Some turning advice for an oak fan vault tas-de-charge.

[Adam W.]

It's time to think about turning the tas-de-charge for the fan vault to spring out from and I'm planning to do this in some green oak, as that's what I have most of.

The tas-de-charge looks like half of this...

And it sits where all the ribs come together on this model of the fan. So it's like a daffodil shape and it gets hollowed out a little to reduce the weight.

Once it's turned, I'll carve the ribs down to where they all intersect, to look like this. Then I'll join the pillaster and the remainder of the ribs with straight joints, much like the ones on the masonry vault, but using tiny pegged scarf joints and m&t joints.

The size is about 10" long, 4"ø at the small end and about 6"ø at the large end.

My question is; would it be best to turn it like a bowl/vase from half a log and then saw it on two or turn it like a spindle with two pieces joined with a paper joint and split them apart and what's the best position to have the tool rest in?

I'll probably use the chuck with the tailstock, but I have a faceplate if that's better.

For reference the thread on the vault is here.......

https://www.ukworkshop.co.uk/thread...-fan-vault-construction-for-beginners.132558/

 

[Tris]

From a fairly amateur perspective if I were doing this I'd turn it as a spindle, leave a couple of extra inches either end shaped for mounting in the chuck, then part through the centre as far as I'd dare.
Saw through the rest of the parted bit, re mount in the chuck, and hollow each piece in turn as far as you wish to go. You'll need some good jaws to hold those pieces, and I'd worry about catches whilst hollowing something that shape (also why I wouldn't turn it cross-grain but that's more me being out of practice and not wanting to test my visor).
The only other thing, and from reading your other threads you'll be way ahead of me, is how to allow for radial shrinkage in green wood?

Oh, tool rest as close to the surface as you can get it, poking inside the hollowed area at that stage.

Regards
Tris

 

[Adam W.]

There's a lot to think about on such a small part. The turned bit is up to where all the ribs separate and become complete, so it's not wildly flared.

Grain direction, I don't want to be joining where the timber is at it's weakest and want the grain to run parallel to the scarf joint as much as possible, which it isn't, but it's more important at the centre of the fan than at the edges.

I'll just allow the shrinkage to happen, as there's not much to be done about it and the scarf joints will keep the ribs in place anyway and I can just carve the moulding to fit.

The timber is half dry, but that's where the dimension change starts anyway, so I'll have to live with it, but the hollowing and deeply carved surface will help equal the drying out and should minimise splitting.

Is it a bad idea to have the toolrest parallel to the tapered surface of the wood or parallel to the axis of the lathe at all times and hang the tool over the rest ?

I ask this one, because the model (pic.1) was turned by someone else and he always had the toolrest parallel to the bed.

 

[Tris]

I think I misread the size as 9” not 6", so was thinking it was nearly double the actual size. Think I'd still go with spindle and end grain hollowing.

The thinking on the tool rest being as close as possible to the workpiece is, as I understand it, to reduce the force exerted on the tool, fulcrum point sort of thing.

 

[Adam W.]

I changed it from 9" to 6" but it might be 8" ø I have yet to work out the exact design, but it's still that shape.

 

[johnnyb]

would it be best as a spindle just for the subsequent carving that would need doing. any shrinkage on the bowl cut would make the joints show on the end grain. especially on green stuff( but realistically on any stuff)

 

[Adam W.]

OK spindle it is. It's 10" long 3"ø at the bottom and 7"ø at the top and looks like this........

I'll be discarding half of the turned item, as the finished piece is over half a cylinder, so I'll need to turn 6. I should have some lumps to take it out of which will enable me to sort the grain direction on the piece I'll need and I don't mind the waste, as I can use it for carving practice.

Off to the wood pile then.......

 

[scooby]

My turning experience is limited (only been doing it since January 2020) but I'd prefer to turn the object as a spindle. If you paper joint, a chuck gripping a tenon would be ideal for a bit of added security.
I've only paper jointed once, 4 pieces of 2"x 2" to have a go at inside out turning. I used pva and brown packing paper, it worked well but a thicker paper might be better to make splitting them easier. Unless you leave it a bit long to allow for a bit of damage due to having such a tight joint.

 

[johnnyb]

I think he's needing more than half so he'll have to stomach the waste.

 

[Lazurus]

As for tool rest position , keep it in line with the shape created to prevent too much overhand

 

[Droogs]

I would create a 3x3 piece blanks using superglue and masking tape to stick them together so that there would be no waste as such. The 2 outer pieces in the wood you want with the centre thick enough to allow the outers to be turned to the circumference I want and then seprate them once done. No under sizing or waste. Then hollow out most of the internal waste using height steps, going out from the centre each slightly lower on a table saw and then smooth out if needed with an arbourtech or if doing handtools a compass plane. I would do all the jointing work on the pieces while still square and fill with scrap while turning using the superglue and tape again

hope that makes sense

 

[Lefley]

Paper joint! But I glue a waste block top and bottom so my live Center is not in the paper joint causing pressure like a wedge , which could cause the joint to fail while turning. If a ring (or square) is paper joint glued top and bottom then joint will never fail and happy safe turning.

 

[Inspector]

Adam when you said a scarf joint are you glueing pie shaped pieces (staves) together to make the cylindrical turning stock or actual scarf joints like when glueing long boards end to end to make a mast?

I would glue a stack of long dry boards together and then crosscut them to length, longer than needed for each piece. Mount it between centres, 4 prong drive in the headstock and live or cup in the tailstock. Turn the small end at the headstock with the wide flair towards the tailstock. Both set up to leave a 3/4" diameter tenon/spigot at each end when done. Make a jig/sled to hold the turning by the spigots to cut the excess on the bandsaw assuming you are cutting lengthwise then plane the cut face smooth. Skip the paper joint, the bandsaw removes minimal material. The spigots are then used to hold and position for carving, loosening and rotating to hold the piece as you carve it. The spigots also allow you to turn the hollow in the ends. Spigots get cut out after all the shaping/carving is finished.

Tool rest gets repositioned as needed to be close to the surface being cut. A wide spindle gouge, a parting tool and a bowl gouge to hollow the ends are all you really need. A scraper will do if you don't have a bowl gouge.

You can minimize waste by gluing bocks around a 3 1/2" x 3 1/2" at the wide end only. It was often done with large pedestal table bases.

Is there any reason you can't farm out the turning to someone local to do for you? I would expect that if turned parts were needed in days of yore they would be done by a specialist, a bodger perhaps, as were metal pieces that went to a blacksmith.

Pete

 

[Adam W.]

Yep, I'm not worried about the waste and the pieces will have some defects in them because I'm turning them from firewood, so that bit will get chopped off and go in the burner.

The first one is done and I'll knock up the drawing plotting the ribs and the cut outs. I'll have to do that straight away as it'll split because the wood is quite wet still. They are made from riven wood so it should be quite stable if I hollow it out and dry it slowly. The carving will increase the surface area, so they should dry fairly quickly.

It stained the bed of the lathe quite nicely.

Pete, the scarf joint connects the rib to its corresponding "spoke" coming off the tas de charge cone. I'm going to make my life difficult by trying to make the model just like a timber vault with pegged carpentry joints, so the ribs will have a tenon going into a mortice on the cone.

I've recently bought a lathe, so there's no chance of me allowing myself to farm out the turning to someone else.

 

[Adam W.]

Thank you for your help, I'll move it over to the MA thread.