If you want a step by step of the RIGHT way and order
(as well as avoiding the "oopses" I made) see this
file before reading this one!

http://home.rmci.net/gregg1/HF-vise2.txt

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.

The first problem is that it's not possible to use
a parallel against the fixed jaw because the ways
do not go all the way to it.

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 your parallels, 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. Let it cure overnight.

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

This is now where I made a "process error". :p

I milled the aluminum bar "flat" to the vise ways and
also took a TINY bit off the ways. I carefully allowed
the mill to just set under it's own weight onto the
way then tightened the quill. The mill will then remove
a very small amount of metal. I got it all nice and
flat but when I turned the vise back perpendicular to
the table it "leaned" one direction .001" over the
width of the vise. (Figured by measuring a chunk of
aluminum milled on two "parallel" edges.)

Ouch! I should've known the various parts of the vise
were probably not quite true. :(

OK, time to do what I should've done to start with.

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 use the gentle
set on technique to 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!

I filed the bottom of the upper part of the vise
like I did the bottom of the bottom.

Then I made another "process error". :(

I removed the base from the table to insert the T
bolts, mounted it back on the table, punched the
pivot pin back in and mounted the vise top back on
while failing to notice that it "rocked" a bit.
I milled the top of the ways and the aluminum
insert until they were flat again, marveling at
how terribly un-flat the top of the base had
been. I discovered my "oops" when I turned the
vise 90 degrees. I could visibly see that it wasn't
right. :( I removed the nuts and spun the top around
and noticed that it would rock back and forth a
bit. That's when I remembered that the center of
the base around the pivot pin was quite a bit
lower than the two outer rings! Out came the pin
and I cut three light passes off the center boss.
Pin back in, top back on and it set flat on the
outer rings!

So I milled the top of the ways AGAIN. They were
VERY much not flat. But now they would be very
close.

This is when the idea hit me of what I should've
done at the very beginning, before milling the
vise throat and after milling the top of the
base flat.

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

Next I milled 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.)

NOW is when you should re-assemble the vise,
mill the throat, make and bolt/glue in the
metal bar then mill down the bar and LIGHTLY
mill the top of the ways. If all went well
it should only need one pass over the ways.

If you follow the _proper sequence_ like I
didn't, you should be all done. :)

What I have to do now is figure out how to
hold down the movable jaw so that I can mill
off a bunch of the sufaces where the retaining
bars mount. For some reason I seem to have a
lot of clearance there. No idea why. ;)

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. I'll remove the jaw face and
go from there. If I get extra ambitious I'll
get a cup wheel and regrind the top edges of
the jaw faces so they'll be level with each other
again.

At least the vise will be (hopefully) far more
accurate than it was originally. It does turn
much easier now. To aid that I'm going to
drill out the very crude pivot hole and press
in a bushing that's a lot better fit to the
pivot pin.

Have fun accurizing your 4" Harbor Freight
Milling Vise (or any cheap milling vise)
and don't make the same "process errors"
that I did! :)

Addendum:

I've bored out the pivot hole in the top
with an 11/16" HSS end mill which poked
right through almost like the cast iron
wasn't there. Then I turned down a bit of
steel tube to .001" oversize and pressed
it in with some J-B Weld Kwick to make
sure it stayed out. I left a bit of the
tube sticking up and turned a bevel on
the inside to help locate it under the
9/16" end mill I used to bore a nice,
straight and smooth hole. That done, I used
a hacksaw to lop off most of the sticking
out part then milled off the rest.
Finally, I turned down the original pivot
pin for a nice fit in the steel bushing.
Spins like a dream now. Had I really wanted
to, this would've been a great time to
ensure that the pivot point was right in the
center of the vise top, but given that the
scale on it could hardly be accurate and
on the drill/mill the base has to mount at
an angle it's useless anyway.

Finally, I've milled off the bottom of the
movable jaw a bit. The two ways are a bit
different thickness so each side of the jaw
needed a different amount carved off to make
it snug.
The bearing surfaces took one light pass to
level them up because there was a high end at
the back on one side.

The last thing to do is mill or grind the
tops of the jaws so they'll be the same
height and level.