update BOM and drawing for fab class

.gitignore

 **.3dmbak
 **.rhl
+**drawing/
\ No newline at end of file

n17_linearPinion/README.md

@@ -4,29 +4,34 @@
 
 **USED IN:** [Little Rascal](http://gitlab.cba.mit.edu/jakeread/littlerascal)
 
+![parts](n17_linearPinion_partsCallout.jpg)
+
+![explain](n17_linearPinion_explainer.jpg)
+
 ![n17sofar](n17-assem.jpg)
 
 Smaller Gantries. Uses 45* bearings in 3DP blocks, with one set riding on flexures for smooth preloaded motion. Cut aluminum or steel rail on WJ with high cut quality, or cut acrylic on laser. 0.25" or similar (6.35mm) stock assumed.
 
-### Development Notes ... 
-- while heat-setting M4 inserts in for the rollers, (1) there isn't *really* enough space to get the soldering iron in (without melting the adjacent pla) and (2) it's difficult to guarantee these go in straight. maybe use a press-in insert? / a hex nut ? 
-- long runs (beams) are not stiff ... try 1/8" waterjet stiffening folded members 
-- in the fullness of time, add an N23 size for torque-y applications like the CAT's bed: this is minimal overhead.
+**shoulder bolts can be substituted for fabricated bushings** 
+
+s/o to Rahul in Kerala for this example: 
+
+![bushings](shoulder-alternative.jpg)
 
 ### BOM for N17 
 
 Type | Size | QTY | Where Used | Vendor | PN
 --- | --- | --- | --- | --- | --- 
-Flat Head Thread-Forming | No. 6, 1/2" | 12 | - | McMaster | 95893A255
-Flat Head Thread-Forming | No. 6, 3/4" | 8 | - | McMaster | 95893A258
-Bearing Shims | 5x10x0.5mm | 22 | - | McMaster | 98089A331
+Flat Head Thread-Forming | No. 6, 1/2" | 20 | Most Plastic Fasteners | McMaster | 95893A255
+Bearing Shims | 5x10x0.5mm | 22 | Adjacent Every Bearing | McMaster | 98089A331
 Shoulder Bolts | 5x10xM4 | 2 | Pulleys | McMaster | 92981A030 
 Shoulder Bolts | 5x6xM4 | 8 | Rollers | McMaster | 92981A146
-Heat-Set M4 Tapered Inserts | M4 | 10 | - | McMaster | 94180A351
-Heat-Set M3 Tapered Insert | M3 | 5 | - | McMaster | 94180A331
+Heat-Set M4 Tapered Inserts | M4 | 10 | Anchors for Sholder Bolts into Plastic Parts | McMaster | 94180A351
+Heat-Set M3 Tapered Insert | M3 | 3 | Threads for M3 Screws to Adjust Motor Tension and Bearing Preload | McMaster | 94180A331
 M3 SHCS | 10mm Long | 4 | Motor Mounting | McMaster | 91292A113
-M3 SHCS | 14mm Long | 5 | Tensioning and Tuning | McMaster | 91292A027
-M3 Washers | - | 9 | Screws without Washers are Sad Screws | McMaster | 93475A210
+M3 SHCS | 14mm Long | 2 | Bearing Preloading | McMaster | 91292A027
+M3 SHCS | 20mm Long | 1 | Tensioning Pulley | McMaster | 91292A027
+M3 Washers | - | 7 | Screws without Washers are Sad Screws | McMaster | 93475A210
 M4 SHCS | 20mm Long | 4 | Belt Mounting | McMaster | 91292A121
 M4 Washers | - | 4 | ibid | McMaster | 93475A230 
 625ZZ Bearings | 5x16x5mm | 12 | - | VXB | [link](https://www.vxb.com/20-625ZZ-Shielded-5mm-Bore-Diameter-Miniature-p/625zz20.htm) or McMaster 6153K113