Given that there are at least three ways to make a metal tube, it’s reasonable to ask which is best. Each manufacturing method has advantages and disadvantages but which matter most depends on the end application. Here we’ll review the process used for making spiral welded tubes and identify the benefits that result.
Tube Manufacturing Methods
Rube Goldberg could probably come up with a dozen different ways to produce tubing, but there are only three that you’re likely to see in practice.
- Extrusion
- Longitudinally-welded plate
- Spiral-winding
Gun drilling is another option, but we’ll overlook it here as it’s only suitable for relatively short lengths – like rifle barrels.
Extrusion is great for long thin plastic tubing, but less so for metal and especially steel. Extruding metals takes high forces and temperatures and there’s a limit to how thin the walls can be. From the perspective of spiral perforated tubes, another challenge is how holes would be put in. It’s possible of course, but at what cost?
Longitudinally welding entails bending flat plate into a circle. The two opposing faces are then welded together to make the tube. This too has a number of limitations.
- Tube length and diameter is limited by the size of the plate. (42” is about the biggest diameter possible.)
- Springback makes it hard to maintain tight diameter tolerances
- As plate thickness gets thinner large diameter tubes become increasingly difficult to fabricate.
- Making perforated tubes means putting the plate through a punch before forming. Punch press capacity can limit the maximum size of plate processed, plus, this becomes a batch rather than a continuous process.
- There’s a risk of cracking along the weld. (This can be exacerbated by choosing an alloy composition for formability rather than weldability.)
Then there’s spiral winding. This is essentially the process used to make the inner cores for rolls of kitchen and toilet paper.
In spiral winding a long strip of material – the technical term is “skelp” – is wound into a helix. The angle at which it’s fed is adjusted to make the side line up with the open end of the tube being created. As the sides come together a welding head, (or perhaps two, the second being inside the tube,) creates a permanent join. To make spiral perforated tubes all that’s required is a relatively small press that punches holes in the skelp.
The Benefits of Spiral Welded Tubes
By now it should be clear that spiral winding creates a different type of tube than the other processes. This yields a number of benefits for both the tube-maker and the user of perforated tubes. Some benefits to the spiral weld include:
- Makes it possible to create large diameter tubes in long lengths. Freed from the constraints of plate and punch press size, a spiral perforated tube can be as long and as large as needed (usually limited by shipping limits to maximum length).
- Enables production of thin wall tubes, with the added benefit of being lighter per linear foot.
- The ribbed structure that results from the helical weld adds strength – 15% or more versus longitudinal welded tube.
- Spiral welding increases crush-resistance.
- Better control over diameter tolerance than is possible from longitudinal welding, especially in terms of ovality.
- The direction in which the skelp is rolled is neither perpendicular or parallel to the pipe axis. This improves crack resistance.
- Superior manufacturing flexibility. A single width of skelp can be used to make perforated tubing in an almost unlimited range of sizes.
- Opens up a range of perforating options as only a small punch is needed.
A Better Way
When you think about it, the process for making spiral welded tubes is incredibly ingenious, the sort of thing that requires Rube Goldberg thinking to come up with. But not only is it clever, it has a number of benefits, as listed here. The result is a cost-effective process that creates a technically superior product. If there is a “best” way to make perforated tube, spiral welding is probably it.