Grinding Machine for Large Telescope Mirrors

I designed and built a grinding machine for mirrors up to 42 inches [1.1m] in diameter. By slipping on and off a couple of belts, the machine can quickly be reconfigured for spinning work or for stroking work. The machine can also work smaller mirrors if the drip pan is moved to the fast shaft on the side of the machine.

I shared a Sketchup model if you want to see construction details.

Here's the 3D model.

A short mp4 movie depicting a thin meniscus 25 inch F2.6 being polished.

Some images followed by technical notes and links.

motor
0.5 horsepower motor with 50:1 reduction = 1725 rpm / 50 = 34.5 rpm output

reductions
2.3:1 from 6":14" 12.25:1 from 4":14" + 4":14"

Hindle/Zeiss mode
main stroke = 34.5 rpm / 2.3 = 15 strokes/min
platter rotates at 15 rpm / 12.25 = 1.22 rpm

Elgin-Waineo mode
platter rotates at 34.5 rpm / 2.3 = 15 rpm
side swing strokes at 15 rpm / 12.25 = 1.22 strokes/min

small table Elgin-Waineo mode
platter rotates at 34.5 rpm / 2.3 = 15 rpm
offset oscillates at 15 rpm / 12.25 = 1.22 rpm

axes separation
42" mirror with 21" radius, 80% stroke length = 17" stroke; 21"+17"=38" axes separation
motor to shaft and shaft to reducer set to 45 deg angle between axes or 27.25"

pulleys
V-Belt pulleys
6.25" dia, 6" 4L dia
14.16" dia, 14" 4L dia
4.75" dia, 4" 4L dia

belt lengths
V-belt 4L
motor gear reducer + 6" pulley to 14" pulley: 76" (qty=2)
14" pulley to idler of 4+14" pulleys to 14" pulley: 73" (qty=3)

motor dimensions
Face Mount Four 3/8"-16 tapped holes on 5-7/8" dia. B.C.
Base Mount Four slots on 5" x 3" centers
Shaft 5/8" dia. x 1-7/8" w/3/16" keyway
Size 8-1/2" x 7" x 8"

gear reducer dimensions
see pdf

paint
brilliant star yellow

analysis
Hindle/Zeiss style
25" stroke polishing should move no faster than one stroke every 4 seconds, or 15 rpm
need 34.5/15 = 2.3:1
rotating platter further 12:1 reduction

Elgin-Waineo
42" spinning should rotate at 20 rpm (20" mirrors at 40rpm, stroke at 5rpm) need 34.5/20 = 1.7:1
stroke can be further 8:1 slower

compromise at 2:1, 12:1
2:1 from 6":14" (2.3:1)
12:1 from 4":14" + 4":14" (12.25:1)

belt lengths
http://www.pyramidbelts.com/calc_beltlength.php

7/8" dia motor gear reducer to 1" diameter primary shaft distance = 22"
pulley diameters of 6" and 14"
belt length = 76"

1" dia primary shaft to reduction shaft 1" dia to secondary shaft 1" dia distances = 22"
pulley diameters of 4" and 14"
belt length = 73"

cost
$1800 (completed 2015)

1/2 horsepower AC motor
50:1 gear reducer
belts
shafts
pulleys
MOM parts list
mirror machine examples with moving graphics
machine figuring action
mirror o matic images
Jeff Baldwin's machine

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