Tag: Gantry Build

After planing the gantry, I realized that the front face, where I intended to mount the Y axis rails, is no longer perpendicular to the base. Since I was planning on using epoxy for the rail mounts anyway, I figured I could just use epoxy for leveling the front side of the gantry.

With the gantry on its back, I made sure the gantry bottom was perpendicular to the machine base, since that is where the X Axis bearing guides will be mounted. To minimize the use of expensive epoxy, I made a dam out of 1/2″ plywood covered with blue painters tape, which releases easily from epoxy, then sealed at the edges with clay. This dam allows the epoxy to flow only to the areas where the rail mounts and the ball screw bearing support mounts will sit for the Y-axis, with extra room allowed for the meniscus of the epoxy at all edges.

Once the epoxy was mixed with hardener, I poured a small amount into another container and mixed in a little acetone to thin it out. I didn’t want to thin the whole batch because acetone can change the strength properties of the cured epoxy. I poured the non-thinned epoxy throughout the area and let it settle for a few minutes, and then poured the thinned epoxy over the top to achieve a very flat surface.

Remember, all of this work is driven by the fact that I didn’t start with a perfectly flat work surface. If the gantry had been properly square, I would have just attached my rail mounts and bearing supports directly to the wood face of the gantry. However, it is a good learning experience and I’m glad to have been able to test out the epoxy leveling method in case I want to use it for my next machine.

My first major problem with this build. After all the glue was dry on the gantry, I rotated it 90 degrees on my work surface (the machine base) to see what it would look like in its final orientation on the machine. One corner of the gantry sat above the surface by about 3/32”. I first thought it had warped after the glue-up, but back in the position where I constructed it the gantry laid perfectly flat against the base. After doing a lot of checks with a straight edge and feeler gauges I found that my base was not perfectly flat as I initially thought. It was flat along the edges in both length and width, but there was some variation throughout the center. So, between the 40 or 50 thousandths of twist introduced where I constructed the gantry, and another 30 to 40 thousandths in the final orientation, I ended up with the gantry having this 3/32” warp or twist in relation to the base.

The way I decided to fix this was to create a large planing jig for my router. Following some videos online, I mounted two long pieces of plywood (with the factory cut sides up) along the sides of my machine base and used a level and two wires running from corner to opposite corner to get the rails level and without any twist. Then I built a sled for the router that would slide along these two rails, allowing me to run the router over the whole surface with a 1 ¾” surfacing bit. I waxed the router sled and the rails, and used some clamps for stops. My wife helped out by holding the shop vac to collect the dust while I routed the entire surface of the machine base. I clamped one end of the base to my leveled workbench so that the corner with the twist would be up the air and would get milled down. If you want to see more details, this is one of the videos that provided inspiration: youtube.com/watch?v=qtkBZHLJyD0

After routing the top, I flipped the base over and repeated the operation on the other side. In theory, these two sides should now be parallel and without any twist. This will hopefully get me back to square one for having a flat work surface to build the machine on.

Rather than re-constructing the whole gantry on this newly flattened surface, I decided to use the same planing jig to get the top and bottom of the gantry to be flat and parallel. This seemed to work well and now the gantry sits perfectly flat against the base.

Having a nice flat surface to work on, I decided to start building up the gantry. I printed out some screenshots from my solid model and wrote in the dimensions by hand, not wanting to take the time to create all the dimensioned drawings in the CAD software. Then I got busy cutting out all the pieces of plywood on the table saw. I used a dado set for cutting the slots in the top and bottom pieces.

I took this image to show how the pieces will fit together. The top is slid back slightly for better visibility:

Next I cut out all of the web pieces from 1/2″ MDF. Here are all the gantry pieces laid in place for a test fit:

Once again, I glued the pieces together in stages to make sure I could keep everything aligned properly. The number of clamps available also limits how much can be done at once.