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Old 8th November 2004, 09:22 PM   #1
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Default DIY Turntable Concept (Easy-to-make)

I'd appreciate suggestions and comments on my DIY turntable project:

Click the image to open in full size.

Briefly, the primary issues with a homebrew turntable seem to revolve around (pun intended) the bearing, i.e. friction and feedback noise, trueness of the bearing, axial alignment, homebrew machining difficulty, necessity for maching accuracy, etc.

This concept comes from spending quite a bit of time coming up with solutions to similar problems with telescope drive systems, especially friction, stability, damping, and alignment accuracy. I notice a similar, but much smaller, solution to mine has been developed by Well Tempered Labs and their turntable products, specifically this spindle housing:

Click the image to open in full size.

The problem for the DIY'er with doing such a small diameter housing and spindle bearing lies in having to accurately machine, super-polish, mount, and align it. I believe Well Tempered machines a spindle that attaches to and passes through an acrylic platter. Not only would it be difficult for the DIY'er to make the spindle housing shown (requires several lathe operations, including deep inside operations with a boring bar), it would also be pretty difficult to turn an accurate and large diameter acrylic platter with an accurately centered and nicely cut center hole. I suppose it is be possible to cut an acrylic platter with a router and circle jig, but it is something I haven't fully considered.

In any case, if I go with my initial and less expensive choice of massive laminations of MDF board for the platter, it would be significantly easier to surface mount a large diameter bearing to the bottom of the platter than to make a smaller diameter unit similar to Well Tempered's design. Further, the possibility for axial misalignment where the platter doesn't turn true and flat increases the smaller you make the bearing--all it takes is quality machining capacity, the thing most DIY'ers lack.

Going instead with a much simpler Delrin disk riding on three equal spaced and against two side-load Teflon bearings submerged in bearing fluid should result in a very low friction, very low noise, true and flat turning table that is significantly easier for the DIY'er to make since the primary tool required by a design such as this is a router and circle jig. Note that the Delrin disk size would have to be adequately large to permit the three supporting Teflon bearings to be spaced far enough apart to prevent the platter from tipping. How much resistance three widely spaced Teflon bearings and two side bearings rubbing against Delrin in bearing fluid would produce is pure conjecture at this point, but I cannot see how it would fail to meet the criteria for quiet, accurate, easy to make, and (if properly spaced) stable.
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Old 9th November 2004, 05:41 PM   #2
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You can edit a post for some period of time (I think it's 45 minutes?) after first posting.

Is there an issue with stress creep using Teflon? It's a pretty soft material.
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Old 9th November 2004, 05:42 PM   #3
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No one?

Other than having a somewhat 'unattractive' bearing (that you'll seldom if ever see), does anyone see a flaw with this design? I'm thinking the Delrin platter bearing should be around 8" diameter, leaving a couple inches of platter overhang around the perimeter.

My primary concern is with friction and possible sticking that could result from having the three Teflon bearings spaced far enough apart to support the platter properly. Teflon against Teflon is well known for being extremely low in friction, as is Delrin with Teflon. This is without going to ball-bearing assemblies, which I believe would be a no-no in a turntable since even the highest quality, smoothest units would probably produce noise that would transmit through the platter. Very small diameter Teflon balls, or rod ends (est. 1/8" to 1/4"), will likely be chosen.

If using Teflon balls/rod ends results in sticking or excess friction, then I plan to use (make on my mini-lathe) all-Teflon bearings such as these (both rod and bearing made from Teflon) submerged perhaps in bearing fluid. The Delrin platter bearing would have a dead flat, mirror-smooth surface and I predict the resulting combination would be *extremely* smooth, quiet, and stable.

Click the image to open in full size.

If no one here has an opinion on this project, I'll just build it and report back in a few months when it's done. I'm planning to go with a Rega RB300 tonearm (re-cabled and upgraded) and am looking at good phono stage options. While the table will be inexpensive to construct, I plan for it to perform up with the best. Also, it will be nicely finished in deep lacquer to ensure it looks the part of a quality turntable.

-coma

P.S. How do you live without an edit function on this forum? I always find typos after I click the submit button no matter how many times I proof read
.
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Old 9th November 2004, 07:00 PM   #4
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It might work. Took me some time to see why only 2 balls pressing against the side and why you call it ball bearing if after all you slide over the balls. But then again it may not be my day. It looks like a simple concept and I don't see any fatal flaws.

