Hi Theo
Off the shelf cutting discs dont react well to impact, they tend to break and
then wobble and shred up, they need a gradual introduction to the item been
cut-see dead metal approach.
A long while ago pre robot wars I experimented using flexible discs, which
when spun up to grinder speeds would cut through Hdpe but would not break
under inpact.
Re previous post -if you want a angle grinder I have a faulty mains 9 inch hitachi
which you could convert to 12 or 24 volt drive yours or anybodys £5.