Rebarreling and Home-Shop Machining

Cross-Arm Schematic

            Photo 1 shows a schematic of the cross arm. The schematic from the right shows a 3/4 x 10 thread per inch speed ball handle. A 6" by 3/4 inch homemade grade 8 stud. The left side will be threaded with the lathe. A 3/8 inch thick by 1.5 inch outside diameter by 3/4 inside diameter spherical washer. A 11" x 7" by 3/4 inch slotted steel plate. The plate thickness adds rigidity and also adds mass in a good place. A 3 x 3 x 3/4 inch thick bulkhead to cap the cross arm. The face of this bulkhead is machined true in the topic grinding and facing the cross arm true in this Journal. A grade 8, 3/4 by 13 TPI nut welded in place with a cap welded on the left side. Two 3/4 inch bulkheads or shear plates welded inside the 3 x 3 x 13 inch by 1/4 inch wall tube to make it stiffer in torsion and finally the ½ inch plate that the cross-arm is welded to that is used to mount the CNC drive motor that drives the vertical spindle travel. The cross arm is welded to the plate on the inside as well as the outside to improve rigidity.

            The slotted plate will be trued on the mill on the left side and mounted with ½ inch socked head capscrews. The screws secure the plate to 1 inch square solid stock that is drilled and counterbored and welded on either side of the opening in the fifth column. I decided to mount the plate from the right to simplify maintenance and shimming the plate true.

Finished Cross Arm

Photo 2 shows the threaded Grade 8 Stud for the Milling Head Cross Arm and the finished and painted cross arm. The thread is a 3/4 inch by 10 thread per inch. There is no gearset for 10 TPI listed on the Shoptask lathe threading decal, but 2.5 mm is very close (a 10 TPI = 2.54 mm thread). The threading went well. I fed the compound rest in .004 inch at a time till the threads pointed up. Then .001 inch at a time until the nut threaded on snuggly. The 10 thread per inch threading gauge fit the resulting thread well. The welded up parting tool that is pictured below in the Lathing Plastic topic worked much better than before on this hardened and tempered steel. I parted 1/8 inch deep and finished off with a hacksaw. Photo 2 has the finished and painted cross-arm on the left, the 3/4 inch stud, a 3/4 inch spherical washer and the 3/4 inch speed ball handle.

Milling Arm and Table Stiffness before the Fifth Column

I did some preliminary test without the fifth column on the spindle flex on my machine by applying a 100 foot-pound torque on the spindle assembly which forced the spindle downward. This is shown in photo 3. I got .018 inch of movement. That’s a lot of movement. I did a similar test on the milling table by twisting it clockwise with 100 foot-lbs of torque. Photo 4. The edge of the milling table moved up only .0015 inch. This indicates that milling table flex will not be the limitation on milling and that substantial rigidity improvement can be made on the milling spindle assembly by using a stiff fifth column.

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