# Specific 3D Printers, Scanners, & Hardware > CTC 3D Printers >  CTC - fixed my Y axis backlash- circles not round problem

## Raymo

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

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## adrianq

Ray,

Thanks for the info!   

I have been looking to replace the plastic gears.  The belt tensioner will also come in useful.

I am getting ready to put in my Drylin bearings so will probably change the gear while I have it open.

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## bigo93

Why a 17 tooth gear? I though the ones needed were either a 16 or 20 tooth?

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## Raymo

nope, ctc uses 17 tooth gears.

But I guess you could switch to 16 or 20 and re-align the steps per mm in software... but easier to just get 17t gears.

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## slippyr4

> I am getting ready to put in my Drylin bearings so will probably change the gear while I have it open.


BE CAREFUL!

On my printer the right hand Y carriage block (the one that holds the X motor) cracked so I had to replace it.

Whilst I had it apart, I changed a whole bunch of other parts, including changing all linear bearings to drylin bushes. And when I put it back together, I had the dreaded non-round circle problem. The issue is the circles are flattened top and bottom. The usual cause is slop in the Y axis - when the Y changes direction, the print head doesn't move for a moment whilst slack in the belt is taken up.

It took me weeks to figure out the problem; I aligned and realigned the X/Y axes thinking they were out of square, but to no avail.

In the end, I figured it out - the drylin bushes on the Y axis had too much friction (static friction - "stiction"). What happens is the when the Y changes direction, even though there was no slack, the bushes stick until the tension on the belt has increased enough to suddenly unstick the bush. The result is identical to the one you get with slack, but it was not caused by the slack.

I put linear bearings back in the Y axis and the problem disappeared immediately.

You might get away with it, but if you end up with the same issue as me then at least you'll be able to solve it quicker than me, I hope. I think the problem is that where the bushes press in to the plastic mouldings, the clips deform the bushes slightly which increases the pressure on them.

The bushes in the X and Z axes are fine though, and I still have those in place.

Good luck.

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## adrianq

Slippy,

Thanks you for the info.  I will definitely keep that in mind!

*crossing fingers*

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## Raymo

I agree 100% about the bearings.
I switched to oil impregnated brass bearings and found that it was hard to get good free motion in the x and y carridges.
The brass bearings have a much tighter tollerance than the linear "ball bearing" bushes and tend to stick, so it's vital to ensure they sit perfectly into the carridge.
Mine needed 1mm cases to make them fit since they were only 14mm and the linear bearings were 15mm... so I printed out cases.

The linear bearing mod only reduces sound AFIK, I don't think it makes prints any more accurate... that only comes from good belt tension and minimal slack in the drive system.

but for sure the metal gears and new belts really improved my circles.

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## PntlssMachi

Hi,
I have also circles not round problem. My squares sides are almost equal (+- 0,05mm) but they are not right angeled, but slightly off. It causes, that circles are not roud, in diagonal. Should also help just replace plastic gear or can be there also some missalignment in axis themselfs? ( they are not in right angle to each other)

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## Raymo

try rotating the model 45 degrees and print, see what the error is then.

I think new gears will improve things for you.

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