Some people spend hours sanding a bowl to get a reasonable finish. But, apart from the tedium, not to mention the cost of sandpaper, 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. This article is based on my own experience, not on any formal research, so includes untested assumptions.
To achieve a good surface without torn grain, you have to cut the wood in a way that allows the shavings to separate cleanly without damaging the underlying material. As the gouge pushes between the shaving and the wood beneath, it acts as a wedge. Side grain shavings have 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.
On the end grain areas, the edge of the tool impacts the side of the fibres and pushes against them. They can break below the surface and pull out. The fibres are not cut, but torn apart. Tearing usually occurs either where the surface is changing from end grain to side grain as the wood rotates, or where locally disturbed grain opposes the cut. These are situations where the force comes from below the fibres, lifting them. It is where a split can easily start and propagate.
To prevent torn grain during final cuts, you need thin, weak shavings that cannot transfer much force back into the uncut fibres. They must only bend through a small angle, and the uncut fibres must adhere to each other strongly enough to resist being split apart.
How to get a good surface
- The first thing to do is of course to sharpen the gouge. The edge is then better able to 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 is 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. They can bend and break easily without much leverage on the fibres not yet cut. Many beginners rush to complete cuts 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, make the final cuts as light as possible. 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 torn grain than a skew chisel when spindle turning.
- Make sure the bevel aligns 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 reduces 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 usually reduce the pull on the fibres.
- Present the cutting edge at a skewed angle. The effective top angle and bevel angle are at a maximum when they meet the oncoming wood square on. If you skew the edge, the wood sees the angles 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.
- If necessary, 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. It removes very little wood on each pass. You can also use a diamond point flat scraper, on its side to skew the edge. It is easier to keep sharp than a gouge, and can be given a rolled burr using a burnishing rod. Shear scraping like this will usually get rid of ‘macro’ torn grain but does not leave a burnished surface as a bevel-guided cut can.
- Use a negative rake scraper flat on the rest. Beginners sometimes unintentionally lift one side of the tool off the rest, which causes minor dig-ins that tear the grain. The shaving has to bend sharply as you cut it, but the top angle is too great to create a wedging action. Provided the shaving is thin and there is no vibration, a reasonable or good surface may be achieved. Too much tool projection can cause vibration, and so can thin walls on the bowl. Most woods respond best to a correctly used gouge.
- Cut the wood ‘with the grain’. Turn the outside of a bowl from bottom to top. Turn 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. Then the gouge can cut them rather than break them off.
- 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 torn grain than others. Their fibres separate more easily. It is possible to apply shellac or other sealer to reinforce the uncut fibres. This makes them more resistant to splitting apart.
If your best efforts still leave noticeable torn grain, you will need to sand to remove the damage. There will almost always be some sanding needed to remove tool marks too. 120 grit counts as 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. Sometimes you can get away with spot sanding with the lathe stationary, then blending it in with the lathe running.