Q. What is the difference between mandrel bending and rotary draw bending?

Standard rotary draw tube/pipe bending consists of 2 to 3 piece tooling utilizing a rotating bend die, clamp die, and follower die. Modern rotary draw bending is normally done with 2-piece tooling with a rotating bend die that incorporates an interlocking clamp arm and an elliptical follower die. The tooling is designed to support the top and bottom of the material to promote a more uniform bend and less tube wall distortion. This is done by elliptical machine tube grooves in the bend die as well as the follower die.

Rotary Draw Bending Applications:
Thick Wall Tube and Pipe
Square and Rectangular Materials (being bent on large radii)
Parts that do not require consistent shapes in the bend area

Mandrel bending consists of 4 to 5 piece tooling configurations. Tube, pipe, and extrusions are commonly mandrel bent in order to bend on tighter radii without material distortion. Mandrel bending tooling consists of a rotating bend die, a clamp die (to hold material in the bend die), a follower die (or pressure die, to push the material in and through the bend die when it's rotating). The next key tooling component that separates mandrel bending from standard rotary draw bending is the internal mandrel. Mandrels come in all different configurations depending on the material being bent and the radius trying to be achieved. The first type of mandrel is a common plug mandrel, which supports the material at the bend tangent. The second type of mandrel is a formed plug which would support the material at the tangent, as well as slightly past tangent. The third type of mandrel is a standard pitch which has from 1-5 links to support the material at the tangent and as far as 90° past the bend tangent. The fourth type is a close pitch mandrel, it can have more links than a standard pitch mandrel due to the distance between the links. This type of mandrel is used for thinner wall materials, to support the tube with more contact points. The fifth type is an ultra-close pitch mandrel, that is used for very thin wall material that is being bent on 1 times the diameter (1xD) or sub 1xD radii. These have the most links out of any of the mandrels, and sometimes will support the material throughout the entire bend up to 180°.

The fifth tooling component in mandrel bending is the wiper die. It's designed to support the tube behind the tangent point and on the backside of the tube. The purpose of the wiper die is to apply adequate resistance to the material as it flows into the bend die. The wiper die will keep the material on the inside of the bend from building up and forming wrinkles. Wiper dies are necessary with most materials when bending tighter than 2 to 2-1/2 times the diameter of the tube.

Mandrel Bending Applications:
Thick Wall Tube and Pipe (bent on tighter radii, 3xD or less)
Thin Wall Tube and Pipe
Square, Rectangular and Extrusions (bent on tighter radii)

Conclusion
Standard rotary draw bending is fairly quick, inexpensive, and effective when bending parts that do not require tight radii, or tube OD consistency in the bend area. Rotary draw bending is very effective for structural chassis parts such as roll bars, roll cages, suspension parts, and other harder materials commonly used for race car chassis fabrication. Thick wall pipe is also commonly bent with ease for hand railing, fencing, and many other basic applications. Mandrel bending requires more involved and extensive tooling setup, and specialized machinery to perform tight radius bending. Because of the additional cost of the machinery and tooling, mandrel bending is in a completely different category from standard rotary draw bending. Most companies who have mandrel bending equipment are more inclined to manufacture a great deal of different parts to keep the machine running to fulfill the capital requirement to purchase and maintain the equipment. Some examples of materials that would need to be mandrel bent are thin wall stainless, mild steel, aluminum, brass, tubing and pipe that require bending less than 3x the diameter. In the automotive industry, exhaust and induction components always require mandrel bending for efficiency.

Q. What is the difference between tube and pipe?

Tubing is measured on the outside diameter (OD), while pipe is measured on the inside diameter (ID).

Q. Should I be using chromoly or mild steel for a rollcage?

4130 chromoly tubing is a superior material for any rollcage or chassis structure. One of the greatest advantages is its strength to weight ratio. You can achieve an equal or in most cases stronger assembly when using an alloy tubing. The weight of an average rollcage can be reduced by almost 40 percent when using appropriately sized 4130 materials. The other great benefit to using alloy tubing is its ability to flex without breaking or bending immediately. The spring-back of 4130 tubing is almost double that of ordinary mild steel tubing, leaving the chassis to flex without breaking. Other than the additional cost, welding chromoly tubing is the only other disadvantage, as it has to be tig welded. Even so, with todays technology of filler materials, as well as square wave welding technology (pulse welding), it is much easier to learn to tig weld. There are also better blends of filler material to create a more desirable weld. All the options weighed, chromoly is the best way to go.

Q. What are some key tips to welding chromoly tubing or plate?

The first and most important factor is to use an appropriate tig (tungsten inert gas) welding machine. The next tip would be to clean the material as best as possible outside the weld area, as well as inside the weld area. Clean material welds better; we generally use cold-drawn material as it is cleaner, and welds better than hot-drawn material. Expansion holes must also be drilled to keep the tube from pressurizing, and blowing back out distorting the weld area at final closing of the weld. These must be drilled in a tube that is going to be closed off. If the tube is not to be closed off (welded shut or mounted at either end) it is also good practice to purge the tubing with argon from the inside. Tiny welds are not always good welds; it is imperitive that an appropriate size filler material be used when welding two sections together. Fusing two thin components together is not recommended: as material is drawn into the weld area, the component will thin out around the weld losing strength. A slightly raised weld seam is much better than a flush weld. A weld should never be concave.

Q. Do the bends in my rollcage tubing make it weaker?

Not necessarily, just because there are bent sections in a chassis structure does not make them weaker- given that the structural design is up to par. When material is bent it actually increases the tensile strength of the material in the bend area substantially, as long as the material has been cold formed. Cold forming is the process of bending or shaping material without heat. Cold forming is one of the only ways to strengthen some types of material. The tensile strength of some material can increase more than 50 percent when cold formed. Cold forming introduces a certain amount of work hardening. Work hardening occurs when material is being stressed but not strained. When applied correctly, work hardening can significantly add to the tensile strength of the material.

Q. So many companies claim to mandrel bend their rollbars and rollcages- whats best?

The phrase 'mandrel bending' is thrown around quite a bit in the aftermarket automotive world. Most rollcage materials like DOM mild steel and chromoly tubing are not suitable for mandrel bending. Even CREW tubing, which is fairly elastic, is not recommended to be mandrel bent for chassis components. When the material is formed with an internal mandrel, it takes on an enormous amount of stretching and compression. The inside wall has to compress and the outside wall has to stretch. When mandrel bending materials that are already hardened or that do not have enough elasticity, such as DOM and 4130, the work hardening that occurs in the bend area will make the component subject to failure from being pushed past its useable limits. When bending round materials for chassis and roll cage structures, the empty bending methods are much preferred over mandrel bending.

Empty bending is bending without a mandrel and with specially machined tooling that incorporates a multi-radius bending die to form the material instead of compressing and stretching it as much. Even the most sophisticated mandrel bending machines with pressure die assist and collet boosting will not have as positive of an effect on the formed area as empty bending. The best way to bend harder materials used for chassis and race car structures is to empty bend them with correctly machined bending and follower dies to form the material with the least amount of wall thinning. Generous radii should also be chosen to bend these materials to limit the amount of stress and work hardening.