DOM Square Tube - Twist and Straightness

Hi, What I've done in similar situations is to bolt-on aluminum or steel mounting pads, then machine them flat, and drill/tap through into the main rail for whatever I'm attaching. While 13' is longer than what I've needed, it should still be manageable, and will give you better strength/stiffness than if you have to machine one side.

The pads can be epoxied down as an alternative, shouldn't be a problem if you drill though into the main tube. A high precision level and careful indicating will allow you to traverse the length during machining and maintain planarity, if you don't have a machine with a big enough throw for a single pass.

Or, just drill/tap, and carefully shim your attached part to get the accuracy you need, if it's not a high load or vibration application.

Toolpost

Square and rectangular tube is almost always done as round tube that is run through a 'turk's head" roller (4 rollers at 90 degrees to each other and mounted at an angle to the direction of travel) I don't think I've ever seen a DOM tube done as a square.

1/4" wall will be done to A500 (A, B, or C) specs, not A513 specs. Most mills will have much better actual product than the specs call for. Call your local supplier ask what they have.

I'd suggest that you will have better luck look> Does the drawing process improve square tube's twist and straightness

Aluminum tube is perty straight. Not those exact sizes, but close--4x4x

0.25, 24 ft L, altho some places will cut. Proly as straight or straighter than DOM. Even the 2x2 and 3x3x1/8 I have seems super straight. Hadco alumn is in NY, PA, FL. Standard tube Sales sells DOM and other stuff--very expensive stuff. They should be able to help you with specs, etc. Located in MA.

As far as I know ALL square or rectangular tube, welded or DOM starts as round tubing and is rolled into the final shape. That means that the twist is due to the rolling process, not the initial tube source. So it is likely that the twist will be about the same for all square or rectangular shapes.

My experience though has been that a much large problem is the angle of tow adjacent sides. Square and rectangular tubing usually has very parallel sides, but the cross section is frequently a parallelogram. Hope this helps.

Gary H. Lucas

Thank you, everyone, for your input. I think a good option is to rivet (or otherwise bond) a hot rolled 1/2" plate to the long side of a 5x2.5x3/16" HSS tube. This plate can then be machined to achieve the flatness required for the profile rail.

I forgot to mention that this is going into a wide format printer, with a production volume of hundreds of units per year.

DOM's cold-worked strength increase over regular tube is not needed - I was hoping it would be straighter and less twisted.

Your help is much appreciated!

It's not clear to me what you mean by "profile rail," I assume you're talking about THK type recirculating ball linear bearings. But, have you considered using the linear guides that Item, 80-20, etc. make? They'd be much less expensive than machining 13' long spars. Look under the "Dynamic elements" here:

One thing you might consider is to try to untwist the tube, or probably better, purchase it to the specifications you need and let someone else worry about selecting the straightest tubes for you.

Dan

You should be able to straighten the tube to be as flat/straight as you like. I have done this kind of thing many times. I set up a dial indicator under my press to measure the deflection, and gradually bend/ overbend, until the bend has been removed. You have to guess the spring back amount and compensate for it. The hard part is measuring the straightness of long pieces. Good quality, long straight edges are uncommon. I use a straight edge to locate the bends, then you could use feeler guages to measure the bend. On shafts, I set them up on rollers, and use a dial indicator.

Using this method, I have straightened large shafts, 150mm dia, 3500mm long to within .02mm. Shafts like this are delicate, if they are dropped, or bumped when loading into a machine, they can bend quite easily. Often they get bent when transported, just from being tied down, or from the pre-stress in a trailer.

.040" would be a walk in the park. You could almost do this with a rubber mallet, by eye I think.

Cheers, Dom.

There is one thing you might be able to do to get straight untwisted tube. Stretch it. Sounds far fetched I know, but this is how they straighten aluminum extrusions after they come out of the extrusion press all wavy. I also worked on a machine once that formed trusses used in prestressed concrete panels. They stretched all the steel longitudinal pieces to straighten them. Clamp one end of the tube down on a large old scrap beam. Apply a large hydraulic cylinder and grip to the other end.

Gary H. Lucas