Introduction

What follows is a basic summary of the main points of making a mirror. I have gathered the information mainly from books, to develop the expertise of make a mirror, and my own experiences as I have progressed in making a mirror. Whether what I have written is good sense or rubbish it up to you to decide.

Preparation

If any bubbles >1/16" diameter are near mirror surface, or if tool or mirror thickness varies by >0.04" then return both to the supplier and complain. If thickness of the tool or mirror >0.01" grind level to within 0.01".

Firstly grind a chamfer on mirror and tool edges at 45 degrees and 1/8" wide. I made it 2mm wide on tool and mirror edge. I had to re-grind chamfer a number of time so starting of with an 1/8" chamfer is good idea. This stops shards of glass breaking off the mirror or tool edge during grinding and causing scratches.

Place 3 cleats 120 degrees apart on solid stand, placed so that the mirror or tool are a snug fit. Note the tool/mirror tightens up because grit fills any gaps between tool and cleats.

Find salt shakers to spread the grades of grit evenly over the tool/mirror surface, one shaker pre grit size. Drill small holes (1mm) in lid of jam jar.

An outline of the total grinding process is summarised in the following table:

Abrasive	Size	Grinding Time	No. Wets
80 Grit	7.5 thou	3hr	As long as it takes
120	4.9	+60 min.	10
220	2.9	+60 min.	10
280	1.6	60 min.	8
400	1.4	60 min.	8
600	1	60 min.	8
900	0.7	40min	5
BM305	0.02	40 min.	5

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Rough Grinding or "Hog Out Mirror"

This means removing glass as fast as possible from the centre of the mirror to create a concave surface on the mirror and a convex surface on the tool by applying the chord grinding stoke.

Sprinkle 80 grit over the tool so that it is covered without grip over lapping so you get an even distribution with no gaps or pills of grit and spray water until all the grit is wet. Lightly spread out with the mirror. One charge will last 2-4 minutes of grinding, which can be extended by misting.

Apply your full weight/pressure on to the mirror centre which should be about 1" in from the edge of the tool moving along a chord as shown in the diagram. The mirror can overlap the tool edge safely by 1". It is a good idea to vary the position of your hand during grinding and get into the habit of not having your fingers curl over the side of the mirror/tool which can have major effects during polishing.

Grind along a cord back and forth in about 1 second a few times, then rotate the mirror clockwise, and move yourself anti-clockwise (or visa versa) onto a new chord. Do 60 to 80 stokes during once or twice around the table. Every 5 to 10 strokes change to a new cord, move clock wise, and the move mirror anti-clockwise (or visa versa). Repeat this process, randomness of mirror strokes across the tool is the secret, moving yourself, the mirror, and grinding along different chords/directions ensures the grinding is even. As hollow develops, grit is retained and progress speeds up.

As the loud grinding noise reduces and the feel of cutting reduces, re-charge with grit and water, i.e. the next wet. You can re-charge the grit with what is wash off the edge of the tool. But do not do this during fine grinding due to the possibility of contamination from previous grits.

After about 6 wets wash mirror and tool, and test mirror against a template shaped to the radius of curvature of the mirror. If grinding correctly then middle of mirror and edge of tool will be ground with the opposite being scratch free. Repeat above for 6 wets.

Measure depth of mirror at it's centre (i.e. sagitta (e)) using a straight edge and feeler gauges, and test with template.

Once 1/3 of mirror is hollowed out then move the mirror centre closer to too the tool centre with the mirror edge over-hanging the tool by about 1/3 it's diameter.

As mirror hollow increases in size gradually move chords to centre so that mirror and tool centres pass over each other, i.e. the diametrical stroke. Use short strokes i.e. <1/2 mirror dia.

Reduce pressure during the final grinding stage to reduce the depth of scratches i.e. use the weight of the mirror + hands only. Now need to make the mirror spherical.

If mirror centre is to deep then grind with mirror and tool centres over each other with strokes of 1/3 diameters.

