Some people spend hours sanding a bowl to get a reasonable finish. But, apart from the tedium, excessive sanding can have a bad affect on the finished bowl. It is not the best way to deal with torn grain. Heavy sanding should rarely be necessary, and there are a number of things to try before getting out the 40 grit. I thought it might be worth setting down some of the things that help to prevent torn grain when turning bowls.
To achieve a good surface, you have to cut the shaving in a way that allows it to separate cleanly without damaging the underlying surface. As the gouge pushes between the shaving and the wood beneath, it acts as a wedge. The shaving has to bend to slide up and over the wedge. If the shaving is stiff, it will not bend easily, and because its fibres extend back into the main body of the timber, the stress can start a split that runs ahead of the cut. The fibres are not cut, but torn apart. Tearing usually occurs either where the surface is changing from end grain to side grain with the rotation of the wood, or where locally disturbed grain opposes the cut. These are situations where a split can easily start and propagate.
To prevent torn grain:
- the shaving needs to be thin and weak during final cuts so it cannot transfer much force back into the uncut fibres
- the angle through which it bends must be small
- the uncut fibres must adhere to each other strongly enough to resist being split apart.
The first thing to do is of course to sharpen the gouge. The edge can then cut the fibres before any gap opens in front of the cutting edge.
Increase the lathe speed (as consistent with safety. A chunk separating from a fast-spinning bowl blank can be highly dangerous). For a given feed rate, the shaving will then be thinner and less robust. This allows it to separate from the timber with less stress on the remaining wood.
A slower feed rate will also remove less wood per revolution, making thinner, weaker shavings which bend and break easily without much leverage on the fibres not yet cut. Many beginners seem to be in a hurry to complete the cut before something goes wrong. Let finishing cuts be slow and gentle.
A lighter cut, like a slower feed rate, will make the shavings thinner so they pull less. For best results, the final cuts should be as light as practicable. A very sharp gouge makes this easier.
Use a smaller gouge. The tighter radius at the point of cut will take a narrower shaving, which again will be weaker and will separate more cleanly. This is why the curved edge of a gouge will sometimes cause less tear-out than a skew chisel when spindle turning.
Make sure the bevel is properly aligned with the surface underneath, without pressing on the wood. If the heel of the bevel lifts from the surface, it changes the top angle and the shaving has to bend more to get into the gouge flute. The tool is harder to control too, and tends to make grooves in the surface.
On the inside of the bowl, a short bevel will fit the curved surface better than a long one. It allows you to optimise the top angle and improves support and guidance of the tool.
A keener, more acute sharpening angle on the gouge will also affect the top angle. Any sharp edge will cut, but a smaller bending angle for the shaving will reduce the pull on the fibres.
Present the cutting edge at a skewed angle. The effective bevel angle is at a maximum when it meets the oncoming wood square on. If the edge is skewed, the wood sees the sharpening angle as smaller and more acute and the shaving slips over the edge more easily. Also, the skewed edge takes a narrower shaving. A traditionally ground bowl gouge (ground square, or nearly square, across) can be used with the wing at a very skewed angle, giving a very clean cut.
Make the final cuts with a very gentle scraping action. The lower wing of a swept-back gouge, with the flute closed and the handle down to skew the edge, will take extremely fine, fluffy shavings. Very little wood is removed on each pass. An ordinary flat scraper can also be used, on its side to skew the edge, and is easier to keep sharp than a gouge. Scraping like this will usually get rid of ‘macro’ torn grain but does not leave a burnished surface as a bevel-guided cut can.
A scraper flat on the rest can take a wide shaving, particularly if it has a curved edge that is similar to the curve of the wood surface. The shaving has to bend sharply as it is cut, but the top angle is too great to create a wedging action. Provided the shaving is kept thin and there is no vibration, a reasonable surface may be achieved. Too much tool projection can cause vibration, and so can thin walls on the bowl. Most woods respond better to a gouge.
Cut the wood ‘with the grain’. In a bowl, this usually means cutting the outside from bottom to top and the inside from the rim to the bottom. Then the fibres approaching the tool are short and running out of the surface of the wood. Any split that begins to form will follow them and exit the surface before doing damage. In addition, the fibres are supported by those below and this allows the gouge to cut them rather than break them off.
Some people wet problem areas with finishing oil or water. It lubricates the cut and softens the fibres to allow the shaving to bend easily. Wood with high moisture content usually cuts better.
Some timber species are more prone to tear-out than others. Their fibres separate more easily. It is possible to apply shellac or other sealer to reinforce the uncut fibres and make them more resistant to splitting apart.
If your best efforts still leave noticeable tear-out, you will need to sand to remove the damage. 120 grit would normally be considered coarse. I usually start with 120 or 180, and the sanding takes only minutes. If you sand just the defect you end up with a depression, so you have to sand away the surrounding high areas too – but sometimes you can get away with spot sanding with the lathe stationary, then blending it in with the lathe running.