Can a 4-knife jointer run with only 2 knives

[Rob Blaustein]

This just popped into my head again reading another thread and I first wondered about it when I ordered my new jointer but didn't order replacement knives. If one knife gets damaged, I would guess you can't just remove it since the cutterhead wouldn't be balanced. But it would be with 2 knives. I would think you might sacrifice quality of cut, but could compensate by feeding slower--after all, many jointers only have 3 knives, and as has been discussed here before a few times, it's not really clear that 4 is better than 3 (may be even worse--an extra blade to adjust). So has anyone done that?

[Jim Young]

I don't see why two opposing knives would be a problem. And ya, you will have to slow down the feed rate from what you are used to.

[Jim Becker]

You might get away with it (assuming you secure the hardware that holds the knives in the empty slots) with opposing knives, but you will hate the results on the wood. If you get a nick in the knives...slide one to the side slightly and reset height if necessary to "fix" the problem.

[Wes Bischel]

The only time I could see one knife getting damaged is during installation. In use, I would think the knives are rotating too fast for a nail or something bad to hit just one.

Is my logic flawed? (wouldn't be the first time.):eek:

Wes

[tod evans]

rob, i would think that a jointers head would behave as a shaper or moulder head in this instance? when setting up moulding heads if you only use two of 4-8 knife slots blank steel must be inserted in all other holes and torqued to spec or you run the risk of distorting the head......02 tod

[lou sansone]

well, I tend to think the radius of gyration will change, but that might not be a big issue, provided all things remain symmetric. There are jointers that are designed with only 2 kinves ( wadkin for example ), but I really don't see what you are gaining by this experiment. You absolutly want to make sure all the other hardware is reinstalled and tight. Personally I think you have more down side potential for lower performance than any upside quality improvements.

lou

[Ken Fitzgerald]

First as others point out, removing the blade would in my opinion change the balance and thus the cut. Depending on the size of the head, the mechanical imbalance at the radius involve could be relatively significant.

As stated by Jim B. I'd try shifting the blade sideways to eliminate the problem or just slightly lowering the offending blade and the one 180* from it so they didn't make contact with the wood. Removing it and running that way.............I don't know if I'd do it......and if I did.........not for long!

[Dev Emch]

THis is done all the time. Martin machines imported into the US all have the optional four knife tersa head and these can be run on two blades.

My schmidt shaper heads have four pockets. Two at 20 degrees and two at 12 degrees. These are two knife heads where you have two different sets of knives. Each set is ground at its respective hook angle. You only use two knives at a time. Other heads have four pockets but all four are at the same hook angle. Again, you can run either 2 knife sets or 4 knife sets. NEVER RUN THREE KNIVES or you will dance the washing machine tango.

Here is the beef. In normal applications with no in place knife honing, the height of one knife will always exceed the others. This one is the finishing knife. The other knives act in support of the single finishing knife. But your finish is dependent upon the number of nicks per inch making feed rate just as important as cutter RPM and knife count.

Most ground shaper knives come in sets of two and are run in sets of two. As long as I back off my feed rate, life is good. But if I install a power feeder, then the higher feed rate reduces my nicks per inch count and the finish suffers. The irony in all this is the higher feed rate gets longer life out of the knives. That is because the increased chip load is transporting unwanted heat away from the knife tips.

The same applies for jointers albeit no one really listens. Jointers have notoriously low feed rates and often are sporting four knife cutter heads. The fourth knife is sweet in that it eliminates the beaver munching that you get with two and lessor with three knife setups. Its a more delicate and a finer cut and at these slow rates, a much higher nicks per inch rate.

But with improved finish comes the disadvantages. The much reduced chip load now allows the knives to overheat the tip thus shortening their life spans. You wind up having to sharpen more often.

Personally, this does not really bother me. Its the cost of doing business. But on machines like my planer, I have an onboard knife grinder with an in place honing stone. Even though my planer has three knives, I still get better than four knife cut performance because all three knives are finishing. As I loose my edge, I just rehone with the stone. I can do this about two or three times between actual grinding.

Right now, my porter jointer has a three knife head. Currently I have to change and adjust knives like everyone else. But I have seen a porter knife grinder equiped with a borazon grinding wheel. This really did a sweet job on HSS! When I get to it, I hope to someday build a clone of this knife grinder for my porter. But two things come to mind. First, I have seen a number of porters but only one porter knife grinder in many many years. Why? Did the pattern shops and cabinet shops not buy the optional knife grinder because the extra tweak it provides was just not worth the money? Or did these things just get lost over the years like wadkin quadrant gages and fences? I dont know?

But in short, you can try to run two knives as long as you CORRECTLY secure your slots and gibs in the unused spaces. TERSA heads are easy to secure as are schmidt heads. But who knows with other makers. I would hate that you run some popular import jointer and have the gibs come off and impale your pin up calendar! Dont blame me for the hole in Betty Garbo's portrait!

[J.R. Rutter]

There was a thread on this on one of the woodweb forums not long ago. As Dev said, running 2 knives is an economical option, since you are getting the finish cut from one knife anyway. Another option mentioned there, though not related to the original question, was to run 2xcarbide and 2xsteel knives. This supposedly gives longevity and a good finish. I've not tired either, since I went to Shelix in my planer...