If whole mirror is deeper than the template do the above but with the tool on top. Note that long strokes tends to grind mirror centre more than the edge while short stokes grinds the mirror edge faster than the centre, and strokes about 1/3 mirror diameter long grind whole of mirror surface equally. Return to top of page

Basic Tests

1. Watch for bubbles in grit and water during grinding being evenly sized and distributed after a few strokes.
2. Clean the mirror, draw diametrical pencil line and grind for 6 strokes and all lines should disappear equally if the mirror and tool surfaces are spherical.
3. Use template with a light to check for gaps.
4. Measure the sagitta (e=r²/(2R), where r is mirror radius, R is the radius of mirror's centre of curvature). Use straight edge and measure depth at centre with ball bearing + feeler gauges or similar (feeler gauge is best). If e this greater than the desired value then grind with tool on top, see above. This needs checking to know at what point of e to stop, and therefore allow for reducing e to the desired value!

Measure Focal Length

1. Scrub and clean everything and grind 2 or 3 wets using 400 grit to make the surface reflective with water on it.
2. Use torch next to eye and view mirror along axis and move both side to side.
3. If image moves in same direction as head then inside radius of curvature.
4. If image moves in opposite direction as head then outside radius of curvature.
5. At radius of curvature the image appears not to move. Measure this distance a number of times and average. Average/2=focal length. If mirror focal length is several inches (>30 mm) to long return to 1/3 diameter centre over centre grinding with 80 grit. and repeat tests. If focal length to short then could accept it and move on to fine grinding.
6. Finally test evenness of spherical shape by catching the image of an unfrosted bulb on the mirror surface. As the angle of the mirror is changed the image of the filament should change smoothly. If not the surface is not spherical. Bright points are high points. Dark points are low areas. Sudden changes means the surface is not spherical and further grinding is needed.

Find Grinding

Start each new wet of fine grinding grit by lightly spreading out the grit using only the mirror weight for several strokes. This will help break down any larger particles and so minimise deep scratches. See table to relate grit size to micron size.

For >700 grit the grit can be deflaocculated. This involves adding the grit to a 3' column of water for a given time allowing heavy particles to settle out. Then draw off the remaining liquid, let this settle, draw off the water, and use the settled powder. This may be worth trying if scratches appear when they should not. Return to top of page

The Process

Thoroughly clean off the previous grit. Spread grit with shaker and wet, do a few light stokes first. Use slightly less than a 1/3 W stroke, i.e. start with a 1" over hang on one side and end with a 1" overhang on the other.

Rotate mirror and move round stand as in rough grinding. Randomising the movement of the mirror over the tool, the stroke, and yourself around the stand between each wets will ensure an even surface is ground. Continue each wet until the sound and feel of grinding stops and a sandy feel starts. Then wash spent abrasive off, add new abrasive, and water. Each wet should last about 6-8 minutes, spray if the wet becomes dry.

Every 2 wets swap tool and mirror, when mirror is on the stand use slightly shorter strokes so that grinding the mirror edge is minimised. Alternation maintains the centre of curvature of the mirror radius.

The previous grit could be ground out in 15 minutes, however it is safer to grind for 1 hour or until all pits are removed. Return to top of page

Testing

Mark the mirror surface with a soft pencil into 1" to 1/2" squares, and use a magnifier to check the following:

1. Check uniform smoothness edge to edge
2. Any pits should be a similar size and shape

I found a number of small bubbles in the glass that intersected the mirror surface. These appear circular about 0.1mm diameter or larger, and can be ignored.

3. Check focal length and sphericity as described above
4. Check bevel on mirror and tool edge is still about 1/8" wide. Otherwise shards of glass break off the tool/mirror edge and cause scratches.
5. If all check OK move on to next grade.

Measure the mirror centre of curvature (R=2f)using a Foucault tester or the rough methods given in Basic Tests above.

Repeat above fine grinding process for each grade given in the table above.

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Comments of Grinding Fine Grades

Risk of scratching increases due to:

1. Contamination between grits, clean thoroughly
2. Glass from chipped edge of tool or mirror, maintain chamfer on tool/mirror edge.
3. Allowing wet to become to dry so it combines into small lumps, add more water using a mister.
4. Tool/mirror seize because of to little grit and water.

The times in the table are a guide only, grind until satisfied all large pits are removed, e.g. find largest pit and monitor its removal. For the finer grades (>120) add 2% detergent to the water. This acts as a wetting agent and deflocculent and makes grinding a lot easier. Give the mirror and tool a good wash after testing, particularly at the fine grit stage, to avoid contamination.

