I started this thread to record my building progress of a new mirror grinding and polishing machine. This machine will be capable of handling mirror sizes up to 22". I have decided that 22" is the absolute maximum otherwise becomes too cumbersome to move around.
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Requirements
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Part One - Machine capability
- With a 600mm turn table, the maximum aperture size = 22 inch;
- Minimum aperture size = 4 inch;
- Be able to polish parabolic mirrors for Newtonian reflector;
- Be able to polish hyperbolic mirrors for Cassegrain secondary mirrors;
Part Two - Machine feature
- Easy to change the speed of rotation, reaching into the machine to change pulley belts is to be avoided at all cost;
- The overarm of the machine must be fold-able upward and does not drop on to the mirror (this feature is borrowed from the Mirror-O-Matic), this way I can easily remove the mirror and tool for inspection;
- Changing eccentric position, as well as overarm position must be quick and precise;
- Purified water supply with 1 micron filter to guarantee good quality water and no impurity that might scratch mirror surface;
- Automatic water drip with controllable dripping speed;
- A built-in timer and laud alarm clock so operator (me) do not have to watch the machine all the time;
- Turntable top can be removed, mirror is securely clamped on the turn table, so that optical tests can be performed on the mirror without having to remove the mirror;
- Easy removal of the tool;
- A splash pan / sink under the turntable to protect machine from grinding access, also be able to wash the mirror on the turntable directly is a required feature; Doesn't have to be circular;
- Pipe connected below the sink / splash pan to drain water and powder;
- A panel on the front of the machine with circular slots cut, so that plastic containers with various grades of abrasive can be accessed easily;
- Optional draws to hold various bits and pieces;
- Weights can be added on top of the overarm to speed up the grinding process;
Part 3 - Machine construction
- The frame of the machine must be sturdy and not to be easily bumped or deform in shape;
- The machine must have lockable casters;
- Vibration of the machine must be minimized;
- All exposed parts of the machine must be water proof, and best if not built of iron to resist rusting; Galvanized steel or iron might be ok;
- 1/3 horse power single phase electric motor, so that I don't need special 3 phase power supply;
- The reduction mechanism must be simple and compact, minimum number of pulleys whenever possible;
- Minimize the number of stress points on the machine, if a stress point can not be avoided, then it must be properly reinforced;
- The finished machine should be at least not "ugly" if not attractive;
- The turntable should be 600mm in diameter (~24", with 2" reserved for fixing mirror clamps;
- Inside the machine, machinery (pulley, motor and gearboxes) and pipework must be in separate water-tight compartments;
- The rotation speed of the turn table is to be from 1RPM to 60RPM;
- The rotation speed of the eccentric drive is to be from 1RPM to 60RPM;
- The rotation speed ratio between turn table and eccentric drive must be variable;
- An on/off button on the control panel for the motors;
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Work in Progress
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28/05/2010
I have won 2 x 1/3 horse power single phase electric motors on eBay, at $45 each. The motors are in great shape and are absolute bargains. What's even better is that the motors come with pulleys attached already. The RPM of the motors are rated at 1350RPM.
I have bought 2 x alloy reduction gears of great quality on eBay, again at $45 each. The reduction gears have ratio of 60:1. Each of these compact reduction gears is made of quality alloy and will give me 20RPM when coupled with the electric motor. These gears again are absolute bargains.
29/05/2010
I have won a coffee table of 120mm x 60mm x 45mm from eBay, at $80. This table is absolutely well made, so heavy that I can't lift it from the floor. I tried to bump the table after assembly, the table did not even move. Absolutely fantastic. Well, I was initially going to use this table as foundation of my machine box (I am a lazy builder), but since this table is so good, I put it in front of my TV as entertainment unit. I am going to bid on a second table so that I can convert that table as the basis of the machine.
30/05/2010
I have built the overarm from aluminum alloy tubes. The hinge is a mini barn door hinge, 2 x 1 inch on both sides. The door hinge however is a bit shaky and will twist and wobble with any sort of load. To solve this issue, I bolted two aluminium tubes left and right of the centre arm so that the centre arm when dropped will sit in the slot of the aluminum tubes neatly. Problem solved.
I have spotted two round tables of 600mm diameter. Each round table is 1-1/2" thick and made of timber. I will convert these round tables into the turntable and the removable table top.
Having difficulty locating a proper overarm with swivel joint at the end. Bought a sizable bolt and nut of 1/2", with washer. If the end of the quarter ball shaped bolt can be polished, I may not need to buy a swivel joint anymore.
Have to say I am not entirely happy with the overarm, may end up building a wooden one of the Mirror-O-Matic type next week.
31/05/2010
The aluminium tubes are perhaps best reserved for other use than building the overarm, I will redesign and build the overarm with steel and iron parts. Now is time to utilize my Arc Welder. I really like the overarm arrangement of the Big-O-Matic, will build something similar.
Originally I was going to multi-speed chain drive + derailing setup to vary speed ratio, this would be cumbersome and easy to break down. (Imagine how reliable the gear changing on the bike goes, sometimes even cause the chain to derail completely and jam the bike).
A much better solution would be to build a speed controller for the motor, one such design I found can vary motor speed from 5% to 95% of the rated RPM, while maintaining torque. This is EXACTLY what I am after! The motor + gearbox + controller will give me smooth and precise speed control for turntable, the stroke arm and the eccentric. Problem solved.
UPDATE (31/05/2010)
The turntable is going to be an easy job now, just won two 600mm diameter round tables on eBay, EXACTLY what I am looking for! Now I don't have to dread myself with the task of making a perfectly round turntable from concrete/wood/metal/whatever anymore, it's all done for me - the tables are perfectly round, with 3inch thick metal support shaft below, all I have to do is to cut off the stand below the shaft, and use the top of the table + shaft as is. The table is water proof so I don't even have to epoxy paint it any more. Sweet!
The water purifier in the requirement is to be dumped, why put filter and unnecessary pipework in the machine, I can just fill the water bottle from my filtered water chiller.
Toying with the ideas of step motor controlled stroke arm and eccentric positioning, controlled by joystick mounted on the control panel, plus a 5 axis controller from the US. If this idea flies, then I will be able to hook up the controller to my laptop later. Then I would be able to automatically control the following:
- stroke arm position
- eccentric position
- turntable rotation
- the water drip speed
- the powder feeding routine
- automated mirror flushing / washing
1/6/2010
Drawn a rough sketch of the machine, top view, enough to conceptualize its operation. It's a cross between MOM and Draper, with an up-folding overarm feature borrowed from MOM. Really like the simplicity of MOM, but like the flexibility of the variable driving cranks of the Draper/Hindle type of machine. So I will incorporate both features. See drawing below:
http://rxl001.fileave.com/MachineTopView.jpg
Another possible layout, the stroke arm pin can be fixed or sliding, so that both a Hindle type of action as well as MOM type of action can be selected. Similar in concept to the first design, but simplified. The rotating eccentric crank is replaced with a re-positionable pin. The draw back is the limited number of stroke pattern possible compared to the first design, but simpler to build and the stroke arm has better balance.
http://rxl001.fileave.com/Machine%20Top%20View.jpg
24" Mirror Grinding and Polishing Machine
Please. I'm more of a visual person hehe.
