Making a lathe chuck back plate can be a difficult task for someone that is new to machining. If you are skilled at machining this project should take you about 6 to 8 hours. This task is very detailed and will require access to many machine shop tools.
Step 1- Create Your Workpiece
The first step is to make a 4 1/4 inch square from a 1/2 inch or 5/8 inch Aluminum Plate using your bandsaw. After cutting out the 4 1/4 inch square, mark the center and mark out the corners at 4 1/4 inch. Then saw off the corners to make an octagon.
Step 2- Lay out the Bolt Hole Circles
Now that you have your workpiece, locate the center point and make an indentation using a #0 center drill. Using the dividers accurately mark out a circle of 1.299 inch radius with a felt tip pen. Next, scribe a circle of 1.654 inches in radius for the chuck bolt circle. This corresponds to a diameter of 3.307 inches as specified for many chucks, double check with the manual for your chuck.
Step 3- Calculating the Bolt Hole Positions
Now we need to mark 3 evenly spaced points on the bolt hole circle. You can use a dividing head or rotary table if you have one handy, but if not, you can measure and mark the points using a dial caliper and divider. There's a simple formula for determining the distance between holes of a bolt circle:
B = D x sin( 360 / (2 x N))
B = distance between bolt holes
D = diameter of bolt circle
N = number of holes in circle
Inner Spindle Bolt Circle
D = 2 x 1.299 = 2.598, N = 3, B = 2.598 x sin (360 / 6)
B = 2.598 x sin 60, = 2.598 x 0.866, = 2.250
D = 3.307, N = 3, B = 3.307 x sin (360 / 6 )
B = 3.307 x sin 60, = 3.307 x 0.866, = 2.864
Step 4- Marking and Drilling the Bolt Holes
Pick any point on the circle and make a small punch mark right on the center of the line. Then, set your dial caliper to 2.250 inches and scribe an arc from that point to an intersecting point on the circle. Then scribe an arc point on the other side of the circle. Repeat this procedure for the outer bolt holes, do not make them too close to the inner bolt holes. Now that both inner and outer bolt holes are marked, use a #0 center drill to make starting holes at each arc and the center. Then, enlarge the holes using a #2 center drill.
Step 5- Tapping the Bolt Holes
Drill the 3 separate 6mm holes on the inside circle with the #9 tap drill. Make sure that the tap is square with the surface. After this point make sure that the holes line up against the spindle plate correctly. If they do not, you should mark a new set of holes and try again.
Step 6- Mounting the Back Plate
Using a parting tool, make 3 studs 7/8 inches long by cutting the heads off 6mm screws, then thread a stud into each one of the tapped holes in the back plate. Then, test mount it on the spindle using nuts. Screw the studs into the tapped holes and then mount it on the spindle plate for facing. Using a cutting fluid and a 3/8 inch drill bit, drill the center hole all the way through. Then use your boring tool to widen the center hole to .875 inches.
Step 7- Make a Spindle Template
Now that you have reached this point, it is necessary to make a spindle template. To start all you need is a 1/4 inch circular piece of aluminum with a 3 inch diameter. Then mark the center and layout a bolt circle 2.199 inch in diameter, just like the back plate. Scribe another circle 2.165" in diameter to mark the edge of the land area. Use #0, #1, and #2 center drills in sequence to center drill the center hole and the three bolt holes. Then drill and tap the holes 6mm. Mount the template on the spindle plate using 6mm studs, then carefully center it. Be sure to only screw the studs in halfway at this point. then face the outside edge of the plate, until the land is about 1/8 inch higher than the surrounding plate. Now, with the compound set at 0 degrees and the carriage locked in place, start removing a little metal at a time from the diameter of the land. Use the recess on the back of the stock 3" chuck to test for a snug fit, then use a file to remove any burrs. Mount a right-hand cutting tool in the tool holder and turn down the outside edge of the template using a rounded facing tool at about 1500 RPM.
Step 8- Boring the Spindle Recess
Using your spindle template, take a light boring cut from the center hole out until the recess is about .130 inches deep. Make the last cuts very light at about 1500 RPM to get a nice finished surface. For these final cuts advance the cutting tool towards the headstock, removing just a few thousandths with each cut. Since we are enlarging an inner diameter, the cross feed must be cranked outwards away from the lathe center on each successive cut. Stop frequently and try to press fit the spindle template land into the recess. You are done with this when it fits snugly. Then, turn your cutting tool to about a 45 degree angle and clean up the sharp outer edge of the center hole.
Step 9- Finishing the Back Plate
Using your cutting tool, face off enough of the surface of the back plate to make a nice, smooth finish. Then, use the boring tool to lightly chamfer the sharp edge of the recess. Now, remove the back plate from the spindle, remove the studs then flip the plate over and reinstall the studs on the opposite side. Mount the plate to the spindle and face this side of the back plate. Then turn the rough edges of the back plate into a smooth circle. Now, remove the plate from the lathe and place it on the drill press table with the recessed side facing up. Drill out the holes for the chuck bolts, that were previously center drilled, using a series of drills up to 1/2 inch. Then using a 1/2 inch 2-flute end mill, mill out the cone-shaped depression left by the 1/2 inch drill until you have a flat surface for the bolt head to bear on. To turn down the outside edge, take a very light initial facing cuts and continue slowly until you reach 2.834, and when the chuck just press fits over the raised land you are done. Use a file to smooth the rough edge.
Now insert the mounting bolts into the counterbored holes and tighten them down. Make sure that the heads of the bolts are entirely below the back surface of the adapter plate so that they will not interfere with the mounting of the plate on the spindle. Screw the studs into the threaded holes and mount the chuck on the lathe. Your hard work is done and you are free to use your new lathe chuck back plate.