The information here will be very familiar to experienced turners. It is written for beginners who may be having problems with their bowls. I have been turning bowls for many years, and in that time I have discovered for myself the difficulties that can arise. Here are some of the problems that people encounter, and what can be done about them.
Tool vibrates on the walls of the bowl
This is a very common problem and causes small spiral ridges and grooves a bit like those made by a chatter tool. The cut becomes noisy and in extreme cases the wall can break. Even if less severe, the sanding becomes more difficult. Possible solutions are:
- Sharpen the tool. This is the answer to lots of other problems too.
- Don't press the gouge against the wood. No pressure is needed, just a light touch that controls the tool.
- As the wall gets thinner, chatter gets more likely, as the wood can flex. Use the fingers of your left hand against the outside of the bowl to support the wood and dampen out vibration while still maintaining control of the gouge.
- Slow the lathe speed. This is where electronic variable speed is very useful.
- Rotate the tool into the wood very slightly, as with the next following problem, so as to let the edge get beneath the chatter marks to cut a clean surface.
Gouge leaves a wavy, rippled surface
This can happen when the bevel is pressed against the wood. You can feel it bumping, and once it starts it gets worse and worse and very soon there is a pronounced ridge on the surface. You should stop as soon as you feel the gouge bouncing and level the surface before going any further. To remove the ridge, reduce the bevel pressure. If a gouge with a swept-back grind, cutting just behind the nose, is rotated very slightly into the wood, or the handle is raised very slightly, so the sharp edge touches but not the bevel, it will be able to cut under the ridge. Don't rotate the gouge too much or it could catch, just enough that the bevel pressure is relieved. The movement needed is very slight. Back off and restart the cut on the surface that was cut before the bounce started. Go very slowly and gently until the tool has sliced off the high spots. Let the wood come to the tool rather than forcing the cut. Many beginners tend to traverse the tool too quickly. A good clean cut comes from a sharp tool resting lightly against the wood. The bevel guides the tool but the wood should not feel any pressure. Sometimes it is easier to remove the ridges and finish the bowl with a heavy scraper.
The gouge is most likely to start bouncing if it is projecting a long way over the tool rest, or is blunt. Move the rest closer if possible, and/or use a heavier gouge. And sharpen, always a good thing to do.
Scratches on surface of finished bowl
When sanding, the first, coarse grit sanding leaves deep scratches in the wood. Subsequent finer grits are meant to remove these scratches. Don't skip straight to a fine grit, work through the stages, and inspect carefully before going on to the next grit. If necessary, go back to a coarser grit and spend a bit longer. Use fresh abrasive so it cuts well.
It is normal practice to rough turn bowls from green, unseasoned wood because it's cheap, easy to turn, and dry wood is hard to obtain in large sizes. Bowls made this way normally dry well, ready for finish turning. They warp, but don't usually split, though you always lose a few. The risk of splits is less if the wood dries evenly. To achieve this:
- turn the base and walls thin. This lets the wood dry without creating such severe stresses. It will move, but usually does not split. Thick walls dry on the outside and try to shrink while the inside is still moist, which creates stress. Bowls can be finished while still green if they are turned thin. If you intend to re-turn the bowl later, aim for a wall thickness of about 10% of the diameter. Much thicker than this is just asking for splits.
- Turn the walls and base to an even thickness
- Avoid or take extra care with species that are most prone to splits, for example plum and eucalyptus.
- Don't include the pith (the centre of the growth rings) or large knots in the bowl
- Use a sealant on the end grain, which is the fastest drying.
- Don't leave the unseasoned bowl in a warm place or in sunlight or drying winds. Slow drying allows moisture time to move from the inner layers to the outer.
- Enclose the bowl in a paper bag or wrap it in newspaper to keep the humidity high at the surface of the wood and slow the drying.
- Finish the bowl in one session. If you have to take a break, wrap it in plastic to stop surface drying.
If a crack does occur, the best option may well be to scrap the bowl. It may be possible to turn away a shallow crack near the bottom, or turn away the rim to clear a rim crack, leaving a shallower bowl.
Otherwise, there are a variety of treatments that can be used to make a feature of a crack or split, such as
An open split can be filled using various materials. Depending on the size of the split and the effect you want to achieve, these can include
- It can be left as it is, perhaps softening any sharp edges
- it can be opened up with a chisel and abrasives to emphasize it
- it can be drilled out or cut out with a saw, leaving a hole or step in the rim.
- matching resin,
- contrasting resin,
- glue mixed with sawdust,
- superglue allowed to harden slowly so it remains clear,
- an inserted wooden wedge or plug,
- you can saw through the crack to make a slot to take a contrasting veneer slip,
Best of all, do something new that you invent for yourself and that no other turner has thought of.
Gouge resists cutting
This happens when the edge is blunt, or when the bevel is holding the edge out of the wood because the tool is not presented at the correct angle. Sometimes projecting parts of the bowl blank can hit the bevel and knock the gouge away from the wood. This happens when roughing an uneven blank and make it difficult to keep to the cutting direction. Correct this by moving the rest closer and twisting the gouge into the wood a little so the bevel doesn't knock.
