You can make a lathe chuck mount yourself using a few tools and a little ingenuity. This job will require the use of several machine shop tools, so check the materials needed list to make sure that you have everything you will need before beginning this project.
Step 1- Measure Your Spindle Nose
Using your thread micrometer, or possibly a 3-wire setup, measure the diameter, length, and length of thread on the spindle nose. Also measure the diameter of the spindle collar.
Step 2-Work With Your Backing Plate Blank
Now, you can either weld up a plate and bar stock to make the backing plate blank or purchase a cast iron blank from Enco or MSC. Or if you are really ambitious, you can start with a billet of stock of the appropriate size and cut it down. The easiest method would be to purchase the blank, the cast iron also helps with speed. Use the Machinery's Handbook to determine the spindle-thread root diameter and grab the chuck-side of the blank in a 3 or 4 jaw chuck and bore a hole front-to-back through the center of the blank. Now without removing the backing plate, cut the internal thread to specification by finishing the thread you began with the lathe with your tap. Bore the threads out to form the internal surface that will face the smooth part of the spindle. This should be close, but not too tight.
Step 3- Facing the Backing Plate
Take a facing cut on the face of the backing plate that will bear against the spindle collar. Then take another facing cut on the large diameter plate that will ultimately bolt to the chuck's back. This is the side that people will see. Remove the backing plate from the holding device and remove the holding device from the spindle. Now thread the backing plate onto the spindle. It should go on smooth and then bottom out at the spindle collar. Take a truing cut across the face of the chuck plate, this is the face you will mount the chuck on.
Step 4- Finishing Up the Mount
Cut the required step into the face of the chuck plate to fit into the recess on your chuck. Make sure that it is a good fit so you will not have any excessive runout. Now, cut the diameter of the backing plate down to meet the diameter of the chuck. Then, place the chuck on the backing plate and use transfer punches to mark the hole locations for the hold down bolts. If you prefer you can also use a rotary table, or bolt-pattern coordinate set calculated using the aforementioned Machinery's Handbook. Then drill and tap the holes to match the supplied bolts. Using a torque wrench, uniformly torque the bolts holding the chuck to the backing plate.
Now place your newly mounted chuck on the lathe spindle, chuck a piece of quality round bar stock and measure the runout a few inches out from the front of the chuck jaws. It should run within 0.002 TIR at worst, and probably 0.001 at best.