Mirror grinding machine construction log
I started thinking about building my own mirror grinding machine several years ago.
Having been on the ATM list now since almost the beginning of it here on the web, I had
lots of of inspiration reading of others projects and viewing their webpages

I started out with wooden table and some bits cobbled out of our scrap bin at work,  I didn't
get to far and put it aside for a while as I built several med sized dobsonian telescopes, A 15"
for myself and a 16" for the NBANB.  Both of these mirrors were bought, but I still wanted
to grind one myself.  I ordered an 8" kit from Willman-Bell but it has sat on the shelf for
several years now.  

The current project really started out as a design exercise for my CAD work,  something to
hone my skills on. About the same time we scrapped a DoAll lapping machine at work.  Looking
over the interior of the machine I realized there was a treasure trove of parts inside, ones ideally
suited to building a mirror grinder with.   The machine contained a large speed reducer (47-1 ratio),
a variable speed mechanism, motor mount and tachometer.  the frame was too small to be able
to fit the over-arm drive in so I left it.  I started out copying all of the existing parts into my CAD
program.  I then sort of filled in the blanks.

Cad drawings

cad drawing 1

eccentric drive

Construction pictures- all picture are linked to larger files, just click on the picture to go to the larger image
                                All pictures taken with a Kodak DC-290 digital camera at high res- 1900x1200 pixels
                                but have be reduced in size and compression to speed download.

Basic frame after welding, 24w x 36l x 36h
constructed of 2 x 2 x 3/16 angle.
mirror grinding stand 1

Various components for the mirror grinder.

motor mount 1 motor cradle  

overarm 1  overarm

 speed reducer  Speed reducer

Here are some pictures I took after assembling the platter drive components (Feb 8, 2002)

patter drive 1

These photo's show the various pulleys and linkages that control the variable speed drive

  platter drive assm 5      

You can see the sprocket and chain drive that moves the pivoting mount for the variable speed pulley

platter drive assm 4    

I spent a sunday afternoon at the shop machining the base for the eccentric drive.  Note the ruled markings on the side
to set the tool stroke.  I still have to put on  the marked divisions.  It would be nice to engrave them, but I will probably
Just use number stamps.

eccentric 1

I now have the frame painted and home in the basement.  The top has been glued up and the hole for the platter is in it.
I still have to drill the bolt holes for the platter and overarm as well as the eccentric mounting area.  I will coat the top
with fibreglas and epoxy resin to keep the water off of the MDF.

These pictures are of the intermediate shaft bearing blocks for the speed reduction of the gearmotor to run the overarm
and the bearings for the eccentric.

bearing block 2

I have machined the bar that the bearing for the overarm reducers mount to.
reducer frame 3

Its august now and I have started to assemble the components together with only a few pieces left to make.
Here is the reducer frame with the gearmotor and intermediate bearing assemble and the standoff
which will mount the unit to the bottom of the MDF top of the machine.



Here I have the pulleys mounted on the reducer frame and locating it to drill the holes to screw it onto
the top.


A closeup of the standoff brackets


The reducer frame attached to the table top and ready to turn over and bolt up to the table frame


Here are the clamps to possition the tool or mirror on the platter.

platter with tool

and the swivel mount to attach the mirror to the overarm

mirror plate swivel

The grinder nearing completion, with all the parts inplace but not hooked up yet.
The Speed Reducer still has to be aligned and the Variable Speed drive must be aligned to the
pulley of the speed reducer.

assemble machine 4

And a closup of the eccentric drive hooked up to the overarm

assembled 4

And a closeup of the eccentric itself.

assembled 5

I have the tachometer installed and the label plate for the speed control wheel attached.
The tach reads to direct rpm and indicates that my platter will rotate from 6rpm up to aprox 60.
I had to install a weight on an extention attached to the motor mount to keep enough tension on the belts,
They are quite wide and thich and the weight of the motor alone was not enough to keep the belts from slipping.

tach and speed reducer    

Here is a closeup of the tach motor.  It runs from a rubber O-ring around the input to the speed reducer.


I placed the blanks on the platter to check the holding ability of the clamps I made up and used a magnetic base and indicator
to check the runout.  It only took a min and I could get the blank centered within .004" and most of that was irregularitys in the
blank.  I will mount a block of steel later to attach to magnetic base to.

blank setup

And some other views of the setup.  The overarm is just resting on the blanks, nothing is glued or pitched together, just posed for the pictures :-)

     setup2      setup3

Next step is to make a tile tool so I don't have to waste a perfectly good pyrex blank :-)  and I have a dishpan that I have to modify to sit under the
platter to catch the grit and gunk.  I have made up set of ply forms to match the mount under the platter and will heat the plastic pan and press it to shape.

first wet on grinder        First wet on grinder closeup

I finally got the drip tray made after a couple of tries.  I found a plastic tub the right diameter and then cut it down to the proper height.  I cut out a center hole
and used some of the remaining plastic to make a dam inside to keep the sludge and water away from the center shaft and bearings.

