One or more Axis losing, missing or gaining steps.

[Dec0y]

I have found the smaller Brush DC Motors cause Electrical or RF Noise as their brushes wear.
If you have one of these DC Spindles with brushes and commutator dump them.
You can Identify a DC Motor or Spindle with brushes and commutator by looking at attached photos below.
They have replaceable carbon brush ports or they can be visible through vents on motor.
There are Cheep brushless DC Spindles out there, but if you are caving or cutting with small Vbits or tapered bits make sure it will do over 24,000 RPM or better.
Also make sure the Spindle is advertised as a Brushless DC or BLDC Spindle.

WHY?
As the brushes wear they cause arcing between commutator and brushes.
The arcing generates hi voltage and frequency spikes that are also present as RF.
This RF and EM will flow through you're CNC wiring and injected or block step signals.

THIS WILL GET YOU OUT OF TROUBLE!

I had one of these.


I added 2 Ferrite beads to the DC supply wires, the old DC motor has since been replaced with a brushless 800w water cooled spindles from Gpenny.
Pictured where the wires with the beads "Light Blue Rings"


Ferrite Beads are available in various shapes and sizes.


you can find them on old computer monitor, printer and power cables like this.


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[mactec54]

I have found the smaller Brush DC Motors cause Electrical or RF Noise as their brushes wear.
If you have one of these DC Spindles with brushes and commutator dump them.
You can Identify a DC Motor or Spindle with brushes and commutator by looking at attached photos below.
They have replaceable carbon brush ports or they can be visible through vents on motor.
There are Cheep brushless DC Spindles out there, but if you are caving or cutting with small Vbits or tapered bits make sure it will do over 24,000 RPM or better.
Also make sure the Spindle is advertised as a Brushless DC or BLDC Spindle.

WHY?
As the brushes wear they cause arcing between commutator and brushes.
The arcing generates hi voltage and frequency spikes that are also present as RF.
This RF and EM will flow through you're CNC wiring and injected or block step signals.

THIS WILL GET YOU OUT OF TROUBLE!

I had one of these.


I added 2 Ferrite beads to the DC supply wires, the old DC motor has since been replaced with a brushless 800w water cooled spindles from Gpenny.
Pictured where the wires with the beads "Light Blue Rings"


Ferrite Beads are available in various shapes and sizes.


you can find them on old computer monitor, printer and power cables like this.

This is the normal thing to do to help with noise with Dc motors, and with some Ac motors application's also, adding a ferrite choke is a common fix, adding a Ground wire will also help, which you don't appear to have, you are dealing with common-mode noise only when you use a ferrite choke like this, it does not help with differential-mode noise which is also present and just as damaging to low voltage electronics

[Dec0y]

I am sure many other forms of interference will or can be present, in this case, the ferrite beads resolved the problem.
I have since rebuilt and upgraded that machine a few times
Hope it helps or gets someone out of trouble.

[mactec54]

I am sure many other forms of interference will or can be present, in this case, the ferrite beads resolved the problem.
I have since rebuilt and upgraded that machine a few times
Hope it helps or gets someone out of trouble.

Adding just a Ground wire, also would most likely have done the same job, the Ferrites are a good addition to any motor like this, that has no shielded cable or Grounding

[WilleeCue]

I think an exception would be the German made Kress routers that run 29,000 rpm and use brushes.
Never had any problem and have been thru a set of brushes already.
Quality always satisfies.

If you are missing steps you motor tuning could be a bit to close to max acceleration.