Grinding an 8" f/6 Telescope Mirror... And making a mirror-grinding machine!
I was dredging through some old boxes a while back, and came across an old 8" mirror that I had ground way back in the early 70's. It brought back a lot of memories, as I had built a big wooden telescope in high school using that mirror. I won the Eastern Iowa Science Fair with that 'scope and went on to exhibit it at the National Science and Engineering Fair.
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September 24, 2001: Received mirror-making supplies that I ordered from Willman-Bell. Order included 1 Pound packages of silicon carbide abrasives: 80 - $9.00; 120 - $10.00; 220 - $10.50; 320 - $11.00; 500 - $12.00. 1 Pound packages of white aluminum oxide abrasives: 12 micron - $12.00; 5 micron - $12.00. 1/2 Pound white aluminim oxide, 3 micron - $9.00. 8 oz Cerium Oxide - $15.00. 4 oz Zirconium Oxide - $9.00. 1 Pound tempered Burgundy polishing pitch - $13.00, and 1 sq.ft. of Cera-Hex-Tol ceramic tiles (1") - $8.95. Total order, including tax and shipping was $137.04. Order was placed on 09/17/01. BTW, this is enough abrasives and polishing oxides to do several mirrors! |
Also ordered some dental plaster today (Kerr, Vel-Mix) 33# for roughly $35. Ordered it from Patterson Dental Supply in New Jersey (973-256-4900) and the very nice lady there said I should have my package tomorrow or the next day at the latest. I will use the plaster and tiles above to build a segmented ceramic grinding tool, if all goes well!
September 26, 2001
Went to a place called Beacon Electric Motor Company up in Keyport, NJ today (732-888-7888) looking for parts for the mirror-grinding machine. What a treasure of a place! He's got all kinds of new and used motors, pulleys, belts, and other stuff. He gets truckloads of cosmetically damaged industrial items and resells them. He has huge ladders, school-type lockers, shelving, compressors, heaters, and about a thousand industrial fans. One big room full of them he calls the "fan club!" He had a big band saw and a metal cutting saw, and all sorts of great stuff. The place is a hobbyist's dream.
I found a great gear-motor to use on the turntable of the grinding machine. It's a 1/2 hp, 480 in-pound torque, 60 rpm job with a built-in fan, totally enclosed. I paid about $200 for the motor, and he wired it with a switch and electrical cord. Also picked up a 5/8" shaft and some pulleys.
The big find of the day was a great shaft and a pair of mounted bearings. The shaft is 1 11/16", keyed on both ends, and drilled and tapped on both ends. Comes with two self-aligning ball-bearing pillow blocks, plus a collar for attaching a load. This thing came out of a huge 5 1/2' industrial fan. My Dad, my brother-in-law and I unbolted the fan blades and the cowling, and left them there for the scrap wagon. We took the bearings and the shaft, and a big, heavy steel table that everything was mounted on. The table is made out of 1/8" steel, and has two vertical plates that will be perfect for mounting the motors and building the skeleton of the grinding machine. The lot, bearings plus shaft plus metal work was had for $100. Total investment on the trip was $483.36. The owner says he gets new stuff every week, so this is one place I'll have to check back on quite often. Great stuff, and a very nice guy running the show!
September 29, 2001: Meeting of the S*T*A*R Astronomy ATM SIG (Amateur Telescope Making Special Interest Group)
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Our ATM SIG met on Saturday afternoon to work on several projects. Among them, we poured a plaster/tile tool for working my 8" mirror. Click here for more details, along with photos and movies of the meeting! |
October 1, 2001
This morning I took my father up to Beacon Electric to buy another gear-motor for the mirror grinding machine project. We found a nice 1/2 hp, 800 in/lb torque split-phase motor, and decided to buy it for the overarm. This motor runs at 30 rpm. I paid $150 for the motor, and the Beacon guy put an electrical cord on it for me.
