Increasing a Harbor Freight 7x10 mini lathe's standard through-chuck capacity to 0.795" (20mm).
Goal: Increase the through-chuck capacity of the 3" Harbor Freight (HF) 7x10 mini-lathe chuck. I carefully measured and determined that 0.807" (20.5mm ) is the diameter of the spindle bore. Through-chuck capacity is 0.625" and shouldn't be bored beyond 0.750". That's still not enough for me, so I super-sized it to 0.795".
Background: Scottie provided Captain Kirk with plenty of miracles. He managed to squeeze more out of that bucket of Spacely Sprockets and Cogswell Cogs than anyone could imagine. So what to do about the lousy standard equipment 3" chuck on a Harbor Freight 7x10 mini lathe? Besides tossing it, along with the tailstock.
Hindsight is always 20-20 and the 7x10 mini lathe is no exception. If I had to do over, I would of bought at least a 10x22. I lost patience with the capacity and quality after the first week. But money only grows on trees at the Federal Reserve and they're not neighborly enough for me to jump fence and run crazy through their orchard. Thus, we persevere with our cheap junk and await a lottery windfall.
Since chuck capacity is nowhere near rated capacity of the headstock, the decision is to either buy a nice 5" Bison or risk trashing the stock one. That's a no brainer since the chuck is nearly useless and a good replacement is about $150. I might as well buy a new lathe (which I should do) or risk trashing the chuck. Modify it is!
Construction: The photos speak for themselves. Ideally 0.750" is the most you should bore to avoid the screws. They're not precisely drilled or counter-sunk, so they do meander a little. You may still scrape one. There's no precision to the product, so why should I care if I cut into the screw heads? They'll still hold fine.
My final finishing cuts took the bore up to 0.795" (20.25mm). My target was 0.800" but I was concerned the screws weren't perpendicular and I could potentially start cutting threads nearer the jaw end. It's still close enough to my final target diameter. CAUTION: Note exposed screw threads in photos from over-boring.
I ground a cutting bit more suited to iron and took no more than 10 mil cuts since my boring bar isn't very beefy (cheesy Indian import). I took it nice and slow, with light lubrication and frequent vacuuming to limit dust and grit (no chips with Iron).
Conclusion: Working with Gray Iron is nasty and gritty, but not difficult from a cutting perspective. My results were quite favorable. It's important to remember that if the maximum capacity is increased for 0.75" stock, it requires re-grinding of the jaws (bite radius) to properly hold the material. However, by super-sizing, 20mm or 25/32" rod can now be used. I'm giving it all she's got captain!
A Few Notes: You'll need to THOROUGHLY clean the chuck once you've completed the machining. It took total disassembly and plenty of kerosene to clean it properly. Iron dust is darn nasty and quite obtrusive. I ran my shop-vac quite a lot in between boring operations, which still didn't keep the scroll gears clean. -12/5/2007-