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A Three-jaw Chuck for the Mill
By Douglas Ripka
A long while ago, I bought a three-jaw chuck with no back plate and recently added a  rectangular plate to the base to make it easier to hold in my milling vise.
I found a use for it when I broke the plastic air filter cover on a small air compressor by letting it fall on its side. Rather than buy new parts that would likely break again, I decided to make a replacement holder out of metal.
 I needed to drill three holes in each plate. Rather than set up an indexer or rotary table, I used the jaws of the chuck to reference the holes. First, I marked one disk with blue layout dye then scribed a line 1/8″ in from the edge using hermaphrodite calipers. Next, I put the disks in the three-jaw chuck with the scribe lines next to each of the jaws.
 A center punch was lined up with the center of each jaw and a punch mark made. From there, both disks were drilled and the rest of the machining continued.
The thread on the compressor is M20 x 1.5 and I used Kim Steiner’s article from the May/June 2007 issue of The Home Shop Machinist to set up my lathe to cut a “close enough” metric thread.
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