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Converting a Grizzly G8689 Mini Mill to 3-axis CNC, Part 2:



Due to other projects on my various tables, this setup wound up sitting for a bit until I found the time to dive back into it.
I dusted off all the parts, cleared a bit of bench, and sat down to take fresh stock of what I had and what I needed to do.



First thing up, I removed the entire head and column and set it aside. As noted, I have an idea
for a brace to help solidify that weak column pivot, but that'll come later.



I revisited the idea of adding 2:1 belt reductions to the table screws, but I'm rethinking that. For the time being,
I've decided to go with common helical couplers and a straight stepper mount, for simplicity.
The belt reductions can always be added later, if necessary.



One of the first things I did was lightly mill the ends of the Y-axis screw opening in the base casting. This gained me
roughly a quarter of an inch of overall ballnut travel, and provided a flat face in case the nut 'crashes' against it.



The mounting bolts at the front of the casting also needed minor tweaking-
one had been tapped crooked, and badly enough that I wanted to fix it.
I set the casting up on an angle plate and rebored both holes from
1/4"-20 (yep, they were imperial) to 5/16"-18.

While it was set up like that, I was able to use the DRO to 'measure' the bolt
center-to-center, as well as locate where the ballscrew would go- to clear
the bearing, I had to lower it 160 thou over the stock location.




I then found a suitable block of 'looneymum, bandsawed off a likely piece, and bored a seat for a double-row, angular contact bearing.



Using the previously-generated spacing data, I flipped it and drilled and countersunk the two bolt holes.



That bolts on about like so, and the screw still clears the casting.



Now, I was going to want the ballnut to travel pretty much as far as possible, to the point of nearly crashing into the
back wall of the slot. (Which is why I flattened it a bit.) I didn't want the nut trying to run off the end of the screw,
so I milled a small notch like so...



So I could cut the screw slightly long- now the nut will stop on the casting some 3/8" before it reaches the end of the screw.



With the front block and bearing in place, it looks like this.






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