Circle dimensions versus square dimensions.

Hey guys!

Just wanted to throw a discussion out there and maybe you can help me understand. First of all, my setup:

[www.reprap.cn]

Printer: He3D-DLT-600
Software: Repetier Host V1.6.1
Board: Arduino 2650 with Ramps Controller 1.6

I've read up on different things about calibration, making sure everything is rigid, backlash, and other subjects. I was wondering if anyone had any new insights as to getting circles to match the accuracy of a square. For example, when I do a test 10mm cube, I am usually within 0.1mm of accuracy which is great! The problem is that when I decide to do a circle or holes, the OD is usually within about the same, except for some minor oval shape which I am trying to address, but the ID is usually around .7mm off. I'm just wondering if this is typical for DIY printers and if others are having about the same accuracy when it comes to circles?

Just as dimensions go, I am printing a ring with a 20mm od and a 16mm id, the measurements usally come in at around 19.89mm and 15.31mm respectively. Just seeing if I am getting too picky or not.

I found that the type of plastic had a big affect on circles. When I printed something in PLA, my holes where too small. When I printed the same object in PETG, my holes where spot on. I haven't printed anything in ABS yet and I can't say I've attempted to measure anything I've printed in ninjaflex yet.

On exterior dimensions circles or squares made no difference and were both dimension-ally accurate in any material.