Having ground three mirrors by hand I was
getting to the point where I though how nice it would be to have a machine
that would take a bit of the tedium out of the work for me. Ok. I was getting
lazy as well. But that's beside the point. I already knew what type of
machine I wanted to build. I was inspired by an article in the Sky &
Telescope November 1995 issue by Tom Waineo. This design called for a turntable
riding on 2 to 3 upturned casters and driven by a geared motor which in
turn drives an eccentric driving a stroke arm. See an original article
by Tom Waineo/Mark T. VandeWettering here and/or try get your hands on the Sky
& Telescope article. The biggest problem for me was finding a suitably
powerful motor to drive the turntable and stroke arm. I must also say I didn't
really want to spend a few hundred dollars on a suitable motor because at the
time I wasn't convinced of my talents to put a functional machine
together. So I had a rethink on how I could put a machine together for a small
outlay in cash. My attention was draw to a small DC motor, specifically, the
sort you find in an automobile driving the windscreen
wipers. They are
small, compact and have a tremendous amount of torque for their size. Best of
all they could be had in a car breakers yard for a few dollars/euros. Would they
work ? There was only one way to find out. I collected a couple of them plus an
auto battery (still in good condition) from the junk yard for about $20. I had
enough pieces of wood and steel piping at home to start on the machine. One
thing was clear. Using such small motors would involve a rethink on the
mechanics of the machine. In the original design the turntable sits on a spindle
supported by the 3 upturned casters and driven by a (approx) 1/3 HP
motor. My little motors possess only a fraction of this power (1/10,
1/20 HP ?). The motor in the original design must overcome a certain amount of
spindle and wheel friction just to drive the turntable, not to mention the
stroke arm. This friction wasn't a luxury I could afford. I would have to
redesign the turntable mount.
I decided to sit the turntable on a ball
bearing race thus ensuring the small motor had as little friction as possible
to overcome in driving the turntable. I made the
central spindle from 3/4" iron water pipes - the kind found in hardware
stores for a few dollars. I screwed a flange to the baseboard and firmly
screwed my spindle onto this. The spindle is threaded on both ends . On
the upper end of the spindle I was going to mount my turntable. It was
going to be designed so I could unscrew it from the spindle. The spindle
doesn't turn. It's just going to support the turntable. The ball bearing
support for the turntable was going to be simple. As I had decided to try
and source all my material from the local hardware store. I wasn't going
for anything exotic. I bought a large swivel caster ,the kind with a big rubber wheel. These
casters ride on a sturdy ball bearing race and are free to turn in any
direction. I was going to hacksaw off the two arms that hold the wheel which
would leave me with a base plate supported on ball bearings. The ball
bearing base was somehow going to be mounted on the spindle and the turntable -
cut from a 1/2" thick piece of waterproofed MDF - would then be screwed to the
base plate. With the caster suitably lubricated the turntable would then be
extremely easy for my small motor to drive.
Where I had cut off
the support arms for the caster wheel I was going to solder on a 'U', a
piece of scrap metal bent into a wide 'U' .
This 'U' piece of metal would enable me to attach the
assembly to my spindle. This took me a couple of tries until I managed to get it
half way right. Bending the piece of metal into a symetric 'U' shape was trial
and error for me. It then had to be hard soldered and centered -important if
everything is going to revolve about a common axis. I eventually got it halfway right. As it works quite well I
decided to use it.
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When the platter was mounted on the base plate of the caster I had a free turning turntable which, when spun by hand, would turn easily on the ball bearing race for many revolution before coming to a stop. Just what I wanted. Now to drive the turntable. Back to Top.
I had two windscreen motors. One was going to drive the turntable and the other would drive the overarm. Driving theturntable was easy enough. I mounted the motor on it's side underneath the turntable with the drive shaft parallel to the turntable. The drive shaft on the windscreen motor sticks out only about 10 mm (~½") and has a diameter of 6 mm (¼"). Onto this drive shaft I screwed an extension nut . Now, I don't know the exact technical term for this thing. Here's what it looks like. This extension nut effectively increases the length of the drive pin.
Into the other end
I screwed a short length (50 mm/2") of threaded rod. Onto this rod I mounted
a thin wooden wheel with a rubber
rim.
