RE: [PSUBS-MAILIST] Sub as panoramic film platform

Polycast sells pre-shrunk cast acrylic. One advantage to purchasing material that comes from an entire sheet that has been pre-shrunk is that if you heat up small pieces hot enough to shrink it can cause an optical distortion in the viewport because of the way the edges release heat. Post annealing (if done at the correct temperature) will not cause distortion or change in dimension. The material only shrinks once.

Greg Cottrell

Project Manager

greg@precisionplastics.com

http://www.precisionplastics.com

P please consider the environment before printing this email

From: owner-personal_submersibles@psubs.org [mailto:owner-personal_submersibles@psubs.org] On Behalf Of Dan H.
Sent: Tuesday, September 21, 2010 9:07 AM
To: personal_submersibles@psubs.org
Subject: Re: [PSUBS-MAILIST] Sub as panoramic film platform

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 -----

From: Brian Cox

To: personal_submersibles@psubs.org

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

-----Original Message-----
From: owner-personal_submersibles@psubs.org [mailto:owner-personal_submersibles@psubs.org]On Behalf Of Jon Wallace
Sent: Monday, September 20, 2010 11:28 AM
To: personal_submersibles@psubs.org
Subject: Re: [PSUBS-MAILIST] Sub as panoramic film platform

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,