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I was in a boardroom and I noticed that one of the clocks had dropped off its nail and it broke the frame in two places. I assumed is was a very cheap clock made with plywood or some other engineered material. To my surprise it was laminated solid wood.

Wood frame clock

The repair for this seems simple enough. Just need to glue it. Issue is that I want to provide clamping pressure perpendicular to the direction of the crack. That is not easily done. Now, it would probably suffice to clamp on sides (shown in the picture as the right and left sides) and get a pretty good seal.

Let's play and pretend that this clock is something more important and I want to be sure the clamping pressure is as close the to crack as possible. How can that be done?

I picture a jig of the same diameter as the circle so that it can't bow out but I wonder if there are simpler ideas. Likely this would apply as well to other things and not just circles... other polygons with a large number of sides.

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What's interesting is where it split. It seems that there is a grain line/seam at about the 11 o'clock position from the top 'screw' in the metal (just above the 12). I would have thought it would have split along that line. – FreeMan Mar 28 at 15:03
    
@FreeMan Me as well. The wood in behind has the 12 o'clock grain so that must have initiated the break and the wood in front closely followed suit. – Matt Mar 28 at 15:46
    
Interesting. Thanks for the follow up. – FreeMan Mar 28 at 16:42
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I don't think that clamp pressure right at the sites of the two splits would be required to achieve good clamp pressure there... easy enough to check :-) But if you did need this I think the ideal way could be with pairs of shaped clamping blocks. If it was acceptable to mark the edges then the clamping blocks can have screw or brad points projecting from them, if not then linked together by cordage or a strap so they couldn't slip towards 12 and 6 as pressure was applied. But in practice I think a ratchet strap is probably the way to go, or, no purchase necessary — use a Spanish windlass. – Graphus Mar 28 at 18:13
up vote 19 down vote accepted

I picture a jig of the same diameter as the circle so that it can't bow out but I wonder if there are simpler ideas. Likely this would apply as well to other things and not just circles... other polygons with a large number of sides.

Lots of people use ratchet straps to clamp odd shapes together:

ratchet straps
(source)

These are nice because they apply a consistent pressure around the shape and will keep it from bowing out.

Otherwise, one idea that might work is a clamshell-type jig where you take a square piece of plywood, cut it in half, cut out the shape of the thing you want to clamp (half on each side) and then use that to clamp the shape back together. This would be like these conduit clamps on a related Question here.

The repair for this seems simple enough. Just need glue it. Issue is that I want to provide clamping pressure perpendicular to the direction of the crack. That is not easily done. Now, it would probably suffice to clamps on sides (shown in the picture as the right and left sides) and get a pretty good seal.

For the size piece you're working with, clamping from opposing sides will probably be sufficient since the pieces are pretty stiff with comparison to the span. You could put a "bumper" clamp next to the splits to control the tendency to bow outward too (similar to a caul for keeping table tops flat during glue-ups).

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I have a simple framing jig that has plastic corners for use with something like this. Regular ratchet straps are more versatile and would conform better to a variety of shapes. An excuse to purchase something! – Matt Mar 28 at 14:55

I have used ratcheting band clamps on curved shapes with very good results. ( I could not find an image using the clamp on a circular form, but the principle is the same.) Rather than applying pressure in a single direction it provides uniform pressure towards the center.

Band clamps

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Why not use 3 regular clamps? You could put one directly over the crack to keep it aligned and then two more crossing at about 90 degrees to push the two sides together. Another idea is that if there is enough thickness you could use something like a bed bolt in the back of the clock to draw the cracks together and hold them while the glue sets. There would be no reason to take them out after either. Just some thoughts.

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