The shrink hole concept makes sense. A good example of why zonal solidification is of concern, and why risers are often a necessity. I have yet to experience any of this, so I thank you for your thoughts. Gives me something to ponder.
Did your sprue attach to the base? If so, I think this all makes sense, as the upper portion, with it's larger cross section, would be slower to cool than the vertical portion. There would be a natural shrink of the barrel as a result. A riser there would solve that riddle, or so it seems. Your comment would be appreciated, as you are the one with experience.
If you have a mill, you might enjoy success boring the indexer that way. You can make a more rigid (and reliable) setup using an angle plate. If not, you can verify the perpendicularity of the indexer by running a DTI across the base with the indicator mounted on the carriage. That wouldn't be of high precision, but would help you determine if you had serious error. You're at the mercy of the wear of your lathe, assuming it has some.
I agree that if the existing hole drifts too far, an insert on the end would allow you to keep the overall diameter down within reason. It could be installed with LocTite, which would yield an excellent repair. So long as the ends are well supported, the center of the spindle isn't critical.
Do you plan a tailstock, too?
H
Homemade indexing head
Re: Homemade indexing head
Wise people talk because they have something to say. Fools talk because they have to say something.
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OddDuck
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Re: Homemade indexing head
Yes, I plan on some variety of tailstock, probably very simple in design. I gated it at the ends of the barrel, actually. The ingate was at the clamp end and I put a riser on the other end. Casting-wise, gating it on the base wouldn't be a great idea, the metal flowing in would waterfall over the edge and possibly cause sand wash and turbulence, which would show up as small bubbles in the casting when it was machined. I cast some train wheels once, and the iron basically fell about 1/2" as it filled the mold. They looked fine just out of the sand, but when I machined them they looked like Swiss cheese, especially near the gate.
"If you took the bones out they wouldn't be crunchy!" -Monty Python's Flying Circus
Re: Homemade indexing head
Thanks. So many things to consider. 
H
H
Wise people talk because they have something to say. Fools talk because they have to say something.
Re: Homemade indexing head
Very few to maybe no brand new lathes are ever exactly in a true alignment with the head stocks C/L both vertically and horizontally and at each position the tail stock might be used down the full length of the lathe bed. A difference in tightening torque on the tail stocks locking assembly or it's position on the bed can shift things enough to be measurable. With a worn lathe then the tail stocks actual C/L can be pointing almost anywhere except where you want it. When I want to drill a larger hole in the lathe that has some importance of being straight and coming out very close to where I'm hoping for I'll single point bore to the next larger drills nominal diameter and at least 1/2" or better deep. That bore helps to start the drill concentric to the head stocks C/L and guide it a lot better than simply drilling from the tail stock and hoping for the best.
Your casting looks like it has about a 3" or longer bore? At 5/8ths diameter that's going to require single point boring a consistent diameter and in line with the bases datum surface almost 5 times it's diameter and do that with a good surface finish. Fwiw I'd say your 5/8ths chosen diameter will be much too small and flexible for the work your expecting to do with that indexing head. Milling because of the intermittent and generally higher cutting loads takes much more rigidity than turning does where the loads are almost always continuous.
Constructively speaking and if it were me and even more so with that void already in the casting I really think I'd cut my losses now and re-design a new pattern that would be capable of using a spindle of at least 1 1/4" in diameter. That solves two issues at once, it gives the much needed rigidity for the spindle and possibly even more important that larger diameter allows a much bigger and stiffer boring bar to be used. A 5-1 bore to length ratio is pretty much into a solid carbide boring bar range. 3-1 is generally accepted as about maximum for tool steel boring bars.Ultra sharp tooling might extend that by a small amount. I'd also add at least 3/8ths or more material to the base area. With a new casting and since the bore is the most important feature I'd only rough in the bottom until it was just flat enough to get it securely bolted down, then drill and bore for the spindle to your finished dimension. With it's location established as a fully finished bore it then becomes a datum surface you can trust. I'd then machine the bottom dead true to that bore. With a spindle of at least 1 1/4 in diameter it still has enough meat in it to allow for a hollow spindle for longer work. It's much better to try to design and build in as much versatility you can than to try and somehow add it in later. And since gear cutting can be done various ways and just one of them is locking the gear blank to a mandrel and holding it between centers so the teeth are concentric to the bore your probably going to need a foot stock as well at some point so you might as well cast two of them.
Your casting looks like it has about a 3" or longer bore? At 5/8ths diameter that's going to require single point boring a consistent diameter and in line with the bases datum surface almost 5 times it's diameter and do that with a good surface finish. Fwiw I'd say your 5/8ths chosen diameter will be much too small and flexible for the work your expecting to do with that indexing head. Milling because of the intermittent and generally higher cutting loads takes much more rigidity than turning does where the loads are almost always continuous.
Constructively speaking and if it were me and even more so with that void already in the casting I really think I'd cut my losses now and re-design a new pattern that would be capable of using a spindle of at least 1 1/4" in diameter. That solves two issues at once, it gives the much needed rigidity for the spindle and possibly even more important that larger diameter allows a much bigger and stiffer boring bar to be used. A 5-1 bore to length ratio is pretty much into a solid carbide boring bar range. 3-1 is generally accepted as about maximum for tool steel boring bars.Ultra sharp tooling might extend that by a small amount. I'd also add at least 3/8ths or more material to the base area. With a new casting and since the bore is the most important feature I'd only rough in the bottom until it was just flat enough to get it securely bolted down, then drill and bore for the spindle to your finished dimension. With it's location established as a fully finished bore it then becomes a datum surface you can trust. I'd then machine the bottom dead true to that bore. With a spindle of at least 1 1/4 in diameter it still has enough meat in it to allow for a hollow spindle for longer work. It's much better to try to design and build in as much versatility you can than to try and somehow add it in later. And since gear cutting can be done various ways and just one of them is locking the gear blank to a mandrel and holding it between centers so the teeth are concentric to the bore your probably going to need a foot stock as well at some point so you might as well cast two of them.