Keeping the Pieces Together

What is all the fascination of bolting together CNC machines? It's as if the entire world were afraid of a little glue. Modern adhesives are stronger than the wood that they are joining. Even metal is glued together in modern airplane construction. Come on people, join the 20th Century.  Epoxy and Titebond III are great to glue together wood even if they get wet. Look up what it takes to be rated as Type 1 water resistant. We are talking boiling for 4 hours, baking for 20 hours at 140F, boiling again for 4 hours, and then testing while still wet. Regular PVA glue is great if you want to keep it inside and dry. These glues even hold pieces of MDF together pretty well, if you are still hung up on using that nasty stuff. If you have no metal fasteners, they can't eat your expensive cutters when you accidentally hit them.  Glue doesn't cause galvanic corrosion. Vibration won't loosen glue like it will bolts. Most importantly, glued structures will distribute  stresses instead of focusing them like bolts do. The proper term for the focused stress is a stress riser. Bad stuff. It makes things warp and fail.

Now, don't start thinking that I am totally against metal fasteners. They have their place. Go ahead and screw a motor down with them. Drill oversized holes and lock them together with bolts to make things adjustable. Bolt on a proximity switch. Have a screw clamp to hold the spindle down. Use a bolt for an axle for a bearing. Just don't forget that glues have their place too.

Which adhesives are best suited? Epoxy and  PVA will come into play the most. I used to like the moisture curing Polyurethane glues (like Gorilla Glue) but that was before I met epoxy. The main difference between these two glues in use is gap filling and open time. If I have a nice clean joint and it is not going to take a long time to put together, I go with PVA. If things don't fit perfectly or I need a lot of time to get things aligned, I go with epoxy. Also, if you are forced into gluing endgrain, epoxy can really be your friend. If you need a little extra support, grab some fiberglass, carbon fiber, or Kevlar. If you need to span bigger gaps, grab some fillers. It is great stuff. Of course someone will point their finger and say epoxy is expensive. Not by a long shot. You want expensive? Go compare epoxy with polyurethane glues on an ounce by ounce basis. Polyurethane glues are much more expensive. Also, they don't fill gaps nearly as well unless you count non-structural bubbles of foam. One thing to note: Epoxy will not be as strong if you clamp it hard and squeeze out too much of the glue. PVA works better with plenty of clamping force. Read the instructions on the bottles even if you are used to using them. I found out some mistakes I used to make with some glues. One example is polyurethane glue. I didn't know for a long time that you should pre-moisten the surfaces with water.

What about comparing the price of glue to the price of bolts? Well, you can look at some of the fastener kits that come with some CNC kits. One kit I know of comes in at $120. Having bought a lot of nuts and bolts in my life, I can say pretty confidently, that in my opinion, they are not marking them up enough to be worth counting them all out and packaging them. They are doing it as a convenience.