Bowling turning question?(HELP)

[Brian Knop]

I have a Jet 1236 lathe. I have been roughing out some green bowl. But I am getting vibration out towards the end on the bowls and as I gouge out the bowl it starts to vibrate, as I thin down the wall, even on something like a 4X6 bowl. Not sure how I’ll ever get a good finish on them. Am I doing something wrong, is something wrong with the lathe or is this a common problem with this type of lathe? Even the face will not flatten out and it has vibration on it.
Thanks Brian

[Glenn Hodges]

Brian, I thought I was seeing double with your post. Are you talking about turning green wood or dry wood? I would imagine wet wood would flex more than dry. Sometimes when you get the bowl to thin, it needs support to prevent it from flexing when you are applying a gouge to it. I don't turn bowls this thin so I don't have this problem. I do not have your kind of lathe, so I do not know if it is the lathe problem. I doubt it is your lathe giving you the vibration, but my wife says I was wrong once before. I bet other guys will have a better answer.

[Gary Max]

Maybe I am reading this wrong but----when this happens I reset my tool rest so it is closer and use a very sharp round nose scraper. Very gently go over the area and smooth it down. If I can get the right angle I can clean it up and save a ton of sanding.
Of course these are my thoughts and someone else will do it another way.

[Bill Grumbine]

Hi Brian

There are a lot of factors which could be a problem here, and maybe even more than one happening at the same time.

Is your tool sharp? I mean, is it real sharp? Turning tools do not come sharp, and require sharpening often to cut well.

Are you using a bowl gouge or some other tool to cut the wood? If you are not using the right tool, you can get vibration, and in some cases, you are just waiting for trouble to happen.

What speed are you using? Too fast or too slow can cause vibration.

How about the angle of presentation of the tool to the wood? Just a minor shift in how the tool is presented can make all the difference between a good cut, vibration, or even a catch which can be very scary even for seasoned turners.

All this might make the whole thing sound overwhelming, but it is not. I started out on the lathe you have, and it will do a lot of good work. Let us know the answers to some of these questions and we can go about giving you some more detailed advice on how to get a good looking bowl.

Bill

[George Tokarev]

Not sure how you're holding the bowl, but one source could be that it's working loose where it's held. Especially important with chucks, as when they hold wet wood, and are put under stress, they can compress the wood and loosen their hold. Make sure you're snug at the base, or the rim will magnify any error.

Second source is a natural thing with the wood. As you hollow, you release a lot of internal bracing, so the wood flexes in the direction of least resistance, lengthening the bowl along the end grain. Two ways of compensating for this include cutting the rim thin while there's still inner support and accepting what comes thereafter, or limiting the flex with a steady.

In any case, about the worst mistake you can make is to press with the gouge, sometimes referred to as "riding" the bevel. Since pressure will exacerbate the natural flex, it can never really improve your situation. What you need to do is remember how to cut a circle, which is to have a firm reference point and swing the piece into it, so it's cut at the same distance from that point. This means a close toolrest, a firm grounding of the tool on the rest, and no more than a guide on the piece. Scraping allows a firm point and allows the wood to come to it, but is a poor remedy for cutting, especially when it's scraping on a broad surface. It takes face/angle grain away faster than pure end, which squirms away under pressure, and you're scraping uphill in two places on the bowl, so you will, to a greater or lesser degree, have to contend with uphill tears. Don't even ask about punky wood.

The absolute worst mistake you can make, inside or out, is to give leverage to the piece versus the tool. Keep the rest close, and you have greater control. I use a curved rest to finish the inside, but I don't use a bowl gouge, either, preferring to cut at a skew angle with a larger radius "spindle" gouge. The larger radius gives me better bevel support, and I can skew at two angles, producing a fair curve in an essentially catchless environment, because I don't have to make room for some longer gouge and handle over the bed, and the gouge stays clear by its own contour.

With a light guide and a gliding cut, you can scarcely feel cutting pressure, which means you're cutting, not chopping. It's chopping that whacks your elbow and the piece.

[Brian Knop]

Sorry I did a poor job of explaining myself. Thinking about it more the bowl is out of round and I can’t seem to get it round. Just trying to get the blank round I turn and turn, get the sides smooth but it not round. I can put my finger next to the blank when it’s turning and can feel the out of roundest. Even of the end face, I get it smooth but it’s not flat. So all of the vibration is coming from the out of roundest. I am using a 3” holding plate with screws. And I do not think that my turning tools are the problem. The farther away I get from the holding plate the more vibration I get.

Brian

[Greg Wandless]

Brian,

I have had similar problems with both green and dry bowls. I have had some improvement by hollowing it out in steps, first hollow out the top 1/4 part of bowl to get edge the thickness you want and then continue in a step wise fashion to the bottom. This way the outer edge are supported by the inner wood and should not flex as much reducing vibration.

It is also probable that as you hollow out your green bowl the wood will begin to dry and warp producing your out-of-roundness. I don't know of any way to correct that if you want to go from green blank to finished bowl without drying in between. I have made several bowls this way and let them warp to produce an oval shaped bowl.

If you want to make a perfectly round bowl you will need to rough out a bowl leaving the walls fairly thick, let it dry, and finish turn it later. I am sure there are more accomplished turners here that could tell you about returning a roughed out bowl.

Good luck,
Greg

[Brad Schmid]

Brian,

Sometimes it's hard to diagnose these issues via this medium, but let me offer a suggestion.

Applying too much pressure with the tool (I'll assume you're using a bowl gouge) often can produce out of round results. If you start with an out of round object and allow the gouge to follow the existing contours of the blank by applying too much pressure, not only does it not eliminate the out of round contour, it often amplifies it or makes it worse. Too slow speed combined with this makes matters even worse.

My suggestion is this:
1) confirm the blank is securely held by the faceplate and the faceplate fully seated on the shoulder of the headstock spindle.
2) make sure the tool rest is close to the workpiece.
3) make sure your gouge is sharp
4) run the lathe at a medium speed (somewhere in the 600-1000 rpm range should be fine for the small size you indicate) that doesn't cause the lathe itself to vibrate or jump around.
5) And this is where it will be hard for me to explain... Present the gouge to the spinning blank very lightly allowing the cutting edge to work the "high" spots only. Use a slow, steady motion, making light passes until the blank is true. The objective is to prevent the blank (and more importantly the "negative space") from controlling the cutting path of the gouge. Assuming the lathe and faceplate are running true and the blank is secure, you should see success.

I hope this helps, and let us know how you make out.
Cheers,
Brad