I recently upgraded to a full size lath from a midi. I decided to turn a large bowl that is17" diameter. I had quite a bit of vibration at the inside of the bowl lip even using a steady rest. Tool was sharp. slowed the speed. What am I missing? I ended up sanding more than usual.
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Large Bowl Vibration
- Thread starter Jim Bellinghieri
- Start date
Beautiful bowl!
With solid blanks I’m careful to step the cuts down so that there is more support, especially on larger bowls. With a segmented piece you don’t have that option, therefore I think just the standard practices apply to avoid vibration (sharp tools, tool rest tight, shallow cuts). But with that large of a bowl I think you’ll get a fair amount of deflection as the walls get thinner.
With solid blanks I’m careful to step the cuts down so that there is more support, especially on larger bowls. With a segmented piece you don’t have that option, therefore I think just the standard practices apply to avoid vibration (sharp tools, tool rest tight, shallow cuts). But with that large of a bowl I think you’ll get a fair amount of deflection as the walls get thinner.
I've found that it's best to "step down" the inside of large bowls and not go back to those areas if at all possible. Even fairly dry bowls will go out of round when being turned and create the vibration you mentioned. All bowl blanks inherently have stress and as we remove wood, we change the stress, sometimes dramatically.
How were you griping the bowl? Tenon, mortise? And what was the diameter of your tenon? You need a big grip to keep something that large steady.
The tenon was 2 1/4. The vibration is at the lip of the bowl do you think a bigger tenon will help that? I'll go bigger next time and see if that helps. Can't hurt.
Definitely too small of a tenon. I would use 4” minimum, and probably larger. You can start with a large tenon, and after finish cutting the upper od and id, ~50% depth, turn the tenon smaller if your design calls for it ( to turn a smaller foot).
“stepped turning” can be done with a segmented piece. You might consider making your segments a bit wider such that there is more material in the id the provide support in the upper ~50% or so of the bowl. Dont “round out” the lower areas of the id until the upper part is finish turned.
These changes will require more wood that turns into chips but will stabilize the walls significantly.
“stepped turning” can be done with a segmented piece. You might consider making your segments a bit wider such that there is more material in the id the provide support in the upper ~50% or so of the bowl. Dont “round out” the lower areas of the id until the upper part is finish turned.
These changes will require more wood that turns into chips but will stabilize the walls significantly.
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Can’t add much to the above advice.
A 17” bowl is quite a nice sized piece. Your bowl looks quite nice!
You’ve learned that a 17” bowl is 10 times harder to do than a 10” bowl.
Many turners benefit by working up to the large bowls. They Do a couple 12” then a couple 14-15”. Then go for the 17-18 monsters.
Odie
Panning for Montana gold, with Betsy, the mule!
How slow rpm?
^^^^^ Marc Is correct.....a bigger grip will help, but there are other factors that will contribute.
I must confess that I've never turned a 17" bowl, but I have turned 15" bowls. I have gone down to very slow speeds for these.....around 3-400 rpm, or about the same speed as I sand at. A tool not as sharp as it could be, will be a major factor. Too aggressive a cut will be a factor, too.....Very light delicate cuts with sharp tools, and slow rpm.
-----odie-----
I've turned a lot of 18" bowls, 24 to be exact. Happy that this patron paid for my Oneway 2436. I sharpen about every 3rd pass as I get near the end. And by sharpening, I mean one pass on a 120 grit CBN wheel. I also use the Hannes grind which gives me a much better shearing cut. Then I move the gouge much slower on the last pass or two. Finally I find that a light grip on the handle makes a huge difference. Let the gouge do the work! Another tip is that shown hardly anywhere for the inside of a bowl. It's a super fine shear scrape with the bowl gouge. Flute is turned over and the bottom of the handle dropped as low as I can go. This lets you feather off any little blemish.
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I'm not an expert turner, but here is another tip. When doing the inside of a bowl I put a large rubber band on the outside to absorb vibration if I'm having trouble with vibration. Tape it on if it comes off. I was taught this when turning brake drums. I use pieces of inner tube and bought the large rubber bands from Woodcraft. I also have turned a couple of segmented bowls and noticed more vibration from them until you get them smooth. I would agree with all the tips listed above also.