You have the 2 side balls acting as guides, as long as your drive belt stays taut. Don't forget that your platter will need to be heavier that you think since your drive belt pulls above your points of contact (the 2 side balls).
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Old 9th November 2004, 07:10 PM   #5
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Quote:
Originally posted by SY
Is there an issue with stress creep using Teflon? It's a pretty soft material.
Virgin teflon holds up well even under tremendously heavy loads. Even large aperture telescopes such as these use three small squares of it (apprx. 2" x 2" x 1/4") for bearing surfaces.

Click the image to open in full size.

Very small Teflon balls do squash a bit under high pressure (such as when they are used on the tips of screws to apply friction to a mating surface, i.e. usually aluminum), but you are applying a lot of PSI to a small surface area under these circumstances.

Teflon is soft enough that you can shape it with a sanding wheel, but same goes for aluminum--I can quickly shape aluminum parts using that method. With the Teflon rod and wheel bearing assembly I sketched out, you couldn't hang the rod over a cavernous gap without at least a little flexure under a heavy load, but there would be 3 of them to distribute the load and even a heavy TT platter wouldn't weigh all that much. If there ends up being a concern with flexure, then perhaps an aluminum or brass rod could substitute while still retaining superior smoothness and quietness to alternatives like stainless steel ball bearing assemblies.

The only concern I see with Teflon is eventual wear over (a very long) time. But I will ensure the pads/wheels/balls are extremely easy to replace when the time eventually comes. The advantages I'm hoping come of Teflon (and a mating bearing surface of Delrin) to the DIY'er are superior silence, smoothness, and ease of construction without resorting to expensive experimental machining. This is the kind of changes DIY telescope making (ATM--amateur telescope making) went through to make high end performance and large aperture instruments available to normal people without machining skills and capabilities.

I guess we'll see how this goes. I won't be able to start building it until after the first of the year as I'm currently remodeling my home.

P.S. There is a little notification window at the bottom of my screen that says I can't edit my posts.
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Old 9th November 2004, 07:11 PM   #6
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Quote:
There is a little notification window at the bottom of my screen that says I can't edit my posts.
Now that you're out of moderation, the editing function should work.
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Old 9th November 2004, 07:38 PM   #7
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Quote:
Originally posted by Havoc
It might work. Took me some time to see why only 2 balls pressing against the side and why you call it ball bearing if after all you slide over the balls. But then again it may not be my day. It looks like a simple concept and I don't see any fatal flaws.

You have the 2 side balls acting as guides, as long as your drive belt stays taut. Don't forget that your platter will need to be heavier that you think since your drive belt pulls above your points of contact (the 2 side balls).
Good points. I fixed my drawing to reflect those details. I had considered I would need to run the belt low on the platter though I didn't sketch it that way. As for them being 'ball bearings'--they may still end up being balls seated in sockets of some kind, but as you noted, they wouldn't roll.

As for possible Teflon stress deformation--while I don't see that as an issue if this is implemented with simple bearing surfaces rubbing against each other, if it is instead carried out with the wheel and rod roller bearing shown I think there could be possible long-term deformation of the wheel if the platter were heavy enough (it would have to be quite heavy) and it were left sitting on the bearings all the time. Perhaps a simple lever to lift the platter from below a smidgen when not in use might be in order. Like I said, though, extremely heavy telescopes ride on top of Teflon pads (the opposing bearing surface is countertop laminate) and they don't deform the Teflon so much as wear it out eventually--and that is largely due to dirt and sand getting between the surfaces and grinding it away.
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Old 10th November 2004, 06:09 AM   #8
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Very OT:
I love your pics of those telescopes. Do you have some links where there is more about those selfbuild telescopes?
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Old 10th November 2004, 07:00 AM   #9
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Google "Dobsonian" - that's the type it appears to be.
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Old 10th November 2004, 05:46 PM   #10
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Default Friction moment

One drawback I can see in you design is substantial friction moment to overcome. You need here five teflon balls minimum: two ball to handle side force from the belt and three other balls to keep the platter horizontally. Three bottom balls shall be reasonably spaced apart, to prevent platter from rocking, so the distance ball to axle is far larger, than conventional bearing diameter. Bearing in mind the friction force moment proportional to the square of distance to the axle, you get way larger friction moment. Worst thing about friction is it never appears completely linear and consistent, so it may be considered as some sort of vibration. To minimize the detrimental effect of it, you need a massive platter, which in turn brings more friction...
Another thing you have to deal with is the fact, that oil clibs the wall within the narrow gap surprisigly easy, so you have to keep the oil level much lower than bearing hight, oteways you may find oil all over the place.
One more point: try to keep the belt and side balls on the same plane, to minimize dynamic wobble.
You may find some interesting info in the older thread "Let's make DIY TT" in this forum.
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