I went through the McMaster-Carr on-line catalog this morning, and put together a fairly hefty order. Like Mike Lindner told me, their Internet site is pretty complete, easy to use, and contains a lot of useful information regarding their merchandise. I called the Dayton order desk to place the order, and had a surprisingly good experience. The moment I finished giving her a catalog number she knew what the item was, and gave me back the price. I placed an order for 10 or 11 items with her in about one minute. I asked if I could pick it up at the Dayton location, and she said, "Sure! What time will you arrive?" I set up to pick up the order at 1:00 today. When I arrived there (30 minutes from my house) the order was picked and waiting for me in a box, and all the paper work was finished! I asked how they wanted me to pay for it, and the guy said my account was all set up from my phone call, and I would get billed for the stuff! So I walked out of there a first-time customer with $250 worth of merchandise and no cash out of pocket! When I placed the order, she did get my company name and address, so I don't know if they would bill an individual, but if not, they take credit cards at the will-call counter, everything except Discover cards. All-in-all this rated as one of my best purchasing experiences. Many thanks to Michael for letting us in on this company!
Here's a partial list of things I bought: 5/8 bore pillow blocks and flange bearings at about $12 each; three 5/8 keyed steel rods at $25 each; an assortment of 6 4L V-belts, from 32 to 42 inch; a 7" and an 8" 4L pulley, both 5/8" bore; two 2" pulleys with 3/4" bore; and 10 split-ring 5/8 bore shaft collars.
October 2, 2001
Called Patterson Dental Supply this morning to see what happened to my plaster order. I placed it 8 days ago, and they claimed I would get it the next day. They called back two days later and needed my credit card number again. Now today the guy researched it for a couple of hours, then called back and told me they were sorry for the delay, and the plaster would ship today. Said someone forget to clear the shipment after fixing the credit card number. In any case, I'm supposed to have it on Wednesday the 3rd. We'll see!
Went to Home Depot to get wood and misc supplies for both the 8" mirror and the grinding machine. Picked up 4x4's for the stringers and legs, and some 2x6's and 2x4's for bracing. They cut everything to length, and charged me about $35 for the works. Some of the lumber was pretty wet, like it had been stored outside during the recent rains.
I also picked up some 2" fixed castors (3) for mounting under the turntable. Bought two fold-down saw horses ($50), and assortment of 3/8 lag screws and washers, a rasp and a carbo stone. Also got some long nuts (rod connectors) and 1/4 by 20 rod for a future Foucault tester project. And I picked up a 28" water heater pan ($16) to use for a splash catcher on the grinding machine. Also bought some T-plates and angle irons to use to strengthen the leg connections on the grinding machine.
When I arrived home, I started working on mounting the stringers onto the steel frame. Had to round one corner on each stringer to match the bend in the mounting ears on the frame. My father also solved an alignment problem on one of the mounting ears by loosening the holding bolts and repositioning the part to square it up. We used 3/8 lag screws to mount the stringers on the frame, and Dad convinced me to make dado cuts into the stringers to more securely mount the 4x4 legs. Have to go to the hardware store tomorrow and get a bit for the router, plus some clamps, to be able to make those cuts. Dad and I also figured out how to raise and lower the cutting bit on the little router he has. I'm trying to build this grinding machine without purchasing either a drill press, or a table saw. The only power tool I've used so far is a 1/2" hand drill, and the router tomorrow. Having Home Depot cut the lumber to length helped a lot!
Late tonight I decided to do a little more rough grinding on the 8" mirror. Set up the catch pan on the saw horses, put down a wet newspaper in the center, and ground with #80 grit for about a half hour or 45 minutes-- probably about 20 or 25 wets. Used MOT with the center toward the edge of the tool, making chordal strokes. After cleaning up, Renee and I both measured the sag on the mirror to get a rough check on the focal length. I measured five times and averaged .0808 inches, while Renee measured six times and got .0809 inches. If the clear face of the mirror is 7.94", that would be a focal length of 48.73 ", very close to our 48" target! The mirror was at 60" before we did about 10 wets at Michaels house. That's a total of about 45 or 50 minutes of rough grinding to deepen the curve. Now it's on to smoothing.
