Brian,
Your going to find that the cast acrylic is
anything but constant in size. Each lens piece is going to
measure a difference in thickness form one side to the other. The
material is CAST not extruded.
I was originally planning to machine me
viewports to fit the lenses and use O-ring seals. I found that
impossible unless I wanted to face the entire lens then polish it back to
clear.
Another thought came to mind when I was
figuring out how to fit the lens. I was thinking of using a radial
seal in a chamfer around the outside of the lens. When I looked at
the coefficients of expansion of the steel housing and the acrylic lens, I
found that the redial clearance could be quite large in cold water and non
existent in the hot sun. An O-ring around the outside of the lens
could extrude inward.
After I welded in my viewport frames they
went out of round and flat so much that my original idea of a simple
O-ring wouldn't work anyway. I didn't have the option of machining
after welding.
In the end I machined the lenses with plenty
of clearance and laid them in using sekoflex. I decided to
stop thinking about thermal expansion and just do what has worked for
others in the past.
Acrylic is easy to machine. You'll have
no problems. Don't run to fast. Use a positive rake angle
on your cutting tool and be prepared to deal with a pile of string
flying off the tool. But you should preshrink and post stress
relieve your material following a preset procedure. If you don't
preshrink and you only post stress relieve your lens may be four or five
percent smaller when your finished.
Dan H.
----- Original Message -----
Sent: Monday, September 20, 2010
6:58 PM
Subject: RE: [PSUBS-MAILIST] Sub as
panoramic film platform
Hi
Jon,
The plexiglass is definately going to be a fraction of the cost of the
cast acrylic. The reason for this is that I have a few thousandths
of an inch differences between the eight viewport frames and I'm
concerned that they will each have to be custom machined to fit.
I'm hoping that the variation will be small enough so I would be able to
just have one lens fit all the viewport frames.
Brian
Hi Brian,
Acrylic and
Plexiglas are one in the same. Given the thickness of 1.5 inches
I can't see any financial benefit to prototyping with a low grade
Plexiglas which no doubt is going to be very expensive regardless of
the lower grade. If it is machining that you want to play with,
you might try calling around to see if you can get a small
remnant. If it is fit-up that you want to play with, how about
using a 1.5 inch Styrofoam panel from Home Depot or
Lowes?
Jon
On 9/20/2010 11:20 AM, Brian Cox wrote:
Greg, I have already machined my
viewports and they are just waiting for me to get the acrylic lenses
to fit in. I was thinking that I could purchase some regular
plexieglass (1 1/2" thick) and machine those so I could play
around with how everything is going to fit together. Can you
tell me if the machining properties are much different between the
cast acrylic and regular "low grade" plexieglass? I
imagine that the cast acrylic would be a denser material so it may
machine and polish much differently.
Thanks,