Peter
Before committing yourself to slots and a key take a careful look at the work envelope of your mill and decide where the indexer is best positioned for maximum versatility. I bought mine to go with a Lux style square column mill and originally planned to fit slots and a key. I'm glad I didn't as such fixed positioning would have been severely limiting. Especially when working on the side or ends of parts. Hafta say I originally thought that just about everything would be worked on from the top so a central position would be ideal but in practice there were plenty of exceptions. Slitting saw work being an obvious example. As it turned out I pretty much always used it with the axis halfway between slots.
Since trading up to a Bridgeport I still find the halfway between slots position preferable despite greatly larger table and work envelope. My positioning aid was to simply mill off one side as close to dead parallel to the spindle axis as could be managed so that a parallel, or two if some offset were needed, could be used to align it with a table slot. Works well enough but a bolt on baseplate with suitably positioned bolt holes to fit either halfway between or line with a table slot has been on the round-tu-it list since the first time I used it on the Bridgeport. 12 + years and counting so not something greatly missed!
I'm ambivalent about T slot keys. If made tight enough to be really accurate it can be a pain to get things mounted and properly snugged down. Especially on an older machines with imperfect slots. If slack enough to be easy to use accuracy is probably little better than simply pushing the vice, indexer, rotatab or whatever hard up against the fixing bolts or studs whilst tightening down. Thats all I do with the vices I use on the Bridgeport. Combined parallelism and repeatability is in the couple of thou per foot region. Swivel bases make it easy to get them dead right when needed. But I have the space for the bases, folk with smaller machines may not. Another one on the round-tu-it list is to investigate re-engineering the T-nut and stud system to have interlocking, self aligning, upper and lower components. If the top part were made a snug fit int the vice slots it should be possible to get accuracy comparable to a well fitting keyed alignment system in a more easily assembled set-up. Hafta re-work the vice (or whatever) slots to be accurately parallel of course.
Clive.
Edited By Clive Foster on 03/10/2017 18:01:27