October 3, 2001
My plaster showed up with the UPS guy this morning. So I'll be ready to cast a base for the lap for the 8" mirror in a few days. Patterson finally comes thru!
Headed out to Sears after lunch. Picked up a couple of router bits so I can cut the dado joints for the MGM (mirror-grinding machine) frame. Big difference between the good ones ($15) and the cheap ones ($6). I always like to buy quality tools, so we'll see if the good ones are actually better! Found a great foot switch for $20, a tidy black unit that will handle 8 amps. You just plug your load into the 3-prong outlet on the unit, and click away with your foot. Should be a nice feature on the machine. Bought a 24" aluminum straight edge ruler as well. Picked up a kit for the router to be able to cut circles, and do a few other things. Also got a wire brush for cleaning the rasp.
On to Home Depot for more lag screws. Needed some 3/8x5-1/2 inchers for putting in the legs, along with washers. Got a pair of safety goggles. They gave me a left-over 2x4 piece a couple of feet long... I'll use it to practice the dado cut.
Got in a good day working on the MGM. Learned how to run the router, and put in the dado cuts. Got the legs drilled out and fastened to the stringers. My father was a big help. Kept me from doing at least two stupid things! I did end up putting the legs in a different place than I originally planned. Had a moment of panic when I realized I had pre-cut the 2x6 braces, and assumed they would now be too short. But luckily, they ended up being longer than the separation on the legs. I'll just leave them like that, and extend the bottom shelf a little toward the turntable end of the machine. I went ahead and marked the holes for attaching the braces to the legs, but I'll have to wait until tomorrow to actually put them in place.
October 4, 2001
Went to Home Depot today to purchase more lumber. Got three 4x8 sheets of 3/4" plywood to use for the MGM base shelf and table top. Also got a 2x4 piece of 1/2" for an end plate to stiffen the machine. Picked up an 8' 2x4 to use for a rip fence to cut the plywood with my circular saw. Also purchased 2x2 stock (2 8' pieces) to use for cleats under the top to attach it to the 4x4 stringers. Found a couple of good 2'x2' 3/4" plywood pieces to experiment with cutting circles on the router. Picked up fasteners galore to put everything together. Most importantly, I found out that you CAN put 3 sheets of 4x8 plywood in the back of the Honda Van!
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On the machine itself, I finished attaching the 2x6 skids to the bottom of the 4x4 legs, and Renee and I mounted the legs on the metal frame. It's now standing in the driveway, looking great! The frame has absolute rigidity in the long direction. Hopefully the end plate will provide the same strength in the side-to-side direction. |
Talked with Michael Lindner tonight after the S*T*A*R meeting and we discussed different aspects of our designs. Michael warned me about warping in my top if I try to laminate the two sheets of plywood with the facing grains running in the same direction. Also he and Andy Zangle warned me that my idea for using a turnbuckle as a linkage between the overarm cam and the driving lever may not work very well. Thanks, guys!
October 5, 2001
A rather hot day today. I cut out the end panel from 1/2" plywood with the circular saw. Got the machine leveled and square and drilled and bolted that stiffener across the legs on the end. Made an amazing change in the stiffness of the table. Need to buy a deeper router bit and get some more C-clamps before I can continue on tomorrow.
October 7, 2001
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Finished making dado cuts and attaching two 2x4 supports in the top of the frame. |
October 8, 2001
Cut the lower shelf from 3/4" plywood, and got it attached to the frame.
October 9, 2001
Added 2x4 crossbars on the ends of the 4x4 stringers. Also cut the double 3/4" plywood table tops. Will need to route out relief pockets in the lower decking to go over the metal frame connecting "ears."
October 12, 2001
Took measurements on most of the parts that I've picked up to finish the MGM. Worked on design features to be able to pack everything onto and into the frame. Things should be spinning soon!
