ERW vs DOM vs CDS Tubing: what actually changes—and how to choose

ERW vs DOM vs CDS Tubing: what actually changes—and how to choose

Mechanical tubing looks deceptively simple: it’s “just a round tube.” But the manufacturing route (welded vs. welded-then-drawn vs. seamless-then-drawn) drives the things engineers and fabricators care about—dimensional control, inside surface condition, consistency, machinability, fatigue behavior, and cost. The three labels that get compared most are:

• ERW (electric resistance welded) tubing
• DOM (drawn over mandrel) tubing
• CDS (cold drawn seamless) tubing

They’re related—but not interchangeable.

The short definitions (so you don’t get fooled by the acronyms)

ERW tubing is made from strip/coil formed into a tube and electric-resistance welded along a longitudinal seam. Mechanical tubing under ASTM A513 is a common ERW family.

DOM tubing is ERW tubing that has been cold-drawn through a die and over a mandrel after welding (typically after annealing). It’s commonly produced to ASTM A513 Type 5. DOM is not truly seamless—it starts as ERW.

CDS tubing means cold drawn seamless: the starting tube is seamless, then cold-finished (drawn) to improve tolerance and surface. In North America, a frequent spec is ASTM A519 (seamless mechanical tubing). In Europe/UK, “CDS” commonly points to EN 10305-1 (seamless cold drawn precision tubes).

How ERW mechanical tubing is made (and what that implies)

Most ERW mechanical tubing starts as flat steel strip. It’s progressively roll-formed into a round and the edges are heated by electrical resistance and forged together to make the seam.

Two seam-related details matter in real designs:

• Outside weld bead is typically removed, leaving a smooth OD.
• Inside flash (ID bead) may remain unless specified; some producers can control or remove it, but you must call it out.

If you’re buying to ASTM A513, the standard explicitly covers ERW mechanical tubing in multiple shapes and size ranges (including round, square, rectangular).

Practical consequence: ERW is usually the most economical way to get mechanical tube, but you must pay attention to ID condition, weld seam orientation, and the tolerance/straightness class you need.

What DOM really is: ERW + anneal + cold drawing over a mandrel

DOM is best understood as a finishing process applied to welded tube:

1. Start with ERW tube (commonly carbon steel strip such as ~1020/1026).
2. Anneal to normalize grain structure and reduce forming stresses.
3. Cold draw through a die and over a mandrel, improving:
   • concentricity and wall uniformity
   • ovality control
   • surface density/finish
   • achievable tolerances

This is why DOM is often chosen for machined parts and tight-fit assemblies (bushings, sleeves, pins, automotive and motorsport components): you’re paying for more controlled geometry and a better “machining starting point.”

Key reality check: DOM originates as welded tubing—the seam is heavily worked and typically far less “seam-like,” but the process lineage is still welded-first.

What CDS is: seamless tube that’s cold-finished for precision

“Seamless” means the starting hollow isn’t made by rolling strip and welding edges—it’s formed as a hollow by a seamless route, then often cold drawn to hit tighter tolerances and surface requirements.

Two common specification families illustrate how CDS is used:

• EN 10305-1 explicitly covers seamless cold drawn tubes for precision applications.
• ASTM A519 is a widely used seamless mechanical tubing spec where close tolerances, smooth finish, or definite physical properties are important selection factors.

Why engineers pay for CDS: no longitudinal weld line, often excellent ID quality after proper cold finishing, and a strong foundation for honing/skiving/roller-burnishing when the tube becomes a cylinder or precision bore component.

Mechanical properties: the “cold work vs ductility” trade you can quantify

A persistent myth is “DOM is always stronger than ERW” or “CDS is always stronger than DOM.” In practice, heat treatment and delivery condition can dominate the outcome.

A clean way to see the trade-off is in EN 10305-1 grades and delivery conditions, where minimum properties are stated for conditions such as +C (as drawn), +SR (stress relieved), and +N (normalized).

How to use this: when comparing ERW vs DOM vs CDS, don’t stop at the process label. Always ask: What grade? What delivery condition / heat treatment? What mechanical minima are specified on the certs?

