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.
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.
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.
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.
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).
