A Segmented Bowl
Segmented turning
The formal definition of segmented turning is "gluing shit together first". That's it really - if the thing you turn on the lathe is made of several pieces of wood glued together, you're doing a segmented turning. The complexity of the gluing process varies from simply slapping two pieces of wood together, to painstaking marathons of assembly involving weeks of effort and thousands of carefully measured, cut and glued blocks of wood, put together in intricate formations and producing stunningly complex patterns. This bowl of mine (my first segmented bowl, although I have done plenty of segmented spindle work) fits somewhere in between these extremes. Actually it has to, doesn't it, or they wouldn't be extremes :-)
So a mate of mine had her birthday party on National Cherry Day, and she asked her guests to do something cherry-ish, such as wear a dress with cherries on it or wear earrings in the shape of cherries or bring cherry-based puds etc. Unfortunately I don't have any cherry-themed dresses or jewellery and my puds-making skills aren't exactly bra-popping (note to self: investigate whether high level of puds-making skill can cause spontaneous bra failure), so I decided to make her something out of cherry wood. Original, eh!
Cuttage and Glueage
.jpg) | | 6" x 2" and 7" x 2" cherry bowl blanks |
Now here's the thing. I was originally just going to turn a nice big bowl from cherry wood, but my timber supplier didn't have anything thicker than 2". I wanted to make something rather deeper than this, so I got two 2" thick blanks, intending to glue them together and turn the bowl from the resulting 4" thick piece. This is an advantage of segmented turning in general - you can make big things even if you don't have big enough pieces of wood. All you have to do is glue together whatever pieces you do have until you've made a blank of the right size. The other big advantage to doing it this way is that it's easy to find a properly kiln-dried piece of wood 2" thick, whereas properly dry 4" thick pieces of wood are incredibly hard to come by - and when you do find one, it's very likely to have cracks in it.
.jpg) | | Spigots turned for gripping in 5" engineering chucks |
These bowl blanks, as supplied, don't have faces flat enough to just glue them together. They could be flattened on a jointer or a sufficiently large belt sander, but I have neither of these things. I do, however, have a nice big engineering lathe, which can do the job perfectly. But to mount the blanks in the engineering lathe, which has a 5" chuck, it was necessary to turn spigots onto them which the 5" chuck could grip. Easily done - whack a hole in the centre of each blank, mount on a screw chuck on the wood lathe and cut the spigot by hand. Here are the blanks with spigots cut.
.jpg) | | Engineering lathes don't cut wood very cleanly. But when you need something done very accurately, they're the daddy |
Each blank was then faced on the engineering lathe to produce an accurate, flat surface. Tungsten carbide tipped tooling (as you can see in the toolpost here) doesn't cut wood cleanly, but taking the cut nice and slowly prevents damaging tear-out of the grain. The slightly rough finish is actually advantageous because it gives the glue a "key" to hold on to. A light rub with medium-grit sandpaper gets rid of any seriously nasty craggy bits and leaves the surfaces ready for gluing.
.jpg) | | Lovely flat surfaces = lovely strong glue joint. Note the holes from the screw chuck used on the wood lathe earlier when cutting the spigots |
Since the middles will end up being hollowed out, I made an extra facing cut to create a recess in the middle of each blank. This leaves an annulus which will form the glue joint. Recessing the middles reduces the chance of crap getting caught in the glue joint and preventing it fitting together snugly and, trivially, it saves glue. Visible on the table here is some crappy trigonometry that my mate Pete and I indulged in one night after getting disgracefully pissed drinking responsibly. Pete stated that the neighbours' gigantic satellite dish was pointing at a satellite in geostationary orbit and I argued that it wasn't aimed high up enough for that. As it turned out we were both right - the dish had an elevation of only about 5° instead of the roughly 40° it would require to point from South-West London to geostationary orbit; but then we noticed that the receiver wasn't aligned to the axis of the dish and was, in fact, about 20° below the axis, meaning the dish was entirely suited to pick stuff up from geostationary orbit. We both have degrees in maths but due to our near-lethal slightly elevated blood alcohol levels, it took us a long time to figure this all out.
