Expanding Mandrel for turning large tubes

[neilg4dbn]

Cutting a large thick-wall tube to make antenna support rings on my lathe was a big challenge. Southern Comfort and too much coffee resulted in an internal mandrel fixture that works quite well. I can now hold large heavy tube and machine the outside and both ends with ease.
I can't claim any originality for this fixture. It's probably got a proper name but I don't know what to call it. "Expanding Tube Mandrel" will have to do.



I don't have a steady rest, but machining the outside and ends of a tube when using a steady rest means a lot of extra steps, and this thing can stay with the workpiece if I part off a thick ring then want to machine it before cutting another. Once the lathe is clear again, I can just pop the tube back on, complete with the mandrel still inside. Also, finding a big enough steady rest to fit 7+ inch tube on my Colchester 1800 would be amusing.

This is Part One of the video, second part to follow soon.

Now I've mentioned steady rests, I want one of those as well. Too many projects, too little time.

[Claudio HG]

This is a pretty good idea.

[Toolmaker51]

As Frank S will say "That is thinking outside the sphere!". It certainly is. Concept of 'the box' actually has limited coordinates, whereas a sphere contains unlimited points of reference.

That said, I've run a lot of tube, most often from paper manufacturing and printing industry. It has to be supported at each end, for concentricity, lack of taper, and plain old safety. The go-to holding method(s) securing outboard end, always limited to a] false center, b] steady rest, c] bell center. Job requirements often relied on a tailstock mounted 4 jaw first, getting both ends centered up and turning a band for steady rest bearings to run. An interesting complication; some rollers have rubber bonded outside. Good luck spinning that in a steady rest! (Hint; tooled up clamp-on rings).

Thinking back how many times your mandrel would have offered a solution OUTSIDE those mentioned, without discounting some of those rollers were 10 and 12 feet long, several hundred pounds, one of a kind - no spares, late at night - no replacement material......or killing whole production line.

It's tough in job shops, when management can't see value in work-saving projects that can be used over and over.
But 'We' know it, Shop Truths, Phrases, Tales; and Outright Lies

[Toolmaker51]

Ooops. Accidental double post. Hello 'Select All-Delete'!

As consolation, your fixture has a name; it's called a Spider. Related version commonly used outboard the spindle end, supporting small material.

[neilg4dbn]

That makes sense, it's sort of like an inside-out spider. Thanks very much. I wondered if there was a name similar to Cat's Head.
This is the three-screw spider I made to support mostly long hex bar through the headstock. It's got more holes now, so I can use four screws or three. Much easier to align round or square stock with four screws.



Now I need to bore a two-inch hole in the outside wall of the shop and fit a bearing and spider to it so I can turn REALLY long workpieces through the headstock without any danger of whip!

[Frank S]

I Like it. I made a set of internal collets a long time ago to allow me to bore the Id and sometimes thread the ID of long hydraulic cylinder tubing on a relatively short lathe. To do this I used these ranging from 2 1/2" through 5"
click on picture for clearer view the grub screws allowed for tightening to the ID of the tube while serving as an internal 4 jaw

[neilg4dbn]

I like that grub screw expansion method, my smaller mandrels use a tapered shaft and nut to open up a sawn cylinder, but the grub screw method is almost like a Taperlok fitting, very neat and easy to adjust from the end.

[Frank S]

I like that grub screw expansion method, my smaller mandrels use a tapered shaft and nut to open up a sawn cylinder, but the grub screw method is almost like a Taperlok fitting, very neat and easy to adjust from the end.

The disadvantage to them were I had to make one for each ID hydraulic cylinder tube I machined. At the same time it proved to be nice to have so many sizes. We purchased our honed tubing from a supplier in Turkey in quantities of batch lots of 10 to 30 6 meter random lengths for each size at a time they provided honing inspection certs. to tolerances of -0/+.05 mm or less. so 1/4 turn of each screw went from slip in to fully locked in place and centered every time we had a dedicated lathe with a collet chuck and left either a 40 or 50 mm collet in it at all times

[MasterMaker]

I Like it. I made a set of internal collets a long time ago to allow me to bore the Id and sometimes thread the ID of long hydraulic cylinder tubing on a relatively short lathe. To do this I used these ranging from 2 1/2" through 5"
click on picture for clearer view the grub screws allowed for tightening to the ID of the tube while serving as an internal 4 jaw

With that design I think I would have just used some sink-head screws with part of the countersink bridging the slot and some extra relief under the head in the main body/threaded part so they would push on the tab with the taper when tightened down.

[Toolmaker51]

With that design I think I would have just used some sink-head screws with part of the countersink bridging the slot and some extra relief under the head in the main body/threaded part so they would push on the tab with the taper when tightened down.

Mechanically speaking, a flathead screw could perform that wedging action, counterbored deep enough to apply that about mid point.
The drawbacks?
At least two limitations; no flathead screws are dependably concentric to their threaded body, and a large enough screw thread (and corresponding wrench size) might have too large a conical section to fit between fixture shaft and ID of part.

That said, an expedient inside grip is achieved on short parts, sawing an arbor into 4 segments after tapping center for a NPT pipe plug. Another way turns OD of socket cap screw head to about 7°-10°. With same setting to compound, bore a recess same angle into the plug. Fine threaded fasteners work best.