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Fun situation... I have a 5" 3 jaw chuck and zero way to grab my stock. I bought a 7"x7"X1" slice of hot rolled plate steel. I did some reading just now and they say cast is better.... but, this is what I have. I asked the guy at the metal supply his opinion and this is what he came up with. And they are a full out machining shop.
So here's my plan, please critique it.
I have a 1" thick piece. My lathe spindle is 1 1/2" 8tpi. The shoulder to the tip is just less than 1". So I think I'm fine on thickness. Would be happier thicker, but again, I'm using what was avialable and affordable.
I am going to centre drill on the drill press to the largest hole I am capable of, (which I believe is 3/4"). Make a jig to mount this between centers on the lathe and carefully hog out about 1/2" and form a rough collar where the threads will be. Just taking enough material off to allow me to then grab that collar with the chuck jaws. Then bore out the spindle hole and thread it. Mount it on the spindle and from there basically machine the final profile of the back plate to my satisfaction... i feel doing the final machining with the plate mounted on the spindle gives me the best hope of being true with a minimal runout.
in my relatively inexperienced mind, (aside from the safety issues of slinging this chunk of steel across the garage...) i think this works...
I'll deal with the fact that the mounting bolt holes are not thru holes so I can't use a drill hole punch, later. I have to measure and scribe and hope for the best or make the holes big enough to... essentially... throw the hot dog down the hallway... and do the final truing with the chuck mounted with a dial gauge and tighten once satisfied.