I started grinding on the 22nd of July with an 8" pyrex blank from a Willman-Bell kit I bought several years ago.  I will start with an f8 curve to start and see
how the machine works.

I also made a short animation of the grinding movement  in AVI(1.7mb)   or as a animated GIF(2mb)

GRINDING LOG-----------------------------------------------------

July 22, 2003               Machine settings - Platter set at 15rpm - Overarm speed - 6 strokes per min (fixed), Stroke+/- 2.375"
                                     aprox 10 lbs weight on mirror.  Started at 6:10pm using 60grit SiC.  Applied grit at aprox 5min intervals
                                     using shaker for grit and water in spray bottle.  Increased platter rpm to 23rpm and increased weight
                                     to 15lb after 15min and ran for total of  1hr.  This cleaned most of the molded features off of the blank
                                     and gave me a sagitta of .002" on the 3.5000" spherometer.
                                     Ran 2 more 1 hour sesions with wets every 5min.  Molded features all cleaned up.  Sagitta to .004"

July 23, 2003               Increased weight to 15lbs, platter at 23 rpm, stroke at +/- 2.375"  60grit SiC
                                     2 more sessions of 30min each.  Sagitta at .0058

July 24, 2003               Increased weight to 30lbs, platter at 23 rpm, stroke at +/- 2.375" 60grit SiC
                                     4 sessions for a total of 2hrs final sagitta of .0119 or a ROC of 128.74"

                                     Clean up-remove tool from platter and clean all parts to all grit.  removed tape from mirror and replaced

                                     Started on 120grit SiC - Machine settings - platter 31rpm, stroke 6rpm(fixed)  Stroke +/- 2.0" 15lbs weight.
                                     2 session totalling  1hr.   Most of the pits from the 60grit gone.  The outside 3/8" still a bit rough.  It will
                                     probably take another couple of hours to finish with the 120 grit.  Sagitta increased slightly to .0124 or
                                     a ROC of  124" 

July 25, 2003               Two more sessions of 30min with 120grit SiC,  platter 31rpm, stroke 2.0" 10lb weight. 
                                     I applied some Dykem layout dye to the outer 1/2" of the mirror and a couple of bands across the mirror.  
                                     After the first 30min most of the Dykem was gone but you can still see an area on the outer 1/4" with
                                     some blue specks.  There are still a couple of pits but the dykem was all gone after the hour of grinding. 
                                     Total of 2 hrs at 120grit so far.

Dykem test              Tile tool

July 27, 2003               4 session of 30min with 120grit, platter 31rpm, Stroke 1.625,  weight 30lbs.  Sagitta increased to .013.

July 28, 2003               Stripped setup down, and switched to TOT.   Started on 220grit SiC.   Platter 32rpm, stroke 1.25"  Weight 15lb
                                     3 sessions of 30min, sagitta slowly coming back, 0.0128".   Switched to stationary overarm with occasional
                                     oscillation and switched from one side to the other at 5min intervals
                                     2 sessions of 45min, sagitta 0.0125
                                     Mirror seems uniform so I switched to 320grit SiC.  2 sessions of 30min  Sagitta 0.0124

July 29, 2003               Cleaned up and washed down platter mirror and tool.  On closer examination found a half dozen pits. 
                                     Switched back to 220grit.
                                     2 sessions of 30 min, 220grit, platter 32rpm, stroke 1.25, stationary overarm with occational oscillations. 15lb weight.

July 30, 2003               Started on 320grit SiC.   2 sessions of 30 min,  platter 32rpm, stroke 1.25, overarm stationary, weight 15lb.
                                     Sagitta slowly coming down to goal.  Now at 0.01215".   Very uniform across mirror

July 31,2003                switched to 500grit SiC.  Mixed grit with water and drop of detergent in squirt bottle.  Settles out very fast,
                                     have to shake to mix every couple of minutes.  Platter 32rpm, stroke 1.25.  Overarm stationary.  Weight 15lb.
                                     3 sessions of 20min each.  Sagitta now at 0.01205" 
                                     Cleaned mirror, platter and tool, dried mirror and check with low angle light.  No Pits visible. 
                                     Very uniform surface under 7 power loupe.

