NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Paul Dolkas
Date: 2013 Mar 6, 18:45 -0800
Paul-
Is there any problem with lack of homogeneity by using a filled plastic? I.e., if the glass fibers weren’t’ uniformly dispersed in the plastic, you might end up with one part of the frame having a different CTE than the rest. So rather than minimizing warpage, it would exacerbate it. Since there is such a large difference in CTE between filled & unfilled polymers, I would expect a little lack of uniformity would have a large effect.
-Paul
From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of Paul Fithian
Sent: Wednesday, March 06, 2013 6:59 AM
To: paul@dolkas.net
Subject: [NavList] Re: Davis and Astra head to head
From my experience in the plastics industry, I would guess that the Mark 15/Mark 25 main frame is made from a glass filled ABS or Polycarbonate. This kind of material is widely used for parts like camera bodies. Another clue is that it appears the parts are held together with a solvent weld, much like PVC pipe.
If someone was interested, they could analyze the material to find out for sure and also determine the actual glass content.
Davis has been making these sextants since the late 60's, probably from the same molds. There may have been a difference in the frame material for the Mark 15 and Mark 25 at one time, but the cost difference in the actual material would be almost insignificant. And it would be easier to make sure all the dimensions were correct if the materials were the same.
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