> As always when designing such machinery, 'success lies in the 
> details'--the
> proper bearings, the elimination of mechanical backlash, etc.  I'm curious 
> as
> to what type of threaded drive shafts you plan on using?

The drive spindles are 12mm trapezoidal with a 3mm pitch 
(http://www.worldofcnc.com/products.asp?recnumber=61) running in bronze nuts 
(http://www.worldofcnc.com/products.asp?recnumber=47). The rails are 12mm, 
16mm and 20mm ground shafts 
(http://www.worldofcnc.com/products.asp?recnumber=14) running through linear 
bearings (http://www.worldofcnc.com/products.asp?recnumber=16).

I bought a job lot from eBay. It included four spindles, three pairs of 



> Something to be aware of (and this is from my own personal experience) is 
> that
> using *multiple* threaded shafts to drive a single axis can create 
> problems and
> headaches in trying to get things to work reliably (not insurmountable 
> ones, of
> course)-- the shafts need to be as parallel as is humanly possible--in two
> axes--so the moving platform doesn't 'lock up' at some point. Perhaps 
> you've
> taken that into account already.

You make a good point. Originally I was going to drive the two Z axis 
spindles with two stepper motors. By tying the two Z axis drives together 
with the toothed drive belt I don't beleive that I should have any problems 
such as you describe.

I have changed the system from driving the spindles so that the spindles are 
now locked in position and the nuts are rotated around them, thus pulling 
themselves along the spindle. This should remove any problems from the 
spindles wobbling as they turn and also removes the need for bearings at 
both ends!

I think the problem of "racking" on the X axis is more of a worry. Although 
not shown on the image I have decided to add a cable system like that used 
on large draftsmans drawing boards to stop any rotational movement as the 
carriage moves backwards and forwards along the X axis.

Rarius

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