How straight is pipe?

[Henrik]

How straight are mild steel pipe sections generally?

I'm thinking of a little project that will require about a meters worth of 3-4" pipe with 5-6mm wall thickness, but I'd also want the pipe to be reasonably straight and round.

Most of the erw style pipe I've seen is visually distorted (squashed looking sectionally), but I've only really looked at smaller pipe thus far.

Is cold drawn better for trueness?


Alternatively, anyone hazard a guess as to the cost of grinding a section of pipe at some engineering shop? Lathe cut finish would work too, as long as the wall diameter isn't compromised too badly.

 

[Jan]

I know that cold drawn wires are sent through a series of pressure rollers after each drawing/thinning mandrel to ensure precise straightness. I would assume that it's the same for hollow tube. The main difference may be that wire is then wrapped around a drum, while I would expect that tube would either be done in specific lengths or simply cut into stock lengths from a continuous feed process and stored horizontally. However unless the whole length is fully supported rather than at several points along its length there could be some slight (?microns?) sagging at intermediate points.

Many years ago you could watch a continuous casting being carried out at Tinsley Wire industries (before it closed down) where molten metal was poured into an open bottomed water cooled 'mold'. As long as it was supplied with a constant feed of fresh molten steel there was no limit to the length of billet that could be produced. As the centre of the billet was still liquid it could be bent around 90 degrees and fed on to another process; eg 'endless' wire.

 

[pedrobedro]

Do you mean pipe or tube ? There are different grades of both with different tolerances. You can get CDS for hydraulic use which is made to tighter tolerance and also annealed but it sounds like what you are looking at in RHS is tubing with a welded seam. Steam pipe is usually thicker wall to stand higher pressure and it is made to tolerance. You can get DOM tubing (drawn over mandrel) as well but it is more expensive and you might struggle to find someone who will sell you the amount you are looking for.

 

[hotrodder]

http://www.hublebas.co.uk/products/products.aspx the data sheets for all the different flavours of tube contain tolerances for straightness, ovality etc as well as OD and wall thicknesses it's commonly available in. The DOM mentioned = CEW
Googling 'ASTM pipe tolerances' will get you detailed info for pipe, tolerances and charts listing the dimensions for different schedules of different nominal bore sizes

 

[steviec_lj]

5-6mm wall shouldn't be distorted. If it is, send it back, simples..

 

[chevy2]

I often get my cds tube from hublebas as they're local to me and the quality is usually far better than regular chs seamed tube.

 

[Maker]

There's a page on here with a list of steel suppliers, one might be near you. I doubt there's many places that will sell you a meter though, my local only stocks 7.66 meters and bigger. Maybe someone on here has some they might part with for beer money, put a post in the used equipment wanted section.

 

[Henrik]

Thanks guys.

The part would be for a little cnc controlled lathe's bed.

I'm under no illusions as to accuracy, but it sounds like a fun project for a raspberry pi (or some other embedded solution)

Anyway, the idea would be to have the carriage slide along the cylindrical bed (drummond round bed style), but with a keyway to locate it circularly. The tail stock would have a similar attachment.

With that in mind, it'd be good if the bed was at least semi circular, and roughly the same dimensions across the length. I don't think a couple of "thou" would be too bad, so that's the sort of accuracy I'm aiming for.

I'll do some googling on those tolerances, thanks for the links.

I've got a small lathe already and a horizontal milling machine, and I basically need a little project for both of them, and as I "work with computers" I figured something cnc operated would be good.

Again, under no illusions as to the accuracy of the machine, and the hardest material I want to cut is aluminium.

Btw reason for round bed would be that it at least shouldn't twist too badly, and with only one keyway, I think things will be easier to align.

 

[Henrik]

That hub le bas link is awesome, finally some supplier that publishes accuracy

 

[stormin]

The problem with using a round bed type lathe is accomodating some type of adjustable gib or bearing surface to your saddle to accomodate wear and allow fast travel without sticking. Particularly important with cnc machines.

The best solution would probably be round linear bearings but at 3-4" dia they will be very expensive. You may be better using two smaller linear rails with smaller bearings. You could build your saddle smaller with less inertia and source very accurate linear rail and bearings to increase the accuracy of your completed machine.

 

[Henrik]

The problem with using a round bed type lathe is accomodating some type of adjustable gib or bearing surface to your saddle to accomodate wear and allow fast travel without sticking. Particularly important with cnc machines.

The best solution would probably be round linear bearings but at 3-4" dia they will be very expensive. You may be better using two smaller linear rails with smaller bearings. You could build your saddle smaller with less inertia and source very accurate linear rail and bearings to increase the accuracy of your completed machine.

Hmm, you might be right on the sticky bit. I'm not quite sure how the drummond deals with it.

