Mirror-O-Matic grinding machine
When I decided in 2009 after a ten-year break to get into mirror making again, it quickly became clear to me that I would need to construct machine, as hand grinding big mirrors is not something very beneficial for the hands of a musician. I started to look around and quickly found Dennis Rech's website about the M-o-M (Mirror – o – Matic) that he developed and generously offers to the ATM community.
The M-o-M is a marvellous machine due to its simplicity and yet refined design. It is extremely flexible accommodating mirror sizes from 4 inch up to 20 inch (with slight modifications) and can be used for all three steps necessary in producing a mirror. That is you can use it to grind, polish and figure your optics.
One problem I face during construction was that all measurements and plans are done in imperial units since the machine has been developed in the United States. I had to make a choice between building using imperial units with all the problems associated like hardware and machine standards, or convert the plans and metric units. I opted for the latter and I will eventually post the modified plans on this web page once I find time drawing them up.
Another difficulty was to find certain hardware which might be very common in the United States but very hard to find in other countries and in particular in Spain. In the sidebar of this page and in the links section of my website you can find some references pointing towards shops and online stores that might stock those items.
I slightly modified Dennis' design in particular concerning the choice of motors and motor control. You can find those modifications documented in the gallery section of this page. I would also like to draw your attention to the
M-o-M Yahoo! group
that offers extensive support for construction and operating that type of mirror grinding machine.
Make sure also to checkout my M-o-M videos that show the machine in action!
- Image Gallery
- Videos and operation
- Construction plans
- The M-o-M is purposely based on simple parts. You can see for example in those photographs the self lubricating bronze bearings for the turntable and eccentric drive shafts. No measurements are particularly critical even though in converting the plans to metric system, I had to take care to keep dependent measurements in line.
As you can see in the images the main housing is basically a plywood box to support the drive shafts and provide storage space for the motor-block.
I mounted my M-o-M on wheels in order to be able to move it around my basement.
- Those are the main parts of the M-o-M:
- The turntable is made of two 19 mm plywood discs and sits atop one of the drive shafts I conveniently drilled holes and inserted T-nuts to provide predefined settings for the mirror support cleats.
- The drip pan is a standard flower pot drip pan. The largest, I could find was 600mm in diameter. It is made of polypropylene which made gluing other plastic parts to it a bit difficult. However, Loctite™ produces a special glue for sticking polypropylene to other plastics.
- The eccentric moves the grinding/polishing arm. It can be adjusted from zero to Max. swing.
- The drive arm
- The turntable and eccentric have pulleys mounted to them to facilitate attachment to their respective axis. I used a 40 cm pulley for the turntable to give sufficient support over a wide area.
- The drip pan is an ordinary flowerpot saucer. It is 60 cm in diameter and has been drilled in order to provide a water outlet. The central hole for the axis of the turntable is protected from the water with a piece of PVC pipe that has been glued to the drip pan. The drip pan is made of polypropylene and gluing anything to the material is quite tricky. After some trials I found a special Loctite glue that bonds difficult plastics like polypropylene, Teflon™ etc.
- One characteristic of the M-o-M is that the motor block forms a completely separate unit that can be taken out from the housing for servicing, adjustment or cleaning. Also, to switch from grinding/polishing configuration to figuring operations, the motor block is simply turned around in order to invert the speed ratios between turntable and eccentric.
I am using cast iron pulleys as those were easily available where I live. Other people commonly use aluminium pulleys and some even make their own wood pulleys. The M-o-M can be adapted to anything that is available.
The same goes for the motor itself. The construction manual asks for a single phase AC motor but I decided to use a three-phase motor instead, as they are cheaper, more easily available and more robust. In addition, by attaching a variable frequency drive, I now have different speeds available for grinding, polishing and figuring.
- The variable frequency drive is controlled via logical inputs which in turn are activated through the control panel located at the front of the machine.
Controls available through the panel are:
- 3 predefined motor speeds
- Continuous motor speed (minimum and maximum speed predefined)
- Ramp up speed
- Ramp down speed
- The grinding/polishing tool is held in place by the quill of the M-o-M. The end of the quill is comprised of a swivel nut that fits into a socket at the back of the
I turn those sockets on my lathe from aluminium and usually keep several of them handy in order not to have to change the sockets during the different grinding/polishing stages.