An inner tube is much different than a rubber band. I'm very skeptical that a rubber band can change the harmonics of an 18" bowl with 3/8" wall.
My experience with thinner wall bowls and more so on larger ones, is that a curved wall will not deform as easy as straighter walls.
Large straight walls will flex quite readily, and yes a bowl steady will help some, also using a smaller bowl gouge will cause less deflection, plus steadying with your hand helps also.
Large straight walls will flex quite readily, and yes a bowl steady will help some, also using a smaller bowl gouge will cause less deflection, plus steadying with your hand helps also.
You didn't mention the species, but I find that 15"+ maple blanks that have been shelf dried will move aggressively when finish turning. As mentioned above, turn the rim to finished wall thickness and don't try to go back - it will have moved. Work down the wall in 1" - 1-1/2" steps. Works well with the Big Leaf Maple we get locally.
All good advice. One more thing to consider is the shape of your tenon. If you have dovetail jaws and the angle on the side of the tenon is not the same as the angle of the jaws, then you will be gripping only a small portion of the tenon. If the bottom of the bowl isn't flat and perpendicular to the axis of the lathe where it meets the face of the jaws, you will not have maximum support of the blank.
Rule of thumb - tenon size - 40% of diameter of bowl. Lathe should be bolted to floor or weighed down to decrease vibration. Sharp tools and highest RPM until vibration starts , than back off until it stops.
@Paul M. Kaplowitz I think your profile photo is the perfect credibility-verification for this thread. Nice one!
Russ Braun
TOTW Team
That’s one big beautiful bowl Jim! If I recall correctly, my widest segmented piece was in the 14” range, 7” max depth. I like to take my wall thickness to under 3/16”. Once I turn the outer rim, I’m done with it. There is no going back. I use a caliper and go 3/4” at a time. Use a pencil to mark where the caliper sticks and go again. After the second 3/4” section, I run the NRS over the two; continue working my way to the center/ bottom of the vessel. I do like to not let the speed get too low; that’s a feel thing. Sharp tools are a must. To add to the pressure, my vessels are typically 144 segments per ring. These have months of glue up time and they are real knee knockers to turn. The vessel in my picture is 18.5” tall. This was made in three separate sections. Take your time and you’ll be happy to know that your turning skills will improve the more you do it. Most cutting is with a bowl gouge, scrape/ finish cuts with the NRS. Tight radii are done with an East Wood Tool, Round NRS cutter. Have fun!!
Jim, consider if you used a 17 inch grip,,there would be zero flex from the wood. Only the flex (really none) from the spindle and chuck. Grab that same bowl with a 1 inch tenon, and you would have whip so bad the piece would likely come off the lathe. we all have to decide wherein that continuum we feel comfortable.
My practice is to figure a foot about 1/3 of the rim diameter. I turn my tenon to be a bit bigger than that. To complete the bottom of the bowl I trim my tenon to become the foot. So, my 17 inch bowl would have a 6 inch tenon.
I hope this helps.
btw, it’s a beautiful piece!
I think just one of the reasons solid wood bowls deform, more so at higher speeds is because of grain orientation, and it will deform more along the end grain than on the side grain. Since your bowl is segmented, the grain orientation is the same all the way around. As others have said, a bigger tenon, I remember hearing 1/3 to 1/4 the diameter of the bowl. For a piece like that, I would probably use a waste block, solid wood, not plywood, or at the very least high grade cabinet plywood.
Having that feather light bevel rub is huge with any wood bowl over about 10 to 12 inch diameter. It takes astonishingly little bevel rub to get the bowl do deform. Turning the inside down in stages, like 1 to 1 1/2 inches of finish cuts at a time really helps.
robo hippy
Having that feather light bevel rub is huge with any wood bowl over about 10 to 12 inch diameter. It takes astonishingly little bevel rub to get the bowl do deform. Turning the inside down in stages, like 1 to 1 1/2 inches of finish cuts at a time really helps.
robo hippy
Im late to the question and I'm certainly not an expert but I have turned many bowls 19-20 inches. The walls of the bowl will vibrate less if they remain parallel inside and out so if you cut the rim of the bowl thicker than the curve to the bottom that is thinner in the curve it flexes too much if you return to the rim. Unfortunately this is a problem I have had to deal with too much. So, what to do. Sharpen a scraper wider the better. I use a negitive rake but not sharpened like a skew I simple have two bevels, the top on is much shorter than the 70 degreeish bottom. However, instead of using the grinded burr, or what is left from grinding, I roll or burnish a burr on with a carbide rod. This makes a sharper longer lasting burr. I then hold the scraper turned up in a steep shearing angle and work very light. Time consuming but better than sanding!