For grades >=400 can either shake on grit (make sure the powder/grit is all wetted before starting to grind), or mix with water + detergent and apply as a liquid suspension, let the mixture settle for a few seconds before applying, mixture must be a solid colour. First move mirror lightly on tool, if you can not hear crunching/grinding then carry on, otherwise lift off mirror clean and start the wet again.

For grades =>600 seizure and scratching is more likely therefore wash after each wet and apply abrasive from a suspension with plenty of detergent in it. For first few strokes hold mirror off tool and check for large particles/contamination and evenly distribute grit across mirror/tool surface, if OK complete the wet using mirror weight only.

For grades =>900 bubbles stop contact between mirror and tool, remove by rotating mirror while pushing it across the tool. Once removed carry on with normal grinding stroke. Never leave mirror resting on tool as it likely to seize.

Last wet should take twice as long, with mirror on top, to allow any fine pits to shallow out making easier to polish out. Should be able to read 10-12 font sized print through the mirror. Return to top of page

Polishing

This involves making a polishing lap by forming pitch on the tool, polishing the mirror to a spherical shape, and then polishing to transform the spherical shape into the form of a paraboloid.

Making a Pitch Lap

If purity of the pitch is suspect then melt the pitch and strain through a few layers of cheese cloth. Pitch hardness can be measured using your finger nails. Press nail into cold pitch with a force of about one pound and time how long it takes to create a mark 1/4" long. For Pyrex use 40 second pitch, or greater if in doubt.

Warm tool and mirror to 55 degrees C in water. Make collar around tool about 6 to 7mm above mirror edge. Make a creamy mixture of water and rouge, add detergent until you can feel it is there between fingers, could just use a water and detergent mixture. Clean the tool surface with white spirit to help the pitch stick to the tool.

Break up enough pitch to fill mould and melt in a clean pan over a low heat. Once liquid (not boiling) stir and pour into mould until full. Allow to cool say 10 minutes. Remove mould, paint rouge or detergent mixture onto the mirror surface and lap surface. Place mirror onto tool and move about until both are in intimate contact. Watch that the mirror edge does not dig into the soft lap, add more rouge mixture if needed without removing mirror.

Leave to cool/harden for about an hour. Slide mirror off lap surface and place tool in dust free environment, e.g. plastic box with lid, and allow to fully harden over night.

Trim lap edges square with tool edge. Find tool centre, mark out channels 1/4" (5mm) wide to produce squares of pitch so that a square is offset from the tool centre, and cut out channels and clean.

Cold Pressing

This ensures the pitch surface is formed into the shape of the mirror so that they are in intimate contact.

Paint on thick layer of rouge mixture, place mirror on top of tool and seal in damp bag, evenly distribute 5kg over mirror back and leave for 4 hours or overnight in a cool place. Mirror should be in full contact with the lap. If not warm tool and mirror to 30 degrees C apply rouge mixture and work mirror over tool using body pressure until good overall contact is achieved. Clear channels.

Since it is reckoned that the polishing action occurs at the edges of the squares of pitch, then if the number of edges can be increased the polishing action will be speeded up. One method is to paint on hot bees wax onto each pitch square. Once cooled place a fine meshed material over the tool, paint over with the rouge mixture, and cold press until an impression of the mesh material can be seen through the mirror on all surfaces of the tool. Another method is to simply scratch the surface of the lap with some sort of sharp knife, clean off, and cool press.

There are many other methods of making a polishing too, search books and the web, see link page

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Polishing Process

There are three distinct stages of the polishing process:

1. To remove all traces of pits or scratches remaining fine grinding
2. Then remove surface defects to produce a truly spherical surface
3. Finally to figure the spherical surface in the form of a paraboloid, the basic goal.

1 Removing the Pits from Fine Grinding

• Room temperature should be reasonable constant at 20 degrees C +-1, not so important when simply removing pits.
• Since polishing heats up the mirror will need to let it cool for 60 minutes or so before testing, or cool in water.
• Freeze pitch by cooling in cold water before polishing
• Increase humidity to reduce the evaporation of the rouge mixture during polishing
• Cold press the mirror and tool before each polishing session. Could warm lap and mirror before pressing.
• Mix 1 part rouge to 3-4 parts water plus some detergent. Apply mixture to lap squares around the edge and centre, spread out over all squares and let mirror rest on the lap to embed the rouge into the pitch. Mist with water to reduce drag. Add rouge mixture as needed.
• Use 1/3 diameter W strokes working slowly with reasonable pressure on the mirror, ie 1 second per stroke, for between 1/2 and 1 hour. Vary stroke direction slightly to reduce likely hood of mirror digging into pitch.
• To pack up for the night mist down lap, add some rouge and wrap in clean plastic bag. Or place mirror on top of lap and bag up. Warn to slightly and cold press before starting the next day.