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The stroke arm in the original Waineo desktop machine is driven by a crossbeam connected to a vertical rod connected to a threaded horizontal rod (turnbuckle) connected to an eccentric plate (whew !) on the drive shaft of the main motor situated underneath the turntable. This drive mechanism is much too convoluted and inefficient for my small motors. I know because I tried to design my grinding machine just like T.W.'s. using the two motors but it didn't really work well. For grinding it worked well enough but for polishing it was a disaster. The little motor driving the stroke arm just wasn't powerful enough to drive the eccentric arm and lost too much energy on its convoluted way to the drive pin. I figured if I was going to be able to polish with these small motors the overarm would have to be driven directly with maximum efficiency and minimum loss of power. This is how I did it. Back to Top.
I mounted the second motor at the same height and on the right hand side of the turntable . I mounted it lying on its back with the drive shaft pointing up. I found the orientation (in what direction it was lying-front to back, left to right) of the motor to be very critical in my setup. I had originally mounted the motor lying from front to back. During polishing the forces over a time caused the motor to rock back and forth - following the back and forth motion of the strokearm. I then reoriented the motor left to right (looking from the front of the machine) and the problem was solved
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Having read through the ATM archives I noted that it was preferable to mount the drive pin as close to the polishing surface as possible. My tool is made from three stone floor tiles cut round with an angle grinder and epoxied together. I carefully drilled a 12 mm hole centered on the back surface of the tool to a depth of about one tile. Into this hole I epoxied a piece of copper tubing. The rounded drive pin of the stroke arm sits in this short piece of tubing. A squirt of grease in the copper inset ensures that the tool can rotate freely about the drive pin. From experience, when the stroke arm is loaded with weight the drive pin will not pop out of the hole even if the dragging force of polishing becomes too great. Instead, the the motor driving the eccentric tends to stall and stop. The drive pins' vertical motion is anyway somewhat constrained due to its design. Looking at the picture on the left it's easy to imagine the revolving eccentric plate pushing the tile tool (here on top) back and forward. Back to Top.
These are just little fellas. They can only do so much but with a good design they can do a lot. I run the two of them off a 12V car battery. When polishing they draw about 5-6A altogether. This was measured before the stroke arm was loaded. With a loaded stroke arm the current drawn could be upwards of 10A. The motors working with this load will get quite hot. I was concerned about this. I mounted a 12V cooling fan (the type that sits in a PC) behind each motor. The fan just blew cool air over the motor. This was more than enough to keep the motors cool.
My turntable originally turned at a lazy speed of about 18 RPM. The stroke arm was about 2.5 times faster. According to all sources this is the wrong way around ! Professional machines have a turntable (spindle) speed of about 5 times the stroke rate. But strangely enough I was getting good results polishing with these speeds. I did end up with some zones once but I traced that back to how I had wrongly repositioned the mirror on the turntable after some maintenance work. Mirror positioning is critical ! Try to keep the mirror cleats in the same position on the turntable while polishing out the mirror.
I just happened to have some small gears (cog wheels) lying about so I decided to try and gear up the turntable speed. It was quite simple. I now have a turntable speed of approx. 50 rpm. As a quick solution I connected 5, 1 Ohm/5W resisters in series to slow down the stroke arm speed to about 12rpm. I have since replaced these resistors with a 5-6 Ohm potentiometer which makes it easier to fine tune the stroke arm speed. I will put a potentiometer on the turntable motor as well some day. These potentiometers sure do run hot ! The machine now runs at the 5:1 turntable to stroke arm ratio and I'm very happy with it. I am impressed and enthusiastic at what these little motors can achieve. They are cheap and versatile. Back to Top.
Grinding and Polishing with Small Motors.