October 13, 2001
Build a mount on the shelf that will hold the flange bearing for the drive shaft. Moved the metal panel that the motor will be mounted on. Also hit the stores for a new handheld jigsaw and additional hardware.
October 14, 2001
Put in some time tonight putting on a 3/4" plywood stiffening panel on the business end of the machine. After the lag screws went in, the machine really stiffened up. There is now no motion at all when you lean against the frame in any direction.
October 15, 2001
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Bolted the double 3/4" plywood table top to the frame. This is built to be able to be removed. I'll have to take it off to install motors and other equipment under the surface. |
October 16, 2001
Made a quick trip up to Beacon Electric to trade in some V-Belts. Mine were too long. Worked on designing the motor mounts, overarm drive and other features.
October 17, 2001
Not much done today... picked up bolts and threaded rods at the hardware store, and purchased a 1.75" Forstner bit to drill relief holes for the flange bearings. Finalized the design of the turntable motor mount.
October 18, 2001
Cut a motor mount plate from 3/4" plywood. Used the router to put in a couple of slots to allow moving the motor to remove slack in the belts. Got the motor on the mount, and the mount on the frame. Turned on the motor for a "first spin" test! Found a new local hardware store that carries pulleys, belts, and such. They had a nice solid-body 5/8" bore pulley that should be usable for the overarm driving cam. Plus they were a good source for 7/16" motor mounting hardware.
October 19, 2001
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Made good progress today. Drilled through the table at the point where the turntable will be centered. Dropped a plumb bob through the hole to mark the position on the bearing support. Then Dad and I used a Forstner bit in the hand drill to drill out a relief for the bottom flange bearing. Did the same in the underside of the top to mount the top bearing. Dropped the 5/8" shaft down in, with a couple of collars to keep the thing in place, along with a pulley wheel and a V-belt. Got the whole thing together, then we loosened the motor mount and Dad pushed it over to tension the belts while I bolted down the mount. Turned the motor on, and were were turning the spindle! |
October 20, 2001
Not much accomplished today. Hit the Home Depot to pick up some small sheet metal screws to use to attach the TT bearing to the TT. Also got some larger wood screws to use in fabricating the TT itself. Picked up a carpenter's protractor to use for measuring positions for the TT support wheels. Also got some keys for the shafts and pulleys, and most important of all, I got a special pencil sharpener to sharpen the big flat carpenter's pencil that my wife, Renee, gave me as a present! That is all...
October 22, 2001
Another day with very little progress. Went to Sears and bought a new router. Got a plunge router this time to make it a little easier to cut circles. Got it home and my circle attachment didn't work on the base. Got started with a cut, and the base came loose. Screws just don't hold very well in the hard, smooth plastic from which it is made. I think I'll have to look for another circle cutting tool, or just make one myself.
October 23, 2001
Bought a new circle-cutting attachment for the new router. Had to drill and countersink mounting holes on the plastic base to match my router. Then I used it to cut two 24" circles from pieces of 3/4" plywood. They'll be fastened together to make the turntable.
October 24, 2001
Another day with little accomplished. Visited the local hardware store/lumber yard and found they carried a "rubber" step runner that should work well as a pad for the glass on top of the turn table. It's about 1/4" thick, with one side smooth and one side with small parallel ridges about 2mm apart. Thought I would put it smooth side up and give the glass a gentle ride! Bought 3 feet of 27" wide runner for $6.
October 30, 2001
Finally made some progress today. Worked on the turntable parts, cutting a 12" square to support the bearing, and cutting out a matching recess in the table top. Had to unmount the top to do this, and it made me rethink the attachment of the table top to the frame. I may experiment a bit tomorrow and see if my Father's idea for attaching it through a connection on the inside of the 4x4 stringers will work better. I also added another crossbeam next to the turntable to give the top a little more strength. With the 12" square cut out, all the weight of the turntable will be resting on just the bottom 3/4" plywood, and it might sag with the original design. But with the new 2x4 dadoed into place, it now has a solid support. Also marked the TT motor mount to add another slot. Dad and I bought a new tool chest last night, so I got that assembled and started collecting my tools from various locations.
