Cutterhead balance

Solving the problem of a vibrating planer/moulder. January 16, 2002

Question
I have a Powermatic/Bellsaw 12" planer/moulder that vibrates more than it should. It seems that the cutterhead is the source--when the main belts are unhooked, the vibration is almost none, and it doesn't change when the belt to the feed is unhooked. This is with no knives in the head.

I'm going to contact the manufacturer about getting it re-balanced to a tighter spec. What should the tolerances be for a 3 1/4", 4500 rpm cutterhead? Being a lower cost tool, I wonder if the tolerances are not tight enough to be balanced properly. Who else would be able to check/balance this type of cutterhead?

Forum Responses
Most good tooling companies will check the balance of a head for you. The precision of the balance is very important. For example, at 6,000 rpm 1 gram (about the weight of the cap on a bic pen) equals about 22 pounds of rotational force. This will cause vibration and bearing problems.

Balance of the heads and the knives must be correct. The quality of the knives will also play a roll on the balance of the knives. Each time that you have the knives ground, check the balance.

Dave Rankin, forum technical advisor


I own a Belsaw that is about 20 years old. It also vibrates like there is no tomorrow. I don't know that it can be pinpointed to the cutter head, remembering that this is a lightly built machine with a rough pulley and feed system. Is it effecting quality of cut or moulding? I hate to see you go through a lot of work and expense to gain nothing. Maybe you could compare your machine to another in your area.

From contributor C:
The new Powermatic/Belsaw machines are stamped steel; not the heavier cast steel like the old Foley/Belsaw. They have a rather large diameter head on a small diameter arbor. With this in mind, your problem may be more of a bearing issue, rather than a cutterhead out of balance. These machines are not really built for production work. If used mainly as a moulder, it is imperative to keep the tooling balanced. If running one knife and counterweights, confirm the balance of the set. If running three knives you should have less problems. In either case, balance as a set to within 1/10 gram. The machines seem to run better as planers from a bearing wear standpoint.

To determine if the bearings are the culprit, unhook your belt and using a dial indicator, pull up and down on your cutterhead. Up and down movement indicates worn bearings. If that's the case, your best option is to replace them, and keep a good watch on your balance afterward. It would be rare, due to the shallow pocket depth of the knife slots, to have a head out of balance. Obviously, if there's a chunk missing from your head, it could certainly be out of balance. If you determine that it is indeed the head, I suggest confirming with the facility you choose to re-balance your cutter that they use a dynamic balancer (such as a Hoffman) rather than a static balancer. The Hoffman is capable of balancing on all axes.

I'd check the bearings first, especially on this machine, knowing its track record.


From the original questioner:
The bearings are tight, but I found the pulley on the cutterhead was out of round by .010", the shaft itself was within .002". I think the thing to do now is take the cutterhead and pulley to the local machine shop to have them turn it as a unit. The cutterhead has drilled holes in it where it was balanced from the factory, so that makes me think there is some possibility for a balance problem, also.

It vibrates enough to leave chatter marks in the wood. Add to that a one-knife setup and you have a less-than-ideal finish. I'm getting some three-knife sets for a job coming up soon. Hopefully this will help. 1500 lbs of cast iron would really help!


Your say you have only one knife in the head. What is the head configuration? How many knives and knife slots? Obviously any change in the number of knives, size/weight of knives and position in the head can effect tool balance. Maybe someone has changed the original setup?

One knife in a three-knife head will definitely throw the head out of balance. You can not expect a three-knife cutterhead to be balanced with only one knife. You should also check the weight of the three gibs and the three knives. The gibs and knives should weigh equal amounts.

The holes in the cutterhead indicate that the factory balanced the head, but you still have to balance the knives, otherwise you will continue to have balance problems.


I am afraid some of you are unfamiliar with the Belsaw machine. It is basically a three-knife planer. If you decide to mould with the machine, you remove the planer knives and gibs and normally install a single profile knife and they supply two counter weights for the other pockets. The head turns relatively slow and the feed is slower yet. Running three knives is probably a futile attempt, because there is no easy way to reference the cutting circle and alignment is very difficult. You do obtain decent cuts per inch with a single knife. On the plus side, these machines are somewhat versatile with a little creativity, and can do a reasonable job even with a single knife.

