In my last post I described some mistakes made in the Z axis design – particularly that I had not made a secure enough clamping setup on the LM12UU bearings and had also selected aluminum stock for the weight bearing arms that was not up to the task of supporting the build platform in a stable enough manner.
I have decided to correct this before proceeding with testing rather than waste my time fiddling with a clearly broken setup.
My general solution in the redesign is to overbuild the Z axis – this will remove any weight restrictions on the build platform (within reason, of course) and also allow the machine to move smoothly which should naturally translate into higher quality prints.
The second revision of the Gemini prototype used four 12mm hardened steel rods for the Z axis and moved the platform using two leadscrew motors in a similar vein to the RepRap Mendel Z axis with the difference being it used two guide rods instead of one on each side.
This design proved to be difficult to maintain level on the X plane due to the weight of the build platform and the ease at which a nicely machined leadscrew will turn. On the Mendel printers the nut-and-threaded-rod doesn’t turn turn easily with the limited weight applied to them and thus they do not tend to move on their own when powered down.
The third revision addressed this by using a cantilever design guided by two rods at the rear of the printer, leaving me with two spare 12mm rods. I am re-introducing them into the design by doubling up the linear rods on each side to create more rigidity on the axis.
I plan to replace the aluminum angle with 3/4″ square aluminum tubing which has much less flex, particularly at the lengths involved. The two square arms will be well constrained and might possibly lend itself to a three-point levelling system for the build platform as well.
I won’t have time tomorrow to work on this redesign due to other commitments, but I am attempting to get as much of it created today as I can and already have the lower rod holders being printed on my Prusa Mendel i2 as I write this.
Stay tuned, I will have pictures later this week and hopefully will be able to report some success.
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