Rotary Draw or Crush Bending are typical production bending approaches. They are cost effective but often allow the tube to flatten, deform or even wrinkle in the bend which decreases the effective inside diameter and causes restriction and aesthetic issues.
The alternative is mandrel bending of tubes. Both Rotary Draw and Mandrel bending processes use heavy concave dies outside to hold and form the pipe as it is bent. They both use hydraulic pressure to bend the pipe around the bend die. But, mandrel bending adds an additional step, a mandrel that supports the tube from the inside as it is formed and works to prevent or drastically reduce flattening, deformation and wrinkling in the bend. This mandrel is mounted on a long shaft that keeps the working area positioned at the apex of the forming process.
Segmented Mandrels typically have one to four or even five “donuts” that are ever-so-slightly smaller than the inside diameter of the tube and are held to the tool by a ball and socket or a cable system.
Plug mandrels or Spoon mandrels are also used when appropriate. These are a similar to the segmented mandrel description but without the “donuts”. They can support the tube adequately in many applications.
Mandrel Bending done the Ever-Roll Way
Ever-Roll utilizes some unique materials for mandrels that can often produce mandrel bends at a value. This is the balance between quality and cost that yield the best tube shape yet supports high production rates.
A burner tube is produced efficiently with mandrel bending. Wrinkles in the bends would cause hot spots. The tube usage is a couple of thousand a year and is an oval tube. Tube is purchased round and then flattened in a press to make it oval. A Spoon mandrel is used to support the oval tube in the bending operation to produce a smooth bend.
Chair legs for a pew chair are produced with mandrel bending. The square tube formed in a lazy “U” is ubiquitous in stackable seating. Mandrel bending allowed this customer to use a thinner wall tube that previously allowed. The mandrel bending maintained tube integrity and strength.
A grab handle for a high end exercise equipment company is made from 1 ¼” heavy wall tube. The bends are tight and the handle is one of the “jewels” of the exercise machine. Mandrel bending while not strictly needed allows for a near perfect bend that produces a part that is aesthetically pleasing.
A coolant tube for an agricultural diesel engine needed to be bent on a very tight bend coming out of the heads and high coolant flow needed to be maintained. A mandrel bend was used to produce this bend and then bending was completed using standard rotary draw equipment.
As the tube wall vs tube OD ratio grows larger it is more likely that a mandrel bend would be desired. As the bend radius vs tube OD gets tighter and bend angle increases it is more likely that a mandrel bend would be desired.
For tubes under 2” OD (50 mm) with a wall of 0.049” (1.25 mm) or more, bends equal to 1 ½ X tube diameter can most often be made without mandrel bending but a mandrel might be used if perfection is desired and cost is acceptable.