Trouble keeping lumber core substrate flat.
I’m playing around with lumber core using materials I have in the shop, and I’m having trouble keeping my substrate panels flat. These are small panels: a nightstand top and aprons.
I used 1 3/4” wide by 1/4” vertical grain pine for the stave core that is dry and acclimated to my shop. Then a sandwiched the core with 1/4” birch plywood ( which is actually around 3/16”) on either side for cross banding. I glued up these three layers using plywood cauls since I don’t have a vacuum bag. My substrates are coming out bowed, or even twisted at times. I plan to use 1/16” shop sawn veneer on both sides of the substrate, but have not proceeded to that step yet.
Any thoughts on what to change? My parts seem flat before glueing up. Perhaps I am introducing warp during the gluing/clamping process?
thanks in advance.
Replies
First thing I'd try is a non-water-based glue for the lamination. The core might be taking in water unevenly.
I agree with MJ, consider trying this with epoxy. There is a long tradition in boatbuilding of laminating skins over some core material (including balsa and foam). This might avoid warpage introduced by a water based glue.
I had to look up lumber core. I searched 4 different sites to see if lumber core was a recommended substrate for veneer. It wasn’t listed on any of them.
Where was the idea to use lumber core obtained?
Thank you. I’ll try different glue.
I got the idea mainly from Tim Coleman and Craig Thibodeau. Tim Coleman has a YouTube video that briefly outlines his process and he wrote at least one FWW article, including ‘Curved Doors Simplified’. Craig Thibodeau has written about it both in his book and FWW. My understanding of the benefits are that panels are lighter than Baltic birch plywood for example but still take screws and splines better than MDF.
I was hoping to have some success with flat panels before moving on to curved ones.
You might also consider a more stable core material. I have used lumber core plywood in the past and the core material was poplar.
It's a battle making that work. At the least, I would go with a hardwood core like poplar. If this helps in your decision making a "staved lumber core" door is never in a new door spec. As of last year 2 MAJOR door suppliers stopped offering it at all. Not even custom. Particle core, or a structural composite lumber core (think strand board but longer thicker strips) are the standard. 1/4" over the diagonal of a 42x84" door is the max allowance. Staved lumber core just can't meet specs like this.
What is its proper name. It doesn’t come up in a web search to purchase. Must be dirt cheap to go through all that?
I note that the cross banding layer that Coleman uses (https://www.finewoodworking.com/2015/09/30/curved-doors-simplified) is a single veneer, much thinner than 1/4 ply. Also his staves are narrower than yours, and thicker. The wider, thin staves will behave like solid thin wood, prone to warping. I think of it as you're tricking the solid wood into not behaving like solid wood by spreading out the wood movement across many tiny gaps, with each separate piece firmly attached only to the crossband veneer. Maybe try thinner plywood or a shop sawn veneer 3/32 or less made of poplar. And, thicker, narrow staves?
You probably saw it, but there's also a video by Coleman: https://www.finewoodworking.com/2022/11/03/tim-colemans-lumber-core-process
Alright. So I’ll try narrower thicker staves out of poplar or something similar and use thinner veneer crossband. Thanks everyone.
I don't think there is anything wrong with using vertical grain pine for your core. I would definately avoid the 1/4" plywood though. Cheap commercial veneer will work much better for cross banding.
If I needed something wider for crossbanding, would you use 2 ply or paperbacked veneer or just tape together a few narrower sheets of regular commercial veneer?