We had a mill head break the other day. Its an old bridgeport three axis vertical. A guy was doing angle work and he sheared the bolt for the control of rotation about the y axis. The thing is a shaft with a bolt head, that uses a key to hold a pinion gear on the shaft. We got it ground out and are replacing it after romving the head, but when we had the head off we noticed that where the four bolts go into the head there is a t-slot ring in the casting that is just all torn up. There is a lot of casting crap that we don't want to touch because we tried filing it in one place and it came off in a chunk. there are a lot of chunks out and we left it to just cleaning up the burrs on the edges. Anyone else experienced this or have any ideas about a more permanent fix?

That sounds like a tough problem. Is that a machined Tslot, or just a cast T slot? The best thing to do is to weld up the broken areas. You might be able to form a piece of graphite to slip into the Tslot as a backer for the welding puddle. Otherwise, you'll have to remill the Tslot. Use a machinable nickel alloy filler rod.

HU flung, as far as i can tell it was cast with some machining done afterwards...I think, the idea of welding it up scares me a little so I think the present plan is just to leave it alone after cleaning it up...has anyone else had similar problems, it looks like a relatively weak spot in the design because it allow the whole head to just torque arm off the front of the casting and absorb all of the vibration

Welding cast iron can be extremely tricky because of air pockets in the casting. I would suggest to stick the head on another mill, and mill out pockets 1/4' deep, and wide enough so that you can bolt a piece of 1" CRS to either side an form the t-slot that way. Drill and tap for 1/4" countersunk screws. Then take and machine the slot opening to final size if necessary. 1/4" CRS should be plenty strong for the needed purpose, and it is accurate too. If you want it even more accurate, stick the CRS on the mill and face each side first. JB weld on the mating surface may be a nice overengineered what if safety precaution, as well as threadlocker on the bolts. Just a thought.

Correction to the above.
"I would suggest to stick the head on another mill, and mill out pockets 1/4' deep, and wide enough so that you can bolt a piece of 1" CRS to either side an form the t-slot that way"
a piece of 1/4" by 1" CRS strap.

If you don't weld it up, then you cannot swivel the head again, can you? The Tslot bolts must have a flat bearing to tighten against.

Don't be scared. UTP 85FN welding rods make short work of welding up lots of cast iron goodies. That part of the casting is likely quite sound, since it was near the outside of the casting.

In that location, the cast is not too liable to be oil-soaked, which is what really makes welding difficult. Preheat the weld zone to 500 degrees or so.

OK problem solved with word from above, the part that broke (the shaft with pinion gear) will be replaced and it will go back together with only deburring that i spent a few hours on...not my personal favorite solution but o well supposedly this also happened a couple years ago too.

Lol sorry to frag this thread up again, but jus thought I would share that it happened again tonight. Exact same spot on the shaft, its where the shaft through the worm gear narrows down rightunder the head. Anyone with a bridgeport might want to take xtra precautions when doing angled head work I know I will be

The first BP I bought had the same broken bolt. I replaced it with a new one. Now I always have someone hold the head when I'm putting the head at an angle so all the weight isn't on the Bolt. I've never had the problem again.

Why not check on Ebay for a "knuckel" that has the T slots. In the long run, it will be the cheapest and safest way to get it repaired. You are lucky as the newer heads have the shaft for the gear going through the gear and back into the casting. This requires the shaft to be drilled out and it is hardened a bit. When I install a new shaft, I cut it off at the bottom of the gear so if it breaks again, the gear and broken shaft acn be easily removed.

George W.