Please help experts! PLA printed cylinder (10 cm) is not perfect circular!

Hello guys! please i need your help!
im trying to have a perfect cylinder ( radius 5cm, high 5mm ) when i print it, it looks perfect circular, but when i measure it i see its not perfect. (for example X: 101mm, Y: 99mm) some times the fail is about 1mm.
i need it to be perfect circular! what is the problem?
im using Prusa i3, 220 degrees for the extruder and 60 for the glass heated bed.
is it something with the belts? or in the main board? or what?

thank you in advance!

Edited 3 time(s). Last edit at 12/26/2014 05:50PM by rajaakahel.

Sounds like calibration. I am not an expert but you need to tell it to move 100mm and see if it moves exactly 100mm in that direction, if not you need to adjust the steps in firmware. You do this for all 3 axis.

Other possible causes might be slipping (try a much slower speed to see if it still happens, if it does it's not slipping).

You can search for proper calibration technique.

Firstly as the above post mentions, calibrate your printer, make sure it is same hight between bed and nozzle in XYZ positions. Secondly when you printing, make sure that the printer do not shakes when it prints, this cause unstable prints. Third, make sure that the nozzle is fastened and not loose, fourth and not least, the speed. lower the speed where u think is difficult to print.

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thank you dear tmorris9, actually no one told me about this non physical calibration! i mean for the X Y axis. i found that the y axis is not correct, but i dont know how to correct it through a software! i googled it but they were differents ways, and all of them are not starting explaining from the first step! so i dont know hot to start and what to download and how to know my printer's info ( hardware & firmware ) ! i feel so stupid ! can you help me please?
this is the printer i bought :
[www.aliexpress.com]

Edited 2 time(s). Last edit at 12/28/2014 04:38PM by rajaakahel.

thank you dear oklok, actually my prints are stable and everything is fine, but the problem is only with the Y axis its moving less than the correct distance, about 1mm per 10 cm.

Hello, my belts were not so tight and that was the problem for the difference between X and Y printing size. Now I fixed this, but I need to configure the actual printing size for all axis. I will tell you what happens then. Thank you