October 31, 2001
Put in a little work on the motor mount for the overarm today. Got the 3/4" plywood plate cut out, and used the router to put in the top two slots to allow for taking up the slack in the pulley belt.
November 2, 2001
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Finished building the overarm motor mount, and got it installed on the frame. Fired up both motors simultaneously for the first time. |
November 7 & 8, 2001
Over the last two days I've pretty much finished up the turntable. I glued and screwed the two 3/4" plywood disks together, and got them mounted onto the TT bearing. And I got the bearing mounted on the 12" insert that I built to into the recess I cut into the table top. Got a dog attached to the bottom of the assembly, so it is ready to go.
Also finally got the table top attached in a satisfactory manner. I ended up using 8 big 3/8" bolts through the top, with a lock washer and nut directly underneith the top to keep it solid. Those bolts then drop down through 1/2" holes in the frame, and are held in place with washers and nuts on the bottom. The holes had to be drilled out larger to fit the bolts, which were not at exactly right angles to the top. Ended up working quite well.
After getting the top on, I re-assembled the front motor and drive shaft and started spinning the table. The basic idea works fine, but I'm having a hard time getting the pulley belt tight enough not to slip.
November 9, 2001
Studied the problem with the belt tension tonight. It looks like the pressure from the belt pulls on the corner of the motor mount, bending that edge of the 3/4" plywood away from the metal frame. I drilled out three new mounting holes in the 1/8" steel frame, so the motor mounting bolts are much coser to the motor mount holes. That should reduce or eliminate the lifting of the corner of the motor mount. Will know tomorrow after re-assembling the motor and drive shaft.
November 10, 2001
Yesterday's efforts didn't pay off. I still have problems driving the turntable. During experiments, I also cracked off part of the driving dog. Back to the drawing board...
December 6, 2001
We finally experimented with the turntable until we figured out what the problem is. It's the lazy susan bearing. The bearing works great as long as the load is directly over it. But if you put any radial force on the turntable, the bearing binds up and stops cold. On to the backup plan.
December 9, 2001
Installed three castor wheels under the turntable to provide a support for it. The turntable is directly attached to a 9" pulley wheel that is keyed and set-screwed to the 5/8" drive shaft. Thought this would work, but tests show that there is far too much friction in the castor wheels. You can hear them creaking under there, and the TT is far too easy to stop in this configuration. Next idea...
December 11, 2001
Decided to replace the castor wheels with low-friction skate wheels with ball bearings. This required a big change to the TT end of the table top. The skate wheels won't fit in the castor housings, so I had to manufacture my own supports for the wheel axels. I used 3/4" aluminum "L" channel, and cut off 4" pieces to make wheel supports. Nearly finished them tonight, but it'll take an hour or so tomorrow to drill out mounting holes and attach them to the table top. Anxious to see how much this reduces the "sticktion" on the TT.
December 12, 2001
Finished drilling the mounting holes, and attached the skate wheels to the machine's top. Had to use a Dremel tool to deburr the cuts and holes in the aluminum. Also had to expand the mounting holes into slots so I could adjust the tension on the bearings in the skate wheels. Everything is mounted, tomorrow I'll remount the TT and see how this works.
December 13, 2001
Finished remounting the turntable today. Everything works GREAT! The skate wheels fixed all the binding problems, and the TT worked fine in a quick test under a 75-pound load, with lateral pressure on the edge of the TT. I'm a little worried about the 1/4" washers I used as shims in mounting the wheel bearings. The washers can touch on the revolving part of the bearing, and my either heat up a little, or create a little friction. I can't tighten up the mountings as much as I like. So I'll look for a narrower washer to replace those currently on the machine. Not a big problem, though, and the machine will probably work fine as-is. On to the overarm assembly!
