Thin bowls have never really done much for me. The thinnest bowl I’ve made was a small maple (Acer saccharum) bowl with a black band, 135mm in diameter and 60mm high, as a bit of a challenge some years ago, out of kiln dried timber to an even 3mm wall thickness. They are a great test of skill but are pretty useless, which is why I have never really gone for them.
Following on from the editor’s challenge from last month, we decided it could be interesting to see how thin I could go with green timber. The editor suggested I use a light to gauge the wall thickness – something I have seen done with some success, but have never tried and, for no real reason, have always thought rather sceptically about.
The theory
At some stage we’ve all turned a bowl rather too thin and experienced light passing through the walls, whether accidentally or on purpose. The theory of using a light to gauge wall thickness is that, by the colour or brightness of the light coming through the thin, wet wood you can tell how thick the wall of the bowl is – do you see why I am sceptical about it now? Many turners use this technique though, so despite my brain’s unwillingness to accept how well this technique might work, I’m going to give it a go myself and see if it really is any good, so we shall soon see!
Health and safety
While these bowls are relatively small, the danger of cutting through the side and potentially exploding them is much higher than it usually would be. Because of this it is sensible to wear a face shield at all times when turning this kind of work. This is a more advanced technique, so please be sure you are ready to try this out. I always wear an air-fed helmet and face shield when turning. I would also recommend the use of LED lights rather than traditional bulbs for this kind of work to minimise risk.
The plan
Last time I visited George Watkins, I collected enough green logs to do last month’s article and some for this one, so my plan is to try to turn at least one, but probably several small thin walled bowls. By ‘thin’, I don’t actually know how thin it is possible to go, but I intend to find out using a combination of an LED lamp and callipers. Could I make a bowl with an even 1mm wall thickness?
Permission to fail
The first thing I’ve done is not physical, but mental. I have given myself permission to fail. Let me explain: like everyone, from time to time things go wrong and a piece of work ends up in the firewood bag, but generally speaking I have been commissioned to make something to a plan, I work out how to do it and I then simply need to do it within the time scale. Failures are wasted time, wasted wood and wasted money, so one of my focuses is to make sure my failure rate is as low as possible.
With these thin bowls however, I doubt I’ll find out the limits (both of my skills and of the wood) without breaking at least one bowl, so I’m going to try and do the most counter-intuitive thing a turner of bowls can do; not worry about cutting through the side of a bowl.
Wood
I have some green sycamore (Acer pseudoplatanus) for this work, although to really test out the lamp theory I also have a piece of hawthorn (Crataegus spp.) left over from last month’s project, so I’ll try this too. You might remember that the hawthorn is around 30% moisture content, which is a pleasant level, being wet enough to make turning easy but dry enough that the free water has left the wood, meaning I don’t get particularly wet when I’m turning.
The sycamore on the other hand is 41%, meaning I’m prepared to get a soaking and I begin by coating my lathe bed with a protective wax lubricant to prevent rust. It is worth mentioning that I am using quite small logs, none of the finished bowls will have a diameter over 110mm. All of these bowls are end grain, with the grain running along the lathe bed, rather than the more traditional cross grained bowl.
Bowl one
I cut off a piece of sycamore and mount it between centres, quickly truing up one end and forming the outside of a small ogee shaped bowl. I reduce some of the waste and cut a chucking spigot, without leaving enough space to properly finish the base of the bowl, as it turns out, but as this is only the first bowl, it won’t be a problem. The outside is turned and the bowl is held in the chuck. I’m also increasing the difficulty level by seeing if I can keep a natural edge on the bowl. I make a start shaping the inside and as soon as I begin to get thin near the top edge I position the lamp.
LED lamp
I bought this LED lamp years ago as a work light for the lathe, but never really got on with it. The base is magnetic, meaning it can be positioned anywhere on the lathe, but actually it works best on a horizontal surface, which slightly limits its use. I also found the LEDs produce a bright but narrow beam of light, making it good for small detail work, but not so good for illuminating a wide area, so it has mostly sat on a shelf unused since I bought it.
