Madrel Bending Vs. Crush Bending
Mandrel Bending vs Crush Bending
The advent and mass production of steel made a profound impact in a very short time. Various metals of all sorts had been produced for thousands of years with relative success when applied to tools and weapons but when steel was introduced as a building material it fundamentally changed humanity for the better.
The simple act of adding carbon to iron produced steel which was lighter, stronger, and had a higher malleability point meaning that it could be made into more complex shapes with less of a chance at cracking or breaking. One of those complex shapes was the extruded round tube. Once their manufacturing process had been mastered it meant that frames of all sorts could be made extremely cheap for the amount of strength that they provided.
Round tubing makes up the backbones of many different products and structures such as chairs and tables, but all the way up to larger structures like sheds, garages, even automobile seats. The strength lies in the way that weight is distributed along the flat sections. As long as the flat section is supported, the tube will support great weights. The fail point comes in their junctions or endpoints.
Two types of bends are popular in construction, the first being the mandrel bend and the second being the crush bend. Each offers both advantages and disadvantages. Depending on the applications one can be more advantageous over the other. The specialty metal supplier Ever-Roll has a great definition of the two types of bending techniques.
Crush bending involves applying pressure to the surface of the tube to force it against a bending die and essentially bending the tube section by sheer pressure. The bend itself can be done in stages where the bending dies can cause the tube to bend even further, but there is a limit to the degree of bending. Due to the tube being forced against the bending die, as well as the internal structure being unsupported, crush bending can cause deformations on the tube.
Bending veterans have ways around the deformations but for the uninitiated, this method has limits. But all is not lost, crush bending does have some very interesting advantages. First and foremost, crush bending is a very affordable way to achieve some rather complex bends to fairly large tubing. The machines themselves are easy to operate, dies are cheaper, there are all kinds of tips and tricks online about using them more effectively, and some adventurous folks have made them out of little more than steel plate, a few dies, and a hand-operated hydraulic ram.
Crush bending is what exhaust shops had used for decades to make replacement exhaust pipes. Their ease of use is also a deciding factor. A majority of crush benders are manually operated, allowing for a certain degree of control when the tubes are being bent. There are electrically controlled hydraulic crush benders on the market but a manual crush bender can be had for a few hundred dollars, allowing an operator to create fairly consistent bends without investing too much time or money.
Mandrel bending is a fairly complicated process, but professionals that shape metal for a living swear by it. Simply put, mandrel bending is the far superior way to consistently get accurate or extremely tight radius bends that will not have any deformation of any sort. It is actually a method of rotary draw bending, where the inner diameter of the tube is supported to achieve a tighter bend with no damage. Mandrel bending gets its name from the device that sits at the end of a controllable ram, with a series of up to five segmented steel donuts that form a flexible end that supports the inner walls of the tubing. This support allows for a much more complicated bend that will not be deformed, leading to a better-looking final product as well as repeatability which is key to manufacturing a quality product.
Once the operator loads the bend and tube parameters into the CNC-controlled bending machine, the action happens very quickly. The mandrel itself extends to the predetermined location while the various dies hold the tube in place. Then the tube is ready to be bent by having the mandrel start to retract as the bending dies do their work. The angle of the bend is controlled by both the electrical component of the CNC-controller, as well as the physical die.
While the action of taking a piece of metal tubing and forcing it against a curved die to achieve a bend sounds like crush bending, that is where their similarities end. With mandrel bending, the mandrel itself is supporting the inner structure of the tube to prevent any dents or deformations in the bent. The very nature of the mandrel also allows for much tighter bends to occur, and when paired with the computer control system it allows for a very accurate bend to occur over and over ensuring repeatability. Mandrel benders also allow for multiple bends to be performed on a single piece of tube, say for an exhaust kit or for the four bends that form the frame of an aftermarket car seat.
There are some downsides to using a mandrel bender though. First and most preventative is the cost. A professional-grade crush bender that is either manual or electric/hydraulic powered can cost somewhere around $2,000. A CNC-controlled mandrel bender can cost as low as $20,000 or all the way into the hundreds of thousands of dollars. The great discrepancy in price is made up in the consistency of the bends, the complexity of the machine, and the accuracy that it can provide.
The way that a piece of tubbing is bent can make a difference in its final look, its ability to hold weight, and its final cost. The differences between how mandrel bending machines versus crush bending machines achieve their goals could not be more different but that isn’t to say that one is better than the other. What level of apparatus is used really depends on what type of bend is needed