I was thinking of possibly having a saddle riding on three ball bearings (well two sets of three), as that would have less friction than a sliding mechanism.

It's a good shout about bed wear, though. The groove would need something fairly soft sliding in it (brass?)


Another option would be to use an old lathe bed and fit supported bearings to it, however that would give the problem of parallelicity

 

[Maker]

Sounds like a clever and interesting, keep us posted. I agree with stormin, a round bed really isn't ideal. If it were me I would get some supported rails and linear bearings to fit them and have them fixed to some heavy box section.

 

[MrFluffy]

I owned a more modern round bed production lathe that originally was designed as a button lathe with collet chuck and turret tooling etc, it dealt with it by having a replaceable bronze sleeve in the bed saddle, not the most ideal of arrangements and it did droop from wear when I measured it. The round bed was 70mm steel bar with a mechanism a bit like a guitar nut inside to twist the bed into being more accurate (never stripped the bar itself, this is just a guess seeing the adjusters). I scrapped it in the end after using it for a mach conversion due to the droop and lack of rigidity causing innacuracies, I have the turret spare for my regular lathe when I want it, the headstock is now my milling machine centres and the cabinet has a honing rig on it, so it didn't die in vain.

Remember also, you need something at the rear to run along thats going to be true also, or there's nothing to stop your saddle just sliding round the round bed under cutting forces. The S2S lathe had a cast iron ground shelf at the rear the back of the saddle located on. Its not the most inherently fantastic layout compared to two linear rails or a proper bed...

If you want to persist with the round bed, I'd recommend looking at if you can get a old thickwalled hydraulic piston as they have to be straight, rigid, and hard, then possibly filling it with something afterwards (concrete, resin etc).
If you want a really sucessful cnc conversion, the best bit is a really rigid cast steel base, and ebay etc is full of rigid steel castings already made, they just need ballscrews and some conversion from their existing state of being a manual lathe

 

[Jan]

If this is a 'ground up' build of your own design or modification of an existing design might it be possible to have 2 parallel tubes as the bed? The lateral stability would be orders of magnitude better than just a single tube - at least in theory.

 

[MrFluffy]

Pic of the little roundbed just fyi as theyre not well known, the second tube below the main one is a rack for the saddle to be fed in.
Also of note, there's no handwheels, cross slide by a sensitive feed and diameter of cut set by the stops.
This is a production line lathe, it was made to bang out a zillion button sized parts turning close to the collet chuck before cnc became common. The accuracy fell off quite dramatically on longer pieces but that didnt matter to it for its job it seems.

Also correction, I meant like a guitar NECK nut, they have a mechanism in the rod to straighten the neck alignment, this seemed to have the same sort of thing but on a much more robust level.

 

[Henrik]

Jan, it'd be a ground up build, however I might look into using a used cast iron lathe bed instead.

That round bed lathe looks cool, however I think you guys are right, it is probably better to go with either two bars, or indeed use supported rails and pillow blocks on a flat piece of metal. Just looked on evilbay for flat aluminium bar (100mm wide), and it's not *TOO* expensive, although a used lathe bed would work out cheaper (but be much uglier)

 

[Jan]

Often an old used bed might be the best bet. By now it should have had all the warpage taken out of it and even if it needs to be reskimmed it could work out cheaper and more effective than using new components which could warp severely over time.

 

[Maker]

Jan, it'd be a ground up build, however I might look into using a used cast iron lathe bed instead.

That round bed lathe looks cool, however I think you guys are right, it is probably better to go with either two bars, or indeed use supported rails and pillow blocks on a flat piece of metal. Just looked on evilbay for flat aluminium bar (100mm wide), and it's not *TOO* expensive, although a used lathe bed would work out cheaper (but be much uglier)

If you want to machine aluminum then I think you would want something stiffer I.E. steel.

 

[Henrik]

Just to give a bit of an update on this thread, I've gone and bought myself an atlas 10f lathe "for spares or repair"

It basically looks pretty wrecked, with parts missing, no changewheels / gearbox etc etc, and the bed looks a bit wrecked too, so basically perfect for my needs The bed is also flat "the whole length", i.e. there's no funny business with lining up a new headstock, as it will just need to replicate what is on the lathe at the moment.


Haven't got it home yet, but I'm assuming it will fit into a hatchback with seats folded down (focus). The plan is to dismantle it at site (remove the headstock, saddle and tailstock) and man-handle the parts into the car boot / rear seat. Anyone want to hazard a guess as to the weight of the bed? It's the short bed version, which makes it 36" / 91cm.


The lead screw also looks perfect for fixing the lead screw on my milling machine as well, so bonus for that

 

[Rig Pig]

A rough guess from the last time I had to move mine would be 120 - 150 kg with the headstock, saddle and tailstock in place, my back really enjoyed shifting it!