Balancing moulder heads

      A discussion about out-of-balance knives in a moulder/planer. June 13, 2001

I use a Powermatic planer/moulder to run small amounts of trim. The larger knives that I purchased with the machine don't seem to be balanced with the counter weights. My theory is that the weights coincide with the weight of the knife, but the centrifugal force, due to the large size of the knife, is the problem. Does anyone know the formula to size the counter weights, taking into consideration the dimension of the knife from the fulcrum?

Forum Responses
I don't think adding weight to the other cutter locations will help much. Centrifugal force probably is not the only issue. Air resistance against the larger knife is a major factor. On our W/H moulders, we can hear a definite difference in machine noise (more of a fan-blade-thumping-the-air sort of sound), as well as more vibration with larger knives, and you run the W/H's with matched pairs rather than 1 knife and counterbalances.

Quite frankly, no matter what the machine manufacturer tells you about being able to cut with only one knife, it's well worth the investment of an additional 1 or 2 knives to make up a complete set. This will be easier on the machine as well as give you a cleaner cut with fewer passes.

I discussed this one time with a friend who builds engines. He had a chart showing the change in force for various distances from the center of a shaft, at different RPMs.

I have a small moulder, too. The bigger the knife, the worse it vibrates! I just haven't run it enough lately to figure out the knives.

You definitely want to use two knives in your cutter head. They should be no more than 0.5 grams difference in weight, if at all possible. They should be in direct opposing slots. Total safe knife projection should be no more than 3 times steel thickness if using 1/4 inch steel, and no more than 4 times thickness if using 5/16 or 3/8 thickness of steel. The two other slots if using a 4 knife cutter blk. should be filled with dummy knife stock. These also have to be balanced to each other. Balance them from either of two ends. You should also make sure your gibs as well as gib screws are properly balanced. Inspect your head for damage or out of parallel ends, as well. All of these things are critical to the quality of finish on your product, as well as protecting your machine from possible spindle and bearing damage. Be safe!

That knife projection should begin from the bottom of the gib.

Balance is incredibly important. Consider that a tool that is out of balance by 1 gram at 6000 RPM would be equivalent to you twirling a 22 pound barbell over your head. That's gotta hurt! Save your machine bearings wear and tear by properly balancing the profile knife (with the aluminum gib for weight on this one pocket only). The other (2) two pockets need steel gibs and counterweights. The length of the gibs usually are the same. You can adjust the length of the counterweights in relationship to the height of the profile knife. Balance all 3 to within 1/10th gram.

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

Comment from contributor A:
There is *no* substitute for balancing the assembled head. Everyone knows what happens to their car if they lose a wheel weight, and car tires only run at a few hundred rpm. If you are having vibration problems with a head/knife combination, find someone with a balancing machine and trim by grinding off the inside (bottom) of the knife. Once you have run a properly balanced head, you will understand why balance is so important. Why live with bad surface finish, blown bearings, noise and vibration? Invest in balancing moulder heads to 1 gram.inch each end.

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