The magnetic base and flexible stem do make it ideal for this project though, as long as the light is bright enough to penetrate to wood. The added advantage of an LED is that it gives off very little heat, making it ideal for working close to green wood; LEDs are also much safer to use than a traditional bulb, should a bowl explode or break up close to it.
As I cut into the bowl it quickly becomes apparent that the light from my LED lamp is going to be strong enough, despite my concerns. As I work into the bowl I stop and check with callipers to give myself an idea as to what the colour of the light means. It seems that the whiter the light, the thinner the bowl.
As I progress I am staggered by just how good the feedback from the lamp light is. I expected it to work, but I didn’t quite expect it to work this well. The picture (see above right) shows the light shining through the wall of the bowl. I have cut to a point and there is clearly a visible colour difference between the thickness of the two cuts. Putting my callipers on this it shows that the difference is less than 0.5mm. At this point the lighter area is 1.5mm, the darker is just shy of 2mm. I find that as I work, the bright areas show some darker streaks where my cut is slightly less even than it might be, but this is easy to adjust with a light shearing cut with the wing of my gouge.
Failure!
Just as I originally suspected, to find out just how thin I can go I need to cut through the wall of the bowl, and as it always does, it happens suddenly with a slight ‘pop!’ The brightness that I had been looking at suddenly shows as a beam of light and I stop the lathe to inspect the damage. I study the damage which is a tear, less than a quarter of the way around the bowl.
The amount of flex in the bowl is surprising (it probably shouldn’t be, but remember I usually work with seasoned timber which is much more stable) and I decide that I probably cut too deep into the bowl, too early in the hollowing process, which allowed the bowl to flex under the tool, causing this tear. In the next one I will make sure I don’t get ahead of myself so far. There is still a good bit of the bowl left, so I decide to cut off the bad part and carry on. I work down to the base and part it from the lathe.
The real test
Part of giving myself permission to fail is that I also don’t need to produce a finished item; this is really a learning and experimental exercise rather than a project. Callipers are great, but nothing beats actually seeing how thick your bowl is, so I decide that everything I make today will go through the bandsaw to reveal just how even my wall thickness is. For a first go it’s actually pretty good at 1.4mm, just thickening towards the bottom, which I think is quite natural to account for a foot or base of a bowl.
Bowl two
Straight away I put in a second piece of timber and remember the lessons learned form the first bowl: give myself more working room at the bottom and don’t get ahead of myself when hollowing, focus on a small area at a time. I already know both of these things but in my eagerness to get on with it they managed to slip my mind.
I go for a similar ogee shape on bowl two; once again I will attempt to keep the natural edge in place. This one comes from a little further along the log and is a little larger in diameter, so I make it deeper to suit. This time I put everything I learned (and was reminded of) into practice and manage to successfully complete the bowl. With these being end grain, I turn them as far as I can with my spindle gouges and switch to my Hope Pro-carbide tool to reach the bottom.
As with the first, number two thickens slightly toward the bottom, but not as much. This bowl has taller straighter sides – perhaps too straight – which had a thickness of 1.8mm, so good, but moving in the wrong direction. I’m hoping I can make a bowl with 1mm walls.
Lamp position
I found the best place to position the lamp is at the back of the bowl, which gives a good visual as I work. I tried it in a few different positions out of curiosity and in an attempt to get some good pictures to go along with the article, but settled on the light being at the rear of the work shining through the wall. There is little doubt this is a more advanced technique as good tool control and an awareness of the tool is essential. With the light shining through the back of the bowl it means that my focus is on this rather than on the tip of the tool, so it is vital to be comfortable and familiar with tool presentation to do this successfully.
Bowl three
Having parted the second bowl off, the remaining waste block is calling to me to make a rounded in-curving bowl. I’m aware this will be a much more difficult shape to achieve the even thin wall I’m after and that repeated practice is best done on the same or similar shaped bowls to begin with, but after the success of bowl number two, I am up for the challenge!
I cut a spigot on the exposed surface, flip it and mount it in the chuck and quickly shape the outside. This sycamore is full of knots, but this doesn’t seem to be a problem for the light technique and they don’t seem to be troubling me during the turning like the knots in some timber would, so I ignore them and carry on. This shape really highlights the grain direction of the bowl and I find cutting into the end grain with my spindle gouge much more difficult than on the previous two bowls so I switch to the Pro-carbide tool, which handles it much better, being more closely related to a ring tool than a gouge.
