Originally Posted by vroemm  Hi :-)
I have a few hardened steel tubes.
.......Thanks. |
Hardening makes stress in the tube.
Yes, almost certainly.
Does this stress reduce over time? And does it lead to inaccuracies? Which results in a bend frame?
Probably no. As an example on some commercial machines the bedways are hardened and ground; similarly linear ball guides and ballscrews are hardened and ground and they do not change over time.
Are hardened steel tubes suitable to make a frame for a cnc router?
Maybe yes, maybe no.
There is no particular advantage to hardened tubes unless you are using them as the ways and running bearings along. That is the application where hardened material is preferable.
The hardening does not alter the stiffness it simply means you can bend the tube and it will spring back where a soft tube might bend permanently. But provided you do not bend far enough to make a permanent bend both hard and soft bend equal amounts for equal force.
There could be some disadvantages:
Welding hardened tubes may not be a good idea as they could crack and the heat from the welding will certainly release some of the stresses and they will distort in unpredictable ways.
Drilling hardened tubes is going to be difficult or impossible.