Securing the blade at any given position in a router plane is a simple but critical task the tool must perform. It should be easy, fast, and require no tools as it will be adjusted often. It also needs to hold the blade securely so it doesn’t shift during use. To accomplish these goals a few basic elements of the tool need to be understood. As far as I know, all metal-bodied router planes secure their blades using one of two methods: either using a thumb screw to tighten down a blade-locking collar (typically on larger planes) or by driving a screw directly against the blade shank itself to clamp it against the body of the tool (common on smaller scale planes). Note that wooden-bodied routers often use a wedge to lock the blade.
Next is the geometry of the blade shank itself: round, diamond, and square.
A round shank allows the blade to be positioned at any angle (which is rarely, if ever, necessary), but it can rotate unexpectedly during use which is completely undesirable. It’s the cheapest method of manufacturing, however, as it requires only a simple hole in the body with a screw running into the side of the shank to lock it down. And while this isn’t typically seen on larger tools which see much higher cutting forces in use, it does appear on many small scale router planes where the reduced force is usually not a problem and the blade won’t spin in the body… much. If you are having trouble with a round shank that spins, scuff up the sides of the shank along its length with course sandpaper, that will typically do the trick.
The Diamond shank (where the shank face is rotated 45 deg to the cutting edge) is the most common configuration for large router planes for two reasons. First, the non-roundness of the body means it won’t rotate during use. Second, the diamond, which gets drawn into a V-notch in the body, is self centering and self aligning. It can’t rotate, tilt, or shift side to side.
The only downside is that when the collar is loosened so the depth of cut can be adjusted, the collar tends to fall down the body, sometimes binding on the blade making the adjustment a bit of a headache. Modern manufacturers have resolved this in two ways. Veritas uses a spring-loaded collar so that while the clamping pressure is removed during depth adjustments, there is enough pressure to hold the collar where it belongs and it functions very well. Lie-Nielsen did away with the collar entirely, opting to apply pressure to the shank directly with a brass screw which again, works perfectly. Preston fixed their collar problem by trapping the collar in position with a locating pin which is incorporated into the collar locking screw itself.
The Square configuration (where the shank face is parallel to the cutting edge) is rare. Lie-Nielsen uses it, but they drive a screw against the edge of the shank, not its face. This pushes the blade into the back corner of the body, essentially clamping it against a V-notch just like the diamond shank blades. Preston, however, typically used a square shank with a collar that simply pulls the shank tight against its back face.
The problem with the Preston method is there must be clearance between the side faces of the shank and the notch in the body. As a result, nothing constrains the shank except the friction between the shank and body which is produced by the collar. During heavy cuts, the blade can shift laterally or tilt slightly, neither of which is acceptable. For our design, we’re using the preferred Diamond configuration, but we are going to utilize Preston’s clever pin locator on the collar screw to keep the collar in position when loosened.
Next time, blade positioning. It’s exciting stuff…