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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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Glad to help. 
I'm looking forward to the computer controlled grinder as well!
Best,
Mark
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rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 8334
Loc: Bremerton Washington
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Quote:
Glad to help.
I'm looking forward to the computer controlled grinder as well!
Best,
Mark
Your help is in essence validation.
This coupled with quality technique results in a quality product. Since I've only been at this for a couple of years..I'm pleased with my progress. I just have to get past making stupid mistakes and not properly protecting my work. From everything...sometimes even myself.... 
Thanks
Rob
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www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.
http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.
www.vimeo.com/6014031
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Glig
sage
Reged: 10/02/05
Posts: 370
Loc: Baltimore, MD
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Man this thread is a goldmine and I almost let it sink into the west!   
I don’t yet have a plan to build a grinding machine but I think I need to start by figuring out how to build a turntable.
What is the most practical way I can build a turntable that runs true? I would like turned aluminum for the top surface. Should I use wood layers underneath? Running on Castors? What kind of castors? (Note - I'm a perfectionist and I'll pay for quality)
Should two pillow blocks support the shaft?
Should the shaft alone support the table?
If so, how exactly how does the shaft link with the table?
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Richard Caldwell
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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Two methods I know work reliably:
1) Good casters supporting a machined flat top plate. This is what I did with my curve generator and it holds tolerance under a lot of pressure. The casters were used from a metal scrapyard, some sort of nylon or tough plastic, and I trued them with a disc sander before use. The plate is centered with 3 ball bearing races inside a machined ring attached to the plate. Works great.
2) Shaft supporting a top plate through some sort of hub, thrust bearing on the bottom, pillow block near the top. Longer the better, so it just fits in the machine is ideal. Either have the top trued to ths shaft properly or figure out a way to adjust it so you can zero it in for use. A laser shining off a centered mirror or flat piece of glass on the table (on the support layer you'll use, like the ubiquitous throw-rug pad / shelf liner mesh) should show no deviation at all when hitting a wall beyond. You need it to be that good.
Shaft linking with table is variable - do it like the MOM plans with cogs fitting a pulley (works), attach it permanently but be sure it's accurate (obviously works), build it so it can come on and off and be adjusted (best - I'm redoing my older machine this way and I'll post some pic when it's finished). That way you can take the table off and outside and clean it properly when needed.
Best,
Mark
PS Nominate it for "Best of ATM" if you want it preserved!
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Glig
sage
Reged: 10/02/05
Posts: 370
Loc: Baltimore, MD
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"...The plate is centered with 3 ball bearing races inside a machined ring attached to the plate."
Can't visualize this one. What are the races used for? If ball bearings were in them, what would turn the table?
Here's something I found on eBay.
If something like this were bolted on an aluminum plate and the lathe were locked on the shaft, a turned top would do the trick, right? But then I wouldn't be able to lift the turntable off easily.
Ideally I would like to be able to remove the turntable by just picking it up. ( I don't know how to properly make the cogs - too much of a perfectionist to try) But then, what about using a lovejoy coupling to do the same thing? ( 3-jaw or spider coupling)
Also, I need an easy way to center the mirror every time I place it on the turntable. I was thinking about having the machinist drill a 1/16th hole dead center of the platter
and magic mark the bottom of the mirror to line the 2 up.
What do you think?
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Richard Caldwell
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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Quote:
"...The plate is centered with 3 ball bearing races inside a machined ring attached to the plate."
Can't visualize this one. What are the races used for? If ball bearings were in them, what would turn the table?
Arggh, no pics of this without tearing it apart.
OK, it has a top plate (5/8" aluminum machined) and it has, attached underneath that, a 1/2" thick aluminum ring. There's a smaller ring beneath that to which I've got 3 bearing races on short posts, and they fit inside the larger ring to fix the location of the table. The table turns on 3 casters which I described, they lie outside the 1/2" ring, and bear directly on the 5/8" table top. So there's no variation in vertical position for the workpiece as the table is rotated. I drive one of the casters directly with a geared down stepper motor via a rubber drive wheel in use.
The cogs in a MOM that locate the table on the pulley can be made out of almost anything, you just trace the pulley's cast arms onto the thing you want to cut out to make the cogs and then attach them to the table underneath.
I don't find it too hard to center the mirror - yeah you can have a center mark (or just draw one with a Sharpie when the tables turning - or even a whole series of them at various diameters (I've done this), but the trick is just put the mirror on, start the machine, and note where the misalignment is. Fix that and try again, it doesn't take long to center it up.
It will be hard to see through the mirror sometimes to the base.
I wouldn't want anything subtle to connect the table to the shaft - either it's permanently attached and trued (like the business end of my generator grinding setup - the 1-1/2" thick aluminum carrier disc was made .001" undersized for the stainless shaft, and then the shaft was frozen while the carrier was heated, and the two joined in a press. It ain't ever coming apart...) or you have two layers with solid pins of some sort making the connection when you lay the top table back on. The problem is always the same - the table takes a lot of force, and if it can fail or get misaligned it will. What's worse is (what just happened to me) you won't notice that for a while and it'll make you crazy until you figure out the problem.
Best,
Mark
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Glig
sage
Reged: 10/02/05
Posts: 370
Loc: Baltimore, MD
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O.K. I have a new obstacle in my path for building a grinding machine. It belongs on this thread because I want to use caster wheels for turntable support. I need good quality wheels and I don't know where to get them. I need the rigid frame, right? That complicates things. It gets better: I need ball bearings on the axles, right? And I need a wheel that's runs true without noise. I figured that a skate wheel caster with a rigid frame would be perfect, but no one sells this anywhere in the world. I can buy the swivel kind, several companys sell them. Would that work? I was hoping to buy a suitable size rigid caster, lose the wheel and substitute a skate wheel. If it fits, and that's a long shot.
Can anyone tell me a perfect caster I can buy for turntable support?
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Richard Caldwell
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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I get caster wheels at a scrapyard where they have endless numbers of them for about $10 each. They don't need to swivel. On the generator I used poly-something wheels (ball bearing) and turned them true with a clamped down hand drill and sanding pad before putting them to use. Performance is excellent. People have used skate wheels before. You could have rigid discs machined pretty inexpensively if you like. They don't have to be exactly the same size. You could even just use ball bearing races directly on a shaft, that's certainly an option.
When I build the 30" grinder/polisher I'll probably use casters or ball bearings for the support (to a flat table), as supporting the overhang loads off a central shaft is just a bit troubling.
Best,
Mark
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Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
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You want to try my favorite place.... Harbor Freight.
Have a look here....
http://search.harborfreight.com/cpisearch/web/search.do?catPath=All%2BProducts%252F%252F%252F%252FUserSearch1%253Dcaster¤tPage=1&lastPage=6&isNext=false&isPrevious=false&category=&attributeValue=&attributeName=&requestedPage=2&resultsPerPage=10&resultsPerPageBottom=
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