Resonance when turning bowls

[Prashun Patel]

When I'm turning bowls, when the walls get thinner out towards the rim, my gouge seems to cut ok, but I get this awful resonance or humming sound from the cut. I can minimize this by finagling my pressure and angle, but I'm curious what causes this? Is this common? Do I need a bowl steady? Or do I just need to sharpen more often?

[John Keeton]

Prashun, some of this is difficult to control as the bowl loses its concentricity, but it does sound like you may have lost your edge, as well. You will need an extra keen edge as the bowl gets thinner, and the least bit of pressure will further cause a loss of roundness. You can use a little hand pressure on the back side to ease the situation ( a thin glove might be in order) - or, get a single arm bowl steady.

[Dan Hintz]

Also, don't forget to leave as much bulk in the lower portion of the bowl as possible. In other words, don't try to hollow the entire bowl to a uniform thickness with each pass... work on thinning out the top third first, then once you're pretty happy with it, move on to the middle third, etc.

[Gary Conklin]

Using a a moded gouge with a very narrow bevel will help as well. Just take a regular gouge and grind back the heel leaving a bevel of 1/8" or less.

[Reed Gray]

Tool control on thin bowls is really a 'touchy feely' thing. When you 'rub' the bevel, there is almost no pressure on the bevel. If you have too much, the bowl will start to vibrate. It is a lot easier (and technically the wrong tool to use) to get this resonating sound if you are using a scraper flat on your tool rest. You can use your hand on the outside edge of the bowl as a steady rest. Again, very gentle pressure and tool pressure = hand pressure. If your hand is getting warm, then you are pushing too hard. Think of a soap bubble on a wand. If your hand is at rest, the bubble stays still. A tiny wiggle by your hand will start the bubble moving.

ROUND OVER THE BOWL RIMS BEFORE YOU EVEN THINK ABOUT PUTTING YOUR HAND ANY WHERE NEAR THE RIM. IT WILL SLICE YOU TO THE BONE IF YOU DON'T.

It is almost impossible to take a final inside cut from rim to bottom on a bowl that is of any size, especially if it is thin. This is why we normally turn the inside down in stages, and blend in the start and stop points. Wood moves.

robo hippy

[Sean Hughto]

The sound is from the outer edge vibrating. A sharp tool - a light cut - a different approach angle or grind (same thing achieved in a different way) - and cutting the edge to final thickness while plenty of meat remains on the lower portions are all things you should consider. And how thin is thin? The larger bowl in the picture below is 13-14" in diameter and around 6" deep. It's maybe a quarter inch thick throughout. I consider that thin for the size, and that's what I'm talking about in my suggestions. Perhaps you mean something radically thin? I've done some of those in smaller sizes; there are other strategies that can be brought to bear - like turn the inside first, then use a custom jam chuck to support the form while you work on the outside.

[Prashun Patel]

Thank you, Everyone.
Sean, my bowls are no thinner than yours @ 1/4". Often, I'll get this vibration at even 5/8". I will try everything below.

Truth be told, my gut tells me i'm just not sharpening enough, which might be leading to slightly more pressure than is appropriate.

I already normally turn in sections, with the rim going first.

[curtis rosche]

for very very thin peices, under an 1/8 thick, i stead one side with my hand so that the gouge doesnt just push against the wood and shatter the peice

[Bob Bergstrom]

To turn thin one should use a fairly pointed bowl gouge. As been stated, sharp and little bevel is important. A bowl with a good natural flowing curve will also help. The JoHannes Michelsen grind is a good example. http://www.youtube.com/watch?v=OYeTaROD_jk His hats are about as thin as any. Graeme Priddle is another noted turner with a longer pointed gouge similar to this oneIMG_2584.jpg
IMG_2583.jpg
Of course the narrow the tip and bevel the more likely the ripples and possibility of going right through the side wall. There is a learning curve to turning thin, so don't try it on any wood you value. Maple and other stable woods will also help.

[Reed Gray]

Sharp tools and a light touch. I watched Mike Mahoney and Stuart Batty turn a bowl at the Symposium this year. Maybe 8 inches deep, 12 or more wide, maybe 1/8 inch thick. No steady rest, no hand steady, no long pointy bevel and swept back sides. Light touch! You can do it, buy you have to FEEL THE FORCE, and only the back/right hand applies any pressure.

robo hippy