Without getting into the specifics of the rules of various sanctioning
bodies, the answer is yes, you can do it yourself.
The aim of almost all the rules regarding roll bars/cages is to prevent
kinks and flat spots on the bends. In this case, the mandrel is simply
what is used to make the formed curve. If you've ever used a Hickey to
bend electrical conduit, you've used a mandrel, it was the part that
formed the bend.
Really nice equipment works by perfectly fitting the curvature of the
pipe or tubing, and bending it along its length in the exact same
rotating manner as the electrical conduit Hickey does.
You've seen advertised those cheap pipe benders from various folks like
Harbor Freight and such. They to can work, but it takes a lot of work
by you to do it.
First, the dies tend not to match up well to conduits and tubings.
This can be overcome by shim stock or by making your own die or
modifying one of the oem ones.
Second is the way this unit makes the bend. It pushes the center,
dragging the outer sides by those fixed rollers. You've got to help
stuff the pipe up through there to prevent collapsing the inside of the
wall that the hydraulic ram is acting on. Many times packing sand is
necessary. You're not going to kink the pipe/tube/etc with this
hydraulic ram, but you're very likely to flatten it some, which will get
the cage kicked out on a tech inspection.
The DOM specification is to distinguish it from the cheap seamed
pipe/tubing. That stuff will not hold up to the stresses of a crash and
is prone to splitting along the seam. DOM is frequently the only form
of seamed pipe/tubing allowed in cages by those bodies that allow any
seam at all. Seamless pipe is preferred. It is stronger still, and of
course the most expensive.
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