How to make a temporary joint in item being turned?

[Robert Marshall]

I am considering assembling a plywood pentakis dodecahedron (12 pentagonal pyramids making up a 3-D geometric shape), and then turning it to a sphere. I would like to be able to do that with a temporary joint along the middle of it, so that after turning, it can be taken apart into two pieces. Here is a wire-frame diagram of the geometric shape.

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My plan is to assemble the pentagonal pyramids first, making sure they are identical to each other. Then, glue together 6 pyramids into half of the final shape, and glue together the other six pyramids into the other half. Then, temporarily join the two halves, to be put on the lathe and turned into a sphere. So my question concerns how to accomplish this temporary joinder between the two halves, for turning, so that after turning, I can take the two halves apart.

Here is a screen-shot from a YouTube video about building a plywood dodecahedron. That shape differs from the one I intend to build, in that each of the 12 pentagons is a single flat piece of plywood, whereas my project would be using pentagonal pyramids instead of flat pentagons. I don't think that difference matters, however, with respect to my purpose. As you can see from this photo, the half-assembled dodecahedron consists of one pentagon at the bottom, and five pentagons joined to it (and to each other). It gives you a five-pointed zig-zag "equator", to which the other assembled half should match up to form the full dodecahedron. It seems to me that the zig-zag shape of the joint will itself impart some strength (rotational resistance) to the temporary joint, as compared to something with a straight "equator."

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So, how would you suggest I temporarily join the two halves of the dodecahedron to each other, in order to place it on the lathe between centers for turning to a sphere, and allowing it to be disassembled into the two halves, after turning? I've thought about double-sided tape, or hot glue; looking for suggestions on these or any other temporary joint ideas you might have.

Thanks.

Robert

[Chris A Lawrence]

Some people use paper joints to attach glue blocks. You glue a piece of paper between the 2 pieces. It is supposed to allow the joint to be broken apart at the paper. I never tried it but others have said it works.

[Grant Wilkinson]

+1 on Chris' suggestion. A paper joint should work.

[Mel Fulks]

The paper has to have a little thickness to it. Too thin and the paper won't split. Do some tests with scrap.

[roger wiegand]

Too thick/weak and it will split when you don't want it to :mad:. Definitely test! Double-sided carpet tape can also work, it can be hard to get apart though.

[Paul Williams]

A different approach might be to cut it apart with a thin saw after you have turned it round. Obviously the joint would be straight and perhaps harder to hide.

[Robert Henrickson]

Paper joints are very useful. They are generally used between two flat surfaces (planes). In that use, they hold together very securely, yet can be separated quite easily. I use brown paper bags, the larger sizes for slightly greater paper thickness. Some highly skilled users of paper joints recommend newsprint, but I've never tried it.

I've never tried a paper joint in any zigzag sort of join. It might work, but each segment of the join is at an angle to its neighbors. Separating the two halves might be very 'interesting' since you as you try to wedge one segment apart, you are pushing against other surfaces. The segments are partially interlocked along the joint. I would certainly recommend some trials with scrap before making a serious attempt, especially if you are intending to make each of the faces pyramidal.

[Kyle Iwamoto]

You have that pattern on the inside? I would think that would be plenty enough to drive the item without any adhesive provided you turn between centers. I'd use the Oneway safety drive, in the event that one of the zigzags catch, you may save the item. Well, at least the drive won't break it apart.
I'm also with Robert above, that the pattern may not do well if separating a paper joint. Plus, you'd have to get rid of the paper. Not like you could sand it away.

[Sam Force]

I think tape the circumference on the outside at the center of the "sphere" 90 degrees to your bed and as you rotate the sphere change the tape. Always keeping part of the joint taped and pressure from head and tail stock

[Robert Marshall]

Lots of good ideas! Thanks, all.

I think I'm going to try taping around the "zig-zag equator," one half at a time. One half of the joint is left of the true equator, zig; the other half is to the right, zag.

Tape the zig, turn and true up the zag. Remove the zig tape, tape the zag, turn and true up the zig. High turning speed combined with gentle, non-aggressive use of the tools. And with the drive center and the live center keeping moderate pressure against both ends of the sphere. Ought to work. (Famous last words.) If it does work, will post some pictures; otherwise, you never heard of me.

One more use for gaffers tape.

Again, thanks.

[Reed Gray]

I am guessing that you want to pre-turn sections of the pentagonal dodedahedron. I made custom footbags (aka hacky sack the brand name) for years. With the pentagonal dodecahedron (there is a rhomboidal one as well) we used to sew them in a hemisphere first, then sew the two halves together. It could also be done, with the wood, as 4 segments with 3 pentagons each. Not sure if that would help or not, but some times smaller pieces might work. I have seen footbags from 1 to 320 pieces.

robo hippy

[Paul Williams]

Building on Kyle's idea of driving without taping the joint, you could put some small dowels in the joint to make sure there is no slippage.

[Bob Coates]

Possible problem????
I would be more concerned that you material is not thick enough to turn. I might be wrong, but if I connected the point and then made a circle touching the center of each such side, I think you might loose the points.

Bob