3.5" spherometer              4" spherometer

Aug 5, 2003                 I was off to the Nova East Starparty for the weekend, a good turnout but lousy weather.  We were rained
                                    out for 3 days. :-(
                                    Started on the 12micron grit today.   Platter 32 rpm, 10lbs weight, alternating from one side to the other
                                    of the platter with 1.25"stroke.  Leaving the tool at each side for 2-3 min then switching sides. 
                                    One session for 15min then Near Disaster :-(  The screws holding the tool to the pivot plate came loose
                                    after I let it get a little to dry.  Luckily I got it stopped before any damage was done.  One of the main
                                    causes was the lack of grooving as the perifery of the tool was ground down.  I drilled out the holes
                                    where the screws had been and inserted some plastic wedges normally used in drywall and used some
                                    longer screws.    I then grooved out the tool to let the grit and water circulate a little better.  I removed
                                    some of the outer partial tiles rather than have them fall out on the mirror.   This also made the tool easier to
                                    remove from the mirror without having to slide the mirror to far to the side.
                                    I started back on the 12micron grit doing another 2, 10min sessions

Aug 6,2003                 Did another 15min session with the 12micron grit.  Surface seems to very uniform with no pits I can find with
                                    the flashlight.   Cleaned up and then switched to 5micron grit.  2 session for total of 30min,   same conditions
                                    as previous.

Aug 7,2003                 Finished up with the 5micron with another 30min today.  Cleaned up the machine

Aug 11.2003               Started my pitch lap today, what lovely stuff to work with :-)  I have little peices of pitch stuck all over the
                                    place now.  Its even on the body of my mouse here.  I have an extra blank so I used that for the pitch tool
                                    instead of making a new dental plaster tool or using the existing tool (just in case)  I melted the pitch and
                                    preheated the blank in hot water.  I wrapped the blank with masking tape and then poured  the
                                    pitch and got the mirror on before it cooled to much to form properly.  It didn't form conpletely so I dunked
                                    it back in hot water for a while and hot pressed to for a couple of hours.  Then came the fun part, grooving
                                    the tool.  I tried the heated brass strip but it was too slow.  I used the slicing element on my Weller soldering
                                    gun to melt out the grooves.  Messy but effective.  I used the soldering gun to fill a couple of large
                                    bubbles.  Hot press the tool again and left it overnight with about 30lbs on it

Aug 12, 2003             Started in with the CeO on the pitch lap.    32 rpm, 10lbs, 1.25 stroke, Mirror on Top.  Stationary overarm at
                                   the stroke limits.  Starting to get some vibration on the downward side of the stroke.  The drag on the tool is
                                   pushing toward the overarm pivot on the downward side and causing it to jump.  So I am doing most of the
                                   polishing on the up side. 
                                   I did 3-10min sessions.  The polishing seems to be going very fast. 

Aug 15, 2003             Cold press the lap after last night with the netting in the Willman Bell kit.   Did  another  10min today. 
                                   The drag is much higher with a lot more vibration on the downward side of the overarm.   Finish is getting
                                   much better, only a slight haze with the flashlight and even then is hard to see unless pointed toward you. 
                                   Left the mirror on to cold press for the night.  Wrapped it with plastic to keep the water from drying out and
                                   make it easier to get apart in the morning.  (Didn't realy help that much)  Getting close to time to start testing,
                                   contacted a friend with a foucault tester to borrow it.  Hopefully this weekend.

Aug 16, 2003             Did 3 10min sessions, cold pressed for the afternoon and did 2 more 10min sessions.  Center very clear now,
                                   no spot with laser in the middle1/3.

Aug 18, 2003             Started and did 5min but getting a lot of grabbing and the facets are getting really closed in, I decide to recut
                                   the lap, takes about 1-1/2 hours.  Gawd what messy crap this stuff is.  So I cleaned it up and rebeveled the
                                   lap edges as well and brushed off the debris.  I soaked the lap in hot water and then hot pressed it for an hour. 
                                   Put the mesh in to cold press for the rest of the night.

