Mill with auxilarry spindle

That isn't surprising that it would not load. When you change a stepgen from position to velocity mode, it changes what the pin names are for that stepgen. So the pin named stepgen.3.position-cmd no longer existed causing a hal file error and Linuxcnc not to start.

Thanks again for your reply and as you can guess my knowledge of these things is woefully inadequate; please bear with me. I studied the HAL tutorial last night and am now a little wiser but not wise enough.
I found another Linux post, "4th Axis rotational angle limit problem" where the issue was similar to mine and changed the hal file in a similar way but without success.
I attach the modified hal file.
If you can help i would be grateful.

Doesn't look like the file got attached. Also the debug message was 'axis.3.motor-vel-cmd' does not exist.

setp stepgen.3.velocity-cmd [AXIS_3]SCALE
setp stepgen.3.steplen 1
setp stepgen.3.stepspace 0
setp stepgen.3.dirhold 36000
setp stepgen.3.dirsetup 36000
setp stepgen.3.maxaccel [AXIS_3]STEPGEN_MAXACCEL
net avel-cmd axis.3.motor-vel-cmd => stepgen.3.velocity-cmd
net avel-fb axis.3.motor-vel-cmd => axis.3.motor-vel-fb net avel-fb axis.3.motor-vel-fb => axis.3.motor-vel-fb
net astep <= stepgen.3.step
net adir <= stepgen.3.dir
net aenable axis.3.amp-enable-out => stepgen.3.enable
Also attach the errror part where it says
debug info

I added, "net avel-fb axis.3.motor-vel-fb => axis.3.motor-vel-fb" to the hal file and the resulting debug message was:-
./ Mill-Lathe.hal.103: Pin 'axis.3.motor-vel-cmd' does not exist 4004.
The hal file is attached.

That is because that pin does not in fact exist
There is a pin axis.N.joint-vel-cmd. (but not a motor version)