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# Platonic Gantry Design - LC


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| what | pn | etc |
| --- | --- | --- |
| Wire Rope | 8930T54 | |
| Loop Compression | 3897T31 |(100% rope capacity) |
| Stop Compression | 3926T42 | (40% rope capacity) |
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## Laid Up Structure

To build rad machines w/ less-rad machines, we need to base the straightness of our structures on some quality reference. I.E. if a shopbot is straight to ~ 0.01" over 12", this is the best we could ever hope to build into a new machine. However! If we reference a structure that is straight to 0.001" over 12" during lay-up, we can do about this well.

To do this, I steal accuracy from the optical tables in our ~~dungeon~~ basement at the CBA, laying up each axis with a reference:


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The 'rails' protruding on the beam constrain one DOF ... this little tag constrains the other side, so I lay it up with a reference straight. Here, that's just a run of the mill centerless ground rod, but [these]( types of reference edges are also available.
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## Capstan Drive

Everyone loves a nice cable transmission. Steel Rope is stiff, strong, available around the world, and can be routed easily around a machine. Idlers are simple, and driving elements can be featureless (i.e. we don't need to machine teeth). This is all a result of the most-excellent [capstan equation](, where the loads we can carry on these pulleys goes in the exponent to the number of times we have wrapped the cable around our capstan.

One trouble with capstans is that we typically need lots of capstan length... as in this image:


The capstan needs to be 'tall' enough to cover the entire length of rotation. That helix is baked in, we can't shear the cable along the length of the capstan during rotation. This makes it difficult to implement a capstan drive in the way we normally handle belt-and-pulley transmissions.

Many capstans feature helical grooves to keep the cable in order in this helix. This brings some determinism to where-all the cable will roll on to the capstan (along its length). To do this without a helix, so that the capstan always rolls on in the same location, I use this double idler system, with grooves on both:


For a better view of this, I have a video:


This is just-neat for one axis, but is really cool when we want to whip the drive element around a machine chassis, a-la the LC-SMF machine: