16.2-Inch-Mirror-Blank-Project

I originally started with a 16.2 inch diameter mirror blank with circular web back pattern. This was a mistake due to stress being concentrated in the narrowest part of the webbing. This being said a 16.2 inch disk was worked and a good figure attained in spite of what might be considered a risky anneal. The picture on the right is a jpg conversion of the original dwg format file.

Original AutoCad file:Media:16-inch-blank.dwg

What does this tell us? Because my anneals have gotten much better and the circular webbing moved away from that our end product is workable. Whether picky anal astro types wince or not is not the issue. The issue is not emotion but rather IF something can be done and with what level of difficulty. The answer is YES it can be done and at reasonable and comparable effort levels for the mirror design at hand.

The ronchi test of the finished processed mirror is shown below left. This was a test done by the guy who finished the processing job. I myself then tested the mirror using the focault test with an eye towards detecting astigmatism. I could detect none. Astigmatism is the most common problem resulting from poor annealing.

Master Mold Generation Process Notes

Below is an overview of the mold making process. It will be altered somewhat for larger diameter pieces but is representative of the process and is thus presented: The method for this mirror was as follows

• 1) Circular nub biscuits
  

---a)make a tapered circular nub on a milling machine. In the photo a hex nub is shown because I now longer have any round ones. A 5 degree taper from base to top is cut into the nub to facilite de-molding processes.
---b)use the nub to make an elastomer nub master mold
---c)mold numerous plaster of paris nubs using the rubber nub negative mold
---d) sand each nub to exact fit

• 2) case a dog bowl out of plaster of paris around a suitable diameter circular object and sand to finish to exact dimensions
  
• 3) Glue nubs into dog bowl using full sized paper template printed out on Kinkos gigantic printers. This insures VERY good placement. Its pretty hard to do with out this template. This forms a plaster of paris negative of the mirror. It is a positive of the refractory mold desired
  
• 4) Make a rubber mirror positive mold using the plaster of paris hand made mirror neg / refractory pos. This is done in a vacuum using a relatively expensive two part stiff silicon.
  
• 5) Use the rubber mirror positive to case refractory material against in a tube to retain material from without and the rubber mirror within to shape the desired.
  
  

Advantages and DisAdvantages

Advantages

• Cheap Quick and Easy. By and large a highly effective method to generate refractory molds on an ongoing basis.

Drawbacks:

• The all in one piece makes for a heavy and hard to separate situation. It is manageable for 16 inches but even in the case of 20.5 inches another method was required. Thus a multi-part refractory mold design was used for the larger 20.5 inch mirror blank.

State of the work

• The work is sufficiently advanced to make 20.5 inch blanks. We should start with that and make everything with the same method as we intend to use with the 48 inch design. Thus we commence with this as a starting point for 20.5 inch.
• mold material should be contually worked. Designed in crushability should be worked on using mixed in saw dust is the next step. During slow temp rise the wood will burn out of the mold leaving voids making the refractory material crushable.
• hot tear / mold adhesion has been entirely solved.