Here are some more pics. The ID of the main circle is 6" roughly. I'm not sure what it was originally, perhaps some hydraulic cylinder cut off, it was scrap from a large machining company. The section I got was about 8" long, so I had to cut it down, I scribed a line around it using my height gauge, and cut with a torch, allowing some room for machining down to the scribe line on a rotary table. Then milled out the slots for the fingers to slide in with a 3/4" endmill, simply using the degree marks on the RT to find the 3 points. I then mounted it in the 3 jaw on the lathe, and machined the base to a sliding fit on the ways, pushed the base up to the circle and scribed a line for the flat on the bottom of the circle. The fingers are pretty self explanatory, machined from solid CRS to match the slightly oversized slots in the circle. I then used a hand file to get a nice smooth sliding fit in the slots. As you can see in the pics, the bearing slots were cut, , then an endmill used to create a flat spot where the bearing retaining bolts go, then drill and tap for the 5mm bolts (only bc those were the smallest size bearings I could find on eBay). Then I made the finger adjustment screws, just 5/16 all thread. The ends I turned down to 3/16, just a nub for a pilot hole on the bearing-end of the finger, the other end I turned just the threads off to press into the knobs, and used a groove tool to cut slots for the E-clips that push the fingers up and down. I did make small washers that go between the finger shoulder and the E-clips. I made the knobs next, and not having a good knurling tool, I used a 3/16 endmill to make the finger grips you see, I kind of like that look better than knurling anyway. Knobs are a press fit with high strength loctite on the adjusting screws. Next was the finger locks, just three pieces of hot roll, I used the 3/4" endmill again and slotted all three at once. Then marked and drilled all the holes for the cap screws, but mounted the fingers to the circle, clamped the locks on them, and used a transfer punch to mark on the circle. At this point, I number stamped all the pieces so they all go back to the same spot. Drilled/tapped the circle, countersunk the locks for the cap screws and tapped for the lock bolts. Then it was time for the washer that goes on the 1/2" bolt that holds the base/circle/way lock together. I didn't have much meat left on the bottom of the circle, so I needed to match that arc with the bottom of the washer. My hack-n-slash brain of my former years came into play, so I set a piece of CRS under the circle, scribed a line using the arc, and attempted to use a grinder to match the arc. Yeah right. Even my belt sander wouldn't get it right. So my newly acquired machining brain kicked in, and came up with a better way. I used a piece of 1" CRS stock, lined up the washer on the end of it and welded them together. I then centered it on my RT, same distance from the spindle center as the radius of the circle, and side milled the arc. I quickly found out where I was off with the grinder lol! Anyway, perfect fit with the arc in the circle. The way lock is simply a 3/8" plate with a slot milled in the center, and the 1/2" bolt I milled the hex into a T to fit in that slot. Lastly, the circle and base have alignment dowels between them.
Changes I'd make: the slots that the fingers slide in are too thin, as in the bottom of the slot, which the finger relies on to keep perpendicular to the work, needs to be thicker. This would mean a thicker wall pipe for the circle, or to cut one from thick plate steel. This would also give the advantage of doing away with the arc washer and allow more material to machine a flat into the bottom of the arc on the ID. Also, I would align the circle to the base, and bolt the two together, would make handling a lot easier. I may do that still anyway.
This was a very fun project for a beginner hobby machinist, one I think anyone could do. The only advantage I have is my 12" rotary table that a lot of hobby guys might not have. But there are ways around that. I do recommend the method I used to mark the base to the circle, mounting the circle in the chuck, and sliding the base to it and scribing, it assures perfect height.
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