Machine Tool Spotlight - Modifying the Hardinge Lathe

Since I finished recommissioning my old Hardinge production lathe, it has given me many hours of reliable service. It was extremely well-designed and robustly built, and operates much more reliably than its 70+ years of age, and previous hard life on a factory production line, would lead you to believe. However, I have occasionally run into a problem when performing a specific machine operation, and I wanted to solve it before damaging either the machine or the parts being machined.

The tailstock of a lathe has a tapered bore, which matches the taper of the tools that are placed into it. This tapered mating surface has a large area of contact, and the the surface friction between the tool and shaft keeps them locked together. The friction is increased when the tool is driven into the machined material, because the force presses the tapered contact surface tighter.

Normally, this system works very well. However, since there is no positive means of locking the two, when a large drill bit is used, or a soft material is drilled into, the drill bit can "catch" on the material and pull forward out of the tailstock.

I decided to add an internal pin to the tailstock to prevent my drill bits from spinning. In addition to the taper, most tools also have a notched end which can rest against a pin and provide a positive lock. 

Tapered drill bit with a notched end

Unmodified

Marking the length of the tooling inside the tailstock tube.

The black line shows the length of standard tapered tools without a keyed end. The scribed line shows the eventual location of the lock screw.

Starting the machining process with a center drill

Tapping threads in the hole

Turning down the diameter of the screw head

Resized screw head

Checking the clearance

Installed with Loctite