Surface Grinding

Find someone with a surface grinder.

You will need a professional lapping machine and appropriate /custom made fixture for perfect 90 deg angles . for 500mm is pretty long , not many machine shop has this equipment

You really need to provide more information for appropriate help . Like material brass , steel , alluminum etc !!!!! parallelism , flatness

required

fabricate a perfect forms to hold and clamp the 4mm square bar in 500m long

Any machine shop can do this if the chuck on the grinder is big enough. But you asked about how so here we go. I will assume that this will also need to be square on all long edges as well as // on opposing sides. Debur the part at sharp edges after each operation. Clean up grind only until all sides are square. The overall accuracy of this job will depend on the condition of the machine and chuck. As this is a long piece you will most likely encounter variations in the mag chuck. You may need to grind in the chuck before starting this job.

Clean chuck.

Lay the stock on the mag chuck, turn it on, grind first side.

Remove piece, clean dust from chuck, place side one face down on chuck.

Turn on chuck and grind side 2.

Remove part and clean chuck.

Set up (align) 3 or 4 precision vises on the chuck.

Turn on mag. chuck.

Place appropriate parallels in middle vise(s) to elevate the part just above the vise jaws. If you do not have parallels you can sweep the top of the part with an indicator or use a very good level. Assuming the machine is level that is.

Clamp the part with sides 1 and 2 facing the vise jaws.

Grind side 3.

Remove the part and place it directly on the chuck with side 3 facing down.

Turn on chuck and grind side 4.

Remove, debur , inspect / measure.

Finish grinding can now be done directly off of the chuck. Pick a pair of sides and grind to thickness. If there are some surface defects that did not clean up before this is a good time to get rid of them. Or at least reduce them. Repeat for second set of sides and you are all done.

Keep in mind that when you are using the vises that it is easy to machine in a bow due to tool pressure. Support the work piece well with the vises, especially on the ends. The parallels are only needed for set up. So you do not have to have them in each vise. The accuracy of this set up , when using vises, is also dependant on the vises having matched heights. The indicator is the best method for checking run out in your set up. If square corners are not needed then skip the vises altogether and work directly off of the chuck all the way through. Simply rotate the part and grind the next side until it is cleaned up. Then work in pairs and grind to size.