The plastic limit is the point beyond which if you deform a material it will not return to its original shape. The porthole glass exceeded that limit upon reaching the floor after having experienced gravitational acceleration through about three feet. In other words, I dropped the tool. John Dobson calls that booboo number two. Booboo number one is dropping the mirror. I purchased a 16" diameter piece of 1/2 inch plate glass to continue. I glued it to a 1" thick plywood disk using RTV. That piece of glass got pretty thin and so I purchased a second piece of glass. I thought it would be easy to separate the original glass from the plywood, but NOOOOO. I ended up putting a band saw blade in a vise and having a friend stretch it out. I then moved the tool across the blade, acting as a human band saw. It took about an hour because the RTV would clog the blade, and the blade would stick in between the glass and the plywood. After the separation was complete I epoxied the two pieces of glass together. I had not heard of the plaster of paris and tile tools. However I did make one of those later on to grind the back of the mirror slightly concave to eliminate astigmatism. I learned a lot grinding the 22. ________________________________ From: Chuck Hards <chuck.hards@gmail.com> To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Sent: Wednesday, February 15, 2012 5:07 PM Subject: Re: [Utah-astronomy] Porthole glass mirrors I'm pretty sure I'd use one of my own ceramic-composite tool foundations for any effort along these lines, unless I could score a second porthole for little cash outlay. Please expand on the plastic limit on your floor, LOL! I'm expecting a good story... On Wed, Feb 15, 2012 at 4:44 PM, Brent Watson <brentjwatson@yahoo.com>wrote:
John Dobson gave me some porthole glass that I used as a tool to grind my 22 inch. It was 16" in diameter and worked well until it exceeded the plastic limit on my floor one evening.
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