added thread test

thread_experiment/README.md

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+No, not THAT kind of thread. Threads for nuts and bolts and screws! Neat!

thread_experiment/ThreadExample.scad

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+use <threads.scad>
+use <measurement.scad>
+
+render() difference() {
+    
+    ScrewThread(5, 5, 0.99, 55, 0, 0);
+    $fn = 50;
+    translate([0,0,-0.5]) cylinder(6, 1.9, 1.9);
+}
+
+color("red") x_measurement(3.8, [-1.99,0,2.5]);
+$fn=100;
+//#cylinder(5,2.5,2.5);