im new to cnc forums thought its about time

hi all i have been using cnc basic machining since 2010 for a company i am self tought so learning all the time. i usually use the cnc for basic 6mm routering a sheet of 6 mm acrylic but want to try new things and thing this forum is a great place to start for advice on 3d machining.
i currently am trying to do some area clearance milling at a depth of 3mm which is vital it stays at 3mm for my project. the problem i have is as the machining moves to the right half of the cnc it tapers slightly so only milling a depth of 2 mm:grumpy: . i have measured from the bed to the bottom of the cross arm on the left and the right and its out. so i have diagnosed the problem just unsure how to fix it. either side of the arm at the base by the cogs are what looks to me adjusting bolts and nuts 2 at the top and 1 at the bottom. i want to start messing with these but thought ill double check before i break the cnc and loose my job lol. thanks in advance. how do i post pic to help