[boss4]

I have procured rather cheaply a lathe spindle from a 14" 1960's U.S. made lathe. This is a rather stout piece about 24" long with 2 pairs of bearings 130mm and 110mm sealed ball bearings respectively. The fwd bearings are ND3215 separated by a spacer and the aft bearings are 212KD.

My intent is to modify the spindle and for use in a shop made aluminum headstock as a 4th axis on a CNC mill and as a light duty CNC lathe (lathe tooling on the mill spindle or column with a custom tool holder). This will be driven by a shop made timing pulley by a surplus 1.8kw servo.

I am hoping to get some advice on where to cut the spindle. From a practical standpoint the ideal case would be to cut the spindle behind the fwd bearings, the drive pulley for the axis would then be attached there. This would allow the overall setup to be short and light enough to be easy to handle but I guess the real question is would the spindle still be stable without the rear bearings (I think the rear bearings are primarily thrust bearings)?

I am an electrical engineer by trade and I don't really have the intuition behind spindle design.

[firepong]

While the front bearings are most of the load that will be seen on the spindle, it's still worth the time and money to maintain the rear bearing as well. This is because it keeps the spindle steady (to an extent) at faster speeds, which provides rigidity.

If possible, I would try to make a mounting platform such that you maintain all bearings on the spindle. On a lathe, you want as little runoit as possible. As such, again, maintain the back bearing as long as possible until you know you cannot keep it. At that point re-think how you want to do it, as once you cut the end off, there is no going back.

[boss4]

Maintaining the rear bearings would heavily influence the overall design. If I use the front bearings only then I will probably make the headstock from a single block of material as I can readily bore a hole to that depth. If maintaining the rear bearings I will make the headstock from 1" thick plate that would then be pined and bolted together as I will not be able to bore that deeply. I suppose that I could rough bore the holes and then take the assembled parts to an outside shop larger equipment.

The bottom line is I think the overall design and limitations external to the spindle will have more influence on the achievable accuracy with this part than maintaining the rear bearing or not.

While the front bearings are most of the load that will be seen on the spindle, it's still worth the time and money to maintain the rear bearing as well. This is because it keeps the spindle steady (to an extent) at faster speeds, which provides rigidity.

If possible, I would try to make a mounting platform such that you maintain all bearings on the spindle. On a lathe, you want as little runoit as possible. As such, again, maintain the back bearing as long as possible until you know you cannot keep it. At that point re-think how you want to do it, as once you cut the end off, there is no going back.

[Jim Dawson]

Just a thought. Modifying the original headstock might be an option and use it as the main housing. I thought about doing this to make a 4th axis for my mill. You might even be able to stuff the servo motor inside of it.

Also if you have a large enough lathe, you can line-bore the housing by mounting the housing on the carriage and building a boring bar that is supported by the tail stock and driven by the chuck.

[boss4]

The original cast iron headstock casting has many complex features and is also a gearbox and is just way to heavy to make something that will be portable which is my intent. I believe it is in excess of 200kg.

The more I think about this the more I think that I can be pretty accurate using aluminum tooling plate even if I don't line bore the housing. If I cut the bore to the finished size while the plate is disassembled then I will have the mounting flange at the bottom to register all of the surfaces too, each section of plate would be cut on all sides with the bore and all work completed in one operation from one side only.

Just a thought. Modifying the original headstock might be an option and use it as the main housing. I thought about doing this to make a 4th axis for my mill. You might even be able to stuff the servo motor inside of it.

Also if you have a large enough lathe, you can line-bore the housing by mounting the housing on the carriage and building a boring bar that is supported by the tail stock and driven by the chuck.

[RCaffin]

Imho:
Chopping the rear bearing off is a very good way to remove nearly all value from the spindle.
It's a NOT a good idea.

Cheers
Roger