Gouge vibration is a problem that besets people when they are learning to turn bowls. The gouge bounces on the wood, and no amount of pressure seems to stop it, however strongly they grip the tool. The more they continue, the worse the vibration gets, and unsightly ridges appear on the turned surface. It usually affects the outside of the bowl. This gouge vibration has two causes.
Irregular bowl blanks
This is the most obvious cause of gouge vibration. When roughing out an uneven blank, it is very easy to push the gouge forward into a low spot. When the high spot comes round, it hits the tool bevel and knocks it back. This immediately sets up an in-out rattling vibration. To overcome it:
- Increase the speed of the lathe (as consistent with safety, and not so fast that the machine shakes or there is any risk of a chunk flying off, which would be highly dangerous). There will be less time in each revolution for the gouge to move forward.
- Lower the gouge handle or twist the gouge so the cut is more square to the edge. This lets the wood be sliced off before it gets to the bevel. But this could be risky if the tool pushes too far into the gap and the cut is too heavy.
- Take care not to push the gouge against the wood. Apply any pressure necessary to stabilise the tool downwards against the tool rest.
- Reduce the feed rate – allow the high spots time to come to the tool and be sliced off.
- Adjust the tool rest closer and use a bigger gouge. Small gouges can flex and set up vibration. If the gouge reaches too far over the tool rest, it magnifies the effect of incorrect technique. The gouge is harder to control.
- Make sure the gouge is sharp. Blunt tools make for hard work.
- Cut the bowl blank closer to the finished shape.
Pressing the bevel on the wood
More baffling is when gouge vibration begins for no obvious reason part way through a cut that has been going smoothly. As the cut continues, the vibration rapidly gets worse. The cut becomes noisy, and when you examine the surface, there are spiral ridges.
The cause is pressure of the tool bevel on the wood. Any attempt to control the gouge vibration by pressing harder will fail. It is often said that the bevel should rub the wood, but this is not strictly true. The bevel is aligned with and in contact with the cut surface, but should not press against it with any significant force. Pressure compresses the softer parts and when a harder area comes round it throws the gouge out. The vibration is slight at first, but each time the hard parts come round the effect grows. The softer parts are cut deeper and the ridges get bigger and bigger.
- When gouge vibration is felt, stop or adjust the cut immediately.
- Make sure the gouge is sharp, and adjust the tool rest if necessary.
- Move the tool back to a point where there is no vibration and the bevel can rest quietly on the surface. Align the bevel in the direction of the cut, then lift the heel of the bevel very slightly so that the wood contacts the cutting edge before the bevel.
- Make sure there is no pressure on the wood, but stabilise the gouge by pressing down against the tool rest and holding the tool handle to your body. Don’t extend it too far over the rest. Restart the cut, moving the gouge forward slowly enough to remove the ridges as they come to the tool.
- Sometimes taking a heavier, more positive cut will help eliminate the problem.