Spindle stalling at max RPM without being under load

[SamD]

Hello all,

I am having a problem with the spindle speed on my 1994 Starmill PC, running on Mach 3.

In short, if I run the spindle at max RPM (3000), after a few minutes of running, it stalls. Even when not under load.

I have also noticed that the speed is varying (hunting) when at max RPM.

If I run it at 2500 RPM, there is no problem. I have verified the RPM with an external laser tacho.

I have removed, inspected and, cleaned both the brushes (which looked totally fine), and the commutator on the motor, with no effect.

I have re-greased the spindle bearings, and it turns freely.

I am wondering if the problem is with the Sprint drive or the interface with Mach 3. The Mach 3 controller communicates via a DIYCNC breakout board and a DIYCNC spindle board.

With regards to the spindle RPM on Mach 3; generally what I ask for is what I get. 1000 RPM equals an actual of around 970 RPM. 3000 RPM equals around 2900 RPM.

I was considering fiddling with the MIN, MAX and CURRENT pots on the sprint drive to see if that has any effect, but I am nervous about that.

This machine has had very little use before it was converted by someone else to Mach 3. I have run it for probably 12 hours, most of which at 3000 RPM for some engraving work, and not had a problem until recently, now I can’t run it over 2500 RPM for fear of the spindle stopping mid cut.

Any tips are welcome! Thank you.