update

README.md

 # Roller Coaster Gantries
 
-Parametric, low-complexity fabrication linear axis designs using ubiquitous hardware.
+At the CBA, we make a lot of our own tools. These are shared design elements and ideas that we use when we're spinning up custom machines: they're parametric, aim to be reconfigurable, and we can make them in the lab with a minimum set of stock hardware.
+
+The goal is to provide a set of components from which most other automation equipment can be composed. 
+
+## Usage 
+
+This README should help you figure out how to build your own. A typical machine-wide workflow is described [for machineweek, at this link](https://gitlab.cba.mit.edu/jakeread/machineweek-2018). 
+
+This repo contains the CAD files, BOMs and Drawings that you'll want to peruse.
 
 ## Gantry for NEMA23 Motor, 0.25" Aluminum
 
@@ -72,18 +80,29 @@ Countersink Bit | Flush Mounting |
 
 Type | Size | QTY | Where Used | McMaster PN
 --- | --- | --- | --- | --- 
-Button Head Thread-Forming | No. 6, 3/4" | 8 | Connecting Lap and Tab HDPE, and Belt Blocks | 99512A265
+Button Head Thread-Forming | No. 6, 3/4" | 10 + (4 * rail tab) (lots) | Connecting Lap and Tab HDPE, Belt Blocks, Chassis | 99512A265
+Button Head Thread-Forming | No. 6, 1/2" | 2 | Belt Blocks | 99512A259
 Flat Head Thread-Forming | No. 6, 3/4" | 8 | Flush Mounting HDPE | 95893A258
 SHCS | M3x40 | 2 | Used *only* when pre-loading bearing rollers | 91292A024
 SHCS | M3x30 | 2 | Used *only* when pre-loading bearing rollers | 91292A022
-Belleville Washer | 3.1mm ID | 12 | Used *only* when pre-loading bearing rollers | 96445K157
-Locknut | M3, Nylon | 2 | Used *only* when pre-loading bearing rollers | 90576A102
-SHCS | M5x10 | 1 | Connecting Nema 23 Motor |  
-SHCS | M5x16 | 3 | Nema 23 Motor through arcs | 
+Belleville Washer | 3.1mm ID | 24 | Used *only* when pre-loading bearing rollers | 96445K157
+Locknut | M3, Nylon | 6 | Used *only* when pre-loading bearing rollers | 90576A102
+SHCS | M5x10 | 1 | Connecting Nema 23 Motor | 91292A124
+SHCS | M5x16 | 3 | Nema 23 Motor through tensioning arcs | 91292A126
 Shoulder Screw | 8mm Shoulder x 8mm x M6 | 10 | Guide Roller Shaft | 92981A198
 Shoulder Screw | 8mm Shoulder x 16mm x M6 | 2 | Belt Guide Roller Shaft | 92981A202
 Bearing Shim | 8mm ID x 10mm OD x 1mm Thick | 38 | Roller Separation | 98089A381 
 
+## Shared Purchase Parts
+
+What | Spec | QTY | Where Used | Link
+--- | --- | --- | --- | --- 
+608ZZ Bearings | 8x22x7 | 14 | Rollers | [VXB 10](https://www.vxb.com/608ZZ-Shielded-8x22x7-Miniature-Bearing-Pack-of-10-p/608zz10.htm), [VXB 1000](https://www.vxb.com/Wholesale-Lot-of-1000-608ZZ-Ball-Bearing-p/608zz-wholesale.htm)
+GT2 Belt | 10mm Wide, Length Dependent | 2 | Belt! | [Amazon](https://www.amazon.com/Timing-Pulley-Teeth-6-35mm-Printer/dp/B07BS4DVR5/)
+GT2 Pulley | 10mm Wide, Motor Bore Diameter | 1 | Transmission! | [Above, Combo](https://www.amazon.com/Timing-Pulley-Teeth-6-35mm-Printer/dp/B07BS4DVR5/)
+Stepper Motor | NEMA23 x52mm | 1 | Torque ! | [StepperOnline](https://www.omc-stepperonline.com/hybrid-stepper-motor/nema-23-bipolar-18deg-09-nm-1275ozin-2a-36v-57x57x52mm-4-wires-23hs20-2004s.html)
+Stepper Motor | NEMA23 x76mm | -or | Torque ! | [StepperOnline](https://www.omc-stepperonline.com/hybrid-stepper-motor/nema-23-bipolar-18deg-19nm-269ozin-28a-32v-57x57x76mm-4-wires-23hs30-2804s.html)
+
 # REF
 
 Find [Open Assemblies Hardware Reference](https://github.com/jakeread/openassemblies/blob/master/hardware.md) for shorthands etc.