awerby: I don't think that I will easily find those ground steel tubes in my area, but your advice made me rethink my design. I have a supplier of heavy duty steel tubes in walking distance from my home, and also a CNC manufacturing business where I could mill the grooves for mounting the rails at a relatively low cost. I will bolt everything together with screws, at least 3 for every joint, contact surface should be enough. I don't have a welder, and even if I had, my welding skills are non existent. I hope this will be rigid enough.
[Bolts will work, if you use enough of them and seal the threads so they don't loosen up. In some ways they're better than welding, since there's no heat to distort the steel. Milled channels will work for mounting rails if there's enough thickness for them; you don't want to have the steel warp from uneven stress relief. See if your CNC shop offers grinding services as well. The other way to go would be to bolt some steel onto the tubes and mill the channels in it for attachment to your rails.]
Regarding the Z axis, having already bought the entire set which has the linear rails and ball screws for the Z axis as well, I am going to use those. I am trying to keep costs down (I have mouths to feed
).
[It sounded like you hadn't decided what to do, but sure, that should work.]
So, I came up with a second design. The gantry is made from a 200x100 mm 8mm thick stell box tube with 200x100x5mm legs. The base is made of 80x80x5mm where the rails sit and with 100x40x4mm traverses.
A static stress test shows almost half of the deflection for steel tubes with a 5mm thickness in comparison to a standard aluminium extrusion of the same external dimension. So I get a lot more stiffness and with less money as well.
[Right; steel is a lot stiffer and cheaper, if weight isn't a factor. You can also beef it up by filling it, either with epoxy-granite or fitted spacers bolted in place.]
One more thing: regarding the closed loop steppers. I knew that if they loose steps, they will compensate and recover, but how does this "shut down" work. They will give feedback to the motion controller when the lost steps reach a certain amount and cannot be recovered in time?
Re-zeroing with high precision to be able to recover a part isn't that complicated if done manually with dial indicators.. am I right?
Please let me know what you think.
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