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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'
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
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.
All text, photos and graphics
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