chatter, vibration while turning

[Bob Strand]

I've got a Powermatic 35/20 and on many, if not most, turnings I get a chatter, vibration if the tailstock isn't applied. I'm currently turning a 7"x 1 1/2"
rosewood bowl and when I make my cuts to the inside I get this vibration. Its got a 2" piece of hickory waste block glued to the bottom and is tightlly secured in a VicMark 120. But this happens on many of my works and its puzzling. Wonder if I might get some input on this.

 

[john lucas]

A couple of thoughts. How are you gripping the waste block. If it's square or round and your just gripping it like a dowel it can induce vibration. Mount your block between centers and turn a tenon on it that has shoulders for the chuck jaws to ride against. This increases the holding power quite a bit and keeps the bowl from rocking. I don't remember what kind of wood you said or even if you mentioned but some woods that have soft and hard spots will tend to chatter, especially if you push on the bevel hard. Ideally you should put as little pressure on the bevel as possible. That alone will often reduce chatter. A good sharp tool of course is essential.

 

[robo hippy]

Well, one reason for vibration could be the 2 inch waste block. That is a bit minimal for a plate that size. 1/3 the diameter or more. Tool pressure can be another issue as in 'the bevel should rub the wood, but the wood shouldn't know it'. Most of us push on the bevel way too hard. Left hand just rests on the tool shaft, not the tool rest, and right hand gently pushes. No white knuckle grip. Another is that on shallow forms, like plates, you are cutting directly into the grain, rather than down through it.. So, when cutting into the grain/cutting across the wood, you cut with the grain, against the grain, with the grain, and with the grain on 1 revolution. When you are cutting against the grain, you are kind of head butting it, so there is always some bounce. Shear scraping is a great way to finish up since you don't rub the bevel. Once you start hollowing out a bowl, you remove mass from the inside, and the walls start to flex, which is why many won't take a long single pass from rim to bottom. Especially if you are turning at high speeds, the wood can flex just due to grain orientation.

I have a bunch of bowl turning videos up on You Tube, under robo hippy...

robo hippy

 

[Bob Strand]

Thank you for the input. I suspicion the waste block may have been a bit small and without shoulders. The grain info was also well taken. I don't feel I'm putting too much pressure on the bevel but definitely worth checking on.

 

[Richard Coers]

We don't know what kind of tool you are using, nor what rpm you are turning. A little more info please. Do you use the same size block on all the turnings you get the vibration? You aren't supposed to bottom out anything in a chuck. Cut a shoulder so you work off the front face of the jaws.

 

[hockenbery]

I've got a Powermatic 35/20 and on many, if not most, turnings I get a chatter, vibration if the tailstock isn't applied. I'm currently turning a 7"x 1 1/2" rosewood bowl and when I make my cuts to the inside I get this vibration. Its got a 2" piece of hickory waste block glued to the bottom and is tightlly secured in a VicMark 120. But this happens on many of my works and its puzzling. Wonder if I might get some input on this.

With a classic hemisphere shape bowl the curve continues from rim to bottom inside. As you cut following the curve you are always cutting across the grain and get clean cuts since each fiber is supported by the fibers below it when it is cut.

In a very shallow bowl 1 1/2 " high if is quite likely you are cutting parallel to the grain or nearly so.
Your chatter may come from this:
When the tool edge comes to the endgrain it is trying to cut strait into it and it can't so the fibers come over the tool edge instead cutting and as the wood spins around the tool and the wood get pulled. The wood tears and the tool bounces a bit creating the chatter.
It is generally worse with harder woods. Many rose woods are quite dense.

I have this problem once in while when I have crotch figure in the bottom of the bowl with grain running every which way some will climb in top of the cutting edge instead of cutting. I will take lighter cuts and be sure to grind the heel off the gouge to shorten the bevel reducing the bevel drag.
But once the chatter starts it can only be cut away by cutting the layer of wood beneath the chatter

This is a time where I use a round nose scraper,from the center out smoothing the surface.
The scraper facing straight into the long grain will do quite well and remove the chatter.

 

[Bill Boehme]

There is a slight possibility of a problem with the wood or in you turning technique: hidden flaw in wood, thin walls, dull tool, incorrect tool rest height, using a bowl gouge as a scraper, not making bevel gliding cuts, wood warping, etc. However, I'm leaning towards a machine problem and not a user problem.

I strongly suspect that there is excessive axial play in the spindle. Go to page 18 of your owners manual and run through the procedure for checking and correcting axial play by applying a preload on the spindle bearings. There is also a slight possibility of radial play. There can be some misleading interaction between axial and radial play, but there shouldn't be any free play. The cure for radial play is to install new bearings, but i seriously doubt that is the problem. I do believe from what you have described that there is axial play ... and, it doesn't take much axial free play to cause vibration. The fact that the vibration goes away with tailstock pressure applied reinforces my belief that the cause of the vibration is axial play. Heed the warning in the manual about applying too much preload. If tightening the nut 1/16 of a turn fixes the problem then stop right there ... more is NOT better.

 

[Bob Strand]

Thank you Mr. Boehme, and all the rest of you, for that matter, but that axial play sounds like it may be the answer. Not too long ago I changed my belt so your reference may be my answer.

Later,

Bob