We grind our own knives on the rare occasion we use the machine, and counter balance with steel slugs weighing carefully. My machine, which is also stamped metal even at 20 years old, has most of the feed works on bronze bushings, and I know they are worn. To be honest I haven't the desire to start taking it apart, so I just use it the way it is, vibrations and all.


Balance issues exist with the large machines as well as the small machines. When using 3 knife heads with 1 knife and 2 balancers, the problem that exists is the cutting circle varies between the knife and the balancers. This is true on small combo machines up to large moulders. There are still many people that only grind 1 knife and use a balancer on moulders running 7200 rpm. The science is the same. In all cases, the use of knives in all pockets will reduce the vibration unless parts are worn, bearings are bad, belts are bad or loose or something else is out of adjustment.

Dave Rankin, forum technical advisor


Did you take all of the knives out of the machine and run it to determine if the vibration is due to the knives? The machine should run smooth without the knives. The sheave driving the head needs to be running true and in aligned with the motor sheave. Check the motor sheave and make sure the motor is mounted down to the machine base and not on any kind of rubber mounting bushings. Replace the belt with a new one so that this is eliminated for the equation.

If the machine is running without vibration with NO knives, the balancing of the blades should be looked at.


Dave Rankin hit the nail on the head. Even if the counter weights are exactly the same weight as the knife, you have to have the center of mass of all three at the same radial distance. That means that you may have to put up with the vibration, or get out your calculator and figure out the radial offsets required. I suspect it is a sheave or bearing or loose belt.

From the original questioner:
Right now it looks like the culprit is the out-of-round pulley. I have had the knives out so they aren't the main problem, but I am getting some three-knife sets in soon to keep it balanced properly, plus for a wider (5 1/2") profile, it should keep from working that one knife so hard.

Contributor C, why would you want the knife to weigh the same as the weight? I know everyone I've heard (even manufacturers) says to balance them, but it never made sense to me for the reasons stated in the other posts. It seems there would be a formula to find the difference in force relating to the distance from the center to find the weight needed to balance the knife when the cutterhead is spinning. The only problem (or one of the only) is each knife of equal weight will have a different portion of metal at the outside of the cutting circle due to different profiles. So they won't all be exactly the same, but it could be close.

I almost forgot, I also screwed and glued plywood to the inside of the sheet metal panels. That helped reduce the vibration some. Now I just need to eliminate the source. Before I take the pulley to be turned, I'll check the pulley on the motor--it may be the same way.

There have to be thousands of these things out there. Does everyone just put up with the vibration, or is it only a percentage that are bad?


From contributor M:
I would rethink buying three knife sets unless you are of unlimited funds. The Belsaw with its slower turning head, fairly large diameter and small depth of cut will run fine with one knife. I will almost bet you will not gain anything in finish or reduction of vibration going to three cutters. Aligning three knives is very difficult. The gib system tightens by pushing up against the knife, naturally wanting to push the knife out of the pocket. Fighting that along with right to left alignment becomes a lesson in frustration. In moulder work these issues are critical, but with the Belsaw you can't get that much knife extension because you have to maintain a predetermined cutting circle to align with the feed wheels. If you want to experiment, use the small center gibs and add a small amount of weight to the pockets with out the cutter.

One other thing to mention, being a three pocket head, the variations in weight and cutting circle seem to be less magnified as opposed to a moulder head with 2 or 4 pockets. I have made a lot of knives for our machine over the years, and its main benefit is the fact that you need only one knife for that quick or special detail not worthy of setting up the moulder.


From the original questioner:
Contributor C, I understand why you would want to have the knives and weights weighing the same if they are the same size, but the single knife sets I've used only have a weight that is 1/4"x3/4"(or something like that) and sit down inside the pocket so they are on a much smaller circle than the knife projecting 1 1/2" at the farthest point.

