Drawn Over Mandrel [DOM]:

Drawn Over Mandrel is one of many tube milling techniques. There are other similar Cold Tube Drawing methods to size and finish tube to desired dimensions. DOM Tubing starts out as a Hollow or CREW (Cold-Rolled ERW tube).

Firstly, the larger ERW tube is “pointed” in order to be grabbed by a trolley jaw. The trolley is a conveyor-like platform which pulls the tube through the die. After “Pointing” of the raw tube, a carbide mandrel is inserted inside the tube’s bore. Then, the tube is set inside a carbide die. After which, a solvent lubricates the tube’s surface. Then, the trolley jaws grip the tube at pointing end pulling via the trolley hook drawing the pipe through the die.

As a newly resized DOM tube, ultimately the die and mandrel determines the OD (Outside Diameter) and ID (Inside Diameter) of the final tube. In other words, a high-angle die will thin a heavy-wall tube while a low-angle die will thicken a thin-wall tube. Optimum die positioning and its angle determines correct diameter-to-thickness (D/t) ratio.

Multiple Passes:

Multiple passes (Iterations) may be necessary depending on DOM method, tube size, finish, and metallurgical and mechanical requirements. For example, if the amount of reduction necessary for the finished tube is such that it requires several incremental reductions. After which, additional surface treatments may be deemed necessary as well.

Mandrel and Rod Removal:

Now that the tube has completed its first drawn, the mandrel needs to be removed. But how do you get the mandrel, and or rod, out from inside the tube? Well, there is another step that expands the tube. One method is to apply pressure to the rotating tube over a series of rollers. This action stresses the tube and expands it enough to extract the mandrel. Additional drawing process may follow until the correct finish is successful.

Computer Aided Design (CAD):

Unlike in the past, trial and error has been replaced with Computer Aided Design (CAD). With the assistance of CAD, die design has vastly improved. Therefore, drawing processes have experienced many improvements. As a result, productivity has increased, lead times shortened, and costs have reduced. For example, determining the precise die angle, both thick-wall tubes and thin-wall tubes is achievable without wasted time and costs.

Finite Element Method (FEM):

Finite Element Method (FEM) is one of the most important methods to simulate metal forming.” (Neves, Button, Caminaga, Gentile p.426, 2005).2 FEM is being used to compute the resulting product based on temperature, pressure, material, and other production factors. Modeling and simulation technology has replaced trial and error.

One such software that uses FEM is MSC SuperForm. Used by engineers and designers alike, this software can simulate material flow, stresses, forming pressures, and trimming. With that information, engineers and tool designers can determine precise die and mandrel angle and shape, lubrication, and other cost-saving factors.

Cold Drawing Examples

Cold drawing processes’ variables to consider include the die and mandrel geometry, material properties, and the resulting reduction achieved. Below are some of the most common DOM tubing manufacturing methods. Tube Sinking, Rod Drawing (DOM), Fixed Plug Drawing, Floating Plug Drawing, and Tethered Plug Drawing are described below. The latter being a mix between Floating and Fixed. Generally speaking, Tube Drawing is sizing a tube blank by narrowing (or shrinking) a larger tube (or hollow) by pulling or pushing (drawing) it through a smaller diameter die. The mandrel (or plug) maintains the tubes’ shape (and inside diameter) while the die reduces its overall diameter.

Sinking Drawing:

This drawing process provides no internal ID support. In other words, there is no mandrel present in this process. Additionally, sinking is typically comes after a rod drawing. Both plug drawing and sinking work to reduce the tube OD (Outside Diameter) while minimizing the ID (Inside Diameter) measurements.

In some cases, Tube Sinking is the first die reduction in a series of reductions which may include Fixed Plug drawing as the final one.

Sinking Drawing Advantages:

By far the biggest advantage of Sinking is cost. Applications include low-cost outdoor furniture, tent support, temporary shelters, and other structures that do not require exceptional strength or surface quality.