Cylindrical die thread rolling is quick and economical

Cylindrical die thread rolling is a cool method for taking the heat off manufacturing thin, deep helical fins on long lengths of thin-walled tubing. The process is suitable for making a variety of everyday products, achieving fast, economical production when metalcutting machining just doesn’t cut it. One such product is finned tubing for heat exchangers.

Improving the energy transfer efficiency of cooling and heating fluids has been a challenge since the Industrial Revolution. Any device performing work requires a transfer of energy to make the work happen. A portion of the energy goes into the work, and another portion comes out as heat due to efficiency losses in the system. Methods that improve the energy transfer efficiency of the original device or that capture the heat loss for reuse are in constant demand. It all comes down to heat transfer.

Heat transfer can be accomplished by a heat exchanger, which transfers energy from one fluid to another liquid or a gas. One of the simplest methods of heat exchange without mixing fluids is the use of a tube through which one fluid is passed at one temperature while another fluid is passed around the outside at another temperature. Heat is transferred from the hotter fluid to the cooler one through the tube wall. To increase heat exchange efficiency, such a tube is manufactured with fins that protrude radially outward from the tube surface, thus creating a fin tube. Finned tubing can be found at the core of most heat exchanger devices.

A fin tube cross section with internal profiling. Image courtesy of Kinefac

Why a Fin Tube?
The simple answer is that the efficiency or rate of heat transfer is affected significantly by the surface area of exposure. Fluids have a heat transfer coefficient, which is the amount of heat per unit area they can transfer. By increasing the usable area, multiplying the heat transfer coefficient by that increased area results in more capacity for heat transfer. The outside surface area of a plain tube easily can be increased five to 20 times by adding fins to the outside surface.

A fin tube also can have a finlike profile on the inside surface to increase the surface area and promote directional fluid flow or mixing. Turbulence induces mixing of each fluid, which can improve heat transfer for individual fluids. Turbulence can be controlled by the geometry and arrangement of the fins on the inside or outside of the tube. A helical fin path is most common on either or both surfaces.

Before talking about manufacturing finned tubing, let’s look at the characteristics of a typical fin tube product. Fin tubes come in all shapes, sizes and materials with varied fin heights and thicknesses. The tubes generally are made of welded tubing in stainless and low-carbon steels, copper, brass, aluminum and exotic alloys. One important characteristic to consider for manufacturing is length. Some fin tubes are manufactured in continuous lengths of 100′ or more, which then are bent into compact shapes to fit inside various heat exchangers. Other relevant traits are tube diameter and wall thickness, fin height, the number of fins per inch and tube material grade and construction.

In simple terms, a fin tube can be thought of as a hollow, threaded rod. However, the simplicity starts to diminish when there are 60 or more thin threads per inch with an additional threaded profile on the ID over a 100′ length. Two processes traditionally are employed to produce external threads on the outside of a workpiece: thread cutting and thread rolling. Both methods have strengths and weaknesses. Thread rolling is the process to beat, though, when it comes to manufacturing fin tubes at high rates of speed. There are also benefits with material savings, strength, surface quality and the ability to generate internal profiles without wasting material by generating chips.

A cross section of rolled thread grain flow. Image courtesy of Kinefac

When cutting an external thread on a lathe, a single-point cutting tool plunges radially into one side of the workpiece and traverses along its exposed length. Multiple passes at successively deeper depths usually are required to achieve the full thread form. It can take minutes to accomplish this task on a length less than 10″ or 12″. Single-point thread cutting creates a force imbalance on one side of the workpiece due to the forces generated by the cutting tool.

Deflection of the workpiece is always a consideration. The workpiece geometry and cutting forces dictate the limits of exposed length that can be threaded, above which deflection and stability problems will occur. Once the limit is exceeded, rolling is the best option.

Rolling Basics
When rolling an external thread of discrete length, two or three rotating cylindrical dies plunge radially into the workpiece to some depth while rotating it to replicate their geometric surface features into the periphery of the workpiece surface. This is referred to as infeed rolling. For longer continuous lengths of a threaded product, the through-feed rolling process is used by skewing the axes of the rolling dies at an angle to generate through-feeding action of the workpiece.

In either case, the blank material is forced to conform to the die geometry without removing or gaining material. Therefore, thread rolling is a constant-volume process. The starting pre-roll blank is smaller in diameter than the final thread major diameter—usually somewhere near the thread pitch diameter—because material from the blank is displaced radially outward by the dies to fill the form. Material also can and will flow in two other principal directions, axial and circumferential, depending on several factors, such as the degree of form fullness or the ratio of thread depth to root diameter.

A typical machine screw thread can be rolled into a bolt blank in a few seconds. The feeding of the bolt blank into and out of the rolling machine usually takes longer than the rolling process itself. Rolling generates an uninterrupted grain flow around the rolled root, which helps enhance strength to a rolled thread over a cut thread. Other characteristics, such as material workhardening and a highly burnished surface from the rolling action, also help increase the strength and quality of a rolled thread.