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Hi magudaman

it seems like you are wanting to know how to, what I call, 'blocking up'

I am going to assume you are using some type of vertical mill and it cast aluminium plate and not extruded aluminium. Some extrude aluminium has a skin on it and can bow when the surface tension in that skin is removed.

First remove you vice from the mill table and make sure it is free of any bumps and burrs.

next check the spindle is square to the table; you will need a clocking arm fo this. A bent arm with a Dial Test Indicator (DTI) on the end. You will need to sweep the clock across the table in a circular motion and adjust the angle of the spindle until the clock reads zero all the way around.

Now clamp the plate to the table, either suing edge clamps or if you can put some temporary holes in the plate somewhere they won't matter. Mount the long edge of the plate parallel with the long axis of the mill; usually the X axis.

Mount a fly cutter that will cover the 5inch width of the plate and machine the 5inch x 13inch face until it cleans up.

Flip the plate over ready to machine the other side. It is important that you know the plate is sitting flat on the table. I would prefer to sit it on low parallels after first removing any burrs on the plate. this way as long as all the parallels are tight there is a good chance the plat is sitting flat. Just be careful the clamping method you are using is not bending the plate.

if you are not looking for precision flatness you can machine the thickness to size but if flatness is important you may wand to rough machine both side and then do a finishing cut on both side.

Now drop two dowel pins into the front T slots and using the same clamping method, pull the plate up against the dowels and clamp it and then machine the opposite 13inch edge with a suitable end mill. If you are careful where you put your clamp you can machine the 5inch edges at the same time. if not you will need to turn the part through 90 degrees and the clock the 13inch edge so it is square, use a DTI for this. the machine the 5inch edge using the end mill; then repeat for the other 5inch edge.

I hope this helps Brian