By Donald Rothfuss
I have an older Southbend 9" Model A lathe and the 3-jaw chuck that came with it has seen better days. I purchased a Bison 4" 3-jaw chuck and also purchased one of their threaded backplates to match. That is a 1.50 diameter 8 tpi configuration for those not familiar with the Southbend. The normal installation of a backplate calls for mounting the backplate on the spindle and facing the side that goes into the back of the chuck. Then the backplate has a step cut into the face that very closely matches the indentation in the back of the chuck. This is sometimes referred to a "wringing fit".
A sloppy job here will result in excessive radial runout, your chuck will not cut true. Even if you do a first rate job in matching this step there is a very good chance that the 3-jaw chuck setup will have some runout and it will probably vary at different cutting diameters. Since I wanted a chuck capable of cutting in the 'tenths' class, I decided to put on an adjustable backplate.
I purchased a kit from Hemingway that included drawings, one page of directions on how to proceed, a 4" round slice of steel for the Adapter Plate and some 1/4 BSF socket head cap screws. I did not use the BSF bolts as I am not equipped to tap for them. Instead I substituted 1/4-28 tpi instead. I departed with Hemingway's directions in several other ways but feel that it still resulted in a successful project.
There are four main components, the chuck which requires no modifications, a steel adapter plate that is attached to the chuck with 3 cap head screws in the manner provided by the chuck manufacturer, a backplate to fit your lathe spindlle and which mates loosely with the adapter plate, and finally, four radial fine thread screws which provide the trueing adjustment. Hemingway calls for using three 1/4-40 tpi shop made adjustment screws but lacking a tap of that size I elected to use four 1/4-28 tpi fasteners here as well.
Start by mounting the Adapter Plate in the 4-jaw chuck and then facing the Adapter Plate. Reverse the Adapter Plate in the chuck and bring the plate to final desired thickness. Now drill and bore the adapter plate center hole to provide clearance for the through hole in the lathe spindle and the through hole in the 3-jaw chuck. You can now machine the recess for the threaded Backplate to nest into the Adapter Plate. This should be carefull work as these two parts will clamp together axially and poor workmanship will result in an inaccurate running chuck.
Reversing the Adapter Plate once again, the shoulder can be cut to match the concentric indentation on the back of the Chuck. This must be a careful fit and would be done the same way with a conventional Backplate installation. My plate required a little tap with a non-marring hammer to seat and is a snug fir. At this point I proceeded to drill the holes. I had access to a friend's numerical controlled mill which greatly speeded up this part of the process. The 3-bolt circle diameter on the back of the Chuck was determined by carefully measuring the chord length between two of the holes. This process was aided by use of a couple of points which fit on the jaws of a pair of calipers. These points are cylindrical with 60 degree cones on one end. The sides are slotted to allow slipping over the caliper jaws and are held in place with small set crews. The points were then centered in two holes and adjusted until all radial play was eliminated. A direct distance between the two holes was obtained, center to center. A bit of geometry using the good old 30-60-90 triangle resulted in an easy calculation of the bolt pattern diameter and this was programmed into G-code. It could have easily been laid out and carefully center punched had the mill not been available. Clearance holes for the bolts supplied with the Chuck were now drilled. The Adjustablle Plate also has another 3-hole bolt pattern of a smaller diameter to attach the Backplate to the Adjustable Plate. These holes were also drilled to tap drill diameter for 1/4-28 screws. The Adjustable Plate also requires drilling and tapping for the four 1/4-28 screws that make this an adjustable backplate. These holes are drilled in the periphery of the Adjustable Plate and are positioned to maximize the clearance with the 3-bolt pattern that matches the back of the chuck. These 3 holes are spaced at 120 degrees and the radial holes are spaced at 90 degrees. Offsetting 15 degrees from one of the Chuck attach holes yielded the best clearance and these locations were laid out and drilled and tapped on the mill.
The Backplate was then installed on the lathe spindle and faced to depth and turned to diameter that allowed 20-30 thousandths clearance with the pocket already provided on the Adjustable Plate. The Backplate was then reversed and the hub diameter was thinned down to allow room to get at the 3 screws that attach the Backplate to the Adjustable Plate. The three holes were then drilled in the Backplate to clearance diameter for the 1/4-28 fasteners used. These holes were then counter bored to allow room for the fastener heads. A spindle diamter of 1.5 inches doesn't allow for a very thick walled hub and the appropriate dimensions to use must be carefully thought out in advance.
At this point the Backplate was assembled with the Adjustable Plate and the four dog point set screws were installed. Using the ID of the Adjustable Plate as a datum, the assembly was trued using a dial gage and then the 3 screws were tightened in place. It may be difficult to access these screws to tighten them and making a custom wrench may be in order. Shortening the short leg of a long allen wrench is acceptable as well as using a short length of allen key silver soldered into a flat bar, such that all clearances are met to reach and torque these three screws. Now the Adjustable plate can have the OD cleaned up removing any scale, turning to Chuck diameter or slightly less depending on the stock size used.
The next operation involves mating the Chuck itself to the Adjsutable Backplate assembly. Install and tighten the three screws and mark the Adjustable Plate and Chuck with a scribe mark to ensure that they always go back in the same orientation. A similar mark should be made between the Adjustable Plate and the Backplate. Chuck a piece of bar stock and adjust the backplate to minimize the runout. Hopefully, you too will have a 3-jaw chuck that is in the 'tenths class".
If you want more information, contact:Donald Rothfuss