From: Kurt Greske
Sent: Wednesday, March 02, 2005 7:01 PM
To: Ampex Mailing List
Subject: [AMPEX] Headbox Adjusting Using Gage Blocks


        First a little about gage blocks themselves.

        They come in various tolerance sets.  In the one I have and gave the
URL for, each block is within 50 millionths of an inch of its nominal size.
The nominal size is etched on each block so you can keep track of them.  In
fact, for our purposes here, anything within a total of a thousandth of an
inch ( 1000 millionths ) would be good enough as long as all the guides are
adjusted to the same height, so even the cheapest block set is more than
accurate enough for what we want to do with it.

        The blocks need to be kept clean ( no sand or grit on them ) and
lightly oiled ( SAE 30 weight motor oil will do fine, but any other
reasonable oil is fine too you just don't want them to rust and a little oil
film helps them to stick together when you stack them up ).

        They are like "building blocks".  You stack ( add ) them up to get
the required dimension using smaller blocks.  The fewer blocks you use, the
better, but any combination that adds up to the desired overall dimension is
fine.

        If the blocks are clean and lightly oiled, you can press them
together with a twisting motion ( called "wringing them together" ) and they
will stick to each other enough so you can then handle the assembled stack
as if it were one block -- as long as you don't get too energetic with
handling it.

        The tape bottom edge is intended to be 3/4" off of the stainless
steel overlay on the transport ( all of them, even the 35x transports, so
you can use this same system for aligning 35x transport head boxes, but
using .750 for the basic dimension ) as well as AG-440 head boxes.  And the
stainless overlay is 1/32" nominal thickness.  So the dimension from the
surface plate where the pads on the AG-440 head box sit to the top of the
bottom rod of the guide where the tape bottom edge should be is 25/32", or
0.781" to the accuracy that we want to work.  So we need to look for two
gage blocks out of the assortment that is available in the set, the sum of
which will leave us with .x81 in its dimension, like for instance .140 and
.141, or .135 and .146, or any other combination of two blocks that add up
to .281, out of the blocks in that row in the case.  If we subtract this
.281 from the .781 we want, the answer obviously is the .500 block in the
set.  So these three ( any three that work ) blocks are what we want for the
bottom of the stack.  ( For the 35x transports, you just want the .75 block.
The only difference here is that you have to let the head cables overhang
the side of the surface plate and work around that. )  If you later find you
need a block for some other part of the stack that you have used here, then
just pick another pair that add up to .281 and use the block you first
picked for the new stack.  For instance, if you picked the .133 and .148
blocks, you may find that you need the .148 block in the next step, so when
you find that out, just go back and pick something else for this pair like
the .140 and .141 blocks.

        Now you need a set of blocks that add up to the distance between the
guide rods of the tape guides.  This is a bit "cut and try" -- JUST DON'T
FORCE ANYTHING in the process of trying it!!  The nominal tape size is
quarter inch, but you find that the .250 block is too thick to go between
the guide rods easily.  When fitting the block(s) to the guide rod space, it
is better to GENTLY try to insert them in from the end of the guide than to
try to insert them in from the side of the rods because the nearly square
end of the rods will more easily prevent the block(s) from entering if the
block(s) are too thick than the smoothly rounded side of the guide rods
will.  And you don't want to force the block(s) into the guide and distort
it.  It's better to have the block(s) a little too thin than a little too
thick.  I found that a thickness of .248 ( composed of the .148 block and
the .1000 block ) is a little too thin and .249 ( composed of the .149 block
and the .1000 block ) is a little too thick ( this "little" too thick is
okay to push between the rods, but still, you would like it to be a little
thinner ).

        Here, it gets to be a case of "feel" and "judgement", and there is
really no way to tell more accurately than by trial and error, but I found
that for the guides that I have, the combination of the .148 block and the
.1005 block, which produces a thickness of .2485 seemed to be about the
right combination of easy fit without being excessively loose, but I
wouldn't argue with a few ten-thousandths of an inch one way or the other.

