Having been enamored with the 5C collet chuck on the 9×20 lathe, I certainly wanted the same for the big Keiyo Seiki. Not having a whole lot of use for the 9×20 anymore, I decided to simply move the chuck to the much bigger lathe. I looked around online for a suitable adapter plate, but I wasn’t sure of precisely what I needed, so I went back to New England Brass & Tool and Bob had just the adapter plate I needed in stock, and at a price lower than I had figured.
This was a semi-finished backplate, which means that it still needs final machining to fit an attached chuck (more on this later). However, it also had the recess for the spindle’s indicating lug off-center. I’m not sure if this was actually intentional, as the lathe’s indicating lug is right on center with the rest of the bolt pattern:
Well, there were two possible solutions – either drill a new indicating lug recess on the backplate, or drill and countersink new mounting holes through the backplate and use the existing recess. I decided to just drill a new recess, opting for simplicity, as I’m sure I wouldn’t be able to drill 3 new mounting holes with the same accuracy as the existing ones. The indicating lug hole doesn’t have to be super precise anyhow – I think it’s simply there to make sure that the same holes on the backplate match up with the same holes on the spindle with each mounting, ensuring better accuracy.
I clamped the plate to the mill’s table with a hold-down clamp that was almost the perfect size (I filed the tail end of the clamp a little to get it to fit the inside of the plate). I then found the center of one of the mounting holes and set its location as the origin on the DRO.
I then moved around to the other holes and the recess for the indicating lug, noting the coordinates for each one. I then whipped up a quick CAD drawing with each of the 4 points to see how far off the lug recess from the bolt circle was (if anything). It looked to be off of the bolt circle by only 0.002″, which I’d simply consider measurement error on my part. I then determined the X,Y coordinates for a lug recess centered between two of the mounting holes. Back at the mill, I shuttled the table to this location, locked the ways (on my old Tree, locking really doesn’t put a lot of clamping on the gibs, but it helps keep things steady), and proceeded to center drill the spot, then drill down about 0.4″ with a 1/2″ drill. The recess needed to be just a little over 3/4″, but I didn’t have a 3/4″ drill, so I used a 3/4″ endmill to bore the depth.
After bringing the recess to size with a boring head, I removed it from the table, cleaned it off and tried attaching it to the spindle. The screws went in rather tight, and it had difficultly squeezing flat against the spindle. I guessed that my hole for the indicating lug was off by just a bit, and I was squishing the lug.
After removing the plate, I had a look at the lug recess and saw the telltale signs of metal interference:
I clamped the plate back onto the mill table and bumped the 3/4″ endmill up against the marred edge of the recess. I then zeroed the DRO, retracted the quill, moved over about 0.005″, then milled down about 0.3″ to relieve the area that was binding. I put the plate back on the spindle, and the screws tightened up a bit easier this time, so I considered the rear of the plate to be complete.
The front of the plate has a raised boss that slips inside the rear edge of the chuck to keep it centered, and this boss must be cut to size once the plate is mounted to the spindle. This ensures that the boss is cut perfectly concentric with the lathe’s spindle (something impossible for the manufacturer of the plate to do, as every spindle will run just a hair different).
I had to take the diameter of the boss down about 0.060″ or so – I used a carbide bit and took pretty light passes so I could ’sneak up’ on the final dimension without cutting any further than necessary. Once I got close, I’d stop the lathe, clean off the chips (dust, really – the plate is cast iron, which creates more of a coarse powder, like fine sand, rather than chips like you’d get from aluminum or steel), and try fitting the chuck to the plate. After the final thousandth of an inch, the chuck slipped on with no side play, and I fastened it in place with the mounting screws.
Now I was curious to see just how accurate the chuck was – with the backplate cut so perfectly, I should ideally see zero runout on the chuck. I attached a dial test indicator to a magnetic base and had a look.
Before even checking the runout, I decided to see how rigid the chuck and spindle are on the lathe – on the 9×20, I could get the indicator to deflect a thou or two just by pushing firmly on the chuck perpendicular to the spindle axis. I pushed with about the same amount of force with the same chuck mounted on the Keiyo Seiki, and watched the indicator needle anxiously. Not. Even. A. Single. Twitch. This beast is solid. A side effect of such rigidity is that a runout measurement should be a lot more accurate, so let’s see what we have…
Appears to be just under 1.5 thou – not perfect, but good enough for the moment. On the 9×20 I had cut the boss on the backplate a bit undersize accidentally, but the extra slop actually allowed me to adjust away the runout by carefully snugging up the mounting screws, checking TIR, gently tapping the chuck in the appropriate direction with a mallet, checking again, tightening the screws further, and so on to make the chuck run true. Of course, the collets themselves have runout as well, but I don’t worry much about that if I can get the chuck adjusted well. But enough of that for now – time to cut some metal!