The mill is an X2 "mini mill", manufactured by Sieg and imported and sold in the US by Harbor Freight, Grizzly, etc.
The milling vise is a small screwless toolmaker's vise with clamping slots, sold by Little Machine Shop, Shars.com, etc.
Step 1: The Design
I'm making the clamps from some scrap angle iron. The stock is 0.25 inch thick. Each side is about 2 inches long (from the outside corner of the angle to the end of the "leg").
Additional stuff that's needed that I'm not making is a T-nut, a flange nut, and about 2 inches of 3/8-16 threaded rod.
Step 2: Rough Cut the Angle Iron
Anyway, I cut about a 1 inch wide piece off the length of angle iron. The exact size is not very important.
Then I cut off part of one "leg" of the piece, this will be the foot of the clamp later (where it stands on the mill's table). I learned my lesson about the plastic-tabled saw and made this cut with a hacksaw.
Step 3: Clean Up the Long Edges
The pictures show the setup for milling the two long sides. The rounded end of the angle-iron fits into the horizontal V-groove in the movable jaw of the vise. The edge that I'm milling is sticking out 1/4 inch or so past the vise jaws. This setup was plenty secure.
I eyeballed the workpiece to find the part that was sticking out the most, and gently touched off the cutter at that point. Then I started doing cutting passes in the Y direction, front to back, so each pass was done in conventional milling mode (I'm staying away from climb milling for now). Each pass took off about 0.010 inches of material, on whatever parts of the workpiece were sticking out. Eventually it cut along the whole face of the workpiece, and then I did a 0.005 inch finishing cut and called it done.
I used a 3/4 inch 4-flute milling cutter spinning at something like 800 rpm. Feedrate was probably about 5 or 7 ipm. Depth of cut (of each cut) was about 0.010 inch. I occasionally squirted WD-40 on the cutter and workpiece while cutting.
To mill the other long side I flipped the workpiece upside down (as shown in the second picture). The location of the V-groove in the vise jaw and the length of the foot (the short leg of the angle iron) conspire to make this possible. If the dimensions hadn't worked out I could have kept the workpiece in the "foot-up" orientation and slid the workpiece to the other side of the jaws.
Step 4: Cleaning Up the Foot
I clamped the "long leg" of the workpiece in the vise, being careful to clamp on the straight part of the angle iron, not up near the corner where the thickness changes. I positioned the clamp near the center of the vice to keep the clamping force balanced and avoid twisting the movable jaw.
The desired length of the foot (per the drawing) is 0.875 inch. I knew I cut the foot longer than that with the hacksaw, so there is some spare stock at the end of the foot to work with.
First I squared the bottom of the foot by skimming it with the cutter, just like I squared the long sides in step 3 (except along X in stead of along Y). I cut from the left of the workpiece to the right, to stay with conventional milling.
Once it was flat along the whole bottom of the foot I measured the foot height with calipers (I wiped the reference surfaces with a rag so the swarf wouldnt throw off the reading). This measurement told me how much material I had to remove. I took it off in 0.010 inch cuts until I got within about 0.020, then I measured again and took shallow cuts until I got within 0.005 inch, then I called it good.
Step 5: All the Facing Is Done
Now it's all over but the slotting.
Step 6: Slotting Setup
It's nice to have the workpiece roughly centered in the jaws, because it gives even clamping pressure, not so much twisting force on the movable jaw.
I want the slot in the middle of the clamp, so I measured the final, actual width of the clamp, located the edge, and positioned the spindle at the X axis center of the clamp.
Lock the X gibs here, we'll just be moving in Y and Z for a while.
Step 7: Locate the Foot
Step 8: Pre-drill the Slot
When I was done there were three 3/8 inch holes through the middle of the clamp, with their sides just touching.
Step 9: Finish the Slot
Then I widened the slot by about 0.050 or so by moving the milling cutter in a sort of spiral, cutting the full wall of the slot with a depth of cut of about 0.010 inch. I cut the spiral clockwise, to ensure that I was always conventional milling.
(Note: For the pics in this step I'm showing a clamp where I tried a shorter slot with just two holes pre-drilled. It's working fine, though I think I prefer the longer slot.)
Step 10: Cut the Threaded Rod
I cut it to length with the hacksaw and cleaned up the cut edges with a file.