I build a stock size MPCNC with 304 stainless steel tubing. Not willing to leave well enough alone, I had a desire to increase its length to accommodate larger pieces. I bought some 60" long 3/4" schedule 40 galvanized steel pipe.
The 304 steel is 1.003" and the new tubing is 1.062. That slight change in diameter is very noticeable. The trucks accepted the tubing with some gentle persuasion, but the core needed all of the bearing bolts loosened to the point of floppy and was still a tight fit.
The most notable difference is that the new axis with the new tubing is “bumpy”. The surface doesn’t feel that bad to the touch, but on the trucks it seems to grab a bit. The stepper motors aren’t attached, but moving it feels like the steppy resistance you feel in the stepper motors.
I’ve read the tubing can be sanded with 200 grit to knock it down a bit. I’ll do that, but I wanted to see if there are any other suggestions before I continue on.
(I realize I’m at the cusp of just building a LowRider, and that will be in the cards eventually. But this is where we are at now.)
Small chance the bearings smash the tube down into something closer and more useful. Maybe get it set up and write some gcode to run it back and forth in a diagonal a few hundred times and see if it loosens up a little.
My goals were to expand my stock 450x330 machine to 450x1220mm or about 18"x48" This size fits the depth of my workbench and gives me room to cut larger projects.
My desire was to do with minimal additional cost. 304 steel was going to run almost $200. In hindsight this is a relatively small cost, but it’s the process that matters here not the product.
The slightly oversized RMT wasn’t going to work. The trucks are built too rigid to adapt to the larger size and it was binding too much for comfort if forced.
I ended up buying 3 tubes of 1/8 wall aluminum. With the knowledge that there would be sag and I would be using midspan supports regardless of the material. I soon realized that the force to move the core is transmitted entirely by the center gantry rail, which, at this length and material, flops around like one of those inflatable attention getters you see outside of tax preparation stores.
Undeterred and stubborn, I forced one of the oversized RMT tubes through the core just to see what a difference it makes. And what a difference it does make. The attachment to the core with the core clamps can be loosened up enough to make it fit, and while the movement is relatively smooth, the core has some twisting slop because the bolts are not tight.
The final solution was to design and print new gantry to core clamps to accommodate the larger diameter gantry tubing in that axis. In the process, I’ve designed into the model better accommodation for my sockets/wrenches and to use 8mm washers instead of the spacers built into the model print.
I believe there is still a slight height difference, 0.75mm, which would cause the core to be out of square. I think I’ll put this all together and see how well it performs. I have a long 40" 3D cut that should give it a good workout and expose any major issues.