Excessive vibration when hollowing a bowl

[Stefan Antwarg]

I just finished a rough bowl and I seem to be having a problem with a loud vibration. The sound seems to come from the bowl/gouge while I am turning the inside side of the bowl. Then about halfway into the cut, it stops. Then at the bottom part of the bowl, my tool is jumping. I then sharpened my gouge but it didn't change a thing. Any suggestions?

Stefan

[Jim Becker]

Stephan, this is not an unusual occurance, especially if you are trying to reduce the thickness of the entire bowl all at once and particularly on a larger bowl. The reason is that as you get the rim thin, it tends to flex and vibrate.

There are two things you can do to help mitigate this vibration:

1) "Step" hollow the bowl by from the rim in steps so you pretty much "finish" cutting the rim to final thickness first. Bring the rim to final thickness, but only to about an inch of depth (dimenison relative to the size of the bowl) and get it as clean as you can. Then work on the next inch or so and DON'T GO BACK to the part of the rim you already "finished". Repeat to finish the remaining interior of the bowl. You should only need a very light shearing finishing cut with a gouge or scraper to bring it all together.

2) Make or invest in a "bowl steady" to provide support to the bowl's rim on the outside and to the front of the lathe. This helps to stablize bowls that are vibrating as you describe.

These two things can be done separately or together.

[Stefan Antwarg]

Thanks Jim.

I was thinking that this was a problem because when I took my lesson with Bill, it didn't happen. He did warn me that I would be spoiled using his lathe. But you are saying it is a bowl issue - not a lathe issue.

Stefan

[Jim Becker]

Yes, it's a bowl issue, not a lathe issue. A sturdy machine will certainly be less likely to introduce problems of it's own, but we're working with a natural material when we turn things and it has variable density, variable moisture content, etc. The vibration comes from that. In fact, moisture can be a real factor....as you get things thinner, the wood dries faster (even a blank that has been sitting for years will still likely have some moisture to loose) and that tends to distort the wood as you turn...resulting in more vibration as you cut. That's why you don't want to have to revisit that rim once you get it where you want it to be. This all takes practice to blend things together, but it's an absolutely necessary technique, especially when you are turning "thin".

[Richard Allen]

Hi Stefan

I would like to know a few things:

What size and shape bowl gouge were you using?

What size, shape, condition (wet or dry) and species of wood were you turning?

What size of cut were you making?

[Stefan Antwarg]

What size and shape bowl gouge were you using?

What size, shape, condition (wet or dry) and species of wood were you turning?

What size of cut were you making?

I was using a 1/2" gouge with a side grind. It was a 10" wet cherry bowl with the standard rough bowl shape. I varied the size of the cut from a huge slice to just a dusting. The vibration was the same, which I found quite odd.

Stefan

[Richard Allen]

Hi Stefan

That is quite odd. I would go through a mental checklist. Most of the thoughts can be discarded right up front because being there you know the answer without having to think about it. Some lead to a quick check to eliminate.

1. Is the work piece secure to the lathe. Screws can become loose and chuck jaws can work into the wood giving a loose fit, the wood can crack from the jaw pressure and tool pressure.

2. The bevel of the tool should be riding on the wood. Can you ride the heel of the tool all the way through the cut without the cutting edge touching the wood. Sometimes a sharp transition or a deep steep bowl will result in the rim of the bowl getting in the way of the tool being used with the bevel rubbing.

3. Is the wood near the rim too flimsy to support the cut. With the outside of the bowl turned true and smooth have someone apply pressure to the outside of the bowl behind the cut. The pressure needs to be smooth and even. Putting a tool between the ways and leaning the handle of the tool against the spinning wood is a safe way of doing this. If the vibration goes way or is significantly reduced then the bowl is flexing to much when cutting. You can try pressing less with the bevel of the tool. Though the suggestions that Jim gave you are better.

4. Is there some "feature" (defect) in the wood that dulls the tool or significantly reduces the strength of the wood?

5. Is the speed wrong. If the lathe is variable speed try 200 or 300 RPM slower or faster. If it's belts for speed change try the lower or faster belt speed (try to keep the rim under 30 MPH). A particular speed can make the bowl vibrate with even a small external force.

6. Is the outside of the bowl running true and smooth? I generally turn the outside first, On some bowls that have warped a lot I will turn the outside a little and the inside a little working my way to the center till the bowl is running smooth and true. Then I take final shaping cuts working in steps, as Jim describes, toward the middle.

I like Jim's answer because most of the time, cutting well supported wood will give the best cuts.

Sometimes wood will dry differently and one side of a bowl can be a lot heaver than the other side. That out of balance condition is difficult to overcome.

Please let us know if anything actually helps.

Good Luck