My circles are ovals

Hi Community (and Seth). I need some help please, I am obviously doing something wrong, I have tried to carve some circles, but they all come out as ovals? and even my straight cuts are not ending up the length I ask them to be?
I watched a tutorial on calibration and my $100 & $101 are both set at 40.00 (this is where they were set when I bought it second hand from a friend and it was apparently working fine)
I have set up to carve a straight line as a pocket with an X line at 100ml and a Y line at 100ml and yet the X line ends up 95ml and the Y line is 105ml
The Circles are all egg shaped with the longest being in the Y axis?
I used the formula as per the tutorial (after first doing the X calibration) and ended up with $100=42.1052632. (I was told that X & Y MUST be the same number, so I changed my Y to $101=42.1052632) but the straight lines X & Y were still different lengths and the circles were a really strange shape.
I’ve set it back to $100 & $101 = 40, but I don’t know what to do next?
Yes I am very new to this . . . . .
What can I do

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Ah. They actually do not need to be set to the same values. I think that’s the issue.
Looks like it should be $100=42.1053
And $101=38.0952

Also be aware that if the belt tension was not set correctly or if the vwheel eccentric nuts weren’t set correctly prior to the calibration then the resulting values may be inaccurate.

After entering these new values rerun the movement test… ohh and the longer the movement, the more accurate the calibration value will be, so if your cnc can move further than 100mm, i would use a longer movement…

also in case you don’t have longer metric measuring tools it doesn’t actually matter if you calibrate using metric or imperial the measurement units all wash out in the math.

Thanx Seth, yes, when I watched the tutorial I got the impression that the $100 & $101 could be set differently, but a friend who uses CNC machines for a living, told me in no uncertain terms that they MUST be the same numbers, which is why I stated that in my previous post.
After reading some of the posts in the forum about Belt tensioning, I think my next task will be to check my belt tension (I’ve broken a Y belt already) and then do the calibration again.
I will do the X & Y lines at 200mm
Thank you again Seth.

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Hi Seth, sorry to keep bothering you . . . I did two cuts with the original settings, X & Y both at 40.000 and the Y line was perfect at 200mm, but the X line was only 195mm so I have reset my $100 to 41.025641 and did the test again, and both lines (X & Y) were perfect 200mm.
So I thought I’d try some circles, (One small and one 50mm) they both turned out strange shapes and oval?
I’ve attached a picture.
Could it be that my belts are too tight? I haven’t done the tensioning yet, I only just got some scales.

Well, the measurement and calibration process should occur in air, and not when carving because other variables come into play while carving like belt tension, and eccentric nut adjustment which causes deflection if loose. So make sure to check the eccentric nuts on the vwheels as well as the belts, and preform the calibration as shown in that video.