[matridium]

I need to mill some 3/8 angle Iron flat on the back sides and at least one of the insides of the bend. the angels is 4" x 4" and 24" long. The outsides needs to be square or perpendicular to each other. the inside part should be parallel to the outside machined surface.
I was think of clamping the angle upside down to two angle blocks on the milling table and indicating it in, but the surface is really rough and rusty.
i suppose I can try to get it close and take a shallow cut first then indicate the newly cut surface and re-cut, and then flip the cut surface against the angle plates and star the other side. Another idea I had was to clean up as much rust as i can with an angle grinder and abrasive pad. Then clamp that part to the table, and then mill just the 3/8 edge and then flip that 3/8 edge over and rest it on the table when I clamp to the angle plates, this will at last get me close to fat to table.
I have not had to do anything like this on the scale so any incite will help.

[Tom-AMS]

unless I'm missing something in your description, there's no benefit of taking a skim cut and then re-indicating it - unless you think the part moved during the skim cut. Instead
select the flattest, smoothest, straightest, surface on the angle-iron and use it as your reference datum. whether it is on the inside or outside of either leg, or even an edge, dial it in and cut it smooth/flat/straight as needed. All other cuts will be registered from it. You may need to place a few jacks to dampen vibration.

[Superman]

Forget using a vice
- angle iron outer faces are roughly square, the inner faces can be tapered


- Clamp to the table ( on blocks ), indicate to be parallel to X axis, this leaves 1 leg vertical, for you to machine both sides
- Flip 90° to set the machined leg onto blocks.... repeat 1st operation

[Tom-AMS]

Superman's method for holding / machining is excellent, no need for angle plate or jacks, and less vibes... however side milling with 4" depth. even with multiple steps, you'll want to choose a tool with as little "stick out" as possible, and that is rigid enough, so it won't deflect.