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Old 25th December 2018, 09:29 AM   #1971
2wice is offline 2wice  South Africa
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Quote:
Originally Posted by nocdplz View Post
Glad to ear you again 2wice.
This new data are nicely surprising for me, so near to those used to build the Rabbit (see the design in post #1719). Graphically I found much bigger errors, if instead are so small it's really a good new.

Now I do not understand if you're using gears or the previous cams.
And just two, without the idler wheel?

Merry Xmas - carlo
Merry xmas Carlo and all.

Not new geometry, just an optimization of #1651

The idlers are still there in the model, I've just removed them for simplicity.
If you look closely, the 2 circles in the drawing do not touch. And the sum of their radii doesn't add up to the length of the carrier arm.

I'm using gear relations in the simulation to approximate cams as the cam relations tend to slip.

I've found a new way to optimize the geometry, using a spreadsheet and a solver.

There are 5 major variables + 1 minor, it iterates those variables to approach the target output tangency.

I think I can tweak the algorithm to get it even closer but it might mean losing the nice rounded dimensions it has now.
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Old 29th December 2018, 08:02 AM   #1972
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Clear, thanks.

Playing with Linkage simulator i found something a bit better than the original, used for * 3d Rabbit #1950. Same ratio, but the 2 pivots distance increased to 54,5 mm. You may find measures on that attachment.

ciao carlo
*...some day i'll print it. Have you found how to get those cams?
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Old 29th December 2018, 08:33 AM   #1973
2wice is offline 2wice  South Africa
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Originally Posted by nocdplz View Post
Have you found how to get those cams?
Yes, 2 methods, one with a single idler but harder to build but with 0 error all the way through.
Another with 2 idlers as shown earlier with very small errors and easier to build, but I'm concerned about the starting torque that would not make it viable.
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Old 29th December 2018, 09:55 AM   #1974
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Sorry, I was curious how to make them fabricate, with those really minimum tolerances. For the "rolamite like" ribbon, what are you going to use?
c
starting torque: consider that both Rabbit and 3d Toy (and Doug's Bunny too) have no such problems, despite multiple bearings used. Side force + stylus drag aids each other efficiently. Imho 2 idler could work too, more when using jewells as intended
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Old 29th December 2018, 12:19 PM   #1975
2wice is offline 2wice  South Africa
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Originally Posted by nocdplz View Post
Sorry, I was curious how to make them fabricate, with those really minimum tolerances.
The 2 idler version can be turned to +/-50 micron very easy on a lathe as they are round pulleys really.

The 1 idler makes use of non-round cams to gets to absolute tangency but has to be cut with a wire-EDM machine that can get to 5 microns.

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Originally Posted by nocdplz View Post
For the "rolamite like" ribbon, what are you going to use?
Ultimately I would like to use Beryllium-Copper foil as the strip but it is not available in South Africa, so I'll practice with SS foil.

Quote:
Originally Posted by nocdplz View Post
starting torque: consider that both Rabbit and 3d Toy (and Doug's Bunny too) have no such problems, despite multiple bearings used. Side force + stylus drag aids each other efficiently. Imho 2 idler could work too, more when using jewells as intended
That's good to know. There are only 2 jewel bearings that just keep the main pulleys apart, so not part of the load. Tension in the foil in the 2 cam version is what worries me, but not for the 1 cam version as the foil never makes a complete loop.
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Old 29th December 2018, 01:57 PM   #1976
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Beyond cams tolerances (seems all feasible today) what concerns me more is the non extensible metal strip, that can raise the tension from zero to too much in a moment. How it's controlled, with the idler wheel?
+- 50 micron are comfortable even for a junk Chinese lathe and my old eyes: i like to press fit the TA parts to get mechanical continuity, and this needs much better coupling.

c
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Old 29th December 2018, 03:10 PM   #1977
2wice is offline 2wice  South Africa
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Originally Posted by nocdplz View Post
What concerns me more is the non extensible metal strip, that can raise the tension from zero to too much in a moment. How it's controlled, with the idler wheel?
Yes the tension is critical, I'm thinking screw or spring tentioner.
The idlers only reverse the direction.
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Old 29th December 2018, 04:35 PM   #1978
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a springy idler wheel? it's a common solution to get constant tension on belts

berillium copper: something like this could work?
Nastro di bronzo fosforoso / matassina da 4 metri - Opitec
Nastro di bronzo fosforoso | Utensileria Revelli

bronzo fosforoso = phosphor bronze (?)- used for spring or relay contacts making - this is 0,2 x 5mm x 4mt.

c
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Old 29th December 2018, 04:39 PM   #1979
2wice is offline 2wice  South Africa
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No, pulling tensioner on the byrillium copper strips.

The idlers are held captive with the strips.

Bronze is to soft.
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Old 29th December 2018, 04:48 PM   #1980
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and 0,2 is too thick. i kept my feeler gauge (steel): even the first 0,05 seems too strong.
sorry - carlo
phosphor bronze is completely different from bronze: maybe there is silicon inside

Last edited by nocdplz; 29th December 2018 at 04:52 PM.
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