason for using quarter-sawn lumber for molding planes.

If the plane billet had the annular rings running diagonally, the dimensional differential between the radial and tangential wood would cause the plane to lean with humidity changes. It would take little change to cause problems keeping the edge of the iron parallel with the sole.

One of the myths of grain orientation is that the bark side of the billet is the most wear resistant. We believe that the main reason for this is that the quarter-sawn sides are needed for the wedge stability leaving a choice of the other two surfaces. These are flat-sawn and prone to cupping. The surface closest the annular rings with the greatest radius would tend to cup less than the surface close to annular rings with the smallest radius. Remember the plasticity of wood? You'll see that these surfaces do move independently of each other when you tune a wooden smooth plane in the fall.

Wear in wooden planes really isn't the issue so many have made it out to be. If it was a major problem the quarter sawn surface would be on the sole. Look at any floor or stair tread and you'll see that the closer together the bands of early wood and late wood are the less wear has occurred.

Another myth among tool collectors is that the grain orientation along the sides of the plane was set to avoid catching loose grain. Grain ideally should, on the side, run down from the toe to the heel. This isn't to avoid catching any grain; boxing in molding planes has traditionally been run with the grain on a bias so that it would do exactly that. Rather, this is to avoid short grain at the mouth that would make forming that mouth risky for the plane maker and subject to breaking away during use.

There's another thing happening with bench planes. The body of a bench plane is divided into three sections. The toe and heel sections are solid wood and the escapement has only air and thin sides or cheeks that have a triangular shape. These thin cheeks absorb or lose moisture faster than the solid thick sections. As they expand and contract more than the adjacent solid sides, they force the planes of the soles of the heel and toe out of alignment. This is taken care of on the traditional smooth plane by the coffin shape which exposes the end grain of the sides. The sides of the heel and toe thus are able to maintain a similar moisture content as the cheeks. We prefer the 18th Century style longer planes because the height of their bodies is less and it limits this effect.

The toe and heel sections, on a coffin shaped smoothing plane, are kept as short as possible. The heel, bed, breast and toe are all end grain and moisture moves relatively freely to and from them. The center of the mass of the heel and toe sections, because of the angled breast and bed, will be low and relatively close to the sole. The moisture carrying capability of the rays comes into play here and helps keep the center areas of these sections in equilibrium.

During drying times the plasticity of the wood shows up as a hump in the heel section of the sole and a smaller one in the sole of the toe. At the beginning of wet seasons, these locations will be slightly concave. The concavity rarely has much effect on the plane's performance. The humps, however, impair its use or keep it from functioning at all. This is when the user must tune the plane.

Tuning is simply carefully planing, scraping or lapping away these humps. Care should be taken to never remove more than necessary. High spots on the sole of light colored wood can be marked by running the plane over carbon paper on plate glass or a surface plate. A fresh newspaper can be used instead of carbon paper but the marked areas will be lighter. A light dusting of the glass with blue chalk can be used for planes of dark wood.

We think that what has usually been attributed to wear was actually the result of seasonal tuning. Evidence suggests to us that many early carpenters and joiners were too aggressive in this tuning and far more of the sole was removed than necessary.

During high humidity times early in a plane's usable life, the concave areas may need to be dressed in this manner. It's also possible that one may have to adjust the fit of the wedge once or very rarely twice on a new plane. This can be done with a scraper, chisel or plane making float.

A properly constructed and tuned wooden plane should have all the features of a great plane. Firm bedding of a relatively thick iron, a true sole and a mouth that's tight enough for its intended purpose. Wooden planes also offer a wider variety of bed angles and this makes them ideal for many difficult woods because the cutting action can be matched to the wood. Properly sharpened and in the hands of a user who has mastered the basic woodworking techniques, there's no reason wooden planes won't perform as well or better than any metal plane.

Any plane, metal or wood, is essentially a high maintenance tool. Wooden planes are treasured for the feed back they give the user during use. The factors that effect a wood-bodied plane's performance are exactly the same ones that are changing the material these tools are working on at any given point. A competent craftsman can learn to read the subtle changes in his wooden plane's performance and understand what's happening to the material for his current project. Working with a high degree of accuracy requires this knowledge and a wooden plane's capability of providing it is rarely discussed.