Originally Posted by
Hartzell
You have right Wizard. Better to move on and understand that I will never get the perfect machine on the first try with my complete lack of structure knowledge and force calculations and so on.
I will make some changes and hopefully set on a design to start with on monday. Then I will need some assistance to order all the hardware.
You guys are really kind and helpful, priceless.
One thing that might help with understanding the strength of the materials being used is to try to manually bend or twist them. Channel iron for example could be held in a vise at one end and twisted at the other. Given a robust vise and a couple of hundred pounds of torque you should be able to see some twist over 4 feet. Size matters of course just like 1/2" pipe is far more flexible than 4" pipe.
Edit:
When I say "see" above I mean just that, being able to visually see the twist without the aid of a dial indicator or other tool. Channel iron and angle iron can be very twisty if you will. A gantry has to resist twisting forces if you expect to maintain accuracy at the tool.
So what I'm trying to say here is that what you want to do with the channel or U shaped profiles isn't impossible, you just need to be aware of the limitation and extra work involved. That extra work might involve a lot of welding and machining not normally required. Lots of welding introduces its own issues. As I understand it you are using this material due to having access to it which is fine. Given that you might want to consider welding up a box beam for the gantry if you can address the post welding work required.
Also when it comes to your design realize nothing is perfect. You could pull a free plan off the net, that has been tested by dozens and it still won't be perfect. In the end the machine needs to meet your needs.
Edit:
Not knowing your design and build capability I'm going to suggest buying tubing for the gantry beam might be a good idea. You might get good results from channel iron if well equipped to do so but I really believe it will be a lot of effort on your part.