- Bending – The Basics
- The Bending Variables
- Longitudinal Weld
- Tube Bending Process
- Ram-type Bending
- Roll Bending
- Compression Bending
- Rotary Draw Bending
- Good Tooling Techniques
- A Quality Machine
- The Math Of Bending
- Bending the Tubes
- A Guide To Flat Plane Bending
- Offset Bending Guide
- Bending Within Boundaries
When it comes to bending roll bar tubing, many people consider it to be an art form. However, for keen enthusiasts, it’s possible to not only learn the skill at home but also to master it themselves as long as they’re prepared to dedicate the energy and time to it.
Why would you need to know about bending roll bar tubing? Well, if you’ve ever wanted to build tube bumpers or a roll cage for yourself then you’ve probably wanted to be initiated into this dark art. Here, we’ll look at the different equipment and tools that you’ll need, the best ways of using them and understanding the right bends for your particular project.
Bending – The Basics
When you’re ready to bend roll bar tubing, you first need to know the properties of the pipe or tube that you’re going to be working with. Pipe that is used to transport air or fluid will normally be specified by its pipe size, however when it comes to using a bending machine, you’ll need to know the pipe’s key variables including its wall thickness, centerline radius and outside diameter. One thing that you’ll need to bear in mind is that the wall thickness between pipes can vary, and this variation must therefore be accounted for, particularly when using a bending process that requires tight-fitting, precise tooling on a small bend radius.
The Bending Variables
The bending variables you need to know include:
- The inside bend radius
- The outside bend radius
- The centerline or neutral line
- The distance between bends
- The bend angle
One thing to be aware of when bending roll bar tubing is that tubes will experience springback once they’ve been bent, and this produces bends which goes through radial growth. The smaller the centerline radius of the bend and the harder the tube, the more springback and radial growth there will be. It’s also worth noting that steel undergoes more radial growth than copper, but stainless steel goes through the most of all.
While some tubes are seamless, the majority are made using a longitudinal weld. The consistency, size and quality of the weld seam is very important in tube bending as if the joint’s two edges don’t perfectly align or if the bead is inconsistent or too large, the tube will not be perfectly round and this is a problem when creating a perfect bend.
Elongation also occurs when you bend roll bar tubing, with stretching of the outside radius. The bend’s outer surface will cave in, resulting in the cross section being distorted from its round shape. Although ovality works in some applications, if you’re doing precision work it is unacceptable.
Tube Bending Process
There are four types of tube bending process:
- Ram type bending
- Compression bending
- Roll bending
- Rotary draw bending
Ram style benders can be found in most muffler shops and this is a simple and old-fashioned method of tube bending. Using a ram which is hydraulically driven, a tube is forced against pivot blocks or rollers. This method allows you to achieve a CLR (centerline radius) 3-4 times the outside diameter of the workpiece.
In this method, the inner diameter of the workpiece isn’t supported with a lot of stretching on the outer side of the tube’s bend. Ram type bending is ideal for square tube applications and is the most affordable way of bending roll bar tubing but since it isn’t easy to control it isn’t suitable of cosmetics matter.
The method of choice for large construction workpieces, this method involves 3 rolls which are in a pyramid construction, either horizontally or vertically. These rolls then move producing specific radii. Alternatively, a pinch style 2 roll bender can be used for this method. Roll bending can be used for producing spirals.
This method uses a compression die or roller to bend your workpiece round the stationary bend die to compress the tube. Suitable for symmetrical workpieces, this method works best for those tubes which will be bent to a centerline radius which is a minimum of 3 times the outer diameter of the tube.
It isn’t suitable for workpieces that have a CLR which is under 3 times the diameter of the tube. Perfect for commercial and household products, the most common use for compression bending is to make items like towel rails.
Rotary Draw Bending
Rotary draw bending is perfect for precision tasks and for applications which involve tight radii since it allows for maximum control.
Good Tooling Techniques
If you want to get the perfect bend, you’ll require an excellent tooling set up, especially when using the rotary draw method. The hardness of the mandrel is vital – if you’re bending a hard tube you need a softer mandrel and vice versa for the best result. Tooling also needs to take the radial growth into account. Excessive radial growth will result in a noticeably different radius at the start of the bend.
