Here are some possible reasons:
1. A geometrical error in your build. In particular, check that the bearing spacing at the top of each pair of rods (on the carriage) is exactly the same as at the bottom end (on the effector). If they are different then the effector will tilt by varying amounts depending on XY position, which will cause print distortion.
2. The elastic bands you have between pairs of rods will distort the rods (i.e. cause them to bow in towards the centre), which changes their effective length a little. If the amount of distortion varies with XY position (because the bands get stretched more or less), then this will again result in the effector tilting. If you must use elastic bands to keep the joints tight, it is probably better to place them near the ends of the rods than in the middle.
3. You have the wrong tower steps/mm or wrong rod length configured in the firmware. Have you checked that the steps/mm is correct, for example by measuring the heights of your test prints?
4. You are exceeding the angular range that the joints can handle.
5. Is the firmware that you are using known to work on deltas as large as that? 8-bit firmwares in particular might be subject to overflow issues.
Causes #1, #2 and #3 will cause the distortion to appear gradually as the size of the printed part increases; whereas causes #4 and #5 are more likely to cause no distortion until the size exceeds a certain critical value.
Additional suggestions:
1. Print a test piece comprising a set of grid lines covering the whole printable area of the bed. I expect you will see that the spacing between pairs of lines widens at the edges. Is it gradual increase in spacing, or is there no increase until a certain distance from the centre is reached?
2. Mount a bulls-eye spirit level on the effector, so that you can see any tilt.
HTH David
Edited 2 time(s). Last edit at 11/13/2018 04:02AM by dc42.
Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod
Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].