As with the others the turning is quite straightforward and I keep a close eye on the colour of the light shining through the wall to achieve the even wall thickness. I find that this more enclosed shape holds the shavings much more than the open ogee bowls. This isn’t usually a problem but I need a clear view of the light, the shavings quickly cover this, giving a false impression, so regular clearing of shavings is vital to success with this bowl.
Once the inside is complete to my satisfaction I put the light inside the bowl, which I am happy to do because it’s an LED, I’m not sure I’d be comfortable doing this with a traditional bulb, although I’ve seen it done. You can see how the light shines through the bowl, giving me a guide to finish the outer curve and part off the bowl.
As before, I put the bowl through the bandsaw, and just as before I have managed to turn the base thicker than the walls. I’m a little frustrated with this, although I can’t help feeling the thicker base is probably a good thing overall and it isn’t that thick, the walls being 2mm thickening to 5mm in the base.
Bowl four
I decide for this final bowl I will use a piece of the hawthorn I had left from last month’s article. I’m curious to know if the light shines through it the same as it had for the sycamore, with it being so much more dense, tighter grained and that bit drier. The sycamore was quite open grained, having grown very quickly in George’s garden and was freshly cut only a month or so ago.
I went back to the ogee shape of the first two bowls for this one as this lends itself to natural edge work and was a slightly easier option. As before, I shape the outside and then move on to the inside. I use the spindle gouge as far as I can easily reach before switching again to the Pro-carbide tool.
A word on tools
The Pro-carbide tool is a great little tool, very sturdy in use and easy to manipulate. One feature of it though is something of a double-edged sword for hollowing; it has the ability to cut on the end of the tool, unlike some fixed ring tools I’ve used. This can be a very useful feature, especially for removing the little pip in the bottom of a small hollow form or bowl like this, but it can also be a pain because if it touches the bottom of the bowl without you being fully aware of it, the tool will catch and run across the base of the bowl.
After this happened to me a couple of times, although thankfully it didn’t result in losing the bowl, I switched to a different tool, this time the Ashley Iles fixed angle ring tool. I’ve owned this for some years and used it occasionally. This tool cuts well on the side but not at all on the end of the tool, making it a much less catchy option on the lower corner of the inside of the bowl. It isn’t as rigid a tool as the Pro-carbide, but it did the trick here.
Finishing bowl four
As with the other ogee bowls, I turn a small foot to give it stability (as if I was actually going to keep it as a finished bowl, rather than put it through the bandsaw). Looking now at the photo of me turning the base, it shows that the base is thicker than the rest of the bowl, so I shouldn’t be surprised to find this when I cut it. I seem to remember watching this technique in a demo and being told that light will travel through end grain easier than side grain, giving a slightly different ‘reading’.
I’m not sure how true this is, certainly in this instance, as the photos show that the light shining through the bowl and the cut bowl agree very closely on the wall thickness. The other problem is that I still have an eye on the overall look of the bowl and because it is so thin, I can’t go back to adjust the curve, so I produce a curve that looks good, rather than truly following the guidance of the light. On a complete bowl this is always going to be the best option – ‘Form is king’ as a friend of mine often quotes.
As before bowl four goes through the bandsaw to reveal – just as the light suggested – that the walls were a very even 1.2mm, thickening at the base to 3mm before the foot. A very satisfying result!
Conclusion
I am astounded by just how good the light technique is. It is hard to believe just how accurate simply shining a light through a piece of wood is at showing the wall thickness. I knew it worked, but I had no idea it worked that well! Despite my dislike of turning wet wood – and this sycamore was really wet – I have enjoyed making these bowls, even if I don’t actually have a finished bowl to show for it.
Many turners will be horrified at the thought of putting something they have just made through a bandsaw, but I can tell you it is really worth it. Of course not every bowl needs to go through, but every so often I would recommend doing it, just to show you whether or not you are achieving that even wall thickness that you think you are. I know I tend to make the bases of my bowls a little thicker than the rest, and this continued here; apparently it’s difficult to break the habit of a lifetime!
