Does glue need to be considered as an additional measurement, ~1/32, especially when cutting rabbets and dados? I was making a couple of stop blocks using rabbets for the joining the sides to the top. The depth of each rabbet was equal to the thickness of the plywood but yet when I did the assembly, the top was slightly proud, maybe ~1/64-1/32, of the sides. Since this was only a jig, I wasn’t concerned but thought it might be a bigger concern with furniture making or other projects.
Thanks, Brett
Replies
I gave this subject more attention than it probably deserved a while back while gluing up several different laminations. The first was a cutting board glued with epoxy. I over-clamped and learned that epoxy needs to have a slightly "fat" glue joint, or it behaves like a starved joint. What I learned was that different glues need different thicknesses to perform effectively. Having learned that, my next project involved laminating a curve using resorcinal. I researched the appropriate film thickness for resorcinal, built my lamination, did the math and clamped so the glue thickness would be correct. Knowing the glue thickness also helped me make sure the final piece turned out to be the right width; .005 adds up when there are 15 laminations.
All that said, I would only expect glue to contribute to the overall width or thickness when there were multiple joints involved. I wouldn't expect it to produce ~1/64 - 1/32 unless it wasn't clamped.
Or perhaps the plywood wasn't exactly its stated thickness as sometimes happens? Also if the plywood may have swollen from humidity?
Making rabbets slightly deeper than the required thickness of the plywood could be planed flush afterwards.
Regards,Bob @ Kidderville Acres
A Woodworkers mind should be the sharpest tool in the shop!
With ordinary yellow glue, there is no extra allowance for glue thickness. Most likely, you either didn't get the joint pulled in or you didn't cut it as you thought. If it was flush when you did your dry fit, then you didn't get it pulled in when assembling. This can be a challenge and may require some cauls along with the clamps. It is common to try a piece in checking your set up and then not having the joint cut as expected. It's generally, how well the work fits the fence, the method used and you need to check the actual pieces for fit, before assembly. It's not unusual for something like plywood to be a bit different in thickness across the sheet. If you stood your pieces up on a concrete floor, or had them sitting around for more than a couple of days, the end grain may have swollen. If the rabbet does not have 90° corners or the work end or the assembly was not square, the same will happen. It doesn't take much.
This forum post is now archived. Commenting has been disabled