.jpg) | | "Of course, when I was a nipper, we had to do this using just a mallet and our teeth" |
At this point I figured that while I was going to the effort of gluing up a blank for turning, I might as well make it interesting. Also I had niggling doubts about whether simply gluing the pieces of wood together would produce a joint strong enough to withstand the considerable forces involved in roughing out a bowl and (I confess) occasionally getting nasty catches due to over-aggressive cutting technique. Milling teeth into the two blanks and fitting them together is possible but that requires some rather accurate set-up work and some rather tedious cutting, since the teeth can't just be straight slots in that case - they have to be shaped like slices of pie, i.e. their edges have to follow the radii of the blank. It's easier just to cut straight grooves and then make "keys" to fit in the grooves, and this way you get to make the keys out of something different, for a nice contrast. So that's what we're doing here. The blanks are mounted in a 5" engineering chuck on a rotary table on the mill and I'm using a 1" router bit to cut 6mm-deep grooves, 9 of them at 40° intervals. Easy peasy.
.jpg) | | Ready for some serious glue-squirtage |
And here are the blanks with grooves cut, along with the 9 keys that will fit in the grooves, made from purpleheart. The keys measure 1" x 12mm (yeah - brutal mixage of metric and imperial units there) and were cut to shape on a bandsaw after some trial-and-error using scrap wood to set it up accurately. The next step is simply to slather the whole thing in polyurethane adhesive and assemble it with care and patience. Admittedly, a bandsaw is not the tool to make ultra-precise cuts on and some of the purpleheart pieces were a smidge wider than 1", resulting in my having to assemble the pieces with not only care and patience, but also a large club hammer. You know that scene in 2001 where the ape-like thing learns to use weaponry, and celebrates by smashing up a skeleton using a big chunky bone? It was a bit like that, but with less hair and (regrettably) no Strauss playing in the background.
The turning bit
.jpg) | | Ready to be bowlified |
If you were watching this happen in real time this is the bit where I'd toss aside the messy freshly-glued piece, reach under the table, say "here's one I made earlier" and get out a glued-up blank which was clearly made by someone who actually knows what they're doing. Anyway, since this isn't a cooking show with a surprisingly ill-qualified chef, what you see here is the same thing as in the above piccy, after the glue has dried. As usual with polyurethane glue, it's foamed up and made all sorts of chunky shapes all over the surface of the wood. No biggie - that's all going to get taken off when we turn this bowl. Yes, this does mean I used too much glue, and no, I couldn't care less. Glue is cheap, whereas having a blank fly apart at 1000 RPM and spending 14 hours having a surgeon reconstruct your face is expensive.
.jpg) | | All done, except for the base |
So here's the bowl, 99% finished. Since my buddy Joanna (whom this bowl is for) is allergic to nuts, my usual tung oil finish wasn't really an option here. She's not so allergic that simply holding a bowl covered in tung oil would make her drop dead or anything, but then again this is a very pretty bowl and I was worried that she wouldn't be able to resist rubbing it all over her face and licking it and stuff. So the bowl is finished with shellac, using the usual not-quite-French-polishing technique: apply alcohol, apply shellac, apply olive oil, rub until your fingers start to cook. All we need to do now is finish the base. Bring on the shiny new cole jaws...
.jpg) | | These things are pant-browning at 1000 RPM |
Cole jaws are simply big plates onto which you can screw little plastic nubbins for holding a bowl on the finished rim so that the base can be turned. Since the jaws are huge and they don't provide anything like as secure a grip as standard metal jaws, it's important to go slowly. Annoyingly the plastic nubbins are actually quite hard and these ones marked the finished surface of the bowl a bit, which kind of pissed me off. Meh, live and learn - next time I'll put cloth padding in the way or something. Anyway, finishing the bottom of the bowl was a 5-minute job - turn off the unsightly jaw-mounting recess, hit it with sandpaper and rub shellac on.
.jpg) | | Ta da! |
Et voila! I noticed after finishing this bowl that I'd glued the bits of cherry together with misaligned grain. Since wood changes shape primarily across the grain and not along it, I have to hope that (a) the glue I used is bloody strong, (b) the wood was properly dry when I bought it and (c) the shellac seals the wood well against moisture movement. Otherwise there's every chance the bowl will eventually dismantle itself. Oops. Anyway, there you have it - my first segmented bowl. Now to cook up something with much crazier patterns in it...
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