One of the stressful parts of this build is making the curved panels. I always find it a little nerve-racking working with glue you actually have to mix up. When you start out you're not quite sure if you're making too much (which would be wasteful of the expensive glue), or not enough (which would be worse since it takes a fair bit of time to mix it up, all the while you're already mixed glue is sitting around either in the pot or on the panel drying out). And truth be told, I was nervous about whether this was actually going to work.
If you've never worked with precatalyzed urea formaldehyde glue, it's weird stuff; it doesn't just dissolve into the water like you think it would. There is a lot of stirring involved and it takes a while for all the powder to hydrate. I sort of liken it to making pizza dough. Pizza dough doesn't have a nice smooth texture when you first mix it, but after you let it sit and hydrate for a bit and allow gluten to form, it becomes a lot smoother and more plastic. There's no gluten with the glue obviously, but the same sort of concept applies.
I tried to estimate the amount I needed based on the coverage listed on the container, plus 10%. I mixed the first batch up all at once - that is, enough glue to cover the 3 layers needed for one panel. That proved to be a little too big of a batch. I had difficulty getting a nice creamy texture and had trouble with lumps of unhydrated powder. They were a big pain as I had to scrape them off the sticky panel when I was rolling out the glue. The other issue with doing a big batch of glue was I wasn't quite sure how much I could use on the first two layers. I was afraid I was going to run out, so I probably used less than I should have until the final layer. I ended up with enough, but I don't like being to uncertain if I'm going to have enough. There were some anxious moments while I was waiting for the panel to cure in the Curvomatic wondering if the glue would fail and the curve would completely spring back.
So for the second panel I decided to make up three small batches in 3 separate cups - one for each layer. That way I would use an entire cup's worth of glue on each layer. And I made 20% extra instead of 10% (I have more glue than I need, and it doesn't have a long shelf life). Both the small batches and the 20% buffer were wise choices. Doing it in small batches, the mixtures ended up smooth with no lumps and I didn't need to guess about how much to put on each layer.
Here's my setup:
I weigh out enough powder for each layer in the panel in 3 separate cups. You add the water at two different times - 2/3 the first time, and 1/3 later, so I measure out the first dose of water into 3 other cups. I made a spreadsheet to calculate the weight of the powder, and first and second additions of water. I pour the water into the first cup and stir. And stir. And stir until it is smooth, and about the consistency of runny peanut butter. Then I let that sit while I do the same with the other two cups. By the time I'm done with those, the first batch is ready to stir again. Bubbles will have formed in the glue as the powder more fully hydrated and you want to stir those out. Then I measured out the second dose of water into each of the water cups. Then I add it to the first glue cup, and stir. And stir. And stir. Weird things happen when you added the second dose of water, but if you continue stirring, all will go well. Then I repeated the process with the other two cups. By this time they have all been sitting long enough that they are ready to go. With one more round of stirring.
It does not spread particularly nicely. It thickens fairly quickly, though it doesn't set quickly - so you do have time. But after about 6-7 minutes with a 6" glue roller, you can get a nice consistent film of glue across the sheet. Then you place the next layer of plywood on top and repeat...after you've stirred the next cup again, of course. Lather, rinse, repeat and you're ready for the clamps. Or Curvomatic. I like to wrap the panel is stretch plastic (like they use for wrapping pallets) to prevent the squeeze out from getting on anything. You can see the rolls of it in the next picture. I also put down a layer of baking parchment on the Curvomatic just in case the plastic wrap leaked.
The glue instructions are very clear that the temperature of the room and the parts must be above 70 degrees F for the glue to cure properly. My shop hovers right around 70 this time of year, so I took out a little extra insurance and use a couple heating pads with a few old pillows on top. I keep the heating pads and pillows around in my shop for exactly this sort of thing.
I'm never sure exactly how long I should leave the panel in the clamps, so I always err on the side of caution. I left each panel in there for 24 hours. Which means it take a minimum of 4 days to do 4 panels. But it was nice having an excuse for not working on it the rest of the day (though I certainly could have worked on other parts of the build).
When I took the first panel out of the Curvomatic the next day I was very pleased to see there was almost no spring back. Which was surprising - shocking even, given the incredible amount of force I had to apply to the panel to get all the layers to curve all the way. I was leaning on the ratchet straps with all my weight when tightening them up. Quarter inch Baltic birch plywood does not like to bend. But it turned out great. After they were all done, I removed the veneer tape and gave them a decent sanding.
