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How I finally got my smartrapcore alu to print exactly the same in X and Y (corexy calibration guide)

Posted by DjDemonD 
Okay so this might be an area everyone else knows plenty about but I've struggled with this until I recently made some changes to my machine.

The issue was that the x axis on my smartrapcore alu was not orthogonal to the y axis (i.e. not at 90 degrees to it).

So this is how I fixed it. Which I think is the same as other advice I've read but hopefully easier to visualise and to do, and far less abstract. Also it does not require printing calibration objects, as doing this then changing something and doing it again is very slow.

1) With the belts attached, running properly through all the pulleys, but not super tight, move the carriage to the front of the frame by hand, until it hits a physical travel limit, such as your y endstop. Move it back and to into this position (its best to hold the x rod mid way along) to ensure its sitting passively in place.

2) Now measure the distance between the x smooth rod and the frame both on the left and on the right. Mine was around 1.5mm longer on the left compared to the right i.e. my x rod was further away from the frame on the left.

3) Now if the x axis is further away on the left from the frame its because the A belt (left motor) is not pulling hard enough to keep the x axis straight compared to the B belt. Try pulling a bit on that belt to tighten it, the x axis will move closer to the frame. Tighten the A belt/left belt, with my new adjustable motor mounts its easy to do this, but if not move the belt one tooth tighter, then check again, it might need more. If its the right side of the x axis that's too far from the frame, tighten the B belt (right motor).

One thing that makes doing this difficult is that its easy to go from a normally tensioned belt to very tight indeed, but you might need to for example loosen the B belt instead of tightening the A belt, which achieves the same effect. The reason I adapted Realthor's balance tension carriage is to avoid this procedure but ti did not work as expected, however at the turn of a wheel overall belt tension can be adjusted.

4) Now remeasure the two distances. When the x axis is parallel to the frame (assuming your frame is square/rectangular) you are there.

5) Print a calibration object rotated at 45 to the x (or y) axis and remeasure the sides, they should be the same.

As for calibrating everything else here's an overview.

The Y axis is orthogonal to the frame, if the y rods are the same distance from the frame at the back as they are at the front. This is less difficult to get wrong as assuming you are using new upgraded parts the brackets holding the back and front parts of the y rods are the same shape and mounting.

The z axis is orthogonal if the z rods are parallel to the frame, i.e. same distance at the top and bottom.

Use triffid hunters guide for x,y and z steps per mm. But assuming your pulleys are made to spec, are not oval and are attached to the motors well, the number of pulley teeth plus your motor's steps/rotation determines the distance moved per step. So for 20t pulleys and 200 steps per rotation (1.8 deg) motors, its 80 steps/mm (assuming gt2 belt). Use this guide to calculate it for smaller pulleys, 400 steps/mm (0.9deg) motors or different belts.

As for extruder calibration try this object and see how you get on.

Edited 1 time(s). Last edit at 09/06/2016 09:01AM by DjDemonD.

Simon Khoury

Co-founder of [www.precisionpiezo.co.uk] Accurate, repeatable, versatile Z-Probes

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