Making 4th Axis Tombstone- How to get it straight

Hi,

Hope someone can offer some advice. I am needing to make a 4th axis tombstone for a new product we are going to release. Need some advice on how to make it square and not bind. And perhaps I'm overthinking it.

The tombstone is 3.25x3.25" and 20" long. It will attach to the HRT-160 using the 5th Axis Rock-Lock system (we change products on our rotary often and want to stick with this system.) Opposite side supported by an A-frame.

I am worried about getting the end that connects to the Rotary Table square enough so the hole on the opposite side at the A-frame doesn't have runout relative to the centerline of the rotary. In other words, my worry is if you were just to attach the tombstone to the 4th axis that there would be runout on the opposite side, and the A-frame would try to "pull" it straight, potentially causing binding. Or maybe at that length there will be enough flex in the system so not as to worry about it?

My plan is to mill the ends first (the Rock Lock features and the hole for the A-frame) and then face each side of the tombstone once it's set up so it's square with the table. Unless others have a better idea.

What would be a good strategy for getting the right end square, with simple features for the Rock-Lock, and then make the 1.5" hole on the A frame and somehow locate it to be centered relative the the opposite side?

Thanks!

My machine is a Haas VF4-SS.

Probably the best approach to this would be turning from round stock. This will be the most fool-proof way to make sure the outside support features (round I assume) are centered. Then doing the mill-work just as you suggest. The hard part is milling the rock-lock features in a 20" tall part. But, if it was turned, you at least know the ends are square.

I milled a disc with a rectangle pocket in it, OD and pocket in the same setup so it was concentric so to speak ( mine were 2x4 tombstones ). I then put the adapter on the lathe chuck end and a live center in the other, machined one end then flipped them and did the other end. Mine was a 5c indexer so I machined the 5c profile and threads on one end and a 1.5 dia spud on the other my disc had a through hole to clear the 5c profile. I think they are 28" overall. the disc was snug enough I had to tap it on and off with a rubber mallet.

I do as you are thinking and haven't had it be an issue in 20 years. I hold my fixtures between the 4th and a tailstock so I can swap fixtures while the spindle is stopped and parts while it is spinning. I can swap fixtures on the 4th faster than in a vise.