The topic of tube bending always seems to draw the most attention when it comes to chopper frame fabrication. Other than some relatively expensive bending equipment it is not really all that hard to do but that geometry and trig class you slept through has come back to haunt you.
Keep a few things in mind: Tube is not the same as pipe. Different dies are required to properly bend tube or pipe. Pipe is not a suitable choice of material for a vehicle frame. A good low cost bender for tubing will cost in the area of $300 to $700 (+) with the additional cost of $150 to $275 (+) for die sets. With accessories, it is possible to have $600 to $1200 tied up in a bender to do just one size of tubing. Avoid the cheap pipe “bow” type benders where the material is held between two rollers and a die presses in the middle forming the bend. These type benders do not compress the tube to hold the sidewall shape of the tubing.
Let’s bend up a piece of tube and see what is involved.
First step is to mark the beginning of your bend. With most bender dies, the bend starts somewhere inside the die. In the picture is a silver line showing where the actual bend will start and the red line that is lined up with the outside edge of the bending die. The little chunk of pipe around the tube has been squared up and makes a handy tool for drawing a complete line all the way around the tube that is being bent. Next step is to load the tube in the bending die. The second picture shows the tube in the bender. This particular bender is a follow bar type. The 2-1/2” square aluminum block below the tube is the follow bar. The tube is inside the half round groove in the follow bar. The red line drawn in the first picture is visible just in front of the aluminum follow bar. The radius die is just above the tube. The silver line drawn in the first picture is inside the die and follow bar (remember the bend will start 1/8” inside the radius die. The next picture will make a little more sense with what is going on.
There are a lot of different bender styles that are suitable for the small shop. Some are hydraulic and some are mechanical. Most benders bend tube in a horizontal plane. The picture shows a hydraulic bender that bends the tube end upwards. A magnetic base dial angle finder shows the angle of the tube as the radius rotates and bends the tube. The silver line is now visible just below the red line, which is still lined up the leading edge of the die. This tube was being bent to 90 degrees. Since tube has a bit of memory or “spring back”, the tube actually needs to be bent further than 90 degrees. The amount of over-bend required varies for each tube type, diameter, wall thickness and die radius. In this case, the tube required almost 4 degrees more to spring back to 90 degrees.
Below is a picture of the complete 90-degree bend. You may be able to tell where the bend actually started visually but it is usually easier to feel. In the case of this 1-1/4” x .125 wall DOM tube, the inner surface of the tube was compressed .050”. Unless some kind of internal die is used to hold the inside diameter, the typical low cost bender will cause this tube wall distortion. An expensive mandrel bender has the internal die (or mandrel) to support the tube during the bending process).
This 90-degree tube now becomes an important tool for future bends. There are a number of ways to put it to use. One was is to determine tube length. The top rectangle (green…but hard to tell in the picture) on the paper is the start of a full scale bend. The red lower lines are a planned 15-degree bend. The “tool” is laid on the drawing and the bend line is lined up with the bend line on the drawing. A line can now be drawn following the actual bend of the tube as in the second picture.
With the tube moved, the radius lines are now visible. Next, the tube is lined up with the straight part of the lower angle and the radius lines drawn in the last step. Now the bend line can be marked showing the end of the bend.
In this last picture, the beginning and end of the bend has been found. There are a lot of calculating that can be done and software that figures tube length. However, a short piece of flexible tubing (stripped off wire insulation in this case) makes a good tool to lay on the center line of the bend area, the beginning and end of the bend are marked and the flexible tube straightened out and measured. This is the length of material used by the bend. Add on the length of tube before the bend and after the bend and you have the full length needed to make the part. Not super accurate but is really close if you only have a few bends to do. It is always a good idea to leave a little excess tube to trim off after the bend and be aware that the tube length will increase slightly by the compression of the tube during bending. Some type benders can cause considerable increase and a few test bends should be made to learn what to expect.
If the tube is going to have more than one bend, it is often a good idea to tack weld a chunk of flat bar to the end of the tube in line with the first bend (just lay the tube flat and tack the bar flat with the same surface). Now, using a dial angle finder or protractor with a bubble level, you have a reference for rotating the tube for subsequent bends. Clamps are available for purchase from fabricating suppliers (MechWerks has these) that attach to tubing and provide a flat for attaching a dial angle finder. Once the clamp is attached, the tube can be rotated a needed amount and the next bend will be in the correct radial relationship to the first bend. Note that tubing has a lot of built in stress from the forming process of making the tube. Bending causes some of this stress to be released and may result in the radial degrees to be off slightly. It is difficult to compensate for this twist but is something the fabricator needs to be aware of. Sometimes, application of a heating torch is the only way to pull tube back into place after it twists. Sometimes if the tube can be fastened down securely, simple application of force can be used to fix twist and take out small amounts of overbend.