Troubleshooting compression bending

The Tube & Pipe Journal December 2003
January 13, 2004
By: Marsha Blasengame

You can achieve nearly trouble-free bending by being aware of the causes of typical compression bending problems and by correctly operating and maintaining the compression bender. Most compression bending problems are one of three types:

  1. Flattening or collapsing on the outside of the bend.

  2. Crimping or wrinkling on the inside of the bend.

  3. Dimpling on the inside of bend.

Generally you can prevent these problems by implementing these standard solutions:

  1. Make sure to use good quality tubing in the material thicknesses recommended.

  2. Use the correct backpressure settings.

  3. Inspect tooling and replace any worn bushings.

  4. Lightly lubricate the dies prior to bending.

Thin Material Creates Flattening or Collapsing

If the tube material is too thin relative to its diameter, the tube will collapse as the outside of the tube wall elongates and thins during bending. The tighter the bend radius, the smaller the degree of bend is possible, and the more the material is required to elongate.

The use of good quality material is the first step in preventing collapse. In addition, by adhering to standard material thickness specifications, which often are embossed on the tubing, you can prevent pipe flattening or collapsing.

Compression bending is suitable for bending exhaust system components. A wall thickness of 15 gauge is sufficient when bending 2-inch diameter or smaller pipe, and fresh cold-rolled pipe may work for a short period of time.

As a rule, when bending 2 1/4-to 2 1/2-in.-dia. pipe, use good quality 14-ga. aluminum-killed (AK) pipe. How-ever, to achieve good bends on 2 1/4-in.-dia., 3-in.-radius dies and 2 1/2-in.-dia., 4-in.-radius dies, a minimum 14-ga. wall thickness is required. Bending 3-in.-dia. pipe requires the minimum wall thickness to be 14 ga. also.

Material, Backpressure Problems Cause Dimpling

The main cause of dimpling is poor-quality material, material that is too thin, or backpressure that is too low. To avoid this problem, use the recommended material gauge thickness and adjust the backpressure.

To adjust the backpressure, place the radius die and shoes on the bender. Engage the bender to ram out. While the swing arms are opening, the backpressure should read 700 pounds per square inch (PSI). If you need to adjust the backpressure, turn the knob while the swing arms (pivot arms) are moving as required until the desired bend quality is achieved.

Photo courtesy of Meineke Car Care Centers Inc., Charlotte, N.C.

Never increase the backpressure to more than 700 PSI unless the factory specifications indicate otherwise. Afterward, reset the PSI to the factory-recommended setting before starting the next bend on the pipe or before bending other diameter pipes.

Worn Bushings, Misalignment Cause Crimping and Wrinkling

The most likely causes of crimping and wrinkling are worn bushings or die misalignment.

When the bronze brushing in the die carriage is worn, the radius die becomes misaligned with the back shoes. Check for this by placing a screwdriver under the die carriage to see if there is up and down motion in the die carriage. If you can move the die carriage up and down more than 1/16 in., the bushings need to be replaced. Replacing worn bushings can prevent die misalignment.

Die misalignment of the back shoes with the radius dies causes the pipe to crimp or wrinkle on the inside of the bend. The wrinkling can occur anywhere on the pipe. Aligning the radius die will solve this problem.

To align the radius die, you must determine which side of the radius die must be adjusted and whether it must be adjusted up or down. If the wrinkling occurs in the upper left quadrant, move the left side of radius die up. If wrinkling occurs in the lower left quadrant, move left side of radius die down. Wrinkling in upper right quadrant indicates that the right side of the radius die needs to be moved up, and wrinkling in lower right quadrant means that the right side of the radius die should be moved down.

If the pipe is fresh AK 14 ga., you can usually make a bend up to 90 degrees without reducing the backpressure or leaving the gauge setting at factory-recommended PSI. However, AK pipe hardens after it has been in stock for a long time. In addition, a high content of carbon in the pipe also affects the degree of hardening. This will stall the machine if you attempt to bend a full 90 degrees at the factory-recommended PSI backpressure setting.

To achieve a smooth, wrinkle-free bend in 3-in.-dia. pipe, ensure the wall thickness is at least 14-ga. AK. Severe crinkling can result if a standard cold-rolled pipe or a thinner 16-ga. AK pipe is used.

Most compression benders have depth-of-bend scale on the right side of the machine below the pivot arm. Some bender models are equipped with a microswitch, which stops the machine automatically once the desired maximum bend is achieved.

If the pipe is not pre-lubricated, the dies should be lightly lubricated before bending. Always lubricate the radius dies, especially around the rails or extreme outside of the die where it contacts the back shoes.

Marsha Blasengame is director of sales and marketing, Ben Pearson Tubemaster, P.O. Box 5668, Pine Bluff, AR 71611, 870-534-6411, fax 870-534-3177,

Marsha Blasengame

Contributing Writer

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The Tube & Pipe Journal

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The Tube & Pipe Journal became the first magazine dedicated to serving the metal tube and pipe industry in 1990. Today, it remains the only North American publication devoted to this industry and it has become the most trusted source of information for tube and pipe professionals.

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