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