Considerations for Tube Bending
A knowledge of the various tube bending methods, including capabilities, limitations, and economics will help you determine the most economical process for a quality product at the best value. Each process has its own strengths and economics that must be considered. The different tube bending processes are rotary draw bending, crush bending, and roll bending. Mandrel bending is a special application of rotary draw bending when bends have a small radius or maximum tube roundness must be maintained in the bend.
Defining the Part
The initial step in determining a tube bending process is to define the part. This can be done in several ways but using a consistent drawing standard will benefit your suppliers and insure you receive consistent quality.
Tubes are defined by Tube OD and Tube Wall thickness while pipe is defined by ID and a scheduled gauge of wall thickness. Tube material (steel, stainless steel, aluminum, other) must be defined as well as the preferred manufacturing process. Tube can be rolled up from flat strip and welded or extruded. Close tolerance diameters or tube with a very heavy wall for the required diameter may be Drawn Over a Mandrel (DOM). DOM tube is made from strip, then welded and finally drawn over a mandrel and often through a die to smooth out the weld seam and precisely size the OD and ID. Often DOM Tube is pressure tested and certified for high pressure service. Each process has its own cost points and attributes. Some tube can be flared out to a much larger diameter, expanded or flattened without cracking depending on the tube specification such as SAE A513.
Tube bends are specified using the centerline radius (CLR). This is the bend radius measured at the centerline of the tube half way between the inside radius and the outside radius. The preferred method to dimension the part is to show the straight lengths tangent of one end or bend to tangent of the bend follow by the CLR for the bend and the bend angle. For a tube bend that is not parallel to one of the planes of the drawing special views must be added to show the bend and the angle of the view will need to be defined.
Another option is to set up a tube bend chart which actually defines the way a CNC tube bender will process a part. Starting at one end show the length to the tangent of the first bend then bend CLR and angle then the rotation of the tube needed for the next bend. This follows for all required bends ending with a final straight length.
There are several considerations that when appropriate to the tubes application can help reduce the manufacturing cost:
- Tube bend CLR ideally will be 2- 4 times the tube diameter.
- Using a common CLR for all the bends saves on tooling and allows production on a single radius bender.
- Tube wall should ideally be no thinner than 3% of tube OD and no heavier than 10%.
- For ease of bending there should be a straight section between each bend of at least 2X the tube OD.
Obviously design requirements often stray outside these parameters and are still manufacturable. Tooling and bend difficulty are just increased.
No tube bending method is better than another but each is different and each is optimum in the right circumstances. Some bent tubes are economically producible in more than one process.
Rotary Draw and Mandrel Bending
Rotary draw bending is the most common method the most universal and is used for bends as tight as 1X the tube OD, depending on wall thickness to tube OD ratio. To make a bend, the rotary bend die turns with the tube clamped to the front of the die and a pressure die presses the tube into the rotating bend roll controlling the tube bend radius producing the bend. Design of the bend roll, pressure die and precision shape of the groove aid to keep the tube round.
In mandrel bending a support mandrel is inside the tube and held near or even past the bend tangent as the tube is pulled off the mandrel by the rotary action of the bend die. The mandrel supports the tube and keeps it from flattening during bending. Mandrels can be simple or complex depending on the application and the perfection of the bend desired for flow or aesthetics. The process is slower than bending without a mandrel.
Compression bending is a low cost, efficient bending process that forces the bend ram die (bend roll)between two pressure die “wings”. It requires large bend radii to prevent flattening of the tube. As an alternative the bend die is designed with material take up that folds the material inside the bend. Most tubular yard furniture and items like grill legs are produced with this tube bending process. This take up can be seen inside the bends of these items. If tolerances are not tight and flattening or controlled deformation is allowed this can be is a good process to consider.
Roll bending is used for large-radius bending. The tube is formed by three rollers configured in a pyramid. The top roller forces the tube to deflect between the other pair of rolls to form the bend or arc. The ideal CLR is at least 8 times the tube OD but may be as large as desired.
This process is for large-radius bends and sweeps. It also allows for coiling while the other processes are limited to about 180 degree bends.
At Ever-Roll Specialties, we want to be your partner for all your tube bending production needs. Contact us today to learn more about what we can do for your company.
bent on perfection.
bent on perfection.