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Re: [PSUBS-MAILIST] acrylic dimension change

Dan,
         I'm getting closer to purchasing my acrylic.  I was lucky enough to get a referral from Dr. Jerry Stachiw to a guy down in Santa Ana, Tony, anyway he has been very helpful to me and has been working with the material for quite a while.  I was a bit concerned over machining the faces of the acrylic since with the cast acrylic the faces can vary a lot.  He said that the faces can vary as much as 175 thousandths and I would definitely need to machine the faces.  But that is not a problem as they can be machined easily.  ( as long as you have some idea of what you are doing !) , afterwards sand and polish.  He indicated that it is important to keep your machine tool cool as well as the material.  Attention needs to be paid to speeds and feeds and not to chip or over heat the material.  Also only use soapy water as a coolant and do not use any chemical coolants. Do not use alcohol.
 
Brian
----- Original Message -----
From: Dan H.
Sent: Monday, January 23, 2006 19:43
Subject: Re: [PSUBS-MAILIST] acrylic dimension change

Brian,
 
No, I didn't machine the faces of the lenses.  
 
I made my viewport housings deeper then the thickest lens by about .050 of an inch.  Yes, deeper then the lens is thick.  I then ordered some stuff from West Marine that was a combination of urethane and something else. (I forgot what)  It's recommended for sealing both painted steel and acrylic, among other things. 
 
Before installing the lens, I buttered the lens housing with the urethane bonding stuff and pressed the lens in almost all the way, then bolted on the retaining ring.  The ring pushed in on the lens until the outside was flush and the urethane squished out after it filled the .050 plus gap.   PVHO standards say there should be a urethane cushion between the lens and housing anyway, so I have a cushion and it doubles as a seal. 
 
Also, I machined the diameter of the lens so they would be .100 smaller then the housing opening.  I let that gap fill with urethane also.  I did add and O-ring seal to them after the urethane hardened, but it wasn't necessary.  I already had a 45 degree chamfer on the lenses to accept the O-ring from my first design idea.  So I put it in.  Remember, your not going to get the lens out very easy if you put them in like this, but it makes a good seal.   
 
Another note,  If you have any imperfections or scratches on your lenses, put the scratches on the outside.  They disappear when your sub is in the water. 
 
Dan H.
 
 
----- Original Message -----
From: Brian Cox
Sent: Monday, January 23, 2006 8:16 PM
Subject: Re: [PSUBS-MAILIST] acrylic dimension change

Dan,
          That's very interesting about the acrylic faces not being even,  I'm only familiar with the plexiglass that is commonly purchased which is "sheet" acrylic.  The cast acrylic is a new one for me, I assumed the cast was similar to the sheet but I guess not.  So then you probably had to machine the faces perfectly flat and the sand and polish them to get them crystal clear.  How smooth were you able to get the acrylic with just a machine tool?
 
Thanks
 
Brian
----- Original Message -----
From: Dan H.
Sent: Monday, January 23, 2006 10:46
Subject: Re: [PSUBS-MAILIST] acrylic dimension change

Brian,
I first shrunk my viewports with a higher heating process. This was before final machining. I probably did have some thickness shrinking then but I didn't have an exact measurement of the thickness before hand.  When you get your acrylic it won't be exact thickness anyway.  My pieces were all different thicknesses.  I don't remember the exact variations but I'm thinking they varied about .030 thousants of an inch.  They even very from one edge to the other of the same piece. 
 
I was told that thick acrylic sheets is made by pouring the liquid acrylic between two sheets of glass and there is always some bulging of the glass form.
 
If you need exact thickness you should have your acrylic in hand before you machine your housings. 
 
Dan H.
----- Original Message -----
From: Brian Cox
Sent: Monday, January 23, 2006 11:34 AM
Subject: [PSUBS-MAILIST] acrylic dimension change

Hey Dan H,
                   When you annealed your viewports did you have any change in the thickness ?   I'm getting ready to machine my viewport flanges but I haven't purchased the acrylic yet,  I'm just wondering if that dimension is going to change on me.
 
Thanks,
 
Brian