Aug 19,2003              Polished for 2-20min sessions in the afternoon and then again in the evening for 3-20min sessions. 
                                   All but less than 1/2" of the outside is fully polished now. 

ptich lap

Aug 24, 2003              Did another hour of polishing in 3-20min sessions.   Mirror seems to be almost finished, maybe an 1/8" of pits
                                    showing up with the laser pointer.
                                    My buddy Garth Arsenault brought up his tester and gave me a quick course in its use.

foucault test

Aug 25, 2003             I was a little dismayed at the first results of the foucault test.  The Ronchi test was even worse :-(   Obviously
                                   I had some severe zones developed by leaving the mirror at rest to one side as the platter rotated.  A little depressed I
                                   set to work after the testing to repolish.  I set the machine to contiuous oscillation and polished for an hour straight.  I was
                                   amazed when I put the mirror back up on the test stand and all of the zones were gone.  The foucault test was a uniform
                                   green (from the LED) and the Ronchigram using a 50 lpi  I made on my laser printer appears to be almost perfect :-)

Ronchi test

Sept 2, 2003              I havn't started the figuring yet.  I have been cleaning the basement and installing a new workbench I have built.  I have
                                  been doing a lot of research trying to determine the best course of action in parabolizing the mirror.  Presently stuck
                                  between a subdiameter lap and a star lap,  maybe I will try both.  I also want to build a tester for myself that will allow
                                  me to use my camera a little easier.   The test photo's above were taken with my Kodak DC 290 digital camera.  It was
                                  a royal pain placing the camera at the right point of the focus while bumping the tripod around to find it. :-)  I am drawing
                                  up a tester to mount the camera and use a couple of suplus micrometer heads I bought for just this purpose.   I don't
                                  like getting down and squinting, plus the fixed camera is a more accurate, not relying on my head position and judgement.

Sept 4, 2003              I have started on the Foucault tester today.   Here are some cad drawings and some construction photos.

foucault test cad drawing          Foucault tester cad drawing 2           

foucault tester constrution1          ball bearinghttp://www.jamescrombie.com/pics/ftcad1tn.jpg

construction 3          constriction 4

                                  The tester is built from 5/8 melamine coated chipboard, 1/2" baltic birch ply, 1x1x1/8 alum angle, 5/8 ground steel rod
                                  and some 1x1/8 alum flat bar.  No machining involved sofar, just a table saw, drill press and a bandsaw,  but you could
                                  get by with a hacksaw.  The v-guides were cut and then clamped together and filed to final shape to keep then as similar
                                  as possible. The 5/8 bar was drill and tapped in the center and is held in place with a 8-32 x 1/2 screw.  The v-guides are
                                  also drill and tapped  8-32 and you can see the countersunk holes in the top board.  The holes are slightly oversized to
                                  allow for positioning of the y-guides to keep everything square.  A furniture glide rests on the aluminum strip for the
                                  third leg of the platform.  In the middle right picture is the anvil for the micrometer to push on.  in the center of the
                                  position of the end of the micrometer is a carbide ball left over from my 3.5" Spherometer.  Without the ball bearing the
                                  micrometer will grab the aluminum and try to lift it.  You can buy micrometer heads with a ball or radiused end, but they
                                  don't go for $9.99 at Princess Auto :-)  I bought the micrometers, disassembled them, sawed off the anvil portion and then
                                  chucked it in a lathe and turned it round to be held by the baltic birch clamps.   A cheapy X-Y axis stage.

tester picture 6          Tester picture 7

Sept 11, 2003               I have the Foucault test finished,  several coats of varnish and the LED wired in.  I used a blue LED from Radio Shack.
                                  Not wanting to be left without a battery while testing I opted to use a 6v wall-wart from some battery powered tool of some type.
                                  The blue LED is quite bright , plus I ground and polished the end of the LED to reduce the spread of the light source.   I now
                                  have to finish cleaning the basement of all the wood dust from the project.

Sept 30, 2003            Finished up an adapter to use my buddies mirror holder for the Foucault tester.  I don't want to have to remove the mirror from
                                  the adapter for the overarm linkage, so the adapter has a hole in the middle to keep it off the back of the stand.  I used 3-5/8"
                                  hardwood dowels in a circle just slightly larger than the mirror.  I cut 3 sections of rubber hose that fits snugly over the dowels
                                  to act as retainers for the mirror.  The lower 2 dowels are glued in and the top one is a snug fit that jams inplace and then the
                                  rubber hose is pushed into contact with the mirror to prevent it from tipping forward.  So with everything ready I can start figuring
                                  the mirror now. 

foucault tester pic 8          Foucault tester pic 9          Foucault tester pic 10          Foucault tester pic 11

Nov 14, 2003         Havn't done much on the mirror in the last month, I have been busy with some other projects and the mess in the basement
                               has procluded andy serious mirror work.  I did play with a star lap but the results were not good.  I hot pressed the lap with
                               a plastic cutout to relieve the outer 1/3 of the lap with 8 triangular sections.  I should have cold pressed the lap afterward because the
                               results were not to good.  Back to full contact and then clean it up back to a sphere.
                               foucault test oct 3

This page  updated Sept 31, 2003