Polishing starts from the centre and works outwards. If edges have not started to polish after a few hours then consider returning to the fine grinding stage, similarly if the centre does not polish. It means that fine grinding did not produce a spherical surface and will need to work out what when wrong during fine grinding. Use eye piece to make sure all pits have been polished out.

Trim edges of lap and the channels as polishing continues. Check that the additional fine channels are still present, if not then repeat procedure. This would tend to indicate lap is to soft.

Testing

Foacault Test At the centre of curvature (CoC) of the mirror, if spherical, should grey out uniformly across its whole surface and the image appears to be flat, which is the objective of the next stage of polishing. Light and dark areas indicate the surface is not truly spherical and these need to be polished out before figuring. Have a go at the finger test: test mirror, then remove and place finger on the surface for a while, re-testing will show a bump where the finger was. This will help identify whether defects are bumps or hollows.

With the knife edge on your left of the light source the defects are interpreted as follows:

Hollows

These will have shadows on the right and be illuminated on the left. Or visa versa if the knife edge is to the right of the light source.

Hills

These will cast a shadow on the left of the hill and will be illuminated on the right flank of the hill. Or visa versa if KE is on the right.

Note: A paraboloid shaped surface will also show dark and light areas, a sort of donut shaped image that should not be confused with the above images of defects.

Eye Piece Test This tests for turned up edges to the mirror. Test by replacing the KE with a 1" f eye piece and the light source with a pin hole. The pin hole will appear as a sharp disc at the CoC. On moving towards the mirror the image should stay sharp, if fussy edges appear on the image then the mirror edges are turn down. This is formed during grinding by tilting mirror slightly towards end of stroke.

Ronchi Test A fine vertical grating replaces the KE, which should be parallel with the KE. The image of the grating should be vertical, if the bend in towards the vertical centre then the edges are turned down.

Astigmatism Test either 25mm inside or outside the CoC, the knife-edge should enter, pass through and leave the light beam vertically. If astigmatic then entering or leaving the beam will be off the vertical. If so check every 60 degrees by rotating mirror. Starting with parallel surfaced on the blank may help avoid this. Cure start again?

2 Creating a Spherical Surface

This involves polishing out the defects identified in the above tests. If defects are encountered the following may indicate how to deal with them:

Turned Down Edges

Polish for 10 minutes 1/3 dia. strokes. If noticeable reduced carry on after cold pressing, otherwise reduce the stoke to 1/4 dia. Otherwise I guess it is back to fine grinding after working out what went wrong in the 1st place.

Turned Up Edges

not a common fault. Cure by using long strokes, alternating mirror and tool on top to maintain mirror depth.

Dog Biscuit

A rough surface caused by polishing to quickly or for to long which generates local hot spots in the lap which then polish unevenly. Slow stoke speed, and don't polish to long.

3 Figuring

This involves polishing the mirror to convert the spherical shape into a paradoloid shape. Use a weaker polishing solution than above and reduce the polishing sessions to 10 minutes each, then test. There are various methods for achieving this:

1. Carry out th normal "1/3 W" stroke to warm the mirror and lap with mirror on top. Then increase the to a "3/4 W" stroke, covering 3/4 the diameter of the mirror in wide strokes. Concentrate polishing with the mirror centre near the tool edge, this removes material from the mirror/spheroid centre to form the paraboloid.
2. Start as above with the mirror slightly off centre use short "W" strokes pressing on the mirror edge closes to the tool centre to deepen the mirror centre. Then polish with short strokes with the mirror centre near/at the tool edge with equal pressure to minimise polishing down at the mirror edge. This method retains a small zone at the original spheroidal surface at about 75% of the mirror diameter.
3. Use a smaller diameter tool to concentrate polishing in the mirror centre and edge in a similar way as above but with equal pressure. The preferred option of mirror with diameters >10".

Yet to Finish