Quote Originally Posted by deannos View Post
The RSJ's are 178x102x19mm, would it be better to use a smaller size on the gantry, 127x76x13mm.
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Although it's convenient to use RSJ on X and Y, you'd be better off using a different cross section. The torsional stiffness of RSJ is pathetic, hardly much better than a sheet, so especially on the gantry where the cutting forces will tend to twist the beam you would get a lot of deflection. Similarly RSJ is poor under shear stress, which is the case on the X-axis. To an extent you have compensated for that by using a large thickness, but with less material you could use box section and get much greater stiffness.
One reason not to use smaller RSJ is it reduces the vertical spacing of the Y-axis bearings, which is something you want to maximise.

What you've drawn is still fairly strong, so it would work - but that is purely because of the large thickness.

Try folding an RSJ cross section and a box section out of card then try and twist them and you will see how big the difference is. This spreadsheet lets you compare them.