Mirror balancing for laser spirograph

Design of my spirograph is based on a regular cooling fans.
Fan is widely available part and it's easy to mount mirror on its flat back side.  After some experiments, I've developed simple and reliable method to mount, adjust and balance acrylic mirror in order to achieve smooth and quiet performance.

All described below is applicable only to plastic/acrylic  mirrors !!!

DIY  spinning mirror for laser spirograph.

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Rough shaping.

I've got acrylic mirror's sheet from local plastic supplier and
my first step was to cut square piece of proper dimension, corresponding to size of fan.

Material preparation.

Next, I marked circle on raw mirror then formed round shape using file.
For 40 mm fan I used bottle cup with diameter 30 mm as template.

Acrylic mirror is getting its shape.

Mirror mounting.

To attach mirror I use 3/4" double sided sticky tape (mounting tape).
Back side of the fan should be clean  and if there's any label it should be removed. Pure alcohol may be used as cleaning solution.

Double-sided adhesive tape.

Square piece of tape is attached to the back side of fan.
Tape is covered with some sort of backing film. I removed film, trimmed half of it,  folded it in half and attached it back onto tape as pictured.

Fan is ready for mirror installation.

After mirror's installed, folded film forms accessible pocket (5 mm wide) under mirror.
Mirror's tilt can be set by inserting piece of thick paper into this pocket.

Mirrors tilt is adjustable.

Balancing.

I've tried different tools and techniques  and finally developed  reliable and fast method of mirror balancing using drill-press. Dremel's sanding drum is installed into drill's chuck and used to spin mirror.

Dremel rotating tools.

To hold fan I've built  simple attachment using scrap plywood, spring and few screws.
Fan is secured between two crews on a pivoting lever.
Compressed spring pushes lever toward drill chuck so edge of mirror always stays in contact with sanding drum.
While sanding drum spins mirror I use file to shape mirror.
At the beginning I work with coarse file then when mirror stops wobbling and revolutions become smooth I switch to fine file to finish edge.

Mirror balancing.

After some practice, if initial rough shaping done properly and mirror is centered, whole operation may take just few minutes.

Setting mirror's tilt.

To set mirror tilt, small piece of thick paper strip (4 mm wide) should be inserted into pocket under mirror.
Usually angle of tilt is very small so post card or business card works just fine.

Mirror tilt setting.

One thing to remember. FS mirror requires gentle handling and it's better to avoid touching it with bare fingers.
If mirror's surface requires cleaning it can be done with soapy water and soft cotton swipes.
Alcohol should never be used to clean uncoated FS mirrors as it may react with aluminum!
 

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