Vibration Hollowing A Bowl

I was hollowing a Sassafras bowl about 11" in diameter this afternoon. The
walls were about 1 1/4" thick and I was using my Crown PM 5/8" bowl gouge
with Ellsworth grind. Near the rim of the bowl, at the beginning of the
cut, I began to experience some vibration.
I put a fresh edge on the gouge and finished with little problem, but still
with some vibration. Then a thought entered my mind........had I switched
to a smaller gouge, say a 1/4" bowl gouge, which would remove less wood per
cut, would it lessen the amount of vibration? Thanks.
Reply to
Barry N. Turner
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Not an easy question to answer! The situation is somewhat dependent on the reason for the vibration. For example, if you are working green wood, and the vibration arose because of imbalance, probably not. You'd be adding the "flex" of the thinner shaft and might actually have a rougher ride. When I run into this kind of a problem, my usual tactic is to check all the fixings, then try whatever tool I think might help until I come across something that behaves more to my liking and stick with it. Probably not textbook, bt it's the best I can offer: hope it's of some help
Kip Powers Rogers, AR
Reply to
Kip
Barry:
I'd start by changing the speed a little bit and see if that helps. You can run into speeds at which the blank resonates.
I think that a 1/4" bowl gouge is an almost useless tool. There just isn't enough metal there to resist flexing even with a minimum overhang. On the other hand, you might have tried a 1/2" gouge -- it may or may not help.
I'd also look for defects in the blank. If there is a problem with ring shake it often will show up as vibration and you would like to get it to the firewood pile before it comes apart at 500 rpm.
Also, since I happen to know what lathe you are turning on -- make sure that all 4 legs are sitting on the floor.
Bill
Reply to
Bill Rubenstein
Vibration occurs when the natural frequency (or a multiple of) matches a harmonic or rotation. You might eliminate or reduce vibration by slightly changing the speed. Also, a vibration can be reduced or controlled by adding or changing mass and/or bracing.
Reply to
Phisherman
Another friend of mine suggested that I take the bowl walls to final thickness incrementally, say an inch at a time, rather than making one sweeping cut from rim to bottom. I think that may work. I just didn't expect to have vibration when the walls were well over an inch thick.
It's not the lathe. All four (4) feet are solidly on the floor. I don't think it was the speed either, but it could have been. I tend to turn slower than most people I know. The blank appears sound. I have some doubts about the 1/4" bowl gouge too. I have one, but have never used it. Thanks for the advice.
Barry
Reply to
Barry N. Turner
it does seem odd to have vibration with walls over an inch thick - in my humble experience, that ususally indicates that there is a hairline crack and it's time to stop and find it - or if you know there are flaws, then it's your sign to use the sequential approach.

