[bassman2914]

I was looking to make a machine using unsupported rods(they're cheap and don't bent if short and thick). I looked at ground and polished rods and the clearance is +0.000 to -0.001 (diameter) and the bushings are -0.000 to +0.001 inches (innner diameter). what i was wandering is if .002 in diff in diameter is enough to prevent binding or if i should get sloppier bushings. There is a potential for even less than .001 in difference in diameter and I wonder if the rods bend at all it seems they would bind. I would be glad for any insight on this situation. P.S. http://www.speedymetals.com/ these ground and polished rods are cheap and precise.

[Eurisko]

You'd be surprised just how much a steel rod will flex, even under light pressure. A 3/4 in. diameter, 36 in. length rod will easily flex .010 under finger pressure.

The .002 slop is troublesome. I'd suggest using adjustable linear bearings to set the preload on the bearings. I don't recall seeing any closed pillow blocks with this feature.

Check out some Thomson adjustable open pillow blocks.

[bassman2914]

what if i sacrifice cutting area for accuracy and use a 1" rod of 1045 steel for x and y. there would be 23" suspended for the x, about 5lb of weight on each rod and 20" for the y axis with about 2.5lb per rod. It would have a cutting area of 12" by 17". I want to be able to cut aluminum but on low budget. I could use longer rods and use bearing to act as wheels to support the weight on the x axis and y axis. then could i use the bushings with these or should i still get some with more slop...clearance. I did go to homedepot today and played with some rods. the 1/2" bends no problem. a 3/4" (hollow) is pretty hard to bend. with way of 10lb per rod and without clamping the ends it didn't bend anywhere near .1in but was only a couple thousandths. so 1" rod that's solid with less weight and supported(kinda) end it should be fine...i hope. I'm 15 so i have a low budget.