Roger Wiegand
Beta Tester
It seems to me that there are different kinds and sources of "vibration", with overlapping but perhaps not identical solutions. As many have already pointed out any flexibility in the headstock, chuck, or tenon can produce vibration, as can too much tool pressure. Sometime there is varying density in the wood (a knot being a prime example, or asymmetric grain) that will set up a resonance vibration as you turn. This can be addressed first with sharp tools and a light cut, but also by adjusting the speed up or down by even a few rpms. Solid wood bowls can (and do) warp as you work on them with small changes in temperature and humidity, especially if they aren't fully dried to equilibrium, though presumably not a problem with a segmented bowl like the OP's.
Careful as I might be about it I often get a slight eccentricity (and resulting vibration) when flipping a bowl from shaping the outside and forming a tenon to holding it by that tenon and shaping the inside. Have to admit I don't know where that comes from or how to fix it. It does drive me somewhat crazy.
Careful as I might be about it I often get a slight eccentricity (and resulting vibration) when flipping a bowl from shaping the outside and forming a tenon to holding it by that tenon and shaping the inside. Have to admit I don't know where that comes from or how to fix it. It does drive me somewhat crazy.
With segmented work where using one species of straight grained wood there is in cutting each segment a dowm hill cut then with the grain the an uphill cut that is repeated at each segment. The use of multiple species or figured grain can also exasperate the problem and this in itself will cause vibration.
I can't come up with any other way to explain it, but I do believe it is a factor and often you can feel the joints even with the best cut and sanded piece.
Happens sometimes to me also. I dont fully finish the OD until its flipped and in the jaws. I also have not been able to find root cause much of the time. Not getting the tenon right does it but is easy to ID. Many times its very slight. I chalk those up to just a bit different bite from the jaws. A lot of pieces have such swirly grain you cant rotate the piece with respect to the grain to get even clamping. I also probably over tighten jaws vs most, but They dont come out. Many times, when showing someone a cut etc (they tightened the jaws), I’ve had a piece come loose.
Odie
Panning for Montana gold, with Betsy, the mule!
The best rpm is a major contributor to eliminating vibrations, providing the best possible cut is also being taken advantage of. One consideration of note, is to understand that end grain provides more resistance to the cut, than does the long grain. Sharp tools can overcome this naturally occurring rule somewhat, and if your tool is going in-and-out with the rotation of the bowl, and a delicate cut is the goal, then you're taking too much of a bite.....and, it's very possible your tool isn't sharp enough to perform what you're asking it to do. All, or some of these variables may change, if the cut desired is a heavy cut with a big bite.
-----odie-----
-----odie-----
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Agree completely, and will add flute angle - sometimes rolling the flute just a bit open or closed can help.
Hi freq vibes, that leave little “rumble strips” are from resonance. I mostly get this with thin walls. Something within the “system” must change for it to go away. A very light cut with a smaller tool, and slight pressure from a protected hand (or steady rest), and rpm changes are the typical tools. Look up Hanns doing a cowboy hat. (BTW I aint even in the same ballpark as that guy)
Odie
Panning for Montana gold, with Betsy, the mule!
Yep.....whatever it takes that works. Each piece of wood has it's own individuality, and whatever works this time, may not be what works the next time. It helps to have enough arrows in the quiver to handle whatever circumstances you have at the moment.
-----odie-----
Russ Braun
TOTW Team
Lot’s a variables, change the form to wide, then tall, thin wall, thicker wall, wood types, segmented…..Experience in all these things equip the maker with solutions to an issue created by seemingly endless factors. No one reason, no one answer!Yep.....whatever it takes that works. Each piece of wood has it's own individuality, and whatever works this time, may not be what works the next time. It helps to have enough arrows in the quiver to handle whatever circumstances you have at the moment.
-----odie-----