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Reply to
William B Noble (don't reply to this address)
On a rim that thick, at relatively slow speed, I'd inspect the mounting. Applies to wobble both free and with a load. You wouldn't be the first guy, especially on green wood, to have squeezed too hard and crushed some fibers so the chuck's no longer firm. Especially if you're holding wet sapwood. Shake the piece. If it moves, re mount.
If it's not moving on its own, but only under load, it's the presentation of the edge or the pressure on the bevel. With a 1/4" flute, you don't have to be cutting too deep to bury the upper, non-cutting part of the profile, and that will cause you to rip and slide over end and side grain. The pickup fuzz should be obvious on green wood if this is the case.
Last, you could be riding that bevel. That will still give you a relatively fuzz-free cut, but you'll get out-of-round really rapidly due to the difference between end and face grain. If the gouge wants to move in and out, that's what you've got. Sometimes you can even see compressed lighter areas against a wet background where you rubbed the heel of the bevel too hard trying to make a quick curve.
Changing speed is often suggested, but it's really the same as those folks who say if you drive your car faster down a washboard gravel road you won't feel it as bad. You'll still beat the crap out of your car, of course, and increase the impact force with the square of the velocity, not to mention the loss of control as your shocks are no longer able to get the wheels back to the road. Same on the lathe. I turn slow, too. No problem, as long as you sharpen and present properly. You can carve with only the gouge in motion, after all.
A 1/4" gouge is a great tool where the curvature is fast, or the access narrow. Use it where you have to, though mine has almost no grind at the ears, making it more suitable for the above uses.
One last shot in the dark, though this one should have been obvious at the get-go. I once cut a blank from wood that had been on its side for a while, ending up with all the water on one side of the blank. That was a grin and bear it. Not to mention, I had another blank to cut just like it, afterward....
Reply to
George
Thanks for the help. I'm still not sure what caused the vibration, but was able to finish the bowl with little trouble. Thanks to you and some others, I have several theories to check out. The blank was sound. It was green, but had been cut and anchor-sealed several months ago.
I don't believe mounting was the problem. I used a SuperNova with standard 2" jaws snugged up tight on a short tenon. I stopped a couple of times during turning and tightened the chuck. I also stopped to put a fresh edge on my gouge a couple of times, the last time just before the final cuts.
Right now, the theory that seems most likely is the one my friend suggested..........taking the walls down an inch at a time. Thanks for the advice.
Barry
Reply to
Barry N. Turner
I core almost every bowl that I turn. On all of the larger blanks, after it has been cored, as I start to turn the inside, there is vibration. I think that this is due to there being little mass to support the walls. I haven't ever had any vibration when turning the outside. To counter this, I let my left hand act as a stead rest on the outside of the bowl. Gentle pressure: if your hand is getting hot, you are pressing too hard. Also, I take very light cuts nearer to the rim. Taking it down to final thickness in steps, rather than one long cut is easier, but with practice, you can take that one cut from the rim to the base in one pass. Have you noticed that the vibration is gone by the time that you get half way or so down the inside of the bowl? robo hippy
Reply to
robo hippy
There was no vibration for me either when turning the outside of the bowl. And, the vibration was gone by the time the cut reached the bottom of the bowl. Next time, I'm going to try taking the wall down to thickness in increments. Thanks.
Barry
Reply to
Barry N. Turner
Hi Barry
I have had vibration in bowls that I did not think should have happened, however they did and My thinking goes like this, the shape of the bowl and the wood grain and stiffness that have to overcome the force needed to cut the wood, sometimes set up a surging or vibration. I have had more problems with flatter bottom bowl than with curved bowls, especially the higher side bowl liked to start flexing. And yes I think your gut feeling to go for a smaller gouge I think is right, anyway that did help for me, making the force needed to overcome the cutting edge smaller helps most, by either the presentation of the cutting edge or using a smaller gouge and sharpening helps some also in my experience , the problem with the smaller gouge though is of course that you do have to take lighter cuts and you do have to be able to keep the tool rest up close. For finish cutting a dried previously rouged out bowl, going the stepped way of 1" or 2" at the time is one way of minimizing the flexing, but keeping a nice flowing shape becomes harder to do, but it can be done.
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Have fun and take care Leo Van Der Loo
Reply to
l.vanderloo
Barry, Some other answers have alluded to green wood. Two comments.
Wood isn't always homogeneous, there might have been areas in the wood that were denser (or lighter) than the rest, and when you removed enough of the inside of the bowl the difference was enough to cause the vibrations.
The other comment, an 11" bowl does take time to turn, and we all ( I do ) need to take some time out for a 'bio break', lunch, or just sharpening the gouge. And I have a light close to the lathe. Make sure you turn that light OFF when you don't have the piece turning for extended periods of time (what ever that means). The heat from the light can dry one side causing warping, or just enough drying to cause it to loose balance. For long times (lunch) wrap the blank with a plastic bag. This will keep some of the moisture in the wood but it may collect in the 'lower' side of the blank.
mike
Reply to
Mike
I don't know - you may be a lot more skilled than I, but when turning to a uniform but very thin thickness I can never get below about 1/2 to 1/4 inch inch on a 12 to 14 inch bowl without vibration, and if you are using light transmission to get to 3/16 or 1/8 aprox, it is nearly impossible (at least for me) - I have turned (small) things less than .030, but not using conventional tools.
On 29 Jan 2006 09:34:15 -0800, "robo hippy" wrote:
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to contact me, do not reply to this message, instead correct this address and use it
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Reply to
William B Noble (don't reply to this address)
"William B Noble (don't reply to this address)" wrote in message news: snipped-for-privacy@4ax.com...
Seems to fall right about 3/8 thickness, doesn't it?
I've had some success with winding duct tape on the outside of the piece to stabilize it, and burn/cut my hand many times backing a cut, but the best has been the steady the kids bought me. Love it for almost anything I'm turning.
Reply to
George
Good point, Mike... especially with the 500 watt halogen that I use... makes you feel like an order of fries at Dennys Mac
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Reply to
mac davis
You have vastly over-estimated my skills. But, I'm flattered! My walls on this 10 1/2" bowl are something like 5/8" or 3/4". Thick! I can get to 1/4" without much problem, but have not tried to do so on a bowl this large. I just didn't expect vibration with walls this thick.
I've had a Stubby 750 for about a year........but my skills aren't quite Stubbyesque yet. Hey, is that a word? Did I just coin a new word? Where is Mr. Webster when you need him?
I would love to be able to turn a 12" bowl with paper-chip thin walls someday, but I may have to wait a while until my skills develop. Thanks.
Barry
"William B Noble (don't reply to this address)" wrote in message news: snipped-for-privacy@4ax.com...
Reply to
Barry N. Turner
My blank was green but had been cut a while and was Anchor Sealed. I didn't get the impression that warping was the problem, but I may have been wrong. I'll try some of your other suggestions. Thanks.
Barry
Reply to
Barry N. Turner
Jim Pugh, an occasional poster here, gave me a great tip on turning green wood.... when you need to take a break, wrap the work in a plastic bag to slow drying while you're away...
I was turning a bunch of almond last year and it would crack on the lathe if I got a damn phone call!
mac
Please remove splinters before emailing
Reply to
mac davis
Jim Pugh is an accomplished turner and he can offer a lot of good tips. I wish he would post more often to rcw.
BTW Mac, why is it that telemarketing calls altho cut short, cause worse warps and cracks than a call from a friend or re a sale at a gallery??
Turn to Safety, Arch Fortiter
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Reply to
Arch
I think it's because the thought patterns they generate upset the karma of the shop, Arch...
opps, sorry.... I think my karma ran over your dogma!
Mac
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Reply to
mac davis

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