Keeping cutterhead balance

Number of knives and other factors in cutterhead balance. (From WOODWEB's Solid Wood Machining Forum) March 5, 2003

I have a question concerning cutterhead balance. If I have a six wing solid steel cutter block (12" wide and 100 lbs - heavy block) and I set two knives in this block, why is it considered necessary to fill the other pockets with filler stock? I assume as long as the cutters are diagonally opposed and weigh the same, with same projection and alignment, the amount of elastic deformation of the block would be negligible in regard to causing any dynamic balance problems.

Forum Responses
(From WOODWEB's Solid Wood Machining Forum)
From contributor C:
This is not a balance issue; this is a head integrity issue. Steel fatigues over time. Most head bodies have a Rockwell R value of less 30. Softer steels will not spring back like harder steel. Leaving empty pockets puts unnecessary stress on the head itself, and will result in head distortion over time. The cutting circle or bore could also be adversely affected. Proper tightening of gibs screws in both sequence and torque will alleviate unneeded stress.

A 12" head is quite expensive, so it's better to be safe than sorry. I have seen heads with wings torn off as the result of a crash with no fillers in empty pockets. You can use either filler steel, or even old worn out knives with the cutting edge ground down to lower them in the pocket deep enough not to interfere with the cut.

Your head could be beefy enough not to distort, but why risk other potential problems with your head, or your machine?

From the original questioner:
What are your thoughts regarding four knives in a six wing block, having the remaining pockets filled with filler stock (knives being diagonally opposed, etc.)?

Contributor C is partially correct. The integrity of the head is at risk if you run with open pockets. This is very true and many heads are damaged due to this. It is necessary to fill all the pockets with a quality filler material. Many people use old straight knives that are too short to use safely as cutting blades. As long as the filler is locked properly and is deep enough in the head for the gib and filler to be secure, you are good to go.

The other issue that must be discussed is the balance issue. Your question is very much about the balance. In a 6 wing head, you can either run 6 knives or 3 knives. If you run 2 or 4 blades you will run out of balance. 4 knives will also give you an uneven knife mark finish. When you run 2 knives and 4 fillers you have created an egg shaped rotation of the tool. There have been many machines damaged because of running tools this way. Bearings are damaged as well as spindle assemblies. If you need to run 2 knives, get either a 2 or 4 wing head.

Dave Rankin, forum technical advisor

From contributor C:
Without getting into a diatribe on different types of unbalance and just considering that *any* weight that is:

A: Diametrically apposed to another equally balanced weight
B: Positioned in the same plane and
C: At the same exact distance from the center of rotation

*will* be in perfect balance.

Consider the case of a six wing hydro head and then the 2 grease holes and 4 cap screws in the sleeve flange holding the sleeve in place. As long as the three conditions mentioned above apply, then this head balances each and every time.

Dave is correct about 4 knives leaving uneven knife marks and an undesirable finish. 6 wing heads are normally run with either three or six knives (filling any unused pockets with balanced filled knives). We have had WKW, Forest City and other leading cutterhead manufacturers make 6 wing heads with 3 different hook angles on more than a few occasions. These combination hook heads are designed to run only 2 knives (180 degrees opposed) and appropriate fillers in the other four wings.

Again, as long as the 3 aforementioned conditions apply, the cutterhead will run balanced. The basic laws of physics rule.

Bottom line, I'd suggest running 3 knives and 3 fillers, or 6 knives in your head.

From the original questioner:
Dave, I have a couple of queries relating to this issue. I agree with what you're saying about the unequal spacing of cutter marks with four knives in a six wing head. My fundamental question would be what constitutes a balanced block.

Let's go back to the previous example of inserting 2 knives in a six wing block, and filling the remaining pockets with fillers. What you are saying is true. Suppose I keep this block just for running 2 knives. If the block misshapes into an egg shape and remains this way, how does this affect balance (assuming the block stretches equally)? I always thought as long as they are diametrically opposed and of equal weight, projection, etc, balance would be maintained. (Or am I describing the difference between static and dynamic balance?) In your opinion, is this a real issue (damaged bearings, etc can be caused by other factors… in practice if you ask a machine operator how they messed up the bearings you mightn't always get a truthful reply)?

My response is fairly simple. After nearly twenty years of actually repairing machines and training operators on these machines, I have experienced a few things. The real world of tooling is clear. A block with 2 pockets will run well but a block with 3 pockets will run a little better. A 3 wing head in the real world of using of the tool will provide a longer bearing life and a more consistent run. The reasons that 2 kinves have been used for many years include rate of feed versus number of knives and cost. In short run production, 2 knives are cheaper than 3 knives. $ knives at a slow rate of feed is too many while 2 or 3 will provide correct chip load.

In a block that has extra pockets there are a few things that exist that increase the chance of imbalance. The first is air flow difference. The next is the accuracy of the machining of the corrugations in the block and in the tool steel. If 1 filler sticks out just a few thousandths of an inch further than the others, then you have created a balance problem. Dust that is created during the cut can and will find its way in behind the pockets. Since this is not consistent, further possibilities exist. As a long time technician I would not use a 6 wing head nor would I ever recommend one as described by a previous writer. I would be interested in having another technician that is actually experienced in running this makeup of head give his or her recommendations. I have learned over time that many things can be manufactured and sold, but that does not make them a good choice in the real world of production.

If you do decide to run 2 or 4 wings in a 6 wing tool, I would suggest that you keep a good technician's phone number handy so that when, not if, something goes wrong you can get it back up and running quickly.

Dave Rankin, forum technical advisor

From the original questioner:
I think everybody agrees on what constitutes the optimal setup on this block, i.e. 3 or 6 knives, remaining pockets filled, however other factors which have been pointed out dictate what you put on the block.

For example, I have been running reclaimed material for 12 years. The condition of these boards gave short knife life, necessitating knife changes every 30 minutes. Rather than ruin six or even three knives at a time, I run two knives (feed 45 ft/min). I must say I have yet to replace the bearings on this shaft (I maintain this machine myself - other bearings have been replaced).

I admit knowing what the best setup is, but what happens in my case is if you can get away with it, these things invariably do happen.

Just out of curiosity, this 12" head that weighs 100 lb is supported by an outboard bearing, correct? If you do not use an outboard bearing, what is the spindle diameter? What is the make of your machine?

From the original questioner:
The machine I am using is a Robinson with outboard bearings. It is an old heavy duty machine, but I get good results with it.

Is there such a tool (inexpensive) that we could spin our heads on before putting them in the moulder, to check the balance? I use 4 pocket 2 knives/filler stock.

You can use a simple static balancer but this is not accurate enough for proper balance. A misbalance of 1 gram (the weight of a cap for a normal Bic pen) gives you a rotational load of 22 pounds. This means that at 6000 rpm, a heavy load is placed on the tool. The best thing to do is to balance using a dynamic balancer. The cost is high, starting around $30,000 US or so.

Use top quality tools and static balance the knives and you will be okay, but a dynamic balancer will make it better.

Dave Rankin, forum technical advisor