TRO+-+Grinding+the+back+flat

=__Grinding the back of the mirror flat__=

The back of the mirror needed to be ground flat before any work could commence on the front. The back of the mirror was significantly convex by around 2mm. This was unusual for such plate glass an must have been the main contributor towards poor quality during the original making of the mirror.



Image above shows the original signatures on the back of the TRO before they were ground off.

Image above shows clearly the amount of curvature in the back of the mirror. This was removed when the back was ground flat.

Mixing the concrete to make a mould for teh front face of the mirror. The fast setting concrete was found to be too little and too brittle. In the end a slurry of normal fine redy mixed concrete was used. The mirror was covered in a number of layers of plastic sheet to prevent it sticking.





The above image shows the shallow curve that the mirror forme on the surafce. A layer of carpet was then added. The mirror was rotated often on this to prevent astigmatism in the back surface when grinding.





Above shows the 18 inch, 1 inch thick, glass tool that was used to gring the back flat. The edge needed to be beveled first.


 * The video shows Geoff at work beveling the edge**: [|TRO_30inch_bevel_edge2.avi]

A number of center over center, and off center strokes were used to even out the work. A spherometer and straight edge with feeler guages were used to monitor the work. the pencil test was also used near the end.


 * The video shows the machine in action grinding the back flat:** [|TRO_30inch_grinding_back_flat.avi]

The initial work showed that the back of the mirror was very astigmatic. The uneven grinding being the give away.

Image of teh final mirror with it's back now regular and relatively flat.