Add plastic shrinkage note in xyz calibration

calibration.html

@@ -1261,7 +1261,7 @@
         <p>Often people will print a 20mm calibration cube and measure the external faces to see how accurate their machine is. While this is a valid measurement for determining how accurate the output of the printer is, it is NOT the correct measurement for calibrating X, Y and Z steps. This is because the printed part is the result of many more variables other than how far the X, Y and Z axes are moving during the print.</p>
         <p>A simple demonstration of this can be made by printing three 20mm calibration cubes, with no changes to the machine but the extruder flow rate altered for each test. In the image below, the cubes have flow rates of 80%, 96% and 120%. Although they look identical from a distance, there is a clear variation in their external dimensions when measuring with calipers.</p>
         <a href="#" data-featherlight="img/cubeflowvariation.jpg"><img class="thumb" src="img/cubeflowvariation.jpg" /></a>
-        <p>If the cube can vary this much without adjusting steps per unit, it goes to show that printed parts are not a reliable indicator of whether the steps per unit are correctly configured. Yes, we do care about the accuracy of the final part, but we need a better way to measure X, Y and Z movement.</p>
+        <p>If the cube can vary this much without adjusting steps per unit, it goes to show that printed parts are not a reliable indicator of whether the steps per unit are correctly configured. Commenters on the video have also pointed out that the printed plastic will shrink as it cools, and this will differ for different materials and even for different colours/age/filament condition. Another relevant variable that ruins our results. Yes, we do care about the accuracy of the final part, but we need a better way to measure X, Y and Z movement.</p>
         <h2>What to do - Measuring raw axis movement</h2>
         <p>The primary variable we need to eliminate is the extruded plastic. Therefore we want to measure the movement of each axis when not printing, comparing target vs actual movement. This is where our calipers or preferably a dial gauge comes in handy. Our aim is to mount the dial gauge so that when we move an axis, it measures eactly how far it has travelled.</p>
         <h2>Dial gauge mounting</h2>