How to Form Tubular Coils in Production
The Coil Market
It may not be obvious but most folks pass within a few feet of a circular coil every day. Circular tubular coils can be found in beverage dispensing machines, oil & gas processing equipment, swimming pool heat pumps, laboratory equipment, and transportation equipment. The list goes on and the applications are endless. However, coils formed in production are most often fabricated using just one of a few common roll bending techniques.
To form a coil successfully, considerations should be given to matching a specific coil configuration including the end bend conditions to that of the type of machinery available that is used to produce the various coil configurations.
Three Ring Roll Benders
The coil depicted in Figure 1 above is a single level design with straight ends. By far this is the simplest coil to form mainly because the ends are straight.
This coil can be formed on several types of roll benders including the three ring roll bender shown in Figure 2 (above). The roll bender shown in Figure 2 is excellent for low volume production. A high volume options to form the coil in Figure 1 is a three ring roll bender with an integrated bulk material feed system shown in Figure 3 (below).
With the right post bend material support tooling, precision coils can be produced efficiently with a mean coil tolerance of .030” on both types of roll bender.
Drum (Mandrel) Type Roll Benders
From Figure 4 (above), a single level coil is shown with several 90 degree bends formed on the ends of the coil.
This type of coil is common in industrial applications because both ends of the tube emerge from the same end of the coil making it convenient to connect to a larger system. Formed from one piece, the circular coils themselves are typically formed on a CNC drum type roll bender, Figure 5 (below).
One advantage of forming a coil as shown in Figure 4 from one piece of tubing is that the overall coil warranty costs tend to be lower.
If the leg that protrudes thru the inside of the coil shown in Figure 4 (below) were instead on the outside of the coil, then a standard three ring roller as shown in Figures 1 or 2 could be used. Therefore, one application for a drum type roll bender is when one of the coil’s legs protrudes thru the inside of the coil.
The 3 axis CNC drum type roll bender shown in Figure 5 above can also be used for form double level coils. Double level wound coils provide for additional heat transfer making them an excellent choice for OEM equipment with space considerations.
In general, roll benders used to form coils can be found in NC and CNC versions. However, a CNC roll bender has the distinct advantage of producing consistent coils time and time again in a shorter period of time. An NC type roll bender has a tendency to produce more scrap v. a CNC roll bender. This is sometimes related to the focus required by an operator.
Making The 90 Degree Bends
The 90 degree end bends as seen on the coil in Figure 4 above are not produced on a roll bender. Additional means such as hand benders or rotary draw benders can be used to install these bends before and/or after the coil has been formed. Hand benders work well for low volume. As the volume increases, so to does the risk for scrap and worker comp. claims.
George Winton, P.E. designs and builds CNC tube fabrication equipment for Winton Machine in Suwanee, GA. He can be reached at gwinton@wintonmachine.com or 888.321.1499.
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