Stepper Motors Pulsing When Spindle Running [SOLVED]

[SamD]

Hello all,

This is my first post here. I am the recent new owner of a Denford Starmill. I had an issue with the stepper motors randomly pulsing while the spindle was running. I was going to ask on here for some advice, but I have since resolved the issue myself. I thought I should still write a post about how I solved the issue, as it may be useful to someone else in the future.

Background:

I purchased a Denford Starmill that had already been converted to Mach3, some time around approx 2010. It appears to have had very little use, which confirms what the previous owner told me.

The specification of the machine and software is as follows:

- Denford Starmill P.C. Non-ATC (December 1993)
- DIYCNC.co.uk Uniport V2 Breakout Board
- DIYCNC.co.uk Spindle V3 Board
- UC100 USB CNC Motion Controller
- Mach3 R3.043 on Windows 7

Problem:

When running the spindle, between 2000-3000rpm, the X, Y, and Z axis stepper motors pulse randomly. This happens when the X, Y, and Z axis are stationery, as well as while they are in motion, where you can audibly hear random motor pulses happening. These random pulses do not occur when the spindle is off.

If I put my finger on the ball screws while the random pulses are happening, I can feel the ball screw turning a very tiny amount. While I have not cut any metal on this machine yet, I feel that these pulses will be enough to affect the surface finish, etc. In any event, the pulses should not be happening.

When the random pulses occur, the Digital Readout on Mach3 does not change. This leads me to believe that the pulses are occurring due to electrical noise generated by the spindle.

Solution:

Firstly, I thought I should check the spindle, as I believe this is where the noise is coming from. I found some threads on this forum where people have had similar stepper motor pulsing issues, and replacing the motor brushes (or complete spindle motor) seemed to be a possible solution, as well as making sure that the spindle was not running over speed and starting to spark, causing noise (viewtopic.php?t=5302).

As the machine has had very little use, and getting to the motor brushes looked like it was going to be fairly labour intensive, I chose to explore other areas first, and come back to the spindle as a last resort.

Next, I looked in the cabinet, paying close attention to the after-market breakout board and spindle board. As these could be the source of the problem as they are not standard Denford parts. The maker of my after-market boards (DIYCNC.co.uk) has since dissolved, however, using the Wayback Machine (wayback.archive.org), I was able to look at archived versions of the website and find some documentation on these boards. Unfortunately the data sheet for V3 spindle board was missing from the archive, but after reading the V2 and V4 data sheets I found that it is important that the board is powered by an independent 5V supply, unless an optional isolated supply version board was used. This also echoed someone else's similar problem on the same above forum thread (viewtopic.php?t=5302), where fitting an independent power supply fixed his issue. On closer inspection, I found that my board did have the optional isolated supply fitted, so there should be no concerns there.

However, while I was looking in the panel, I spotted a shielded cable from the Breakout Board to the Spindle Board, and I noticed that the shield wire had not actually been earthed. Whoever fitted the Breakout Board and Spindle Board also left other wires messy and tangled like a birds nest, so I spent some time tidying and securing loose cables, then earthed out the shielded cable, and for good measure, I also put some clip-on ferrite rings (that I harvested from some old spare various USB cables) around the wires from the Breakout Board to the stepper motors, and around the power cables to and from the Spindle Board.

The net result is that I have fixed the issue, and there is no more pulsing coming from the stepper motors while the spindle is running. One of the things I did fixed it, but I believe it was mostly down to earthing out the shielded cable (which by the way, should only ever be earthed at one end, not both).

Thanks,

SamD