Glue up question

[smorris]

I'm building an 4' octagon shaped table, everything is cut, planed and jointed and I'm ready to do the glue up. I've never done a panel this large before and looking at it I decided I wasn't sure what the best process is.

I'll try to describe it...the center is 6 11/16" padauk boards 50" long by 3.5" and 5", then 3 more ~4.5" boards on each end of that of diminishing length. A total of 12 pieces. Comes out about 50 x 54 inches. Leaving it sit out for a couple days after I machined it, I'm quite pleased with the stability, no twisting or bending I can see.

My question is this, considering the amount of time to apply the glue and then position everything would I be better to glue up the center 6 first then add the other 6 afterwards or do the whole thing in one shot? I plan to use TB III with biscuits and to use cauls to hold everything flat while it is clamped up.
Looking at the whole thing now has me a little concerned about pot life if I try to do the whole thing in one shot. The flip side is I'm also concerned about getting the flat surface aligned properly if I do it in 2 steps, but biscuits might help there.

Your thoughts would be appreciated.

[pecker]

I would do it in sections. Even if you could get everything lined up before the glue starts to harden, it would be difficult to squeeze 4ft of boards together with typical clamps. The boards are gonna resist the pressure to some extent, and you could end up with a whole bunch of joints that were never pulled tight.

[LarryG]

I'm like pecker ... I'd do it in (probably) three sections, partly because of the limited working time of the glue and partly because there are just so many pieces to deal with -- meaning so many places things could go wrong. Using a slow-setting plastic resin glue (Weldwood or similar) would eliminate the first problem but I still wouldn't glue up that many pieces at once unless the sequence of the assembly left me no other choice.

[smorris]

OK, thanks guys. I hadn't considered the compressive force I would need for that many pieces but it would be substantial the get all the joints glued evenly. 3 sections it is then. Thanks again.

[poolhound]

Whatver you choose to do (I would also reccomend NOT to try it all at once) do a dry test run. Make sure you have all the clamps, cauls and whatever else you may need right there. Then time yourself. I know it seems crazy but even TB III has only around a 10 min open time so its worth a rehersal or two before you start spreading glue.

I had to do an 11 joint glue up on saturday and there was no other option they all had to be done at once. By the time I was spreading glue on the last joints and biscuits the first ones were hardening. It was a really PITA. Always go for building sub-assemblies if you have the choice.

[JimD]

Biscuits will help with alignment if you cut the slots correctly but you do not need them for strength. Simple butt joints for edge glueing work just fine.

Jim

[poolhound]

Having re-read your OP I am rather puzzled. How are you creating an octagonal shape with 12 pieces?

I was going to add a suggestion that you start with the center and use a bandclamp to hold everything together. I just cant figure out how you end up with a regular 8 sided item without the pieces being diviisble by 8. Can you spost a photo of the peices laid out dry?

[smorris]

Photo as requested. I'll glue it up then cut about an 1/8" over. I have a template I made that I'll use with the router to trim the edges to exact size. Then build the pedestal. Probably add some bracing underneath but have to leave it able to move a bit without cracking. This was a dry run to find the center line and work out how the pieces will all go together.

The small chalk marks are to indicate a board that must go that side up for the best pattern, or a small defect on the other side.

[poolhound]

Ahhhhh - light goes on in brain

Now I understand. I had thought you were creating an Octogon by gluing up triangular or segmented pieces. You are creating a large board glue-up and will then cut out the octogon from that.

Given the photo I would do three glue-ups first. The center section and the 2 sides spearatley. When they are dry glue the three panels together. And yes use biscuits to help with the alignment.

Going to need a few 60" pipe clamps I can see. FWIW the 10' lengths of 3/4" pipe are around $12 at HD. one 5' pipe is over $8 - go figure. You can then have them cut them down and thread them to make two 5 footers. I just did this last weekend.

[big tim]

I had the same problem as Poolhound, but having seen the photo, I would definitely do it in sections.

Tim

[TheRic]

... I had thought you were creating an Octogon by gluing up triangular or segmented pieces. You are creating a large board glue-up and will then cut out the octogon from that.
....

Thought the same thing, amazing how pictures can totally change what you had pictured in your mind, and what was really meant my the person.

[Workman]

I agree with other posts on multiple glue ups. Your post had other questions, I believe. If you initially oversize board thickness (just a little) you can then plane the subassemblies on the surface planer. Following final glue up, if you have any "ridges" old fashioned hand planes/scrapers are still the answer. I've found I sometimes have these ridges even with cauls. An alternative to biscuits for alignment which does help the "ridges" problem, is a glue line router bit (it must be set up correctly centered). When I use biscuits for edge gluing I do not put glue in the slots or on the biscuits, but just on the edges. If you glue the biscuits make certain that they have plenty of time to dry before surfacing because the water in the glue causes the wood to swell and if planed too early can result in a depression as the wood dries again.