Tolerances and straightness: where DOM and CDS usually earn their keep

If the tube is just a structural spacer, “tube-ish” is fine. If it becomes a bearing surface, a slip fit, a honed cylinder, or a precision sleeve, dimensional control becomes design-critical.

ASTM A513 distinguishes tolerance tables for different conditions, including tighter tolerance regimes for sink-drawn and DOM product forms.

Even straightness is called out in A513 dimensional guidance. For round tubing, a commonly cited straightness tolerance is 0.030 in per 3 ft up to 8.000 in OD and 0.060 in per 3 ft at 8.000 in OD and above.

Rule of thumb: choose ERW when the assembly tolerates more variation; choose DOM when you need better concentricity/ovality control; choose CDS when you need seamless behavior and/or a precision-bore pathway.

ID surface condition and weld flash: the most common “gotcha” in ERW vs DOM

Engineers often specify ERW tube and then discover too late that the ID weld bead/flash interferes with:

• inserting rods or bushings
• piston seals
• flow characteristics (in some pneumatic/hydraulic layouts)
• internal machining passes

While the outside bead is commonly removed, the inside flash may remain unless you request flash removal or controlled flash. DOM’s cold drawing step usually improves ID uniformity, but if your design is sensitive, you should still specify ID requirements rather than assume them.

Fatigue and critical load paths: when “seamless” is the safer engineering decision

For many static structures, ERW performs extremely well when properly specified and fabricated.

Where CDS can be preferred is when:

• the part sees high-cycle fatigue with unknown or multi-axial stress states
• the seam location would be hard to control relative to peak stress
• you want to reduce seam-related uncertainty in fracture-critical parts

This doesn’t mean ERW fails or DOM is unsafe—it means that when the margin is tight, removing a variable (the longitudinal weld line) can be worth the extra cost.

Cost and availability: why ERW is common, DOM is selective, CDS is deliberate

Even without quoting volatile market prices, the cost drivers are stable:

• ERW is efficient: strip → form → weld → size → cut.
• DOM adds annealing and a cold drawing operation (more processing, yield loss, scheduling).
• CDS starts with seamless hollows and then cold-finishes, which is typically the most process-intensive route for precision mechanical tube.

Availability follows the same logic: ERW is ubiquitous; DOM and CDS are more dependent on mill/distributor programs and lead times.

Application-driven guidance (what experienced shops usually do)

Structural frames, racks, guards, general fabrication

• Typical choice: ERW mechanical tubing (A513 family)
• Why: cost-effective, good weldability, broad availability
• Watch-outs: ID flash if anything must pass through the bore

Precision sleeves, machined spacers, tight-fit pins, automotive/motorsport components

• Typical choice: DOM (often A513 Type 5)
• Why: better geometry/concentricity, improved machining starting point

Hydraulic/pneumatic cylinder components, honed tubes, high precision bores

• Typical choice: CDS to EN 10305-1 (or seamless mechanical tubing like A519 depending on region and requirements)
• Why: seamless starting point + precision delivery conditions; property control and consistency for bore finishing

How to specify correctly (so purchasing doesn’t “value engineer” the wrong tube)

For ERW mechanical tubing

• Standard: ASTM A513 (type/condition as required)
• Dimensions: OD / wall (or OD / ID)
• ID flash requirement: as-welded, flash controlled, or flash removed (state it)

For DOM

• Standard: ASTM A513 Type 5 is a common baseline
• Call out: grade/chemistry expectations, heat treatment (if required), critical tolerances, straightness

For CDS

• Standard: EN 10305-1 (Europe/UK “CDS”) or ASTM A519 (North America)
• Delivery condition: +C, +SR, +N, etc., aligned to your forming/welding/machining needs

Comparison matrix: ERW vs DOM vs CDS at a glance

Bottom line

• ERW is the practical default for fabricated structures—as long as you manage ID flash and tolerance expectations.
• DOM is ERW tube upgraded by cold work—often the sweet spot when you need better geometry and machinability without paying for full seamless.
• CDS is the go-to when you need seamless behavior and precision finishing pathways, especially for bore-critical components.