Placement of Z-smooth and threaded rods?

Hello, I am rethinking my 3D-printer at the moment, it's a MendelMax 1.5/1.6 (1.6 being my own design at the moment with vertical X-axis).

I cannot find out much about this subject, but I am wondering, why are the Z-smooth rods always placed on the outer sides of the machine?

And second to that, why is the X-motor positioned to 'hang' a bit on one side of the X-axis? For me it would be more logical to have the X-motor directly next to the smooth rod and to have the belt-gear protrude beyond the smooth rod on the backside of the smooth rod so the belt doesn't touch it.

There are so many 'whys' in my head, why this, why that? I can imagine most designs are made with a very important reason, but I can also imagine a lot of designs to have parts places in very illogical ways.

Currently drawing up Rhino parts, and an planning to start designing in Grasshopper because in that way I can start parametric design which will make prototyping much faster.

Z smooth rods outside threaded rods makes the overall machine narrower than having it the other way around. If you had the threaded rods outermost, the machine would be at least the width of a stepper motor wider, for the same X travel. I doubt there would be any noticable difference in performance.

X carriage design has a lot of variances. That is the great thing about reprap - you can easily change the design and print new parts to try different ways of doing things.

Neil

I think that design choice goes back to the original Sells Mendel, which had a single Z motor and a belt to drive the other Z rod. The smooth rods were placed outside the threaded rods. I think all subsequent Mendel variants have done the same, except since Prusa Mendel there are two Z motors instead of a belt.

The X motor used to be vertical and centrally aligned with the belt around both sides of the X carriage, I think Prusa flipped it to horizontal and moved the belt to the side of the carriage.

So I think whatever original design reasons there were may no longer apply after all the design evolutions, and it may well be worth rethinking the design to see which principles still apply.

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Thanks bobc, that was what I was thinking of too.

It looks like 'heritage' from the earlier models.

I see a lot of people designing 'around' the existing placement of rods and axes, which makes me question if it could be improved.
Don't really like the extrusions for Z-rods by the way.

Currently I am redesigning my Reprap Air 2, but I have to admit, because I am a bit lazy, I am using 'sort of' the same design choices the other machines have so I can use parts that are made by other people freely. This means the rod-distances are somewhat uniform.

The uniformity of a design and a legacy that comes with it in the heritage is worth a thing because you can tell where things came from and how they developed. But maybe it's time for something new

However, the rod distance may maybe be the same in my new design for the Air 2, I am designing a very, very kinky X-motor-end with a 'twist' I haven't seen before

Edited 1 time(s). Last edit at 01/19/2014 10:06AM by Ohmarinus.

Here are a couple of considerations to keep in mind.

- playing with the forces in my head, having the z smooth rods farther apart means that more force, imparted by x movement, is required to deflect the z axis. this also depends on the distance between x smooth rods, and i imagine that for a given length x and given length y there is an optimal ratio of distance between parallel rods to limit torque force. (in other words, if the x rods were spaced as far apart as possible, forming a square with the z rods, that would not be very sturdy, but neither would having them as close as possible, like an H.)

- Since the x rods have to be placed adjacent to the z rods, the x axis CoG will be offset +/- the center of the Y axis. in addition the extruder motor is usually not at the CoG of the X axis. So there is usually a CoG bias toward one side of the X axis that you can compensate for by hanging the the X axis motor to the opposite side.