Using VR Milling V5 to drive VMC 1300 - G28 problem

I'm learning to program with VR Milling V5 and driving my VMC 1300. I've got to the point where I want to make a manual toolchange during a program.
I'm mostly using "G and M Programming for CNC Milling Machines" as my reference.
When I complete the moves with first tool, I programmed the toolchange with
M05
G91 G28 X0 Y0 Z50
M0 T06
similar to page 9 example
When the VMC gets to the G28 line, the machine starts to go up in Z, but makes a really bad noise and stops moving without going home.
I can just do the M0 T06 line without the preceding G28 line, but that makes a non-vectored return home which could crash
I tried variants of the G28 line like G91 G28 X0 Y0 Z0 without any joy in homing.
Of course I can G91 G00 X0 Y0 Z50 then M0 T06, but I'd like to know what I'm missing on G28 if there are any VR Milling users out there

I'm now working on the theory that the noise I hear is due to the Z Axis brake still being on when the machine tries to home. I am experiencing this problem when I try to do a toolchange without a prior G28 too. I can see the Z Axis brake state on the Mint i/o debug screen as Output 15, so I'm going to step through some programs and watch the Z Axis state.

Can't help with the noise/Z brake problem but I am slightly concerned about the "G28 X0 Y0 Z50" line. This means (or, at least, it would when used with Mach3) "goto the tool change position but go to X0 Y0 Z50 first". It's a kind-of "go via" command. The concern is that if the tool takes a straight line from where it is to X0 Y0 Z50 it might well hit something on the way. I use a "G28 Z0" command (with the "use machine coordinates" G code thing as well but I forget what that is without looking it up) so that the tool moves vertically upwards first to max height (my machine is configured to have Z0 at the top of travel); it then moves to whatever is defined as the G28 XY coordinates with the best chance of not wiping the tool out on the way!

Just a thought, as I know that G code interpretation does vary from machine controller to controller so your machine might be different.

Neale, there is a g91 in front of the G28 so it's just an incremental z 50 move

Ah, couldn't remember of the top of my head what that was. However, I'm not sure that an incremental move up is always going to be a safe thing to do - what if the Z axis is less than 50 from its limit position? Absolute, not incremental, and machine not work coordinate move to top Z value might be safer. And making sure that right units are being used, although maybe this controller is fixed metric-only. I'm used to Mach3 where units might change and my own tool-change macro uses more code checking and setting/resetting machine parameters than it does doing the actual work involved!

Very good point. Didn't think of that one

I've diagnosed my problem. It is the Z axis brake as I suspected. The logic is working, that is, the brake is signaled to be disengaged, but I think the power drops to the brake disengage solenoid when all three axis motors start up in rapid. I confirmed by removing the brake from the end of the Z axis stepper.