Hi Ross ,
the parting tooling your photo is only is roughly shaped and needs sharpening
as it is it has a zero top rake that's OK for brass but needs to be reground for steel or aluminium
to give you a positive rake , also it looks a little too wide
while at school in the 70's , parting off tools usually broke before I could cut one part off
years later while making some electrical repairs to an EMI-MEC "plug board" automatic capstan lathe
watching them part off every few minutes , I realised why I broke so many parting off tools on the Myford lathes
first a 2.5 to 3 mm wide parting tool is too wide !! for small lathes - they are under powered and not ridged enough
you need to part off as close to the chuck as possible .
once you start you need to keep going , the feed rate needs to be constant , no stopping halfway through
I've recently re learned the art on a smaller version of your lathe ( machine mart CL300M)
first time I tried ,the tool deflected downwards, as it metal bar climbed onto the tool-,,,,,crunch !!
you could see the toolpost move ,due to the play in the slides !!
I replaced the gibbs on the top slides with brass ones from to reduce the play ,the originals didn't fit too well ,
( with no milling machine I took the easy way out and bought them from Arc Euro Trade )
I reground the parting off tool so its now about 2 mm wide and the top rake about 3 degrees ( angle needs to suit the material)
the tool will get thinner as the cutting edge is resharpened
my second attempt with the correctly fitting gibs was successfull

as other have said its very important the tool is at the correct height
one problem with aluminium is it friction welds its self to the tool
with practice , you will know by the sound from the tool when its cutting correctly
John