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Mini-Lathe vs. Case-Hardened Shaft
While doodling a drag knife holder for the Sherline, I figured a 3/8 inch shaft would hold all the parts and fit neatly into a standard Sherline tool holder, which it did:

Sherline Diamond Drag Holder – installed Having recently upcycled a 3/8 inch shaft from the Thing-O-Matic into a pen holder for the CNC 3018-XL, I cut off another section with an abrasive wheel, then tried to face it off:

Hardened shaft facing – abrasive step Although the mini-lathe’s carbide insert gnawed at the shaft’s case-hardened shell, it obviously wasn’t making much progress against that step.
Back to the abrasive cutoff saw:

Hardened shaft facing – abrasive flattening Which looked better, although it still wasn’t quite perpendicular to the shaft axis.
Back to the lathe:

Hardened shaft facing – lumpy face Well, it’s better, but it sure ain’t pretty.
Put gently, the mini-lathe’s lack of rigidity doesn’t help in the least. The compound was a-reelin’ and a-rockin’ on every revolution and eventually turned a slight tilt into a distinct radial step.
Memo to Self: Dammit, use a brass rod!