If anyone is having the " my circles are not round" issue on their CTC.. here is what I went throught to fix it.

it can be caused by Y axis backlash... i.e. the y axis travel is not 100% the same in both directions, caused by slop in the Y axis pully system.

1. Replace all Y axiz gears, CTC uses plastic and are "press fit", replace with metal 17 tooth type with 2 x grub screws.
http://www.ebay.co.uk/itm/1419470907...%3AMEBIDX%3AIT

2. Replace all belts, not essential but I did anyway cos it was a cheap job and no harm.

3. Print out screw type belt tensioners, the spring type have too much spting in them and add to the backlash.
http://www.thingiverse.com/thing:1071695
These are great. There are two types, use the one for CTC called "Inverted part".

Then tension the belts evenly usaing the screw tensioners.

I managed to fix my circles problem with all above.
also, my 20mm test cubes were x20.6 y19.7 before the work, now they are x20.05 y19.95... good enough.


Hope this helps someone.

Ray