Metal Lathe -- Concave Radius Turning Attachment

 Small radius curves, like the Wheel thimbles and small pulleys are turned using a form tool; a chisel ground to the desired profile. But with metals and small lathes this becomes impractical from a tool pressure perspective at about 0.10" to 0.25", depending on the metal, and about 0.50" for plastics. You can turn larger radius curves in a few other ways:

• Freehand, with a special chisel, like on a wood lathe. The chisels are small and it is very slow on metals. It is also difficult to make repeatable parts.
• Remove most of the material with standard tooling, then smooth it up with a large rat tail file. Tricky.

Or you can use a radius attachment. However, most are made for convex curves (which are much easier to free hand and/or make with conventional tooling), and those that will turn convex won't do deep curves, such as a low friction ring or wheel thimble. So I made my own. 

It covers 11/16-inch to 2-inch and up to 1-inch past the pivot deep. Enough for my needs. 

Somehow it reminds me of one of the robots on the old MST 3000.

It looks dead simple, but it must be very rigid, very compact, and quite precise, so it took a good many steps:

• Careful band saw work
• Milling a slot
• Facing and turning a rod down 
• Concentric boring
  
• Threading several times
• A square hole
• Indexed milling of the hex at the top 
• Fitting the bearing surfaces so that it turns smoothly but with no flex
• Parkerizing of some parts for rust prevention
• Grinding the custom chisel
  

It replaces the tool post and is rotated in use by a 6-point socket with a T-handle from above (see below). The chisel is symmetrical to cut both ways. It uses tiny 1/4-inch blank pieces.

 

I made the 1/2-inch drive Tee handle from #4 rebar with some welding and careful grinding. Why waste good rod? It fits perfectly and is used with other setups on the same lathe.