vibrations?

[Scoony]

I use the lathe mostly for turning chair legs. I use a lot of hand tools (planes and chisels) so I feel that I am getting my tools sufficiently sharp. I am grinding a hollowed bevel on the skew, then honing it on a med india and hard ark, then stropping on leather with the green stuff.

I am using an older PennState 10” lathe with bed extension. It is not bolted to the table, but rather rests on the rubber feet on a solid bench.

When using the skew, I am getting a shallow screw thread effect on the material. I feel that there is vibration/chatter affecting the cut, and understand that dull tools would be a culprit, but assuming that the tools were sharp enough, what are some of the other factors that would cause vibration? Or what I am describing?

I also experience chatter when using a parting tool. I am getting it sharp, and I am sure that I am presenting it correctly, pivoting down into the cut. The parting tool is an old smaller chisel, I think its time to upgrade to a better parting tool.

[Arlin Eastman]

Bolting it down or even putting C Clamps on the feet would help a lot.

Arlin

[Steve K]

A possibility is your riding the bevel too hard causing the piece to flex and spring back some. Just a guess. Someone here said, the bevel should ride on the wood but the wood shouldn't know it.

[Ruler2112]

Steve K said:

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Someone here said, the bevel should ride on the wood but the wood shouldn't know it.

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My wager is that you're pushing too hard into the wood. Let the wood come to the tool, don't force it.


Also, hollow grinds are awesome for stuff like knives & such, but they make control at the lathe more difficult because there's so little support for the cut. The bevel is there to keep you from cutting too deeply and without having a flat bevel, it goes from not cutting to uncontrolled cutting in a flash; very little 'sweet spot'.

[MichaelMouse]

Any part of the tool which is not cutting should not rest on the wood. Doing so allows a bit of compression in the soft earlywood, and a ride up over the harder late. Degenerates into that chatter and spiral you've noted.

For the sake of common terminology, you want a clearance angle, as demonstrated here. http://homepages.sover.net/~nichael/nlc-.../caop.html You anchor to the rest, lay the bevel along the piece, then lift the heel slightly to begin the cut.

Parting tools can be used as scrapers or cutting tools. I stay above centerline and use them to cut. Scraping at center gets you hard-soft-hard grain problems if you're less than sharp.

[robo hippy]

That type of spiral pattern comes from what I have heard called 'whipping'. This means the piece is flexing as you cut. Possible causes are too much tailstock pressure, too much tool pressure, and the piece can be long and thin. A ratio I have heard is 10 to 1 for size, so 10 inches long and 1 inch diameter will be pretty stable. Anything longer and not bigger diameter will tend to whip no matter how light your tool pressure is. You can learn to use your fingers on your off hand as a steady rest. Very gentle pressure, and tool pressure equals finger pressure.

robo hippy

[NCPaladin]

I have heard of screw thread (Alan Lacer) to refer to run back when you loose bevel contact with the skew. This video by Richard Raffan shows concentric ridges from inadequate bevel support at the start and about 4:15 second he show a screw thread. His screw thread is from getting the wrong area of the tool into the wood but the same thing will happen if you loose bevel support while planning.
https://www.youtube.com/watch?v=jOvF5f1phhY

Stuart Batty demonstrates similar poblems. I can't find the one on the skew but the results are almost identical to this with the roughing gouge. Caused by feeding too fast for the rpm.
https://vimeo.com/68784519

Pushing in will cause the chatter. With the skew you try to cut towards the headstock (or tailstock) and apply as little pressure as possible from the perpendicular. This is why you want your skew razor sharp compared to other tools, the sharper the blade the less force needed.

If you can post a picture it will probably help folks with just what the problem may be.

[Scoony]

Thanks all, a little more info:
I am turning hard maple and I am sure that the length and thinness causes part of the problem. I am laying the bevel against the word and lifting the handle to present the blade to the wood. I think I may be trying to ride the bevel a bit much. I am trying to achieve a finished surface off the skew and have been able to do it with other woods in the past. Walnut for example, does not seem to give me these problems.

[AHill]

The advice about not pushing too hard when cutting is good. You said "length and thinness" in your last response. Both will contribute to the piece vibrating, and it may not matter what tool you use. I recommend you investigate getting a spindle steady. It will add some stability to the workpiece. There are shop made versions and store-bought versions of spindle steady's. For really think finials I make on ornaments, I even resort to using my free hand (hand not holding the tool) to support the piece from the back side. I don't recommend this method in most cases. You gotta be real comfortable and have a fair amount of experience to do that. I don't use that method with a skew.

[iclark]

First, it sounds like you are working way too hard on your sharpening. The process that you describe is okay for your first sharpening (and assuming that you are using a grind with a straight cutting edge). After that, though, you do not need to go back to the grinder until your hollow-grind is pretty much honed away. When the edge starts to dull, just use handheld stone or steel to touch up the edge. I use one of Lacer's 600 grit diamond steels. Use the front and back of the hollow grind as your sharpening guide. You will slowly develop bevel flats at the edge and heel of the hollow grind. They are your friend. When they meet is when you go to the grinder.

On the spiral that you are describing, I have run into that with ash and with maple spindles. If the spindle is not round where you start your run with the skew, then the high spot will give the edge a slight bounce.If you do not stop and take care of that high spot, then you get the spiral that you described. Use the skew as a drop-edge shear scraper to get a section of the spindle round. Then use that section as your base to cut right and left from. If you start to get the spiral again, back off to the last previous round section and pick up your cut again.

The flexing of a long and relatively thin spindle can make this effect worse, but I have had it happen on 3" spindles that were pretty short if I got in a hurry and was not paying enough attention.

The Zorro catch that some of the others mentioned is a very different effect than what you are asking about.