Rotary table backing plate

A diversion from the machinist chest – I reached a milestone, got to thinking way ahead, my muse deserted me so I gave my coding muse a boot to the hiney and worked on my Rosetta Code page. Wanted to get back out into the shop but not really ready to get back to the chest; flipping through the Enco sales flier they had a sale on rotary tables. I’ve been wanting (vs needing) a 4th axis for my mill for a while, I have a 5C spin dexer but it only holds parts horizontally. Something like a super spacer, dividing head or rotary table would be nice. The first two are more spendy than I’m willing to justify but a small rotary table and chuck aren’t too bad (as tools go). Stuck them in the cart and wanted for yet another sale. Little while later a 20% + free shipping showed up, that stacked on top of the sales flyer got me some new toys for under $250 (well would have but how could I resist “saving” even more $?). I missed the section in the catalog for the chucks and backing plates for this table, which would have saved some work but cost another $50. And it is a part I can make. So I did. And it took a huge amount of thinking, considering it is such a simple part – a aluminum plate with a center hub, a raised boss/flange and six holes. But my tolerances were well under a thou in all directions so working holding was the major issue.

After several false starts, what I did was mount the plate in the four jaw chuck, facing and boring a hole. Popped it out and stuck a stainless steel tube in and turned it about a thou and a half over size. The plate went into the oven at 400°F to grow the hole and then I attempted to to slide it on the tube (which is still mounted in the lathe). Which it didn’t so out came the hammer. Which moved the tube a few thou so I re-centered it (now running true to with in a few tenths). Refaced the plate and turned down the protruding tube to match the hole in the table (this will all make sense (I hope) when looking at the video). OK, how to machine the back side of the plate? I’m NOT going to remove it from the chuck because I really really don’t want to realign to two surfaces, one I would not be able to reach (it would be against the chuck and I don’t trust the chuck to be flat). I didn’t think far enough ahead to leave space for the tool post between the chuck and part (so I could use a tool on the right side of the tool post). So I put the tool post in the line of fire (of the plate), hung a tool way out and tuned the chuck mounting boss/flange. A fine time was NOT had by all but I got it done. And I hit my tolerances, which is always nerve racking when I can’t so several test fits along the way. I also turned a groove for the bolt circle before I cut the plate off the tube.

[Aside – I figured baking the 6061 T6 aluminum plate would bugger the temper but if it did, it doesn’t seem to matter for what I’m doing.]

Then over to the mill. I faced the cut tube flush with the plate and drilled the chuck mounting holes; ironically without using the rotary table as it was easier than dialing in everything on the table. THEN, finally, I could mount the plate to the table and use the table to drill the hold down holes. I drilled the holes centered on one of the grooves in the table, that way, when I index a T nut on that groove, the bolt will engage, no fiddling (nice since I can’t see it under the plate).

For a first project, I grabbed some scrap and cut six [wrench] flats to make a “nut” and then beveled the top of the nut and chamfered the edge. I did this with a counter sink. It works.

At the end of the video I show a part I bozo’d; the plate has a Morse #2 taper and I wanted a center with a thread on it (for some unknown fixturing). Well, I cut the angle instead of the half angle so that part is now a teaching aid.

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