From log to lathe: breaking it down
Words and photos: Terry Martin
I have written many woodturning stories and they usually begin with mounting the blank on a lathe to begin the turning. However, an important, but often forgotten part of the story of turning is how the blank was created and how that affects the final product. Many turners buy their blanks already cut and that means the decisions about what they can make have already largely been made by a stranger, and that person may have only a vague idea of how to turn wood, or what features of the wood are important.
Recently I obtained a number of freshly cut logs and I was planning how to cut each of them to get the best results. The decisions I made at this stage would irrevocably define what I could make, so I decided to record how I broke down a particular log and what considerations are important to the final product.
The only guides you have when cutting a log are its visible outside features and accumulated experience, which includes an understanding of how different trees grow. This story is not about the turning itself, it is an example of what I did to create a successful turning blank from a quirky log.
1. I began with a large section of mango trunk, 850mm long and 450– 500mm dia. It was freshly cut and very wet, and I estimate it weighed at least 200kg. There were three branch stubs that had either never fully grown, or had been lopped and grown over, and I knew this would mean multiple piths in the wood. The pith is the centre point around which growth rings form in trunks and branches.
2. Looking at the end of the log, the pith in the trunk was not central, as is often the case. Because of this offset, if the wood is left to dry it will split differently around its circumference and this tendency will affect whatever I turn. I would have to consider this when cutting. The reasonably circular inner rings made me think this might be suitable for a wide-mouthed hollow vessel, but I needed to move quickly because splits were already appearing around the central pith.
3. I removed the branch stubs and could see that the branch growth rings were quite developed, so it would be better to remove them from the final turning blank as they would create further tensions within the piece as it dried. I decided to make a cut across the log in the middle of the branch stubs. I marked the cut with chalk and that cut would establish the bottom of the turning blank – at the base of the vessel. The branch remnants would mostly be turned away when I shaped the spigot for holding the blank.
4. After the cut I could see that I had only two branch piths to deal with and they were close enough to the central pith of the log that I would be able to turn them away. I put the other half of the log aside to be cut into bowl blanks.
5. After marking out a rough circle at the top of the blank, I made a series of rip cuts to remove the corners, progressively taking smaller cuts till it was roughly round. This is to reduce vibration once it is on the lathe.
6. Even after removing about 50% of the wood, the blank was still very heavy. I mounted it between centres, positioning my largest drive dog in the centre pith at the headstock end, or the top of the vessel. I then moved the bottom of the blank around the live centre at the tailstock end until I found a compromise position that roughly balanced the wood, but was not so far off centre that it would affect how it dried.
Before turning on the lathe I turned the speed to zero, then gradually brought it up to the fastest I could go without the lathe shaking. A very large blank like this is safe between centres, as long as you regularly wind in the tailstock, because with wet wood the drive dog and live centre may penetrate as the wood spins.
7. I cut from right to left, standing to the right so the spray of wet sap didn’t soak me. I stopped cutting deeper as soon as each section of the wood was fully round. This stage is not about shaping the vessel, but about creating the round shape within which I could work.
8. This is the rounded blank, ready to be turned at full speed.
9. The form of the vessel is established. This was not very different to what I envisaged at the start when I was looking at the whole log. Here you can see that with a gradual curve the tight growth rings are transformed into soft, wavy patterns.
10. As I narrowed the base, the pith remnants moved closer to the centre and I knew I could remove them later when I turned off the spigot. I cut the spigot to 110mm dia for my biggest chuck, large enough to support the weight of such a big piece. Finally, I removed the excess underneath the spigot to the stage where I could take it off the lathe and snap the stub off.
11. Once I mounted it in the chuck I could clean up the top. As I had predicted, the growth rings were wonderfully arranged around the centre. The splits in the centre didn’t reach the rim, so I knew the vessel wall probably wouldn’t split as it dried.
12. I bored out the centre with a large sawtooth bit and then set up my hollowing system to finish the hollowing.
13. This is the final form, ready to dry. From log to finish it took me about 4 hours. It is 350mm thick, all the way to the bottom. If I had rough- turned a salad bowl, for example, it would need to be thicker because it would tend to curl as it dried, but this vessel will contract uniformly and is unlikely to split or warp much.
The drying is controlled at first by placing the piece in a plastic bag, then turning the bag inside-out each day. Once the bag doesn’t collect any more moisture, it can be dried in the open air in a cool place. Mango dries quickly because it is very open-pored, so it will be ready to finish-turn in a few months.
The piece is now 300mm high and 240mm in diameter. It might not seem much to get from such a large log, but one of the mistakes that many beginners make is to think they have to get the biggest blank possible from a piece of wood. I wanted to get the best blank possible, and I think I have done that. When it is dried, refinished and sanded, the growth rings will show as wavy lines around the diameter.
My final decision will be whether to sand and apply a finish, or to char the surface and rub back to create a black vessel with a white heart – but that decision is for another day.
Terry Martin is a Brisbane-based wood artist, author and curator. Learn more at terrymartinwoodartist.com