I used to have the same attachment. I found it hard to set up and unstable. I ended up throwing it away.
I bought a small metal chop saw on ebay for less than £50. It takes 125mm cutting blades and works very well.
I have a few angle grinders all fitted with different discs because I use them a lot. You can get a quick release nut that you don't need any tools to remove the old discs, well worth investing in.
http://www.ebay.co.uk/itm/MILWAUKEE-...QAAOSwZ1BXe6eL
This one is even better but double the price
https://www.ebay.co.uk/itm/Bosch-Ang...26.m2548.l4275
Sorry but no link for the chop off saw