CNC circles not round: ovals, flat spots and facets
An oval aligned with the machine axes means the two axes are calibrated differently. An oval on a diagonal means the axes are not square. Flat spots where the axes reverse mean backlash. A faceted circle means the arc was output as straight segments. The shape of the error identifies the cause.
A cut circle that is not circular. It may be elliptical, have flat spots at the four points where the axes reverse direction, or look visibly faceted rather than smooth.
first
Most likely causes
- Backlash in one or both axesMost common
Flats at the four reversal points are its fingerprint. Loose anti-backlash nuts, worn V-wheels, a slack belt. See backlash. - Steps per unit differ between X and YCommon
An ellipse with its long axis along X or Y. One axis is moving a different distance than commanded. See wrong size. - Arc output as straight segmentsCommon
A polygon rather than a circle. The post processor or the CAM tolerance, not the machine. - Axes not square to each otherOccasional
An ellipse on a diagonal, at 45 degrees to the machine axes, because the two axes are pulling the circle into a parallelogram. See squaring. - Tool deflectionOccasional
Round at the top and oval lower down, because the sideways cutting force changes with feed direction relative to the grain. - Bit diameter, runout or the colletOccasional
Round, but the wrong size. A different problem with the same first fix: measure the bit and clean the collet.
Diagnostic steps
| What you see | Cause | Fix |
|---|---|---|
| Oval, longer along X or along Y | Steps per unit differ between X and Y | Calibrate both axes. See wrong size. |
| Flat spots at the top, bottom, left and right | Backlash. Those are the points where an axis reverses. | Fix the mechanical slack. See backlash. |
| Faceted, made of visible straight lines | Arc output as line segments, with too coarse a tolerance | Enable arc fitting, or tighten the tolerance in CAM |
| Oval on a diagonal, long axis at 45 degrees to the machine | Axes not square to each other | See squaring. |
| Round but the wrong diameter | Bit diameter, runout or deflection | Measure the bit, check the collet, add a finishing pass |
| Round at the top, oval deeper down | Tool deflection under load | Lighter passes and a finishing pass at full depth |
The backlash test, specifically
Flat spots at the axis reversal points are a fingerprint for backlash. At those points one axis is momentarily doing nothing while the other reverses, so any slack in the reversing axis shows up as a short straight section instead of a curve.
- Cut a large circle in scrap, as large as your machine allows.
- Look at the top, bottom, left and right of it, specifically.
- If those four points have flats and the rest of the curve is fine, measure your backlash properly with a dial indicator.
Fixes
- Take up the backlash. Adjust anti-backlash nuts or V-wheel eccentrics, tension belts, tighten couplers. Then re-cut the test circle.
- Calibrate steps per unit on whichever axis measured wrong, in air, as described under wrong size.
- Square the machine if the diagonals disagree. See squaring.
- Enable arc fitting in the post, or tighten the CAM tolerance, if the circle is a polygon.
- Add a finishing pass for a circle that is round at the top and oval at depth.
Why it happens
A circle is the one shape that exercises both axes through every combination of speed and direction, including four moments where one axis stops and reverses while the other is moving at full speed. Anything wrong with either axis shows up in a circle before it shows up in a rectangle, and where on the circle it shows up tells you what is wrong. That is why a large test circle is the best single diagnostic cut a machine can make.
Arc output is the exception: a circle in CAM is usually a true arc, but whether it arrives at the machine as an arc or as a chain of tiny line segments depends on the post processor and whether the controller supports arc commands. Line-segment output with a loose tolerance gives a measurably faceted circle, and a lot of extra G-code. If your circles look polygonal, check arc fitting before you go looking for mechanical problems.
Prevention
- Cut a large test circle after assembly, after moving the machine, and any time the wheels or nuts are adjusted. Keep the measurements.
- Put backlash and V-wheel checks on the same schedule as the fastener check.
- Use a post processor that outputs arcs if your controller supports them.