Contributor M, I may be going overboard on this! A few sets are on the way, so I guess we get to see what happens. For the smaller profiles, I may stay with a single knife if I can work out the weight issue. I did speed it up with the 20'/min sprocket since I was getting the three knife sets. This will get the knife marks/inch closer to what they should be. It will be 18.75MPI compared to the standard 31.25MPI. As far as aligning the knives, I may fix a place to hold a card to make some reference marks of the profile so I can rotate the head to align the next knife. I have an old 2x4 push feed moulder that has a stand to set the knives this way. If it was wide enough I wouldn't be messing with the little one. I also thought I'd feel bad if I sold the Belsaw to someone else unless it was working right, so if all this works maybe I can trade both moulders for a wider four sided one.


The faster that you run the spindle rpm's, the more the balance problem will be an issue. Most machines that run balancers run at 3,600 rpm or they recommend the use of matched knives.

As for the thought of trading up, this is always a good idea if you consider all the costs. The wider the machine, the more expensive those heads will be. If you get a larger machine, get the wide heads with it. When buying these wide machines, most will be of the older style push type moulder. These are a little more time consuming to run than the feed through, but can do an excellent quality on the finish. Most of the machines will not be set up to use balancers unless they are running the old square heads. I strongly suggest that you convert these to round corrugated heads for safety reasons.

The bottom line is even the larger machines can have problems and those problems are the same that the small machines can have. The causes of the problems can be the same, the smaller machines just show the problem worse and faster when something goes wrong.

Dave Rankin, forum technical advisor


From contributor C:
When running a single knife setup, you use a profile knife and lightweight aluminum gib in one slot, and 2 steel counterweights and gibs in the other two slots. The combined weight of the steel gibs and fillers should match your profile knife and aluminum gib. In most cases, this requires the counterweight and gib combo to be longer than the profile knife and aluminum gib. This is not a perfect setup, but is the best way to achieve some semblance of balance.

From the original questioner:
Good news! Replacing the pulleys made a huge difference. The cutterhead shaft was originally turned slightly undersized where the pulley rested, so I had a pulley with an undersized bore turned to fit the shaft. The stamped pulley on the motor was also replaced with a cast iron pulley. The exact sizes weren't available, so the head turns at 4900 rpm instead of the original 4500. I have yet to try a set of knives in it, but the vibration with no knives is greatly reduced compared to before.

One other thing I may do is to replace the pulley that is driven by the cutterhead to power the feed drive. It's also stamped steel and you can watch it vibrate as it runs, but it's a more complex part with bushings and a gear, and it's at a slower speed than the cutterhead, so I'll wait on that one.

A few other modifications I made include latches on the cover to hold it tightly closed, larger hinges on the cover, an additional 4" dust port, and plywood panels inside to dampen sheet metal vibration. I also have a flap sander to mount on the outfeed to soften the edges. I think I should be able to get by with it until I can get a bigger four side moulder.


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

Comment from contributor A:
I operate an old Belsaw 9121 with a huge 3HP GE Repulsion Induction motor turning at 1725 RPM. I get about 4800RPM by using an 11 inch pulley on the motor. I only use it for a planer (3 blades in the cutterhead) and it operates like a charm. No vibration at all. I have replaced the blades, feed rollers, bushings and springs. Also completely disassembled the planer to have the table surface ground as it had a .010 low spot in the center. I set the blades using the Belsaw "sure set" base with a dial indicator installed rather than the provided metal rod. As the postings have said, everything looks good until you tighten the gib screws; when they move they usually move the blade up at the same time, which screws up the alignment. After about one six pack and many unprintable words I decided if the blades were going to move up I should measure how much they move and set them that much deeper in the cutterhead. Bingo, as they moved up about .007 as the gibs were tightened I just set then .007 below the desired height. Have accuracy of about .002 across each blade.


Comment from contributor D:
First thing, run system without cutterhead or with bare cutterhead (no knives) and check vibration. Second thing, check belts for tension, alignment and damage. If you have balance checked, make sure they mark the heavy angle when done. Mount with heavy spot at top. Finally, assemble head and cutters. If it is three pocket head, use three knives but expect that only one will cut. Excessive bore clearance will get you even if everything else is right.
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