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Jim's Mirror making Page 1

Theory

The depth of glass to be removed, e depends on the focal length so for a 6" dia (r =3 inches) 48 inch focal length mirror. 

 3x3/96=.0468 inch

Note R=2x the focal length, 48x2= 96 inch

Tools and equipment.

The mirror kit shown below, was bought from image optics and contained :-

1.) 150 mm mirror blank and tool.
2.) 226 g of grade 180 carborundum, for first stage grinding.
3.) 113.4 g of 400 g carborundum, for second stage grinding.
4.) 113.4 g of grade 303g carborundum, for third stage grinding.
5.) 56.7g of Cerium Oxide, for polishing
6.) 0.45 kg of optical pitch.
(Beer not included)

Also required :4 sponges, 4 screw top jars, and four, 12 mm paint brushes.

Stands and bases

A strong base is required to hold the mirror while grinding and polishing takes place. Any of the following can be used for a 6 inch mirror.1) tripod built from 4x2 timber with lots of cross bracing.2) An oil drum fitted with a ply wood top.3) a corner of a strong work bench.

Preparing the mirror blank

Before grinding can take place ensure that there is a good 6 mm chamfer around the tool and mirror this chamfer must not be allowed to be worn away at any stage of the mirror making process. If it is lost the edge will break and will cause severe damage.

The tool and mirror can be edged using a wet stone. shown below.

You are now ready for rough grinding.

Stage 1 - Rough Grinding

  1. Apply 60/120 silicon carbide to the surface of the tool with a brush.

  2. Apply the stroke shown while walking around the post.

  3. When grinding action stops clean off the carbo and reapply fresh carbo. (wet) 

  4. Measure the depth of the mirror using a straight edge and a feeler gauge set to the value 'e' (as above)

  5. when the gauge fits under the straight edge, rough grinding is almost completed.

The End of Stage 1 - Rough Grinding

At the end of the first stage, the tool and the mirror are not true to each other. clean up the curve with center over center strokes.

Center over center stroke
the tool and mirror positions can be changed to clean up the curve.

Stages 2 & 3

Clear away and clean all surfaces before starting stage 2.

Apply 320 carborundum to mirror as to remove the pits left by stage one.(may take 20 wets)

Use center over center or use W strokes.

 

Check all pits from the previous grade have gone using a strong lamp

Repeat procedure for stage three using the 303 grade carborundum.

When all of the pits have gone you are ready for polishing