Diameter of shaft rod for linear motion

[lete002]

Hello

I'm working on my first machine, and I decided to use hardened rods for the rails of a 36" x 18" x 6" gantry router. I' planning to use tapped rods, they come on 16 and 20 mm diameters with holes every 10", on www.vxb.com
What considerations should I take to select the size of the rods? I understand about the flexing under load, but with support every 10" it shouldn't be a problem (??). Is there any other reason to use larger diameter?

Thank you

[harryn]

Hi -welcome to cnczone.

Hardened rod is vary hard, but it is not really stiffer than any other steel. If the rod is hollow (pipe) or solid, it is still nearly the same stiffness.

Consider to go to a hardware store and pick up a piece of pipe the same size you are thinking about, but 3 meters long. Shake it a little and see if you still think it is stiff enough for precision work.

A rail must prevent flex not just up and down, but also side to side. I was very surprised at how flexible steel really is. Also, read the build log of Carveone. I don't mean to pick on him, but he has tried a variety of methods to build his router, and his build log is a great example of someone that keeps working to make his router even stiffer.

If you have 20mm fully supported rod, then most people would say you are fine for a wood router.

Take care

Harry

[Kipper]

You can always brace it....