Newbie looking at 6040 (China CNC) for polypropylene 15mm sheet

[Snapper]

Are you measuring each piece of stock prior? There can be a lot of deviation on the extrusions, +inaccuracy from the bed and 0.3mm doesn't sound an unrealistic error at all. If you can use stock bottom as part zero and skim the top surface down slightly you will get far more repeatable results.

Also, if milling on a vice, have you trimmed/shimmed it? So that your jaws or soft jaws are square to your axis.

[JOGARA]

The stock is bang on 15mm.
The 0.3mm is across the length of my part.

I milled my soft jaws while in place so if anything it should be near enough perfect?