Compression and Chatter

Pros discuss the meaning and causes of "compression" and "chatter" in moulder operation. October 4, 2005

Question
In an RBI brochure that I have, they mention the terms chatter and compression. Both terms are mentioned in regard to their two-knife setup. I've seen here on this site that the number of knifes has nothing to do with chatter. It seems that RBI is different. Otherwise, why would they mention both terms with regard to a double knife arrangement? So, I ask all those who are a lot more familiar with this than I am, just what exactly is compression and chatter?

I always thought that if you gently ran your index finger over a finished profile, and if you felt very slight ripples, that would be chatter. It seems that there are those who disagree.

People mention slapping belts, out of balance heads and etc. The W&H seems not to suffer this. They use a two-knife system that rotates at 7,000 rpm. The RBI spins at 4,900 rpms and the Woodmaster at 4,200. I've also seen here that the Woodmaster suffers most with chatter. So, my question is twofold - does the number of knives have a direct correlation between what is chatter and any inherent design flaw with a particular machine, and also, what is compression?

Forum Responses
(Architectural Woodworking Forum)
From contributor E:
As far as I knew, I thought that chatter was when the machine couldn't hold the wood tight enough and it moved slightly (up and down) and produced uneven depth mill marks. Mill marks is what the knives leave behind on the wood.

My guess for compression would be as the as the cutter goes into the wood it squashes it before it cuts it and the wood will spring back up making the mill marks more pronounced.


From contributor G:
Confusing isn't it? The number of knives you have really doesn't matter a lot, except for the amount of moulding you can get before a re-sharpening. Whether you have 2-3-or 4 knives, ultimately, one knife does the final finish. The other knives assist. This has to do with the grinding and wear on the stone. It's hard to get two knives exactly the same. If the other knives are ground out too far, then they don't even assist, but just serve as counterbalances, so why even have them? Loose bed boards move up and down causing the knives to gouge and tear out chunks.

Chatter is caused by imbalance.

Causes:
The first is insufficient dust collection. Wood chips collect behind the knives and cause an imbalance. Knives are not properly balanced. I like to get them to 1/10 of 1 gram. The heads could be out of balance, but that's not common.

Another cause is poorly balanced pulleys, and non smooth running belts - the belts being the most common cause. You can make nice moulding with one knife if all the above conditions are met. It seems like these problems creep up more with the lower cost machines than the high priced production machines, but then you get what you pay for.


From contributor S:
I've had pretty good results with my old 18" Woodmaster. I get much less chatter than the stuff I buy from production shops, but it takes me three times as long to run. As it has been mentioned, great care needs to be taken with regard to setup and balancing. Make sure your machine is level and tight on the floor. Of course you're going to run the back relief first, which helps it feed through without binding.

Keep your feed rollers dressed - sharp cutters will help eliminate compression. I try and get the stock as close to profile thickness as possible before moulding it. Keep your stock ends butted together tightly while doing your runs, the beds of the machines require constant even pressure on them to produce decent results. I agree with the previous post - two knife cutters can be a bear to align properly.


From contributor F:
To the original questioner: Like the other respondents said, chatter is caused by imbalance and vibration, and like Leo said, if a molding blank isn’t held down tightly by the feed rollers chatter will occur. It can also be caused by side-play in the cutterhead shaft.

Side play problems become very evident on the sidecut portions of a profile. Compression occurs when the molder cannot clear the cuttings fast enough and they get pounded back down on the finished surface and cause small dents.

Mill marks are present on all wood that has been machined by a rotary cutter. It happens because some places on the wood are aligned with the center of the cutter head while the cutter is at top dead center, and some places on the wood arrive at the cutterhead center while the cutter/cutters is/are in a position other that top dead center. This causes the stock to be machined to slightly different depths all along it length.

The faster the RPM of the cutter head and the slower the feed rate and the more knives (cuts per inch), the high and low spots get closer together and are less noticeable resulting in a better finish.

As Contributor G said, a nice molding can be made with a single cutter. The feed rate needs to be slow to keep the mill marks closer together. I know a man who has all the large equipment and automatic knife grinders. He tells me he routinely will grind only a single knife to use in his production molders on short runs.

In direct answer to you question about the correlation between the number of knives and the amount of chatter, if you have conditions present on your machine that are causing chatter, then a greater number of knives are chattering and will cause chatter to a greater extent than a single knife.


From contributor L:
I'll add a bit to the compression discussion. There is always compression every time the knife comes around and hits the wood. The duller the knife the more compression, no knife starts cutting the instant it hits the wood. It therefore follows that the slower you feed the more compression marks you will have. These show up as stripes in the stain when finished. You will also get random compression marks from re-circulating wood chips due to poor dust collection. The size of the mill marks will decrease as you slow the feed so you will have less distance from the top to the valley making a better looking and feeling molding.

Mill marks are not chatter. Chatter is the result of uncontrolled movement - the wood not being held firmly and feeding evenly, the head moving because of many things. At its worst chatter will actually cause tear rather than cutting. If you have a jointed molder you can eliminate the one-knife-cut effect, but at the price of increased compression due to the land created on the knives. Higher quality machines reduce each of the problems if they are well maintained and setup. We also go for 1/10-gram balance and do a secondary grind to come closer the having two knives cut equally.

There are an incredible number of things that affect the quality of a molding. As to the cutter head speed and mill marks - rotational speed, number of cutting knives, balance, and feed speed all have to be considered and are inter-related. Loose knives not ground in the head will not produce the same quality, and heads that have more slack to the shaft will produce poorer quality (hydro heads will eliminate this effect).


From contributor F:
To contributor L: It is fascinating to me that a profiled knife can be jointed. I was aware that straight knives could be jointed. How is this done by the machine?

From contributor G:
To the original questioner: As I said before, “confusing isn't it”? There is a lot to learn about making moulding, and about woodworking in general. You have to learn to work with the machine that you have. No. 2 machines are exactly the same. The less expensive machines will work well, but some may need a little tweaking.

From contributor L:
To contributor F: The machines are equipped with holders for the profiled stones in such a way that the stone can be lowered into contact with the rotating head. The stones are profiled with a diamond wheel in much the same way that the knives are made.

From contributor L:
Contributor F, on your earlier post you mentioned a man with a production molder sometimes ran only one profiled knife for short runs. It seems like balance would become a real problem? I’ve done that on a heavy shaper, but slowed the RPMs so the vibration was more controlled. Most molders run heads at 6 or 8,000 and the heads are larger diameter compounding the out-of-balance problem. Running out of balance is hard on bearings.

From contributor G:

To contributor L: Could there be some typing mistakes in your post? I have never seen any moulder run at 600 feet per minute. Also, I've never heard of 16 knife heads. I have heard of 6 heads with 4 knives and 60 feet per minute.

From contributor L:
Actually the big Weinigs can run that fast, and yes I've seen the 16 knife heads.

From contributor F:
On my small shop moulder I have some tricks for balancing the specified knife/gib weight by using either aluminum or steel which have differing weights per linear inch and also the number of set screws play a part in the overall weight.

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