Turning large thin-wall tubing

[Tim Wescott]

Large & thin wall as in 4 inch diameter, 1/8 inch wall, acrylic tube. Any attempt to just put it in a chuck results in a severely distorted tube.

I'm thinking that I should either turn a piece that fits just inside the tube or making a cross-bar jack thingie.

The turned piece has bonus of enforcing roundness (it's plastic tubing, so it didn't come round), but I need to use up a bunch of material.

The cross-bar jack thingie would be an aluminum cross, threaded for 1/4" cap screws, that I can use as a jack underneath the chuck jaws so that the tube is pressed between the jack and the chuck, rather than the chuck bearing on the tube and making it go square as I tighten it. It has the advantage that I could use it for other tube-turning projects, and I've got a lot of 1/2" square 6061 tubing, so the material cost is practically free.

Opinions? More options? I'm sure the cross-bar jack thingie has a name, and that real machinists know how good an idea it is.

 

[kwoodhands]

Large & thin wall as in 4 inch diameter, 1/8 inch wall, acrylic tube. Any attempt to just put it in a chuck results in a severely distorted tube.

I'm thinking that I should either turn a piece that fits just inside the tube or making a cross-bar jack thingie.

The turned piece has bonus of enforcing roundness (it's plastic tubing, so it didn't come round), but I need to use up a bunch of material.

The cross-bar jack thingie would be an aluminum cross, threaded for 1/4" cap screws, that I can use as a jack underneath the chuck jaws so that the tube is pressed between the jack and the chuck, rather than the chuck bearing on the tube and making it go square as I tighten it. It has the advantage that I could use it for other tube-turning projects, and I've got a lot of 1/2" square 6061 tubing, so the material cost is practically free.

Opinions? More options? I'm sure the cross-bar jack thingie has a name, and that real machinists know how good an idea it is.

I would turn a disk, wood, aluminum etc for each end if the tube is long. I don't think the cross bar will work as intended because it only supports 4 places. I have had occasion to turn and face copper tube 3" in diameter. I turned 2 wood disks, one one each end and a center hole for the tailstock live center.
mike

 

[SmithDoor]

I use a thick steel split ring to fit size of tube.
Real thing tube takes a plug too.
You made to use a blade in tool post grinder or hand grinder mounted in tool post.

Dave

Large & thin wall as in 4 inch diameter, 1/8 inch wall, acrylic tube. Any attempt to just put it in a chuck results in a severely distorted tube.

I'm thinking that I should either turn a piece that fits just inside the tube or making a cross-bar jack thingie.

The turned piece has bonus of enforcing roundness (it's plastic tubing, so it didn't come round), but I need to use up a bunch of material.

The cross-bar jack thingie would be an aluminum cross, threaded for 1/4" cap screws, that I can use as a jack underneath the chuck jaws so that the tube is pressed between the jack and the chuck, rather than the chuck bearing on the tube and making it go square as I tighten it. It has the advantage that I could use it for other tube-turning projects, and I've got a lot of 1/2" square 6061 tubing, so the material cost is practically free.

Opinions? More options? I'm sure the cross-bar jack thingie has a name, and that real machinists know how good an idea it is.

 

[Ken I]

Quite often I will use MDF (medium density fibreboard) - bandsaw a disk from scrap and turn to finished size - ditto the tailstock end with a centre drilled hole for the live centre.
Here's an example using MDF over a threaded rod mandrel ( on a wooden barrel for my Hypocycloid - but the same principal ) :-

Using the threaded rod down the centre also acts as a stiffener - you can also mount the MDF disks on it with a nut on either side to turn them to get good concentricity - prior to mounting your tube for turning.
Regards, Ken

 

[Steamchick]

Nice idea to use a cruciform jack. But while you'll get a grip of the tube, it won't support the tube while machining. Turned Wood is the way to go. You may manage to use wax to fill the tube and hold it rigid while machining, as pipe benders use to prevent collapse, but wood formers are the way to go without tears! And if you have a warm workshop, the wax may soften, or melt! This may work as a quick fix for just 1 or 2 short bits... I have used it on copper tube, also lead solder.
And use a steady, and / or tailstock revolving centre in a wood plug as the tube won't be stiff enough to resist any centrifgal forces from rotating at the necessary high speed if you just support it at the headstock and the projection is more than 1 diameter...
One job I worked on... (part of the design of a 400kV. Circuit breaker) had pulltruded resin and fibre tubes - 10ft long and 3 in. Dia - with machined ends. (Turned and threaded for aluminium ends). These were Machined on dedicated mandrels. So a wood mandrel should be the way to go, for you.
Enjoy!
K2

 

[MRA]

(If you make the wooden plugs for each end a little conical, you can push the whole thing gently together using the tailstock - but think about things when you're about to break through!)

 

[ajoeiam]

(If you make the wooden plugs for each end a little conical, you can push the whole thing gently together using the tailstock - but think about things when you're about to break through!)

I never found it 'fun' (rather scary most times) but if you cut off with a very well ground tool and your holding, loosely please, the main piece from just before you're breaking through in your left hand and then hit the lever to stop the revolutions with your right, this should work, - - - has for me anyway.
This is not what is usually called high security machining - - - - but sometimes one needs to get the job done. Please - - - this is not likely OSHA compliant and most times I have done this the butt cheeks were more than a bit puckered but it is doable. (Please do be careful though!!!!)

 

[tornitore45]

Wooden, slightly tapered plugs as already mentioned would be the best, easiest and need no expensive metal. Both sides with the TS keeping the thing from wobbling, ne need to tighten the jaws excessively.

 

[Steamchick]

But be careful with tapered plugs.... too much tailstock pressure and they'll split your acrylic tube, or too fine a taper and they may work-in under tailstock pressure and the job will go slack (and destroy your work). I prefer plugs to be a sliding fit into the tube with square flanges to take the slight tailstock pressure.
Different people use different theories and have different skills. Whatever suits you.
With sharp tools, high speed, care and patience many things will work. Otherwise even the "perfect" set-up can go wrong. And with acrylic it is often "explosive" with speed and stresses involved. Sharp flying shards of acrylic can be nasty if they hit flesh.
So do take care, and think about the process you finally choose.
"Think twice, cut once, no failures."
K2

 

[goldstar31]

Whilst I agree with Ken2, the real engineers make up their own LAEGE revolving centres. Sparey was one and Jack Radford of New Zealand was another.

Of course, the World revolves or used to with Metal Box Company in the UK and United Can Co in the States making hollow cans/ 'tins'. I'm not talking about spinning slugs of aluminium but proper Tern steel.
I did a statistics course in TMB's Baker St, London and visited their Acton factory. The NOISE- I felt like Quasimodo.

 

[Patio]

You have not mentioned what kind of feature you are trying to make on the tubing. Soft jaws if you can hold it by the outside or expanding mandrel if you need to hold by the inside, are a couple ways that come to mind.

 

[Steamchick]

Like Goldstar, I have witnessed tin manufacture... for oil filter cans. Deep drawing steel 2 or 3 diameters deep in 3 strikes. On a 3-crank 120 degree press running over 100 rpm making cans for diesel fuel filters 4in dia x 100 in deep.
Also saw brass deep drawing for shell casings in an armaments factory. That was deep drawing!
NOISY!
K2

 

[Steamchick]

Should have been 10 in. Deep. Smell checker keeps changing what I type! And I sometimes miss it!
K2

 

[SteveT]

Joe Pieczynski has several videos on holding & turning thin wall tubing on You Tube which might help