Re: Bending plexiglas to create a coil form (fwd)

---------- Forwarded message ----------
Date: Wed, 29 Apr 1998 14:40:33 -0700
From: Jim Lux <jimlux-at-earthlink-dot-net>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Bending plexiglas to create a coil form (fwd)

Tesla List wrote:
> ---------- Forwarded message ----------
> Date: Wed, 29 Apr 1998 16:02:27 EDT
> From: FutureT <FutureT-at-aol-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Re: Bending plexiglas to create a coil form
> In a message dated 98-04-29 15:51:42 EDT, you write:
> <<
> > > Making your own tube for a secondary form is a very viable option. I have
> > > made large diameter plexiglass tubes like 2 feet in diameter by heating a
> > > 1/4 inch thick sheet and then bending it into tube shape and then joining
> > > the seam with methylene chloride. It's sure a lot cheaper buying the sheet
> > > than an already formed tube!
> >     I never thought you could get it to bend that much!
>   >>
> Nice work.
> I made a secondary form 7" in dia. by 19" high by bending a heated
> sheet of flat 1/8" plexiglas, but what a nightmare.  Parts of the plexiglas
> got too hot and started to shrink, and it was hard to get the seam to
> bend enough.  I had to finish the job with a propane torch.  Eventually
> the job got done and produced a useable, but not perfect form.
> I'm sure with the proper equipment and experience, it can be done
> better and more easily, but I think I'll look for pre-formed forms in the
> future    :)
> John Freau

Forming acrylic or lexan is an art. Temperature control is critical, 
because you want it hot enough to soften, but not so hot it melts. To 
make a big cylinder, you really need a big oven, or a large array of 
heat lamps to provide a very even heating to the whole sheet. Then, 
you'll need a good form to wrap it around, and a good scheme for 
clamping it (we've used sheet metal for the latter). As you indicate, it 
is the ends of the sheet that are the toughest. 

On occasion, we've actually bent more than we needed, at a smaller 
radius than desired (making a spiral), then cut off the ends (i.e. form 
a longitudinal slot)and bend it out (Cold) around another form and 
cement it.

Your best bet is to find someone who does this for a living. They have 
the tooling, and more important, the practice.

I was at a shop that makes big (>4 ft diam) hemispheres. They just heat 
the sheet up, put it over a big hole in a box, and suck it down with a 
vacuum cleaner and gravity. I'll bet they went through a lot of plastic 
before they got the process down, though, in terms of heating time and 
temperature and how much suction and how long.

What is so special about a round coil anyway. Why not make it polygonal, 
with filleted corners. An octagon would be easy with a strip heater, and 
 the corners might not add too much E-field stress...