Increasing moulder knife life

Increasing length of runs between sharpenings. March 21, 2002

I am looking for ideas to prolong the wear of the moulder knives I use between sharpenings. I have HHS knives ground at 25/20 degrees, with feed rates between 28/35 fpm. Species of wood include red oak, hard maple, poplar, alder and a few others, only run a few hundred feet between runs. Would honing the back of the knives with one of those diamond stones gain a few more hundred feet before sharpenings?

Forum Repsonses
Many people hone their moulder knives. This process allows the back clearance edge of the tool to be slightly removed using a variety of different shaped hones or oilstones.

I do not usually hone the face of the tool - just the area that the grinding wheel ground.

For HSS, I have used a tool room assortment that I worked out with an excellent manufacturer of abrasives. This tool room assortment has been used by many folks to help determine which hones worked best for them. The Training Center gets their tool room set from MSI.

Dave Rankin, forum technical advisor

Unjointed knives are very sharp when freshly ground. Unfortunately, their sharpness contributes to their fragility and susceptibility to rapid wear. The sharp point would be better served if it was, in a sense, deliberately dulled.

Joining or "dulling" knives in a cutterhead is a method that more expensive production moulders use to increase the time of runs 10-20 fold before resharpening. The joining stone is introduced to the spinning knife set on the machine so that it barely "kisses" the tips, thereby imparting a slight flat or "heel" to the tip. This heel is almost too small to see with the naked eye without good "bounce" light. Think of the area of "land" of the heel to be equivalent to the width of a human hair. Several more joins are possible as the knife set dulls before eventual removal for resharpening. This occurs when the knives have too much land from several joints and a scuffing or burning of the lumber becomes apparent, not to mention an increase in DBs.

One would think this deliberate "dulling" would adversely affect the quality of the cut. Quite the opposite occurs. The additional steel that joining creates just behind the cutting tip makes the tip stronger and less likely to be damaged by grit or other foreign material in the wood. A nick in the knife is typically jointed out and the run continues without much interruption.

Now to your question: Machines not equipped with joining devices or ways of bolting them on can only be expected to run as yours does. You can experiment with hand filing a heel in the knives and may enjoy some success. There is also another way that was practiced early in the last century by planermen, which is too dangerous for inexperienced hands and I dast not mention it here.

I have predicted for many years that the next significant advance in tooling will be the ability to join knives on any machine with a simple after-market device. I've done it experimentally on a small scale. Given the fact that unjointable machines are said to have bearing sets which are too imprecise to make the benefit of joining effectively possible, there is some benefit to be derived from the process, although admittedly not the ability to increase feed speeds by a factor of the number of knives in the cutterhead.

From contributor C:
What brand of HSS are you using? There is a wide difference in quality from one HSS to another; dependent on the manufacturer. I am surprised that you're only getting a few hundred feet between regrinds on most of the species mentioned. When using HSS to machine hard maple, lower footage runs between regrinds are to be expected. What is the hook angle on your cutterheads? Most operators use a 10 or 12 degree hook head for hardwoods, and a 20 or 22 degree hook head for softwoods. Incorrect hook angle usage may contribute to the shortened knife life. We have customers using the diamond hones you mention, as well as the same Tool Room Assortment that Dave mentions, to deburr knives. You might consider some diamond hones manufactured by EZ Lap and DMT as an option to hone your knives.

From the original questioner:
I am not resharpening every few hundred feet, just changing setups, but I am not even coming close to the footage I should be getting between shapenings. A few hundred more feet would give me lots more moulding time. I am not sure what angle the heads are but I use them on both hard and softwoods. I will check into those toolroom stones and even try taking the edge off freshly ground knives to see if this helps.

From contributor C:
There are many different types of honing stones to choose from. These include silicon carbide, aluminum oxide, hard Arkansas, and diamond hones, among others. Most people use medium (180) or fine (320) grit silicon carbide or AlOx (Aluminum Oxide) hones. The silicon carbide is best at quickly removing nicks from knives, while an oil-filled aluminum oxide stone hones to a better finish. We have a lot of customers using the hard Arkansas natural stone as well. Diamond hones are much finer grit (up to 1200). Some people use even finer grit Japanese water stones with super fine grits in the 1000's.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor R:
Have you tried running different grind angles on your knives? I run birch, maple, oak, cherry and some exotics and I found that a 10-12 degree finish grind with a 15-17 degree back grind in a 12 degree hook produces an excellent finish and almost twice the lifespan of a 20-25 degree grind in hardwoods. Several moulder techs have told me that 10-12 degrees is optimal in hardwoods for a finish grind. I also found that you can buy teardrop shaped honing stones. I leave them in a can of oil, and use them to touch up minor nicks and slightly dulled knives. This adds 150-200 feet more to the life of the grind.