December 15, 2001
Tested the turntable under full load today. It turned a 100 pound unbalanced load with no problem, even with a 20 pound lateral force applied to the edge. I can stop the turntable through additional edge pressure, with the belt starting to slip against the pulleys. I think I'll go ahead and add additional stabilizers to the bottom of the steel plate that the motor is mounted on to remove the movement there when the turntable is stressed. I won't need much reinforcement to satisfy my requirements. The machine will probably do a 24" mirror right now, but I would like to have the capacity for a 30" in the future, so I'll make the modifications right now.
June 18, 2002
Took delivery today on a new 20" Grizzly drill press. This came off a big truck, in a 300-pound package. My Father came over an helped me get started putting the thing together. Talk about huge and heavy! It took both of us to mount the head on top of the column, with my wife, Renee, adding a little muscle and guidance in the final action. Should be able to finish setting it up tomorrow.
June 19, 2002
Finished setting up the new drill press today. All we had left was putting on the spindle and the chuck, and some cosmetic stuff. Everything looks nice and works well, so I'm ready to drill!
June 23, 2002
After a lengthy amount of time away, I'm back working on the mirror grinding machine! Today I installed a bearing near the center of the table for mounting the shaft to drive the overarm. I used a Forstner bit to drill a relief in the top of the table for the flange bearing. Then I drilled a clearance hole for the shaft with a 7/8ths spade bit. I need a block on the bottom shelf of the machine to carry a bearing for the shaft down there, so I screwed together a 4x4 and a 2x4 to build up a thick enough housing. Then I bored out the bearing relief and a shaft relief using the new drill press. SMOOTH!
June 24, 2002
Continued working on the shaft. Got the bottom bearing block drilled out an mounted on the base of the machine. Had to lever up one end with a long 4x4 to level the monster, then slide in underneath to bolt up through the bottom into the bearing block. Worked fine. Had a little trouble getting the 5/8ths shaft through both flange bearings. Ended up having to loosen the hold-down bolts on the bearings, so they could "self-align" themselves relative to the shaft. Didn't forget to put a locking collar, a pulley wheel and a V-belt on this time! Should be ready for a spin- test next session.
I took Emerson to the Home Depot today to pick up some hardware. I have decided to make my machine into a modified Draper, much like the one illustrated on page 40 in "Procedures in Experimental Physics" by John Strong. (BTW, I got this book from Lindsay Publications Inc., a great company. Check out their Internet site!) I have most of the materials I'll need for this kind of overarm, but Emerson and I picked up a giant hinge, and some mounting hardware, as well as some lag screws and such that we needed for the machine.
June 26, 2002
I started work today on the offset crank to run the overarm. I have a 5" pulley wheel that will be used to attach the crank to the top of the driving shaft. The pulley will be attached to the face of a 2x6. I used Dad's dado head on the table saw to cut a 1/2-inch-deep grove down the length of the 2x6. I narrowed up a 2x4 to fit in the groove. On one end of the piece I bored a 5/8ths hole for a driving shaft. Then I drilled a set of mounting holes along the edge of the 2x4. So I can slide the 2x4 back and forth in the groove in the 2x6 to adjust the offset on the crank.
June 29, 2002
Did a preliminary assembly of the offset crank today, and had a power-up test. The crank turns at about 15 rpm's, the way I have the pulleys arranged right now. Nice to see this thing going around finally!
June 30, 2002
Got a good beginning on the overarm today. Sawed off a 2x4. Drilled holes in one end to mount bearings to go over the drive shaft. Cut the 2x4 at the hinge point, and drilled holes to be able to mount the hinge. Also drilled out holes for the drive shaft at the business end.
July 1, 2002
Continued work on the overarm today. Used the router to cut a groove in the end of the arm to mount a 5/8ths shaft to go through the pivot point on the end of the machine. Had to buy a new router bit to do the job, and cut the groove out in three steps to get it deep enough to mount the rod.
July 3, 2002
A bit more work on the overarm. Went to Home Depot and bought some U-bolts to hold down the shaft on the overarm. Drilled out mounting holes for those bolts.