I have polished an 8" mirror with a full sized lap and can report that the motors were well up to it. To a point. These small motors can only push so much weight. It's just not possible to load the stroke arm with pounds and pounds of weight. There is a limit. I have (just to see how far I could go!) loaded the stroke arm with nearly 10 Kg (22 pounds) of weight - weight of lap included and the motors turned merrily. I didn't feel I was at the limit with this load on the machine. It could have taken another couple of Kg. Not at all bad! You'll have to experiment. More weight generally means faster polishing but less is also ok. It just means the machine will take that bit longer to polish out your mirror. As the machine is doing the polishing what's an extra hour or so here or there? Do read the general tips on grinding and polishing Tom Waineo gives in the above mentioned article. They're invaluable and detailed enough to get you started. Another source of tips and tricks can be found in the ATM archives. Enter the search word 'Machines'. You'll get more than enough reading material :)
I would estimate that grinding a 12-14" mirror would be no problem. Polishing a mirror of this size with a sub diameter lap would also be doable. Polishing is done TOT with machines. All my grinding was done MOT- although changing back and forth between TOT and MOT shouldn't be a problem with the machine. For grinding I cut a disc of wood the same diameter as the mirror and bored a hole in the middle not quite all the way through. The disc of wood was then attached to the back of the mirror and held in place with nothing more than duct tape. It held very well. The drive pin sat in the hole on the wooden disc and drove the mirror. Unlike the hole in the tool I didn't line it with copper tubing. The grinding forces are not excessive and the hole remained intact throughout the grinding process. To give me a better view of what was happening to the mirror I cut three large circle out of the wooden disc so I could see through the disc and observe the grinding action and bubbles between the tool and the mirror. I didn't initially hog out the mirror blank on the machine. I did that by hand. I don't honestly know if the small motors would be up to it.
During the grinding process I had already become a big fan of grinding machines. It just made the whole grinding process less tedious and more fun. Polishing was just as easy. I just charged up the lap/mirror with CeOx and left it do its own thing checking on it every 20 or so mins. and recharging the Lap. I had a small plastic bottle with my CeOx mixture in it. To recharge the lap I just squirted the mixture onto the exposed surface of the mirror. Note: you will have to experiment a bit with your stroke length, by adjusting the eccentric offset, to get the stroke arm movement just right. This is why I bored a number a holes in the eccentric plate so that I could experiment with different offsets. See the tips in Tom Waineos document. My longest polishing session was just over 2 hours. The only reason I stopped that session was that I had just about run the battery down. I didn't experience any major problems with the polishing process. I ended up with quite a smooth polished surface with a small trace of a central hill and a minuscule turned up edge. Both of which later responded well to a bit of hand correction.
I got a real kick out of putting the machine together and getting it working. It's true what they say about grinding/polishing machines having a character of it's own. There are so many variables in play, every machine will behave differently. Getting to know it is also part of the battle/fun. Do try and put potentiometers on both drives. It makes it easier to fine tune the (speed of the) machine. The machine went through many modifications and still isn't finished. Only by using the machine did I become aware of problems with the design. I made many adjustments along the way. I tried to avoid any major changes to the machine because I didn't want to stop the polishing process. Back to Top.
There are one or two things I would try to improve. For starters the machine would benefit from 2 (additional) idler wheels underneath the turntable. This would add increased stability to the turntable. The three wheel would then be spaced approx. 120º apart. Driving the two motors with an auto battery is maybe not so ideal. I don't know how the battery would stand up to the abuse of being constantly discharged and recharged. It's still working but I had the feeling that it tended run run down faster after recharges towards the final polishing cycle. As I got it second hand for a few dollars I can't complain. Maybe it wasn't in perfect shape when I got it. Still, I'll check out alternatives during the general overhaul I'm planning for the machine.
I mentioned that I had geared up the motor to increase the speed of the turntable. This was a quick and dirty job and not terribly stable but managed to make it through the polishing phase. A few inflight corrections kept it running reasonably well. All I did was to mount a 40 tooth gear wheel on the drive pin of the motor. This in turn drove a 15 tooth gear wheel to which the drivewheel was attached. This produced a nice increase in turntable speed. I will take some time to put these two cog wheels together in a small stable frame thereby creating a small gearbox. I'll also take a closer look at the turntable. I would like to make it a bit heavier and more stable. Although the ball bearing assembly gave me absolutely no trouble during the grinding/polishing phase I may nevertheless have another look at the ball bearing race to see if it can be improved upon. The next mirror I intend to grind on the machine will be a 12-14" so the machine will have to be absolutely stable for that one.
Back to Top © 2001/2007. Carl A. Brennan. Permission is granted for non commercial use.
