From 0c2fffa2989df5ae0409616eb5d5bd9b91a44457 Mon Sep 17 00:00:00 2001
From: Jake <jake.read@cba.mit.edu>
Date: Fri, 10 May 2019 14:00:06 -0400
Subject: [PATCH] new notes
---
README.md | 59 +++++++++++++++++++++++++++++++++++++++++++------------
1 file changed, 46 insertions(+), 13 deletions(-)
diff --git a/README.md b/README.md
index 9fc7be1..481297b 100644
--- a/README.md
+++ b/README.md
@@ -2,19 +2,33 @@
## Drawingboard Return
-instron:
- redo, this is untenable and no one should build one
- https://www.amazon.com/SFU1605-Ballscrew-RM1605-Housing-Machine/dp/B07DQK2YFN/
- misumi for drive components / checkin ...
- 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.
+
+[ballscrew maths](http://www.nookindustries.com/LinearLibraryItem/Ballscrew_Torque_Calculations)
+
+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.
| Motor | kg | Nm |
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GitLab