Thanks Mike.

Today, I milled two slots!
4mm wide by 5mm deep slots in the steel strips shown in the previous post. The slots will have simple covers placed into them.
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Next I've devised a bit of a mad scheme to drill the array of M6 holes in the bed:
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Then checked that it was sufficiently parallel - couple of whacks with the nearest hammer and it's now within 0.05mm over 400mm.
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Now to continue the drawing...