A smaller radius bend die will often be required. Good welds are also important, especially when rotary draw bending. Inconsistency in the bead affects the wiper die, pressure die and mandrel. The position of the wiper die is also critical – angling it slightly is important so its end can make contact with the tube before the tangent point of the inside radius. The wiper due must also be in good condition and sharp.
When it comes to the clamping die, you need to note that it should be 3 times longer than the diameter of the tube. This ensures that the pressure is spread properly over a larger area. If you’re working on a piece that has a short distance between its bends, this can be challenging however if you use special tooling you can deal with this easily. A multi-stacked arrangement can resolve the problem. Some tube types also require a set of flexible balls to the end of the mandrel and this must be position properly when the machine is set up.
You will need to ensure proper lubrication for the mandrel to avoid friction. A non-petroleum based synthetic lubricant will often be ideal in either gel or paste format, however if you’re carrying out a heavy duty project with tight radii and thick walls, a more concentrated form of lubrication is necessary.
A Quality Machine
When it comes especially to rotary draw bending, an all-electric machine is the best choice since it allows for better control. Many new machines have space to allow for tooling in order to combine several bending processes. Many now offer both roll bending and rotary draw methods in a single unit for extra convenience. There are several factors when it comes to machinery requirements for bending, and these include:
- Wall thickness
- Material grade
- Required CLR
The Math Of Bending
You need to use your math skills. You’ll need to mark the bends properly based on the amount of material within the bend radius. For example, if you’re measuring across the roof from one side to the other, you’ll then have to account for the amount of distance which the bends occupy at either end. You’ll therefore have to work out the amount of area occupied by the tube when it bends around. Here are two formulas you can use to figure out the right measurements:
- Pi x (2x the radius centerline) = the bend’s radius
- The radius of the bend ÷ 4 = the appropriate stretch length of the tube to add to the measurement
- When working out a 90 degree bend you’ll need to determine the bend degree ÷ 90 to find the bend percentage. If the bend is under 90 degrees, you’ll need to work out the tube stretch length x bend percentage
Bending the Tubes
There are 3 bending styles which any DIY enthusiast should be aware of.
- Flat Plane bending (2D)
- Offset bending (3D)
- Bending within boundaries
The offset of a bend refers to the direction which the bend angle takes where the tube bends from its flat plane.
A Guide To Flat Plane Bending
If you are working on a bend design which is intended to remain flat against a surface rather than shifting or bending to another plane, you need to carry out a Flat Plane bend. Bash bars, tube bumpers and main hoops generally use Flat Plane bent tubes.
Offset Bending Guide
If you lay out an offset bent tube on a flat surface, the bend would move either away from or towards your point of view. The bend which turns that direction is called an offset bend. This bending style is commonly found on roll cage front hoops, tube chassis, bash bars and tube bumpers’ secondary sections and strut tower braces. Measuring vertical tubes and then loading them into horizontal bending machines can be confusing.
You need to be aware of the point of view that you are working from. Be aware the offset can change from right to left and from down to up depending on the bending machine you’re using. Pay close attention to the bend’s direction and the way that the tube is drawing around the die once it has been loaded into the bender.
Bending Within Boundaries
This gets tricky, especially if you need the end of the bend to fall in a particular zone or boundary. An easy method for doing this is drawing the piece onto a flat surface like the floor or a table. Cheaters can be fabricated using scrap tubes which will mock up the specific angles and how they fit on a drawing.
This enables you to correctly measure the proper bend starting point to ensure you’ll land right on the mark once you’ve carried out mating or notching. This style of bending is commonly seen in roll cage designs with a recessed harness bar that allows for greater clearance for a driver’s seat below the main hoop. This bending style can also be seen in main hoops which fit inside the chassis.
Follow this guide and you should find that you have a better insight into this complex but highly useful process. Although bending roll bar tubing isn’t going to be easy, it is certainly rewarding when your practice pays off and you end up with a perfect and accurate bend!