        Once you have the right combination of blocks, assemble them so that
on top of the .781 stack is the .248x stack offset endwise enough so that
the .248x stack of blocks can be inserted into the guide space from the
side, sort of like this:

        o         The top glass rod looking at it from the end
       ---------- The .1005 block ( or whatever to fit )
       ---------- The .148 block
        o  ---------- The bottom glass rod and the .141 block
           ---------- The .140 block
           ---------- The .5 block
___________________________________ The surface plate


        Now, loosen the two allen head screws that hold the guide in place.
( DON'T use the ball end of a ball end allen wrench to either finish tighten
or, even MORE importantly, loosen an allen screw!!  Doing so will likely
damage both the ball end of the wrench AND the allen screw head socket so it
becomes very difficult to get the screw loose from then on. )  Set the head
box with the guide in place but loose on the surface plate.  Stick the gage
block stack into the guide from the side ( end of the head box ) and while
holding BOTH the head box itself AND the gage block stack firmly against the
surface plate, tighten the allen head screws.  It helps if you have an
assistant here -- she ( if "she's" a she ) doesn't need to be mechanically
competent here, since all she has to do is hold the head box down on the
surface plate as you hold the gage block stack in place and tighten the two
screws.  So "I don't know how to do this, all I do is chemical engineering
around here" isn't really a valid excuse.  But I don't have anyone but my
dog, whom I have never been able to train to place his paw with that kind of
precision, so I have to do it all by myself -- holding the block stack with
my thumb of one hand while I hold the head box down with my fingers and
tighten the set screws with my other hand.  Sometimes I slip and the gage
block stack comes all apart and I have to start again.

        When tightening the screws, don't try to completely tighten one
screw before snugging up the other one.  If you do that, it may cock the
guide even though you think you are holding everything tightly down on the
surface plate -- especially if you are trying to do it all by yourself
without a helper.  While holding the allen wrench so that the short arm is
the lever, just snug one screw up, then the other one, then go back to the
first one and finger tighten it, then finger tighten the second, then turn
the wrench so the long arm is the lever and finish tightening both screws.

        The guide on the other end of the head box is done similarly.

        NOW comes a slightly more difficult trick!  The sheet metal guide on
the left side of the record shell!  You CAN do it the same way, but it gets
pretty difficult to hold it all in place if you are trying to accomplish it
all by yourself.  ( At least on mine, that guide also needs a different (
thicker ) gage block stack to fit in where the tape runs. )

        Since phillips head screws don't drive very well with the
screwdriver angled to the axis of the screw, I suggest getting some 1/4 inch
4-40 Allen head cap screws to replace the Phillips head screws on this guide
-- and then using a ball head allen wrench to tighten these screws firmly
enough so that the guide plate won't move when you use the other end of the
wrench to finish tightening them.  That almost makes the job easy.

        Also, especially if you are trying to do this all by yourself, it's
easier to measure the distance between the lower guide surface and the
bottom of the guide with a dial caliper and subtract that from .781".  Now
use a gage block stack of this thickness and set the guide on top of the
gage block stack instead of trying to keep the other block stack in the tape
path gap while trying to tighten the screws ( especially if you haven't got
the proper allen head screws to replace the phillips head ones with ).

        The same gage block stack that we started out with above to set the
box end guides can also be used to visually position the height of the tape
heads themselves, to adjust the heads so the tracks are centered on the
tape.

        Also, the end of a 3/4" block stacked ( but offset ) on a 1/2" block
can be touched against the head face to adjust the tilt of the head to make
it perpendicular to the surface plate.  Don't slide the block against the
head or you may scratch the head, just lightly touch it against the head
face.  Then shine a flashlight from the other side of the block and look for
a 'wedge' of light between the head face and the end of the 3/4" block.  The
blocks aren't guaranteed to be 'square', but they are close enough for this
purpose.  BE AWARE that the blocks, being hard steel, MAY be magnetized.
And so touching them against the head MAY magnetize the head and require
that it be demagnetized before playing your good MRL test tape.

        When adjusting the vertical height of the head or the tilt of the
head, try to keep a similar looking space on both sides of the head between
the top of the head and the inside of the head shell, looking at it from the
front, so that the azimuth remains close to the correct position.  Since, on
an Ampex head mounting, changing the azimuth adjustment also changes the
head height vertically a little bit.  And when you get both the tilt and
guessed azimuth position where you think they should be, then go back and
check the head height so the tracks remain centered on the tape.  When
tilting the head, turn the allen head set screws equally in opposite
directions to keep the same height adjustment.  When changing the head
height adjustment, turn both the allen head screws equally in the same
direction and turn the azimuth adjustment nut and the head height adjustment
lock nut in the opposite direction the same amount to keep the azimuth and
tilt the same.  Fortunately, all of these screws have the same thread pitch,
so the same angular turn of any screw produces the same longitudinal
movement although you have to turn the nuts in the opposite direction than
the set screws for the same direction of travel.

Kurt



Kurt Greske