Low Point On Milled Surface

I would look at weight distribution, since everything seems flat except a single corner. If the location of your vise, or where you are clamping a block down to the table, allows the weight of the table to get off centered, if can make things "fall" into the clearance of the gibs and kick up in a small area.

One test would be to run a DTI over the surface held in the quill and see if it will retrace the surface, but not show any variation from flat. This would be a hint that something is moving. If it does show the dip, then the problem is elsewhere.

I have milled perfectly bowed splines on my BP clone before realizing the part was a bit too heavy in the first place, and certainly long enough and asymmetrical weight-wise and caused the table to moved out of planar.
 
I agree with Tony Wells - may have some deflection. You may also have your gibb loose or a ballscrew with some backlash on a axis - cutting steel allowed deflection movement when cutting that area...
 
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Do you have flycutter or a face mill? If your mill is well trammed you should be able to take .002" off with it and get a good flat surface.
 
By chance was it the last pass area that is the low spot? An end mill will try to ride up away from the work and them drop off as it clears the edge. Not sure what dimension you are putting together but if it's the 3.5" then milling that with a 5/16 end mill would take patience. If the last pass had part of the end mill hanging over the side I might expect that whole pass to be low. if the vise was crooked to x travel and the end mill had more overhang at the end of the cut that might explain a one corner low spot. ..it's a bit of a crap shot on mills....I've spent time and tried to do everything perfect....looks great when done....take it over to the surface grinder and realize it's not so perfect.
 
That is alot of passes with a 5/16" endmill. You won't get ir perfect. A flycutter would be the cheapest tool to use Then you would be only making 1 pass.
 
After working on it some more last night I'm almost positive it's cold rolled stress I'm dealing with. Before I milled the face of the block I checked the opposite face and it was flat, when I checked it again last night it was slightly warped. So I milled 0.008" off that face as well, along with the previously milled face. However, the block has warped slightly yet again. It's only out about 0.001" (maybe a hair more), but it's definitely there. How could I stress relieve cold rolled steel so it doesn't warp like this?

I guess the other question is, do I keep taking light cuts to try and get the two faces flat, or is that a fool's errand? I still have about 1mm I can remove.
 
Cold rolled stress- that is interesting on that size piece. I have seen steel move when I heat treat it but never thought about it in a block.

I am just curious, what are you using to measure with?
 
Success! After taking several more shallow passes it's flat enough for my project. Thank goodness. Also, now I know better than to use cold rolled again.
 
On a small block like that I would just lap the .001 error off. More accurate than milling and a better finish than what multiple passes with an end mill would leave behind

Cheers Phil
 
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