I bought an independently adjustable 4-jaw chuck for my lathe. The chuck came with an adapter plate, but my lathe (Emco Compact 5 CNC) is a bit special and didn't fit the adapter plate. The bolt holes for my lathe were half in the center hole on the adapter plate. The solution? Another adapter plate, of course!
I ground some drill rod to make a cutting edge, then heated it with a blowtorch and quenched it in water. Considering I'd never done any of those things before, it went very well. The tool is shown in the above picture ready to take its first cut. I should have ground the heel a bit more, but it did cut and I was able to cut the outer diameter properly.
The stock adapter plate also needed some modification. Remember how I said the bolt holes for the Emco headstock were half in the center hole of the stock adapter plate? Well, I need access to those bolts to tighten the whole thing to the lathe. The stock plate is cast iron, which I'd never machined before. I read up on it a bit, got out my boring bar (first time using this tool too!), and slowly cut openings in this adapter to allow access to the bolts on my adapter. I didn't use a boring head, as the diameter from the boring bar itself was exactly what I wanted.
Then I needed to drill and tap the other 4 holes for bolts that would hold the stock adapter to my adapter. I figured the best way to get the drilling lined up with my adapter would be to use a transfer punch... but I don't have any... so I built one!
Success! These two adapter plates stay bolted together and essentially make one adapter. When I want to change to the new chuck, I just bolt these adapters to the lathe and then bolt the chuck on.