Mandrel Bends What is the big deal about a mandrel bend? A mandrel bend is a tube that has been bent on a bending machine while a “mandrel” is pulled through the tube, keeping the tube from crushing and keeping a constant cross sectional area. Many muffler shops use “crush” benders that actually crush the tube causing the cross section of the tube to ovalize and lose flow area which causes flow resistance in the tube. Here is a photograph comparing a crush bent tube and a mandrel bent tube. You can see how the crush bent tube has a reduction in cross-sectional area – sort of like sclerosis. Here is a video of a mandrel bender in operation. Most benders are hydraulically operated with some high-end benders having precision electrical servo control. (Need video). This photograph shows the various parts of the mandrel bend tooling. The main components of the tool set are the mandrel, the clamp die, the pressure die, the wiper die and the bend die. In this figure, you can see the articulating nature of the mandrel. In this animation we can see the process for bending tube. The tube is slid over the mandrel and is clamped against the wiper die and bend die by the clamp and pressure dies. As the bend die rotates, the pressure die keeps the tube against the bend die while the segmented mandrel remains inside the tube preserving the cross section. As with any machinery, the key to getting good bends is to have a properly trained operator, proper maintenance and keeping the tooling in good shape. The cross section of the bend will ovalize some in the bend area. Sloppy bending practices can result in ovalized cross (see photo) sections and even some wrinkling. It is difficult to fabricate when the bends are ovalized as can be seen in this photo. Burns Stainless will offer wrinkled bends as B-grade bends at discounted prices. How are mandrel bends classified? Mandrel bends are classified by 5 characteristics: 1. Degree of bend 2. Tubing OD 3. Centerline bend radius 4. Tubing wall thickness 5. Tubing material The Burns Stainless part number is a simple combination of these characteristics. What do you mean by degree of bend? The first two characters represent the degree of bend. We stock 15 degree, 45 degree, L-bends (90 degree) and U-bends (180 degree) in many combinations. The bend angle shown in this diagram is the number of degrees that the tube is bent. Here are samples of each of the bends. What about Tubing OD? The next three characters represent the tubing outside diameter in hundredths of an inch. For example, 100 represents 1” OD tubing, 250 would represent 2-1/2” tubing and 300 would be 3” tubing. It gets tricky for the eighths sizes with 163 representing 1-5/8” (1.625”) OD and 238 representing 2-3/8” (2.375”) OD tube What is Centerline Bend Radius? The centerline bend radius, aka CLR is a measure of how tight the bend is and is expressed in tenths of an inch. It is the radius of a circle that passes through the centerline of the tube. It is shown here in this drawing. The smaller the radius of the bend, the tighter the bend. For example, 30 would represent a 3” CLR and 45 would represent a 4-1/2” CLR. It is easy to visualize the CLR of a bend with a U-bend. It is ½ the distance between centerline of the legs. It is difficult to bend tight radius bends. The tightest bends we stock are what we call 1D bends where the bend radius is the same as the outside diameter of the tube. These are stocked in 90 degree bends in 16g 304SS in several sizes. For special order bends, we can bend to 120 or 150 degrees or sometimes more depending on tooling, tube size and other factors. It is possible to have various bend angles using the same CLR as shown here. How about Wall Thickness? Tubing wall thickness is the next two characters expressed in gauge How about Material? The final set of characters represents the material. Burns Stainless stocks mandrel bends in 3 different materials, 304SS, 321SS and 6061 aluminum. We can also special order mild steel as well as Inconel mandrel bends. I hope that you found this video informative. Please stay check back periodically as we will be regularly adding to this series. Cheers

• published: 08 Jul 2015
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