Making a very large bore headstock?

[MarkU]

More of a speculative concept than an actual plan at this point but what do you think about the potential to make a very large bore (e.g. something I could pass say a 3" diameter pipe through) headstock for the Taig lathe? Would that be practical?

I am thinking a simple solid block of aluminum with a section of say 4" steel pipe through it, supported by a bearing at each end. The pipe would project at each end, on the inside end some kind of work-holding arrangement (maybe a series of tapped holes for cap screws although you'd have to be careful about too much shear force on them while turning), at the outside end a groove for a drive belt (maybe with the standard stepped pulley relaying power via an idler shaft so you could vary the rpm).

What would be the concerns one would need to address to make this work? I'm not sure I would actually do it, the idea just came into my head while I was thinking about turning some detail into the end of a 3 foot piece of 2" aluminum pipe, which is so far just a one-off that I might be able to find another way to deal with. But now that the idea has occurred to me it is preying on my mind...

[HuFlungDung]

I cannot imagine that it is practical. What swing is the Taig lathe? How many hp driving it?

There is a lot more to a lathe headstock than a solid block with a shaft turning inside of it.

[Jeff-Birt]

3"? No way. I have a large Clausing 11" mill that does not have a 3" bore...

[MarkU]

3"? No way. I have a large Clausing 11" mill that does not have a 3" bore...

What would be the limiting reasons why it can't be done though? (pretend I don't know anything, you won't be far off )

[eartaker]

Why would you ever need to pass a 3" piece of material through the headstock? Are you trying to thread 3" pipe?

[MarkU]

Why would you ever need to pass a 3" piece of material through the headstock? Are you trying to thread 3" pipe?

in this case I just was looking at a piece of pipe I'm building something with and thinking it would be nice to be able to turn a nice smooth end on it.

[Andre' B]

Reading this thread reminds me of the specialized machines for working on or repairing large parts in the field.
Boring Machines | Climax Portable Machine Tools

Vid of a pipe lathe.
[nomedia="http://www.youtube.com/watch?v=Okfh-0DTj-8"]YouTube - H&S Tool Clamshell.mpg[/nomedia]

[Jeff-Birt]

What would be the limiting reasons why it can't be done though?

Good to see you are approaching this with a spirit of inquisitiveness. I always like to know 'why' as well. The bore of a lathe is related to its swing, i.e. diameter of largest workpiece that be turned. Take a look at your Taig lathe and then mentally scale up the size of the headstock that would accept a 3" bore. Then consider the size of chuck it would take. Oh, and don't forget about the cross slide needing more travel. It becomes apparent that scale of the Taig is insufficient to do the job.

As the previous poster mentioned there are some specialized types of machines for working with larger stock. Also try Googling a 'gap bed' lathe. This type of lathe has a chuck of the Z-axis missing (bed) so that larger work pieces can be turned.

[Swede]

It boils down to the bearings.

To simplify, picture a BIG ball bearing with a FOUR INCH inner diameter race... you'll need that much just to support even a minimal spindle. Now you'll need balls, plus an outer race. When the smoke clears, your minimum bearing size is probably going to be six inches.

Now that needs to be supported in a massive iron housing, and you'll need several horsepower just to turn it properly.

In reality, it's not just a pair of big ball bearings, it is a set of taper-roller or angular-contact bearings that are preloaded and firmly fixed.

You'll need at least a couple of tons of machine to do it properly. There's a reason small lathes can only pass 1/2" or so, at best!

[DICKEYBIRD]

Necessity's a mutha ain't it Mark?

Here's a pic I ran across showing how to do sorta what you want although it's not a Taig and it's not 2 feet long.

You'd have to cobble up some kind of beam to mount your Taig on to get it over 2 feet long and fab up a steady rest and some kind of clamp instead of a big ol' chuck. You couldn't cut very deep or fast but it could be done if you had to.

I'll betch'a machinists in Afghanistan or any number of other poor countries could whip up something in no time!

[S_J_H]

A Taig lathe is a tiny machine, turning on 3" diameter pipe is way out of it's leaque.
To be able to take any meaningful cuts on 3" stock you really should be using a 8" or 9" lathe at a minimum.

I would totally forget about getting the taig involved for turning on stock that large.
Maybe rig up something a lot more heavy duty. I've see some pretty crazy homemade lathe type rigs on the net.

This one is my all time favorite!-
[nomedia="http://www.youtube.com/watch?v=zIC8voYWVWY&feature=related"]YouTube - Homemade 4-Jaw Lathe[/nomedia]

Steve

[MarkU]

Thanks everyone, this has been useful