These can be severe and dangerous in a bowl because of the mass of the wood and the distance of the tool edge from the tool rest. It is very important to wear full face protection. There are some rules to make them less likely.
- Don't angle a scraper upwards while it is flat on the rest. Shear scraping has a different cutting action that allows you to angle upwards.
- Take care with tools in the hand when the lathe is running. It is easy to contact the wood before you are ready.
- Don't use a gouge angled upwards unless the bevel is rubbing. If the tool is perpendicular to the wood surface with the flute facing up, the middle of the bevel offers little support because it has a small area. Angle the gouge in the direction of cut so the wings of the bevel can give more control.
- Don't try gouge pull cuts on the inside of the bowl. You need bevel support to control the gouge.
- Don't use a scraper with its edge in full width contact with the wood, the cutting forces will drag it down.
- Take care not to push the tool in too deep between projecting parts of the blank, so it tries to take a very heavy cut when a projecting part comes down onto it. This is less likely to happen if the lathe speed is higher, but it cannot be run too fast because unbalanced projections can set up vibration.
Some wood species are better behaved than others. Some have grain that easily tears, while others can give a lovely smooth finish straight from the same tool. The surface is more likely to tear when you are cutting against the grain as the fibres have less support that way.
Tearing is caused by heavy cutting with a blunt edge, so the wood fibres are torn out rather than cut through.To minimise torn grain, use sharp tools and light cuts. Two light cuts are better than one heavier one when nearing the final shape. A thin, slicing cut from a sharp gouge will usually get under any rough patches. Sometimes on the outside of the bowl you can use a long bevel spindle gouge to take a light cut that can leave a polished surface. This needs caution however, as a spindle gouge may break if it digs in. You can also usually get rid of rough patches by shear scraping, that is using a scraper on its side, or even freehand without the tool rest, in a pulling cut that takes off very small curly shavings. A gouge with a small flute puts less stress on the wood and can give a better finish. A higher lathe speed and a slower tool traverse will often reduce torn grain.
Scrapers must always be used gently. A heavy cut can arise accidentally when the curve of the bowl is too tight for the scraper nose. This is common on the bowl shape often favoured by beginners, that is saucepan-shaped, with straight sided walls and a sharp transition to a flat bottom. The scraper tears the grain at the transition point. A design that has no abrupt transition point is easier to turn, and may look better too.
Scrapers can tear the grain if they are not well supported by the tool rest. If the lathe has a weak rest that flexes under load, you will have to be even more careful to avoid heavy cuts.
Sometimes it seems that nothing will get rid of the torn grain. In those cases, remember that the power sander is your friend.
Wood usually warps as it dries. When rough turning green wood, you expect warping and should leave the wall thicknesss approximately 10% of the diameter of the bowl. This normally allows enough wood for you to true up the bowl when it is put back on the lathe for finish turning. Some wood species warp so much that they are hard to use. Eucalyptus is an example. Bowls made of such varieties are best finish turned while still green.
If the wood is finish turned before it is dry, the resulting bowl will not be circular.
Bowl has soft, porous patches
This is likely to be spalting, or decay in the wood. Sometimes it can be localised in the wood, or more advanced in one spot. Often the decay adds nothing to the piece, so it is probably time to scrap that bowl. If the piece is worth it, try soaking in lots of thin superglue to harden the fibres. Alternatively, use a wire brush to remove the soft wood, leaving a textured surface. My rule is not to waste time on a blank if I can break out the fibres with my thumbnail.
If the bowl is heavily sanded, the soft patches will become sunken. It is possible to harden the wood with thin superglue, but it tends to show when the bowl is finished. You can also try power sanding the surrounding area with the lathe off to level the surface.
Bowl no longer fits the chuck
This is a result of the warping that occurs with changing moisture content. It is usually possible to hold the bowl in a way that allows a new chuck fitting to be turned. For example, the bowl can be held by friction against a wooden disc or faceplate, or simply pushed against the chuck jaws, in each case with tailstock support.
The reason for this may be:
- Excessive vibration. Either run the lathe slower or adjust the blank so it is better balanced.
- Insecure or inappropriate fixing, The most secure fixing is probably a faceplate with tailstock support, and this should be the first choice for roughing out larger side-grain blanks. If using a chuck, either expansion or contraction jaws can grip well but need to be at their optimum diameter unless the wood is hard and dry.
- Too heavy a cut or a bad dig-in.
Always wear full face protection when turning bowls.
This may simply be due to a bad dig-in. Otherwise, there may have been a crack in the wood - always inspect the blank before starting, and be alert for the sound of the cracked wood as it is cut. Turning cracked and unsound timber is dangerous. A hit from half a bowl blank can cause serious injury or even kill. Don't run the lathe too fast - turning often goes better with high rpm, but slower is safer.
A common mistake is to cut the wall too thin so it separates. You can often hear the change in cutting sound as the wall thins, but stop the lathe frequently to check progress with callipers or fingers.
