Tuesday, June 5, 2007

Chatter, The Wood

I generally turn green wood, although lately I've been turning some parts air dried. There is definitely a difference in the way that they turn. I do all of my green rough shaping and parting tool sizing at a higher speed and then slow down when it comes time to do my detail gouge work or skewing. Air dried wood also seems to turn nicely at higher speeds, but once vibration starts, it can be even more difficult to eliminate. Once again, slowing down can help.

One factor that may affect chatter is the pressure exerted by cranking in the tailstock. Green wood is flexible and too much pressure may not make a difference until you turn the thinner detail, at which point the wood can spring and vibration follows. I apply just enough pressure to capture the piece firmly, remembering that the headstock spurs may sink in as I work and require further tightening.

Adjusting the order in which you turn may quell some problems. I try to leave as much mass on the head stock side of the turning and finish it last so that the torque is transferred through the largest possible area. A smaller section may encourage vibration. Once you have cut a thin detail in the middle of a piece, you can probably expect chatter in the surrounding areas. When I was learning to turn baluster legs, I got in the habit of roughing out the leg and then cutting the lower cove area first. I figured that it was the toughest area and if I was going to screw up, I'd rather it be right away. Now, although I don't have so much trouble with that area, I still cut it first out of habit, bad habit. But I've learned that if the rest of the leg is roughed in and only clean up cuts remain, the vibration is manageable. Also, the relative size of details can make a difference. A tiny cove next to a huge bead may cause issues, as well as looking weird!

One of the pleasures of green wood is the size of cut that I can take cleanly without chatter, but at some point, I can always expect it to creep in. While I'd love to blame the machine or the wood, I have less control over them than I do over my tools and skills. Soon I'll address the tooling and skills that may help eliminate (or more likely manage) this problem.


greg said...

Maybe I'm getting a little ahead of you, but I'm interested in your approach to the taper on the top of the leg. I notice in your leg pictures this detail is not finished. Do you turn it after drying the top of the leg? Or do you use something like one of Elia's taper making tools after drying?

Peter Galbert said...

I leave the tenon end as a rough cylinder and then air dry the whole leg. Then I selectively dry the tenon by sticking it in the top of my kiln. Once dry, I remount the leg in the lathe and turn the tenon to the correct taper. I've found this to be my favorite way to get dead accurate tapers to go with my 6 degree reamer.