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Moulder Chatter

4/17/15       
Kent Holdeman Member

Website: http://covenantmillwork.com

I have a Unimat 218 6 head moulder that I am having a lot of trouble with chatter. I have looked through the threads and can't find anything exactly like I am dealing with. It seems like on any moulding that has a deep cove cut I am getting a chatter mark every 2.5" - 3". Sometimes it looks like it's a single mark, sometimes two or three marks close together. I am having this trouble on a 3" panel mould, a 6" crown and a 8" crown. I have went over the whole machine with a fine tooth comb. Checked the outfeed pressure shoe, chipbreaker, swapped spindles with the two top heads, made custom pressure shoes, I'm about to pull my hair out. It seems like noone I talk to has seen this exact problem. The chatter is definitely in a pattern, you can see it every 2.5" down the whole length of the moulding. Any ideas?


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chatter pic

4/17/15       #2: Moulder Chatter ...
Kent Holdeman Member

Ok, I guess I'm going to answer my own question. After a day and a half of trying to figure this out I was getting desperate but soon after I posted my problem we figured it out. My last bottom head was kind of dull, I didn't see how that could cause the problem we were having so I hadn't done anything about it. I finally took that head out and tried a test piece and it ran perfect. Hope this helps somebody else to keep from wasting a couple of days like I have.

4/17/15       #3: Moulder Chatter ...
David Waldmann  Member

Website: vermonthardwoods.com

Glad you figured it out. That was going to be my suggestion as well.

We recently had a similar problem on our 13" Kentwood. The face (last bottom) had the same type of chatter marks at fairly regular intervals. Turned out to be the last top was too dull...

4/17/15       #4: Moulder Chatter ...
Kevin Jenness

Can you explain why dull knives in one head produce this pattern? If sharpening or taking one head out of play fixed the problem, all well and good, but what is the mechanism? Some kind of harmonic resonance? We don't run a molder, but when I see this sort of problem in a thickness planer I look for a resin buildup on the bed rollers.

4/17/15       #5: Moulder Chatter ...
Kent Holdeman Member

I can't say why, if I could I probably would have found the problem much sooner, I just didn't think that was the problem. I'm thinking it was making it vibrate pretty bad but what made it force the moulding to jump every 2" is a mystery to me. All I know is we finished our run today and no more problems. The dull head was the last bottom which on a Weinig is about 6" past the last top head. Somehow it was causing it to bounce. I think you are right on your idea about what would make it do that in a thickness planer.

4/18/15       #6: Moulder Chatter ...
Gene Wengert-WoodDoc

I agree with the dull knifes. This would include a knife that has been jointed too much.

Here is my guess on what is happening. With a dull knife, the knife sometimes decides to push the wood out of the way (compress it), rather than cut it. This occurs oftentimes with pine, but probably will occur with all wood species.

Sometimes the wood is not easily pushed (especially when very dry), so the feed hesitates just a split moment; this is so fast and so small it is difficult see. So, a second knife comes around at the same spot, removes some or all of the compressed wood from being pushed down (the compressed wood will actually springbuck a little) and after this, cutting continues as "normal."

One characteristic of such a problem is that when you wet the wood, the compressed region will pop back up. Where this causes a problem is that a surface is sanded as smooth as glass, but under the surface there are some compressed wood cells. Then with a water based finish, the compressed cells spring back, creating a small bump. (If you still have some defective pieces, try this water test.) The problem is worse with a glossier finish.

4/20/15       #7: Moulder Chatter ...
Russ

Website: http://www.mirror-reflections.com

Kent
I am glad you found the problem, I too have seen this issue many times, people tend to be forgetful about sharpening the last bottom head because that side of the board is usually never seen.
Here is the reason why it is so important to keep the last bottom cutterhead razor sharp.
The proximity between to top cutter head and last bottom cutter head is very close, it will transmit finnish problems directly to the top head, here is why, if you got a flash light and watched the board as it was being milled you could have actually seen the problem happening.
There is .020" elevation difference from the top head to the last bottom head. a .020" shim is placed under the last bottom bed plate so you can remove any lubricant used on the tables during milling.
When the last bottom cutterhead is dull the motor will always win the battle over the cutter, it actually lifts the board off the table periodically that .020" and winds up indenting the face of the board caused by the top cutter head as you have demonstrated.
Its the same idea with sniping of the ends of the boards, the chipbreaker no longer is holding the board on the bed plate, if the pressure shoe is out of parallel as soon as it leaves the chipbreaker the board lifts up and leaves snipe.
I have a few articles on our website under moulder articles that may help you, but feel free to call if you have any further concerns.
Kudos on finding the issue, I know how frustrating it can be when milling.
All the best
Russ


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