I said _almost_ no spring back. There was a tiny bit. So I am going to remake my brace template to include the new radius. I also recalculated all my volumes - they changed by a fracture of a percent. It probably isn't strictly necessary to remake the brace template, but I want the panels to fit/glue to the braces perfectly. That was why I hadn't yet milled the brace blanks to their final size.
If you've never worked with precatalyzed urea formaldehyde glue, it's weird stuff; it doesn't just dissolve into the water like you think it would. There is a lot of stirring involved and it takes a while for all the powder to hydrate. I sort of liken it to making pizza dough. Pizza dough doesn't have a nice smooth texture when you first mix it, but after you let it sit and hydrate for a bit and allow gluten to form, it becomes a lot smoother and more plastic. There's no gluten with the glue obviously, but the same sort of concept applies.
I tried to estimate the amount I needed based on the coverage listed on the container, plus 10%. I mixed the first batch up all at once - that is, enough glue to cover the 3 layers needed for one panel. That proved to be a little too big of a batch. I had difficulty getting a nice creamy texture and had trouble with lumps of unhydrated powder. They were a big pain as I had to scrape them off the sticky panel when I was rolling out the glue. The other issue with doing a big batch of glue was I wasn't quite sure how much I could use on the first two layers. I was afraid I was going to run out, so I probably used less than I should have until the final layer. I ended up with enough, but I don't like being to uncertain if I'm going to have enough. There were some anxious moments while I was waiting for the panel to cure in the Curvomatic wondering if the glue would fail and the curve would completely spring back.
So for the second panel I decided to make up three small batches in 3 separate cups - one for each layer. That way I would use an entire cup's worth of glue on each layer. And I made 20% extra instead of 10% (I have more glue than I need, and it doesn't have a long shelf life). Both the small batches and the 20% buffer were wise choices. Doing it in small batches, the mixtures ended up smooth with no lumps and I didn't need to guess about how much to put on each layer.
Here's my setup:
I weigh out enough powder for each layer in the panel in 3 separate cups. You add the water at two different times - 2/3 the first time, and 1/3 later, so I measure out the first dose of water into 3 other cups. I made a spreadsheet to calculate the weight of the powder, and first and second additions of water. I pour the water into the first cup and stir. And stir. And stir until it is smooth, and about the consistency of runny peanut butter. Then I let that sit while I do the same with the other two cups. By the time I'm done with those, the first batch is ready to stir again. Bubbles will have formed in the glue as the powder more fully hydrated and you want to stir those out. Then I measured out the second dose of water into each of the water cups. Then I add it to the first glue cup, and stir. And stir. And stir. Weird things happen when you added the second dose of water, but if you continue stirring, all will go well. Then I repeated the process with the other two cups. By this time they have all been sitting long enough that they are ready to go. With one more round of stirring.
It does not spread particularly nicely. It thickens fairly quickly, though it doesn't set quickly - so you do have time. But after about 6-7 minutes with a 6" glue roller, you can get a nice consistent film of glue across the sheet. Then you place the next layer of plywood on top and repeat...after you've stirred the next cup again, of course. Lather, rinse, repeat and you're ready for the clamps. Or Curvomatic. I like to wrap the panel is stretch plastic (like they use for wrapping pallets) to prevent the squeeze out from getting on anything. You can see the rolls of it in the next picture. I also put down a layer of baking parchment on the Curvomatic just in case the plastic wrap leaked.
The glue instructions are very clear that the temperature of the room and the parts must be above 70 degrees F for the glue to cure properly. My shop hovers right around 70 this time of year, so I took out a little extra insurance and use a couple heating pads with a few old pillows on top. I keep the heating pads and pillows around in my shop for exactly this sort of thing.
I'm never sure exactly how long I should leave the panel in the clamps, so I always err on the side of caution. I left each panel in there for 24 hours. Which means it take a minimum of 4 days to do 4 panels. But it was nice having an excuse for not working on it the rest of the day (though I certainly could have worked on other parts of the build).
When I took the first panel out of the Curvomatic the next day I was very pleased to see there was almost no spring back. Which was surprising - shocking even, given the incredible amount of force I had to apply to the panel to get all the layers to curve all the way. I was leaning on the ratchet straps with all my weight when tightening them up. Quarter inch Baltic birch plywood does not like to bend. But it turned out great. After they were all done, I removed the veneer tape and gave them a decent sanding.
I said _almost_ no spring back. There was a tiny bit. So I am going to remake my brace template to include the new radius. I also recalculated all my volumes - they changed by a fracture of a percent. It probably isn't strictly necessary to remake the brace template, but I want the panels to fit/glue to the braces perfectly. That was why I hadn't yet milled the brace blanks to their final size.
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