My recent setback on my GP-4 project (detailed in my other thread: https://www.homebuiltairplanes.com/...n/23299-popped-longeron-joint-repairable.html) has given me the "opportunity" to start over on the fuselage. I've decided to 3D model the aircraft in CAD from the blueprints and cut all the parts on a CNC router table.
In order to do this, I need to account for the thickness of the glue in the joints. The CNC i'm using is capable of accuracy down to .001", so if I was to simply model the pieces exactly to the plans, parts such as crossmembers that fit between other fixed pieces would have no space in the joints for glue. I contacted System Three, and their tech support people told me that minimum bond line thickness for a structural joint is 5 mils, and max allowable gap without a loss of strength is 1/16" - so an acceptable thickness range of .0050 to .0625. They said that if you require precise bond line thickness you can shim the joint, but I'm not so sure that's a good idea since it would reduce the glued surface area. They also told me that clamping pressure only serves to eliminate air bubbles, so as long as there are no voids in the glue and the pieces stay where you want them, clamping isn't even necessary. So for pieces that are built on a flat table, simply holding everything in position with blocks without squeezing them together is acceptable.
Basically I'm trying to decide how much gap to leave in my design for the glue joints, and how to go about achieving that gap consistently when I do the actual gluing and clamping. Any ideas?
In order to do this, I need to account for the thickness of the glue in the joints. The CNC i'm using is capable of accuracy down to .001", so if I was to simply model the pieces exactly to the plans, parts such as crossmembers that fit between other fixed pieces would have no space in the joints for glue. I contacted System Three, and their tech support people told me that minimum bond line thickness for a structural joint is 5 mils, and max allowable gap without a loss of strength is 1/16" - so an acceptable thickness range of .0050 to .0625. They said that if you require precise bond line thickness you can shim the joint, but I'm not so sure that's a good idea since it would reduce the glued surface area. They also told me that clamping pressure only serves to eliminate air bubbles, so as long as there are no voids in the glue and the pieces stay where you want them, clamping isn't even necessary. So for pieces that are built on a flat table, simply holding everything in position with blocks without squeezing them together is acceptable.
Basically I'm trying to decide how much gap to leave in my design for the glue joints, and how to go about achieving that gap consistently when I do the actual gluing and clamping. Any ideas?