The first step for me was to set up the bender. This involved finding a place in the garage with plenty of room, drilling holes into the floor, and anchoring it. Near my workbench and drill press is an area with enough room to swing the long lever on the bender, room in front of it for the tubing (parallel to the bench), and room behind for the tubing to feed bend around. One benefit of this position was that I could rest bent tubing on the workbench, helping me make both bends for hoops along the same plane. Using my hammer drill and Red Head anchors, I anchored the bender to the floor. Unfortunately, I'll due to limited space in the garage, I'll need to remove it when not in use, which will leave the anchors protruding up. I'll have to learn to deal with it.

With the bender set up, it was time to cut and bend my first tubing. In this case, I was to bend tubing between the seat belt anchors on my Miata. Due to space limitations, it was critical to bend exactly the right width; I could leave extra height to be trimmed later. I simply added the height of each upright, as well as the width, then added a small fudge factor to determine the length I needed.

Once cut, I needed to determine where on the tube to begin the bend. Since tubing bends in a curve, where I start a 90 degree bend will determine the overall height. The trick is to figure out where to position the start of each bend; if you get that right, the bending is really easy. JD2 provide pretty accurate rules of thumb that allow you to figure out where to position the tubing. Here are the major components to take into account:

• Height of the bend -- this is the distance from the end of the tube to the top of where the bend ends. Since the tubing changes direction according to the radius of the bend, this is obviously greater than the distance from the end of the tube to the start of the bend. You need to provide this.
• Center line radius -- this is the center line radius of the bender die. On my jd2 bender, the die has it stamped on it. It factors into the equation; in a 90 degree bend, this is the distance that is added on to the height. It is also helpful, as it can help you determine the length of tubing that is consumed in the bend; it's a simple equation -- 2piR / 4 for 90 degrees.
• Tube diameter -- as the tubing is bent across the CLR (center line radius), by definition, the height increase is the CLR across the center. Add 1/2 the tube diameter to get the height to the top of the tubing.
• Bend start -- The bend doesn't actually start at the beginning of the die; the rule of thumb is that it start 3/4" back of there. You need to factor this into the equation.
• Springback -- As you bend the tube, it will tend to spring back some once the pressure is let off. This is mostly apparent in the angle of the bend; you usually need to overbend a little to get the tubing to your desired angle. But this measure of springback applies to how much you need to factor this into the overall height. The rule of thumb is 1/8".

So, take my hoop for example. I wanted a total height of 20 inches. 20 inches - 4.5 CLR of my die - .75 (1/2 of tube diameter) - .75 - .125 = 20" - 6.25" = 13.75". This is where I scribed a mark on the tubing, which was then lined up to the leading edge of the die before the bend.

I fed the tube through the bender, and clamped it in place per the instructions. For good measure, I added a bit of air tool oil (wd40 works as well) to the tube to be fed through the follower, which reduces the chance of flatspotting the tube. When it came to bending, I used a long bar on the bender, and progressively moved it across it's arc. For each travel, it bends the tubing a few degrees, and then has to be reset with the lever arm moved a tooth. It's intuitive. Once I ran out of teeth, I pulled the pin holding the die in place, rotated the die holding arm back, and reset the pin. This allowed me to continue bending to 90 degrees. I have a couple pictures showing this, or use the instructions with your bender to see how that works. Suffice to say that it's pretty easy. It does take some strength to bend the tubing, but it's by no means too tough for someone reasonably strong.

Also remember to bend the tubing a bit past your target to allow for some springback. My bender has a degree ring on it, but no pointer. I attached a magnetic welding triangle to the end of the die, with an edge pointing at zero before starting each bend. This allowed me to get an accurate reading of the bend without having to use a protractor. It's very handy.

With the first side bent, I had to tackle the other. Since I know I needed the hoop to be exactly 40" wide, with the equation above, I could figure out where I wanted the bend to end, but it didn’t provide me with a clear spot to start (assuming I'm feeding the unbent end into the bender, not the other way around). I marked that position, then measured the center line circumference from the "start" mark I had on there previously to a similar ending position. This was about 7 1/8". This accurately provided me with a starting point, as can be demonstrated by the fact that my hoop was perfect the first time out. However, JD2 does provide an equation to help:

• CLR x 90 x .0175

For me, that works out to be 7.0875. Pretty darn close.

This provide me with a bent hoop. I found that the bends are extremely clean, with no flat spots or wrinkles. This is important, as those will greatly reduce the strength of the bend. Note that steel is best loaded in tension only, and that it doesn't resist bending well anyway. Making clean bends at least gives it some strength. For a precision engineered cage, it's equally important that it be designed to tight tolerances. I found I was able to do that with the JD2 bender. Other benders may work equally as well, or even better, but I found this to be reasonably quick, precise, while maintaining the tubing strength. I even found that I liked the ability to "feel" the bend and springback in the tubing -- I doubt I'll upgrade to hydraulic.

Now, on to finishing the roll bar in the Miata. Look for an article specific to the roll cage install into the Miata here at werkz.com: http://werkz.com/feature.php?id=20.