304 stainless steel slide rods

[bertvk]

Hi all,
I'm building my first 3D CNC mill and have a question on the slide rods to use.
I want to use roller skate bearings to keep the cost down and can get 304 stainless steel at 4.5 Euro per kilo ($2.5 per pound). Will 20mm thick rods do for a 300mm by 300mm mill using a moving table (needing 600mm long rods for the X axis)? I'm not sure 304 stainless steel is stiff enough.

I already have ACME rods and zero backlash nuts for the X, Y and Z axis. Also the whole electronic part is ready; not tested yet though.

I'm planning to mill foam, MDF and alu with it. If it all works well I'll add an A axis. The electronics are ready to hook up a fourth stepper.

[jeffs555]

It all depends on what loads you will be handling, and what deflection you can accept. A 20mm diameter 600mm long stainless rod with simple supports on the ends, and a 10lb load in the center will deflect about 0.122mm. With a moving table design, your x slides have to support the weight of the table plus the weight of your material, plus the downward force of your cutting tool.

[bertvk]

Thanks for the data. A 0.1mm deflection is too much for what I need. I'll support the rods at the bottom in some way and make sure the supports can't deflect sideways either.
I'll check the non ferro company, where I get the rods, what kind of profiles they have I can use, otherwise I'll have to use MDF. As an RC model builder I like to find a smart way instead of an expensive one.
I've got a pretty clear idea on how to proceed now. I plan to put the whole development of this machine on my site at http://www.dream-models.com so others can copy it or give comments for improvements.

[Konrad]

All 3 series Stainless are not very stiff, go with something like an alloy 4140H, or some hydraulic piston rod, these are usually case hardened and flash chromed.
Konrad

[bertvk]

My problem is I have no idea where to get that kind of products and I am certainly not going to spend a lot of money buying them. I can just as well get me some linear slides.

If these things are pretty cheap, where would I be looking? Don't say eBay because I live in Belgium and there's not a lot on offer here. I have to find companies that sell this stuff locally.

[jeffs555]

The difference in deflection between stainless and even the best hardened alloy steels would be less than 10%, so I wouldn't worry about different alloy steel. If you need less deflection, you either need to support the rod along its length, or else use rigid fixed end supports. Using fixed end supports on your 600mm rods, your deflection would be about 4 times less than with simple end supports. Or else go to thicker rod. Changing from 20mm to 25mm would cut the deflection in half.

[bertvk]

What's the difference between rigid and regular end supports?

I think I'll go for support allong the axis anyways, so 20mm should be fine I think.

[jeffs555]

A rigid end support is like a cantilever. Rigid means that each end is capable of supporting the rod rigidly by itself without any support on the other end.

[bertvk]

hmmmmm, a bit difficult to do with an MDF frame. MDF won't be able to take the tortional forces.

I'll get me some square rod to support the slide rod and mount that on the base plate.