Post mortem time?

Woooowaaah!
Before going any further, I need to get my sums right and think carefully about some of the issues I faced when I put the first system together. For instance, I had to fettle the ballscrew yoke to get it to fit,  which pissed me off somewhat. I did a load of fairly precise modelling and machining, only to have to get the blacksmith's tools out to get the thing to go together. I intend to do better next time. 

Here are some of my thoughts:

• Assembling the parts onto the machine was a royal PITA:
1. Fit the yoke to the quill and the ballnut into the yoke.
2. Thread the ballscrew into the ballnut, using the special adaptor (careful!!).
3. Fit the lower thrust bearing into the motor bracket, holding it in place with the top hat spacer.
4. Fit the upper thrust bearing (why do I have 2 thrust bearings?) onto the top of the ballscrew.
5. Offer the motor bracket up to the machine head and trial fit the fixings. Bugger about with shims etc to eliminate any backlash, without preloading the bearings. Then finally tighten up the fixings.
6. Fit the driven pulley, key and locknut. Then try to tighten the locknut to a sensible torque with nothing to lock the pulley against. Hmm.
7. Fit the motor to the bracket - only then can you fit the driving pulley to the motor shaft. That's because the pulley doesn't fit through the bracket (why on earth not, fat boy?). 
8. Finally, fit the belt and tensioner (jockey pulley). 
9. Check you haven't forgotten anything that would require it to come apart again....

• The ballnut yoke doesn't need to bottom out in the counterbored fixing hole that is machined in the quill. In fact, a finite gap would ensure that the ballnut's cylindrical face is in contact with the quill. It might also ensure the ballnut ended up in the correct (radial) position.
• I like my current ballscrew design in terms of the cylindrical face that contacts the quill. Unlike the approach used by BMSTECH, I will keep the fixing bolt central to the bracket. This must surely result in a more robust solution. I'm assuming the quill is happy to descend the additional 10mm or so that my concept would enable relative to the original design.
• Some degree of adjustability of the motor bracket relative to the quill would be sensible. Perhaps not quite as much as the 0.7mm or so required last time but if I machine the ballscrew appropriately, I may be able to achieve something approaching +/- 0.5mm or so. One key advantage of having a largely self-contained assembly is that the whole chebanc could be moved as one, relative to the quill, without disturbing said assembly. The key thing is to ensure the ballscrew doesn't end up seeing any significant radial movement over the range of movement of the quill. 
• And the ballscrew only needs to be supported at one end. As I have a pulley / locknut at the bottom end, I can pass bidirectional forces to the (lower) bearing as it is - so the top bearing is completely redundant. WTF was I thinking of? The top bearing loses me the best part of 20mm of precious Z axis travel. What a dickhead.

What next then, Fatty?

So I reckon the sensible thing to do now would be to dismantle the existing assembly and make some very careful measurements. And look more closely into what is required to get a precise fit between the yoke and the quill. I'm suspecting some of my blacksmithery may have been due to miscalculating / mismeasuring the critical dimensions. 

Let's do it...