I am putting this in the wood router forum even though I plan on cutting metal and wood with this machine. Here is my progress so far.

I started with a 24x36x4 granite surface plate. It came from KBC tools and cost about $370 including shipping. I built a stand for it out of 2x4 wood and 3/4 inch particle board. The surface plate is supported at 3 points only. This forms the base of the machine and it was used to lap the other parts flat. I also indicated against this plate to straighten the linear rails.
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The gantry is built from a piece of 8x8x27 inch steel tube with a 1/2 inch wall. I bought ground strips of mild steel and grouted them to the gantry tube with a mix of epoxy and 220 aluminum oxide grit. The ratio was about 75 percent aluminum oxide and 25 percent epoxy by volume. The mix was a little thicker than chunky peanut butter. The ground steel strips were placed against the surface plate to force them to be flat. Spring washers were stuck in the epoxy to provide uniform force to hold the steel strips flat to the surface plate. I used xylene to clean all surfaces to be grouted and roughed them up as well with a sander. I added concrete blocks to increase squeeze out.

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After the epoxy set I flipped the tube and measured the flatness of the steel strips with an indicator. It was out by about 0.002 inches. I decided to lap it the rest of the way. I tried using a small surface plate and two sheets of sand paper but the flatest I could get was about 0.0015 inches probably due to my technique. I decided to lap the gantry against the big plate to get it flatter. I glued four sheets of 100 grit paper to the big plate and lapped for about 20 minutes. This got me to better than 0.001 inches of total flatness. I finished lapping with a sheet of 220 on the smaller plate (not pictured) and got a nice finish and about 0.0007 inches to total flatness error. I decided to stop at this point.



Next was mounting the X master rail. I moved the tube to the floor and brought the drill to it. I temporarily clamped the rails to the tube and used a transfer punch to mark the hole locations. I drilled and tapped the holes in an afternoon. The drill needed to be sharpened a few times because of the aluminum oxide and epoxy layer. I used 5/16-18 bolts to mount the HSR30 rails.



Once the holes were tapped, the tube was put back on the granite table and the master rail was indicated in. I got it straight to better than 0.0003 inches. Once the master rail was attached I verified its flatness with the smaller grade A surface plate. The needle on my 0.0005 inch indicator did not move much at all.



Continued in the next post because I hit the 8 image limit...