Horizontal expansion calibration tool

thingiverse

In an ideal world, if you print a 10 mm hole and a 10 mm post, the post would fit into the hole. In practise, some inaccuracy is present. In my experience this often shows up as horizontal expansion (HE) of whatever you are printing - a bit of lateral expansion of the plastic during extrusion makes the wall slightly thicker than ideal. You can correct for this using the "Horizontal Expansion" setting (under "Shell") in Cura. This print is designed to identify what that value should be set to. If you determine your printer is delivering a positive HE, set a negative one to cancel that out, and vice versa. The post tapers from 10 mm diameter to 9 mm diameter over a length of 20 mm. There are 1 mm gradations along the length (barely visible in the photo on the right, you could fill them with ink if you wanted to increase contrast). The block contains a hole that theoretically perfectly fits the post. Assuming your printer is delivering some horizontal expansion between -0.25 and 0.25 mm, and the X and Y axes are consistent, print both parts, and insert the small end of the post into the large end of the hole. If there is some "flash" around the base of either part where it was stuck to the build plate, you can scrape it off (only relevant for HE < 0). The post should fit fairly evenly (touching all the way around, not hung up on a bump or something), if not you may need to either tune the surface quality of your prints first (for example by adjusting temperature), or just sand them a bit if that's the way you will be using your printer. If the post fits perfectly, you have zero horizontal expansion. If it goes partway in you have positive HE, if it goes through and sticks out the other side you have negative HE. Note that the "flash" mentioned above can catch on the edge and keep it from going through, so this is where it matters. To quantify the HE: First, how far does the post go in? In my case, the initial version went in 8 mm (so 12 mm still sticking out - see photo, left side). Considering the small end of the post, we have a nominally 9 mm circle. It went 40% of the way into the hole (8 mm out of 20 mm). Since the taper is consistent, this means it reached a depth where the hole size is nominally 40% of the way from 10 mm down to 9 mm, so 9.6 mm. But the post is touching the sides, so altogether we have 0.6 mm to account for (9.6 mm - 9 mm) that is not in the design file. Now consider a cross-section, after splitting the print in half from the top. HE is relevant on both the "left" and "right" sides of the post, and also on both sides of the hole - so four places. So the printer is producing 0.15 mm (0.6 mm / 4) of HE - every wall is just that little bit wider than it is supposed to be. The sides of the hole close in by 0.15 mm on each side, making a 9.6 mm hole actually 9.3 mm. The sides of the post expand 0.15 mm on each side, making a 9 mm post really 9.3 mm. If your post goes through and sticks out the other side, your printer is producing a negative HE, and you can do the same calculations based on where the nominally 10 mm end winds up. In my case, setting "Horizontal Expansion" in Cura to -0.15 mm to compensate produced the print on the right, where the post fits almost perfectly into the hole. Note that if you change settings (filament, temperature, etc), your printer's HE value may also change, so you may want to repeat this calibration with the new settings.