Using jointing attachment on moulder

Proper methods for jointing knives on a moulder, and myths concerning their sharpening. February 28, 2001

What is the right way to use a jointer bar for my Mattison 226 Moulder? Will I be able to sharpen the knives through jointing? This tool came with two stones, one rough and one smooth--should they be used in this order?

Forum Responses
Jointing is the process of bringing all the knives to the same cutting circle. Jointing the knives does not sharpen them--it actually dulls them. The jointing of the knives should not produce more than 1/64" of land or flat on the tool. Any more than this will cause the tool to compress the wood behind the cutting edge.

The jointing attachment must be properly tensioned. The smaller the joint the better. The knives should be ground within less than .0005". If they are not ground this close, the joint will vary between the knives.

There are two basic styles of jointing stones. The older style is a much harder type. This stone will very often spark. This has been known to cause fires in blower system. The newer style of stone is the soft stone. This stone can be easily shaped and normally does not create sparks.

The most recent development in jointing stones is the non-sparking stone. Other advances include a new polishing stone that applies lubricant to the jointed edge.

Dave Rankin, forum technical advisor

If you are already grinding multiple knives in a cutterhead to a detail and setting them with a fixture that gets them close to the same cutting circle, install the cutterhead and bolt on the jointer bar, taking careful note that drift pins are used for precise location and relocation.

Employ a soft jointing stone and introduce it so it barely kisses a knife. Hand turn the shaft so the blades cut their shape into the soft "soap" stone. Push the stone in further and continue until all of the stone has the knives' shape on it. The stone should also be shaped somewhat like a chisel.

Back the stone away and set the "bottom out" screw so the stone cannot touch the knife. Turn the cutterhead on. Dive the stone into the cutterhead, watching for sparks and listening for noise. Nothing? Re-adjust the screw and try again. Something? Stop the cutterhead and examine with a magnifying glass and good bounce light. If you detect a flat on the tip of each knife about the width of a human hair, you've done a great job. As the knives wear, several rejoints are possible for sharpening purposes. Stop when the flat area reaches about 1/32" wide, because the wood will burn from too much contact by the "heel" (flat spot) and the noise will disturb all the cats for miles.

Before the advent of the soft stone, four joints were possible by a good joinerman without removing the cutterhead and knives for resharpening. Now 12-16 is common.

Pre-jointers are a thing of the past but you may need one. Be sure to run your lineal fpm faster by the number of knives in the head. Example: 20 fpm now for a smooth finish, 80 fpm with a four knife jointed head.

With the use of a hard style stone, nick lines can be removed when jointing. This, however, does not sharpen the knife. The more joints done, the more flat land is created to the cutting edge. That is why after 4 to 12 joints the tool must be resharpened.

This is a common misunderstanding of the jointing process. Knives can be "helped" to make it through a production run, but a tool that is dull cannot be sharpened by jointing the knife.

Dave Rankin, forum technical advisor

After I form the stone to the knife, I mark the edge of each knife with a magic marker and then joint it. This tells you if the stone is hitting all the knives.

On a jointed knife the cutting edge is produced, by the jointing stone, between the knife face and the joint land (jointed area with zero clearance behind the cutting edge). As the knife dulls, this cutting edge wears, or recedes, effectively forming a small radius which will increase compression of the wood fibers during machining, higher cutting forces, surface glazing and generally reduced surface quality. When the tool is re-jointed the jointing stone removes that radius and sharpens the cutting edge.

Some other important items for the optimum use of softstones in a jointed moulder:

a) Hydro tools.
b) Extreme grinding precision. (Best results often require a change in grinding procedures to improve knife concentricity.)
c) Inbuilt jointers to maintain precise axial alignment after first jointing.
d) Excellent tool balance.