Ball Screw Yoke
One major part of the CNC conversion was to change out the Acme lead screws for ball screws. I found a set of three ball screws ( X, Y and Knee ) on eBay. These were sold as a Bridgeport Clone conversion kit. Lemmee tell you, not all Bridgeport Clones are the same! The bore to bore distance on the Enco XY Yoke was an inch greater than the kit.
Re-machining the original yoke crossed my mind for just under 10 minutes. There was not going to be enough meat left once the bores were opened up to accommodate the ball screw nuts.
Original Enco Acme Screw yoke on the left, Kit yoke on the right. There just wasn't enough metal there to keep the part rigid and prevent breakage once enough metal was cut away to fit the ball screw nuts. Adding to that the bolt pattern on the flange is different and adapters for the ball nut flanges would need to be made. Too many points of flex and play. Plan at that point ( 20 minutes after the inspection started ) was to make a complete new part. This wound up being the most challenging part of the entire conversion.
3D printer to the rescue! I started mocking up a new yoke in Fusion 360. Red part is the initial design for the bottom side of the yoke assembly and was patterned after the original yoke. I planned to beef it up and add webbing later. The yellow print is the pretty much finalized top side of the yoke with the X axis ball screw assembly installed.
3D printed yoke. The part was printed in two pieces and then glued together. It was about an inch and a half too tall to do in one piece on my Wanhao printer. I had widened up the lower part to add more rigidity and got it a little too wide. It fit fine in the saddle opening but the opening in the knee a bit further down was an issue. The ink mark shows where it hit on the opening lower in the saddle assembly. Narrowing the width of the taper at the top by 1/2" solved the problem.
Fusion drawing of the initial design. Fusion was used to generate the .STL files to print on the 3D printer. Part drawing as generated by Fusion 360. There were a couple of minor changes made when the part was machined. All minor in nature. The big block of Aluminum was put in the capable hands of a friend in a nearby town who graciously offered to machine the yoke with his machining center. By the time I got it on a fixture plate, I had run out of room in the X Axis and my current CNC machine is not rigid enough to handle the hogging operations. The block started out 4.5 x 4.5 x 8 inches.
Here are some play by play images courtesy of Chris Lamb...
6/30/2020
When doing the trial fit of the yoke it was discovered that the tip of the yoke would collide with the knee lift bearing housing and flipping the yoke around 180° only created more issues. When the yoke needed to be drilled to retain the ball screw nut, there was precious little metal around the bolt holes. So.... Back to the drawing board computer, so to speak.
Here's Yoke II. Dimensionally it is the same, but the ball screw nut mounts on the face. Due to the dimensions really not changing, I am not going to redraw the plan sheet. When installed the flat on the yoke (and ball screw nut) faces to the left to clear the knee lift drive. There is now plenty of material for the ball screw nut mounting bolts.
The lube line to the X Axis ball screw was formed from 4mm copper line. I laser cut a cardboard template of the underside of the table to check clearances while building. I filled the line with Bismuth to allow bending the tight radius and then boiled the line in water to get the bismuth out.
The Y Axis ball screw line was 4mm nylon with a spiral metal covering to protect it from vibration abrasion. Two of the Allen bolts used to hold down the yoke had to be changed to button head Allen bolts to provide adequate clearance.