M-o-M videos and operation instructions
The following videos show some of the key operations on a M-o-M. They are also available on YouTube. Some of the operation details depend much on the particular machine and most if not all of the basic instructions are covered in the Dennis Rech's M-o-M manual, but I will try to give some guidelines and tips from my experience that can make operating a Mirror-O-Matic even easier.
Spin polishing of 12" mirrorThe turntable rotates with aprox. 40 rpm. I like to grind and polish at fairly slow speeds and rarely go beyond 40 rpm.
You can see that the eccentric introduces a slight swing. It is important to pay some attention to that swing, as it helps us to avoid introducing ring patterns and zones into the mirror figure like the ones you can see on this photograph: Due to the regular pattern of the pitch lap, rings have been "engraved" into the glass.
By adjusting the swing of the eccentric to an amount that is slightly more than the diagonal of 1 pitch square we make sure that no ring zones are polished into the mirror.
Spin polishing 16" mirrorHere you can see a 16" mirror being polished. Contrary to the 12" example in the previous video, very little CeO and low turntable speed is being used. The reason for this is that even using very fine optical grade CeO, as I do, a thick mix of polishing compound will generate a rough surface, as can be seen on this Foucaultgram of a completed 12" mirror: You can clearly make out the roughness of the surface which ultimately reduces contrast and degrades the image. I mix 1 teaspoon of CeO in 3 litres of water which makes for a very thin solution. As a consequence you can here the high pitched whining noise which is a hallmark of very good polishing action.
Figuring with the M-o-MTo use the M-o-M for figuring the motor block has to be inverted, so that the turntable turns slowly and the eccentric turns fast. One of the fantastic features of this type machine is that this can be achieved without having to swap pulleys or motors. Two mistakes can be seen on this video:
- Too much CeO has been used which will result in rough surface
- The tool should slowly rotate under normal circumstances. Instead it describes a "fishtail" movement. The tool had not been pressed long enough in this case and a slightly longer warm press took care of the problem.
Figuring with various offsetsIn this video you can see how we attack different zones of the mirror by changing the "tool offset".
Remember: We call offset the movement of the tool along the longitudinal axis of the M-o-M overarm.
Correction is being put in the edge by constantly incrementing offset, as more and more "lap time" is spent over the outer zones.
The lap is rotating perfectly in this video. Observe also the swirl pattern in the last scene.
- Mirror: 296mm / F5.68
- Lap size: 140mm
Automatic drip systemIn order to allow for unattended operation I am using an automated drip system that I have first seen in a shop setup of Rob Riederrich and Ricardo Dunna. It makes use of capillary action and gravity by letting the CeO mixture or water (depending on whether I am polishing or grinding) flow through thin drip tubes at a controlled rate. Since the tubes can become clogged, I always use 3 tubes simultaneously to make certain that the mirror is kept moist at all times.
I use an aquarium bubbler to keep CeO mixture in suspension for polishing.
Automatic mirror rotationIn order to avoid astigmatism it is very important to rotate the mirror on the machine. This requirement would normally make automatic and unattended polishing difficult to do. However, there are several ways to make the glass turn passively through the friction exerted from the lap. I use a method that I picked up from Ricardo Dunna which consists in placing the mirror on a thin Teflon™ sheet. The low friction between glass and Teflon™ lets the mirror rotate freely in small increments.
Curve generatingGenerating the curve on a mirror blank is a tedious process when done manually or a messy (and dangerous) affair when done with machines. After reading an article by Ed Jones about using diamond pellets for that task, I wanted to try the idea on the grinding machine. For that purpose I glued diamond pellets to a plaster tool and used it like a normal grinding tool on the machine. The tool should not be bigger than ½ the diameter of the mirror. I used fairly high turntable speed and lots of water for grinding.
Curve generatingThis video shows the grinding action after the tool has been broken in. Efficiency of the grinding was not as high as expected and diamond wear higher than expected due to the diamond supporting matrix being too soft. I got another set of pellets with a harder matrix and will give it one more try on my next mirror. Alternative options would be to use diamond saw blades instead of pellets.
Eventually, I will post here metric conversions for the M-o-M and Alibre 3D-CAD models.