The 4" vise that comes with the Harbor Freight 12-speed
Mill/Drill is a solid hunk of iron, but it isn't what
I'd call accurate.

So I decided to change that.

Remove the vise from its base and use a punch to
knock the pivot pin out of the center.

Use a file on the bottom of the base and smooth down
the various bits that poke up and watch how all the
edges smooth faster, indicating they're higher than
the supposedly flat areas of the bottom. I just got
it to the point where all the edges were brightened
up and called it good.

Bolt the base to the mill table and carefull allow
the mill to just set under it's own weight onto the
surface then tighten the quill. Mill the top of the
base until it makes a cut over the entire area. It
should now be flat and parallel to the table, providing
a GOOD foundation like a vise should have!

The center boss that the pin fits into is lower than
the outer rings so cut three light passes off the center
boss to keep it that way after milling the whole top
of the base.

Drive the pin back in and you're done with the base.

Flip the vise top upside down so the ways
are setting on the table. To clamp it down use the
two square washers that are on the bolts
holding the mill to the crate base. They fit
pefectly on the bottom of the ways and hold it
tight with the outboard ends propped up on some
nuts.

Mill the bottom of the vise top flat and parallel
to the topside of the vise ways.
To reach all of the bearing surface you must hang
the fixed jaw over the back of the table and swivel
the head off to one side some.
Position the vise top so that the table can
tavel enough to clear the mill off both sides
at the centerline where the pivot hole is.
There's just enough fore/aft travel to cover
the bearing surface using an 11/16" end mill.
(Use 3/4" if you have a 3/4" holder.) You don't have
to mill the parts that don't touch the top of the
base. (They're just "hanging out" there and have
no effect on accuracy.)

Assemble the fixed jaw to the base and align it
square with the mill table.

This step will extend the vise ways to reach the
fixed jaw.

Accuracy for this step isn't critical so remove the
fixed jaw's face and mill across the vise throat between
the end of the ways and the jaw. Be sure to get close
enough to the jaw so that the face will overhang the slot
when it's replaced.

Replace the face and using a parallel, mill a chunk
of aluminum or steel to the width of the slot. Mill
the face of the bar that will be down against the vise
base so it's fairly rough. (Use a faster feed speed.)

Drill then countersink a hole for a flat head Phillips
machine screw through the center of the face of the bar
opposite the side that will be down. You want the screw
deep in the bar because you don't want to mill off the head.

Now place the bar in the slot and spot drill using the
same bit you used to drill the hole through the bar.
Switch to a bit appropriate for tapping threads to fit
the machine screw, then tap the hole. Cast iron is easy
to tap. Make sure the hole is deep enough so that the
bar can be bolted down very tight!

Carefully clean the metal bar and the vise throat to
remove all grease and oil. Mix up and smear some J-B
Weld* on the bottom and long sides of the bar and also
on the screw threads and underside of it's head. Place
the bar in the slot and bolt it down TIGHT so the
J-B Weld oozes out all around. Fill the hole in the
bar too.Let it cure overnight.

*Often called Araldite in the UK and some other
countries. Moglice or other "plastic steel" should
also work.

Mill the aluminum bar down until it's even with the
ways and mill a _tiny_ bit off the top of the ways
to ensure everything's even. One pass like was done
earlier should do the job provided everthing else
is level and square.

The movable jaw has two surfaces on the top where
it's obvious that it was rested on a milling
table when the machine work was done on its
bottom side. Remove the jaw face and clamp the
casting upside down to the mill table. Lightly mill
the sliding surfaces and across the middle where the
round hole is. You just want to get them flat and
parallel to each other and the flats on top of the
casting. Next you need to mill the mounting surfaces
for the gibs. This will require plenty of patience
as you'll be removing the casting from the mill and
test fitting after every cut. The two sides will
probably need different amounts milled off.

As a final finishing step, I used an 11/16" end mill
and bored out the pivot hole in the bottom of the
fixed jaw part then turned a steel bushing to press
in with J-B Weld then drilled the bushing to ensure
it was straight. The final, final step was to turn
down the pivot pin in my* lathe for a snug but free
turning fit in the bushing. This is also a good way
to ensure the pivot is truly dead center.

*You'll have to use your own lathe. ;)

To finish up, completely reassemble the vise and
see if the jaws' tops aren't level with each other.
I corrected that by putting some J-B Weld Kwick
along the bottom edge on the vise casting then
tightening the screws so the fixed jaw was level
with the movable jaw. (Use some wax or other mold
release on the back and edges of the jaw face.)
Of course I checked to make certain both jaw face
tops were parallel to the mill table.

Alternatively you could use a cup grinding wheel
in the mill to ensure the jaw tops are level.

For a "blow by blow" of how I REALLY did my vise,
see this file.

http://home.rmci.net/gregg1/HF-vise.txt