July 5, 2002
Today I finished the serious work on the overarm. Got everything put together and tried it out. Seems to work fine! Found a couple of spots where I'll need some lock washers to keeps things tight, but I think this design might work out okay.
July 7, 2002
Finished up work on the overarm constraint/support. Cut off the support at an appropriate level, and used the router to clean up my end notch.
July 8, 2002
Began the total disassembly of the machine today. Went to the Sears store yesterday and bought a sander. I'm ready to sand this thing down, and put on a nice finish. My wife Renee wants me to stain it, but I'm leaning toward a yellow enamel covering. We'll see who wins!
July 9, 2002
Finished taking apart the machine, so I'll be ready to start sanding tomorrow.
July 10, 2002
Sanding day! Took all the disassembled pieces and sanded them all with the new electric sander and #60 paper. The Douglas Fir 2x4's were fine, but the pressure-treated wood was a bear to sand. I had to wear a mask and that stuff is HARD! Took a long time to get any material removed at all. Hot and sweaty work.
July 11, 2002
Spent the day staining the machine. Renee won the "finish" contest. Using a gray stain from Home Depot. Bought a gallon, and it looks like I'll use about a quarter of it, I would guess! The stain is pretty nasty to use, as it's very thin and runs and dribbles like crazy.
July 12, 2002
Just had enough time for the second coat of stain on some of the parts that will show a lot. Needed a little more stain to even out the color a bit.
July 13, 2002
Finished staining the back-side of a lot of pieces today. Bought some nice glossy-black enamel at the Home Depot. Used it to paint all of the wooden parts above the table top: the crank, overarm, and the pivot brace and slider. Very nice paint, went on smoothly and easily. So much for yellow!
July 14, 2002
Pretty much finished the painting today. Put a coat of stain on the underside of the tabletop, and balck enamel on the top and sides. Ready to re-assemble the machine, I guess!
July 15, 2002
Started re-assembling the machine today. Had some trouble getting things to fit back together again. Some of the 4x4 pieces of pressure-treated lumber corkscrewed a bit when the tension was let off when I disassembled the base. I had to buy a heavy 36" clamp to force some of the parts back together. Got the base redone okay. Started to tile the top of the table as well.
July 16, 2002
Finished tiling the top of the machine. Put the turntable support bearings back on, and placed the top back on the frame.
July 17, 2002
Reinstalled the drive shaft for the overarm this morning. Used up all my tiles and didn't get to finish the lower deck. Make a trip to Home Depot and LUCKILY they had seven more tiles of the same pattern. They had quite stocking Congoleum and switched to Armstrong, and I found those matching tiles waiting to be thrown away! Lucky me! Also picked up some giant heavy-duty castors to make some rolling tables to move the machine around on. Long overdue!
July 18, 2002
Made a big problem for myself today. Put the top back on the frame, and had the whole thing under a tarp on the driveway. Temperature hit 96F, and the tiles soaked it all up. The tiles on top got VERY wide, and gaps appeared between then. Quickly knocked together a pair of castor bases, put them under the machine and rolled it inside to get it into the shade. Used sponges and water to cool down the top. Apparently the tiles had actually melted down a little, as they didn't contract back to their original size when cooled.
July 19, 2002
Finished tiling the bottom deck of the machine today. Then I re-installed the shaft for the turntable. Much easier working on the bottom parts of the machine when it's lifted up on the castors. Tested two pulley wheels today, and I have 40 rpm and 30 rpm available on the turntable.
July 20, 2002
Started painting trim pieces today. I'm putting 2" trim around the table top, and on the edge of the lower deck. Used standard stuff from Home Depot. Painted it with glossy black enamel, giving them a coat before starting to cut and fit. Dad came over and we re-assembled the overarm mechanism as well. Had a little trouble with the drilled parts. Some paint had run into the holes, and we had to drill them out again to get the bolts to easily clear through. Everything looks nice, with the shiny new hardware against the black enameled wooden pieces.
