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Converting a Logan 11" x 32" Lathe over to full CNC, Part 17:
In late summer of '17, Centroid, a well-respected manufacturer of quality industrial-level CNC
controllers, announced a new DIY/conversion-level CNC controller board called the Acorn.
It promised quite a few pretty significant benefits, one of the biggest of which, for me, was the use of a proper
spindle encoder. Mach 3 could only use a single-point signal; that is, one data point per revolution- this is far
from ideal for threading, and makes rigid tapping basically impossible. The new Acorn board, however, can
take proper encoders- in this case a 2000-line quadrature encoder, for 8,000 data points per revolution.
The encoder needs to be driven 1:1 from the spindle, and since we want to preserve the ability to run bar stock
through the spindle bore, that means setting up a belt drive to offset the read head to the side. On my Logan,
I figured I'd simply replace the original drive gear off the back of the spindle, with a timing belt pulley.
I found and ordered a couple of small toothed-belt pulleys of an appropriate size from McMaster-Carr, and
when they arrived, set up some soft jaws to hold them for boring out to the spindle size.
And done, bored to 1.251" on the nose.
I needed to broach a 1/8" keyway to match, and luckily had this bit of scrap from the bin
that was exactly the right size. I just had to mill a slot to guide the broach.
Once the keyway was broached, the new pulley slipped right into place...
And the collet-closer adapter screws on and holds it snugly.
Now, we have to come up with a way to hold the encoder and the other pulley. The Koyo encoder I bought
is "light duty", and shouldn't necessarily be used as an "axle" to support the pulley and belt. So I pondered,
and came up with what I thought was a workable solution. I decided to set the encoder just a little off to the
side of the spindle, so that if I ever used coolant, it wouldn't drip out of the through-hole onto the read head.
This time, I turned the "step" off the pulley and bored it out to .500".
I then turned a short shaft out of some scrap-box mild steel, gave it about a 0.002" interference fit
and pressed it into place.
That then got carefully turned down on both ends to snugly accept a small sealed bearing on each side.
And now it was time to design a mounting bracket. Not wanting to drill and tap any new holes in the headstock,
I elected to use the back-gear shaft bore as a pivot (the geartrain tumbler used it as a pivot previously) and
will use a second threaded hole to it's left as an adjuster slot bolt. With that in mind, I got out a bit of scrap
cardboard and some scissors and scribbled up something that looked somewhat workable.
All text, photos and graphics
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Keep your fingers away from the spinny blades o' death and you should
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