beams all the way down, overconstrained, and motors in front
nut-in-hole type threading,
10mm width S5M pulley: but check torque spec
- at this width keyless works very well w/ 16mm total width https://us.misumi-ec.com/vona2/detail/110300409350/?CategorySpec=00000045557%3a%3ac
- compare: fancy-ass pulleys with medium reduction, or 1/2" -> 10mm coupling and NEMA34 ? ... consider that reasonable torques for a shaft coupling are '35 - 53' in-lbs (from McMaster search: 2764K424 or 2464K34) - that's about 5.6 Nm, a NEMA34 does ~ 5-8Nm at the upper end, so it's in threshold
- coupling is ~ $100 each for well-spec'd, $72 each for ok-spec'd ... pulleys are probably similar once you've done 2x pulleys and 1x belt, with fancy shit... and pulleys afford smaller motors that you can actually drive,
- if, however, you can find cheap couplings, to spec (probably not) you can do that. get your spreadsheets back out !
## Generating kN
With \#2 feeling somewhat unloved ('both overdesigned and underdesigned'), I'm back at the basics for \#3. There's a few major selections, and decisions to make:
### Material Selection
-> ALU, FR4(G10), Acrylic?
While aluminum is my go-to for machine design, and is ostensibly possible to mill on a shopbot by a motivated user (see [jens](https://github.com/fellesverkstedet/fabricatable-machines/wiki/Fabricatable-axis)), there is some hesitation to use it.
| Material | Young's Modulus (GPA) | Cost for 6mm x 24x24" | Machinability |
| --- | ---: | ---: |
| ABS | 2 | 52 | Not Dimensionally Stable, but OK to Machine |
| Nylon 6 | 3 | 130 | Painful |
| HDPE | 1 | 23 | Easy |
| 6061 ALU | 69 | 87 | Breezy with WJ, Painful on Shopbot |
| FR1/CE (Canvas / Phenolic) | 6 | 81 | TBD, probably WJ Pain and Ease on SB |
| FR4/G10 (Fiberglass) | 22 | 98 | Painful on a WJ, Slightly Easier on a Shopbot |
[data](http://www.acculam.com/data-chart.html)
That said, ALU lands pretty well 1 order of magnitude above Canvas Phenolic ('FR1' or 'CE') for strength, while costing a similar amount of dollars. Fiberglass is a nice candidate, so machining G10 is likely a worthwhile experiment. However, both composites have anisotropic-ness and are sensitive to the size of local features (and to localized loads), making them less favourable.
!TODO: beam equations for the above, to size req' depth
!TODO: shear forces for the same,
### Transmission Design
#### How Many kNs ?
We want lots of force, with very fine control of position. This means a nice linear transmission. To estimate the forces we might want to see, I wrote a quick table of forces required to rip apart ~ 3mm square (0.001mm^2) samples of a few materials.
...
...
@@ -31,6 +45,25 @@ Brinell hardness tests range from 10N through to 30kN (for steel and cast iron)
So, a ballpark of ~ 10kN would be ideal - this is a big number - off the bat I'm going to estimate that 5kN will be a more reasonable target. 1kN is enough for a complete set of plastics, but that's only allowing for a realtively small sample.
-> Ballscrews, Belt Rack and Pinion, Rack and Pinion
Generating kNs of force is no easy feat, especially when we want to do it *very smoothly* while displacing very small amounts.
I will start by mentioning that this is dead easy with ballscrews. With a 1605 ballscrew, (16mm diameter, 5mm per turn) and a NEMA23 with 3Nm of torque, we can generate about 3kN of linear force (per motor) - to land at 5kN total no problem.
However, these are somewhat cumbersome and expensive - and they land in fixed sizes. Towards more parametric machines, we can look at a rack and pinion type axis.
Because tooth geometry very sensitively affects linear-ness of drive, especially where (down below the mm) we will be driving an entire instron test-cycle inside of one tooth-phase, I want to discount a traditional rack and pinion right off.
I am curious about a belt-driven rack, similar to [this design](https://gitlab.cba.mit.edu/jakeread/rctgantries/tree/master/n17_linearPinion).
!TODO: compare by transmission ratios (abstract from motor) and cost of parts.
!TODO: belt spec for hight (huge) load belts: tooth shear, and stiffnesses.
#### Motor Torques
To generate the force required, we're going to need some motor / transmission oomph. Here's a list of typical NEMA size motors, and the torques they can generate. The [atkstepper](https://gitlab.cba.mit.edu/jakeread/atkstepper23) can supply enough current to power any of these.