July 21, 2002
Worked on the trim today. Very tedious work, cutting each piece to fit. Screwed up on two long pieces and made them too short, so I had to make another trip to Home Depot and get some more trim pieces. Eventually got things right, though. Ended up doing six corners, and I managed to get five of them right. Will fix the sixth one with wood filler and a sanding block! Renee says, "spackle and paint makes a carpenter what he ain't!"
July 22, 2002
Use a nail set to pound the finish nails below the surface of the trim, then filled with wood putty. Two hours of drying, then a sanding. Used blue tape to mask off around all of the trim, and then I gave the trim a final coat of glossy black enamel. Looks pretty good!
Tonight was our regular Monday night ATM SIG (Amateur Telescope Making Special Interest Group) meeting at my house. While the rest of the guys were grinding away, Michael Lindner and I poured an 8" plaster/tile lap for use on my machine with his 14" blank, our first test piece. While that was drying, I worked on building a pusher to engage the top piece in the mirror/tool stack on the machine. Ending up building a 3/4" plywood panel with three sliding blocks to clamp onto the top piece and push it around. The unit I build can handle anything from 4" to 10" on top.
Well, very late after all the ATM'ers had left, Michael stuck around to help me finish the pusher, and then we invited Renee outside, and we had FIRST GRIND! Ten months in the making, and we celebrated with Coca Cola all around. The machine seems to work as intended. I need to alter the push block in some way to allow for placing much heavier weights on top. We hung three 2.5 lb weights on top, but about 20 or 30 pounds is needed. Also, we experimented with ways of getting abrasives and water in play without having to stop the machine and remove the top piece. We finally settled on stopping just the overarm at the farthest offset point, while leaving the turntable running. (The top piece rotates around quite nicely in this configuration.) Then we filled a little paper cup with the needed amount of abrasive and just tossed it sideways into the gap. A few squirts with a water bottle (on narrow jet) and the abrasive was in play. Then just step on the foot switch and re- activate the overarm. With the turntable running the whole time, the work never stops!
July 25, 2002
Went back to Sears and purchased some rubber- covered Weider weights to put into my tool pusher. I bought 5-, 10-, and 25- pound weights, all with 2" center holes. (Also bought a circle-cutter for the drill press!) I'll sandwich these between the actual tool pusher plate, and the flange-holder on top, thus adding weight to the stack and improving my grinding efficiency.
July 27, 2002
Had my first grinding mishap today during the "shakedown cruise" on the machine. The overarm crank has two bearings at the top that hold the overarm in place. The overarm was being held up by four set-screws on the bearing sleeves. During the course of grinding, the four screws lost their grip, and the overarm dropped down about five inches on the shaft. When that happened the drive pin came out of the glass stack, and the arm lifted out of the restraining groove. Now the tool was free-wheeling, and it skidded right off the mirror, taking three edge chips as it went. I caught the stack before it hit the ground, and nothing bad happened to the tool.
Upon inspection afterwards, I found that the 5/8ths-inch bolt was 22/1000's undersized, and that might have contributed to the failure. In any case, I have now put a friction collar with a cut and screw CLAMP on the shaft below the arm, and the problem should not be repeated!
July 29, 2002
Made yet another modification to the tool pusher, cutting off the corners. What was once square, is now an octagon! Much easier to toss abrasive under the pusher and onto the mirror without the corners in the way. Worked on two major additions to the machine. First, I wired up a box to power the motors today, with an on/off switch for each motor over a standard plug-in. Also, I routed out a hole in one end of the machine in which I'll mount an industrial-grade fan. The overarm motor runs pretty hot (especially when it's 96F in the room!) and I want to cool it down a bit. Got the hole made, and covered the inside with hardware cloth. Stained the edge of the hole, and trimmed the edge of the entire plate while I had it off the machine. Will remount it tomorrow, and put on the fan.
July 30, 2002
Finished mounting the fan on the end of the machine, and got the electrical switch box mounted on one of the leg posts. Thinking about a modification now to do edging of the blank while I grind. Got a great idea for this from Bob May on the S*T*A*R Web site.