Gouge doesn't cut properly across the inside
This may be due to the bevel angle. A long bevel is nice to use on the outside of a bowl because the edge cuts freely. It can also be used part way down the inside of the bowl. But as the bowl gets deeper, the long bevel cannot rest on the wood.
If you only have one bowl gouge, grind the bevel a little shorter. This will put the handle in a position where it is not obstructed, so you can turn deeper. It will still work on the outside. Alternatively, keep one gouge with a long bevel and another short just for the bowl bottom.
Another possible cause is that the gouge is too thin and flexible to work right to the bottom of the bowl. Deep bowls require robust tools.
Instead of a gouge, you can use a strong scraper to finish the inside bottom. But it should not completely replace gouges. Scrapers work well on the bottom of the bowl, which is side grain, but can easily tear the end grain on the walls, particularly if they are thin.
Gouge skids to the side when starting the cut
There are several possible ways to prevent this:
- Angle the tool rest into the bowl. The rest will hold the gouge back and stop it skidding sideways.
- Start each cut with a scraper to make a place for the gouge bevel to rest on. This is effective, but not very efficient, and it damages the fibres.
- Twist the gouge fully onto its side at the beginning of each cut, with the top of the flute directly facing the centre. The nose of the swept-back gouge enters the wood without skidding and cuts until there is a place for the bevel to rest against. Then twist the gouge back to the usual cutting angle, which can be done without interrupting the cut.
- Put your thumb on the rest to stop the gouge skidding.
There are narrow circular ridges on the curved wall inside the bowl
These can be made by a gouge with a long bevel. The straight or concave bevel does not conform to the curve of the wall. Grind a secondary bevel (easy to do freehand) so the primary controlling bevel is shorter. Sometimes the heel of the long bevel can score the wood. This is caused by too much pressure, but the secondary bevel helps with this too.
There is a bump or a dip in the middle of the inside.
This comes down to tool control. It is important that there are no bumps and dips in the finished bowl, particularly if you plan to use a glossy finish, which will accentuate them. If the tool insists on leaving an uneven surface, a small bump in the middle is better than a dip, as it is easy to sand away. Make sure the sanding disc lies flat over the bump, because this will help to prevent it from sanding a dip. To sand out a dip, you don't of course sand the dip itself, you have to sand the wood surrounding it, ending with the whole surface continuous and unblemished.
Unless you particularly want a flat surface on the bottom of a bowl or platter, it is often better to go for a shallow curve. This can make any irregularities less prominent.
The correct shape of a bowl is one that looks and feels right to you. But it can help to get even curves if you use a needle gauge. This will show you the shape in two dimensions instead of three and help you see bad spots in the curve.
Bowl does not run true when reversed
Variations in the wood often mean that a reversed bowl does not run perfectly true. A slight eccentricity may not be noticeable in the final result. But if it leads to vibration, or the walls are visibly not the same thickness, something is wrong. If only slightly out of true, it may be enough just to take a light cut on the outside to true it up after reversing. If more serious, there is an underlying problem that should be put right.
- If the bowl is on a faceplate, it may be that the screw holes are not flush. Remove the faceplate, level off any burr around the screw holes, and refix it. It can help centre the bowl if, while turning the base, you mark a circle in pencil, or even turn a shallow recess to fit the faceplate.
- If in a chuck, the jaws need a definite shoulder to rest against. This could be the bottom of the chucking tenon or recess, or the bottom of the bowl itself. If there is not a sharp corner between the tenon or recess and the bowl, the jaws may not seat well. If the chuck is firmly seated against a true surface, the bowl should run true.
- If the chuck jaws are not at their optimum diameter, they may cut into the tenon or recess. Any variations in hardness will lead to one side cutting in deeper than the other, throwing the bowl off centre.
- When the bowl is spinning, variations in hardness may lead to compression of the fibres on one side even if the tenon is correctly sized. Vibration makes this worse, until the bowl is dangerously off centre and loose. This can be overcome by re-truing the tenon or recess and making it deeper, giving the chuck more wood to grip.
- The bowl can be forced off centre by a dig in.
- If the bowl develops a slight wobble after some turning, it could be movement in the wood during hollowing. Set the toolrest to touch the bowl wall, rotate by hand to find the high spot and tap the bowl true before tightening the chuck.
No bowl is perfectly balanced. Some blanks, even if truly round, can have patches of higher or lower density. A blank may have been sawn with sides out of parallel so one side is thicker than the other. If you cut your own blanks from logs, you will be very lucky to get them balanced accurately. None of this really matters unless the lathe is run too fast, when vibration will be noticeable and may be dangerous. It is important to keep the speed down when beginning the bowl.
The bowl rim is shapeless
The rim is an important feature of a bowl, adding crispness and character, but its shape can be lost when sanding. If power sanding, try to match the disc contact area to the shape you want to achieve or maintain. Don't dub the rim over. You will have to change the angle of presentation of the disc as you go round the curve of the bowl. A sander is itself a turning tool and needs care in use.
Although the rim should normally be crisp, it should not feel sharp. Edges should be softened with fine sandpaper just enough so they look right and also feel good.