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Subject: No More Super Wedge Backlash!
From: Dick.Green.
Reply To: mapug@shore.net
Date: Fri Oct 20 01:38:52 1995

-- [ From: Dick Green * EMC.Ver #2.5.02 ] --

At long last, after weeks of research, experimentation, advice from MAPUG
posts and countless trips to browse at the local hardware stores, I've
succeeded in eliminating all of the backlash in both the altitude and
azimuth adjustment controls of the infamous Super Wedge. This was done with
readily available parts and no exotic machining (I am neither an engineer
nor a machinist.) Most of the ideas came from Mark (MRWms@aol.com) and the
now-defunct LX-200 newsletter edited by Jim Leonard (copies generously
provided to me by Jack Russell, DuaneJRuss@aol.com.) In the following
description, there is both new information and repetition of previously
posted material. Please forgive the repetition and the lengthy text. I
thought it would be useful to have a complete description of these
modifications in one place.

The first time I used my Super Wedge, I was able to confirm the symptoms
reported by several members of this group. In spite of the massive knobs and
screw rods, it was very difficult to make fine adjustments in altitude and
azimuth. For one thing, the whole mechanism squealed like a pig when
adjusting after tightening the altitude and azimuth lock bolts even slightly
. Worse, there was considerable backlash in both adjustment controls,
especially the azimuth control. Backlash means that you turn in one
direction, then when you reverse direction, nothing happens until some sort
of slack is taken up -- then you can continue adjusting. Depending on how
tight the lock bolts are, the backlash can last from a fraction of a turn to
several turns. The backlash made fine adjustment particularly difficult and
annoying. It was darned near impossible to make a fine adjustment after the
lock bolts were tight enough to hold the mechanism in place. Sometime the
correct adjustment turned out to be somewhere in the backlash range, meaning
that the adjustment control would be floating (i.e., not pressing firmly
against the wedge.)

The Super Wedge adjustment problems appear to be the result of 1) the uneven
mating surface between the bottom of the wedge and the tripod head, 2)
excessive adjustment friction and deformation of the wedge when the altitude
and azimuth locking bolts are tightened, 3) inadequate performance from the
spring washer used as a "bearing" on the altitude adjuster, and 4) tilting
of the Azimuth Thrust Bar when the direction of adjustment is changed. These
problems were corrected as follows:

1.) Thumbscrews to Eliminate Overtightening

All seven locking bolts were replaced with thumbscrews (three 5/16"
thumbscrews for the wedge/tripod and four 3/8" thumbscrews for the altitude
plate). This eliminates the two different allen wrenches needed to tighten
the original bolts and ensures that the screws will not be tightened too
much and deform the wedge (hand tight is plenty.)

2.) Brass and Nylon Washers to Reduce Friction

Two brass washers were used on each of the three thumbscrews that lock the
wedge to the tripod head. For each thumbscrew, one washer goes between the
wedge and tripod head and the other washer goes between the top of the
thumbscrew and the wedge. The three bottom washers have to be placed on the
tripod head before mounting the wedge, making it a delicate operation to
mount the wedge without knocking the washers off. The washers don't have to
line up exactly with the corresponding screw holes because they can be
wiggled into place after the wedge is lightly secured with the large central
knob.

The washers between the wedge and tripod serve two important functions: 1)
they compensate for level variations on the bottom of the wedge and 2) they
greatly reduce friction between the wedge and tripod. The wedge sort of
floats on the three washers. The washers on the top side of the wedge keep
the thumbscrews from digging into the wedge and spread the force of the
screw heads more evenly.

Four nylon washers were inserted between the wedge and the steel washers on
the altitude lock bolts (which, as mentioned above, were replaced with
thumbscrews.) This greatly reduces friction when adjusting altitude and
keeps the steel washers from gouging the sides of the wedge. Most of the
backlash in the altitude adjustment comes from tightening these screws too
much, which causes excessive friction and deformation of the wedge. The
thumbscrews and nylon washers eliminate this problem.

4.) Thrust Bearing in Altitude Adjustment Control

In the original altitude adjuster configuration, the weight of the scope
presses against a spring washer that is held by two nuts against a fixed bar
at the rear of the wedge. This spring washer causes a lot of squealing and
uneveness when turning the adjustment knob.
The fix is to replace the spring washer with a "thrust bearing". A thrust
bearing puts the load of the scope's weight on a ball-bearing race that
makes for quiet and smooth adjustments. Several members of this group have
successfully installed thrust bearings, and each has used a different type
of bearing. I used a common, inexpensive bearing that is probably intended
for a lawn mower wheel. The outer race is a "pan" into which the inner race
is dropped. The other side of the inner race is held in place by a steel
plate. The inner race is flush on one side and sticks out a bit on the other
side. The side that sticks out is seated against the bar at the rear of the
wedge. The spring washer is placed on the other side of the bearing. Since
it is curved, the spring washer holds the outer portion of the outer race,
and does not press against the inner race. The two lock nuts hold the spring
washer against the bearing.

Other bearing types will require different mounting strategies. The whole
idea is to have the weight of the scope (communicated by the screw rod)
resting against one race and to have the other race fixed in place.

5.) Replace the Azimuth Thrust Bar

By far, the most difficult problem to solve was the backlash in the azimuth
adjustment control. This is caused by the Azimuth Thrust Bar tilting when
the adjustment direction is changed. As you change direction, the Bar tilts
away from the perpendicular. After a couple of turns, the screw rod catches
the threads in the Bar, and the Bar tilts back to perpendicular. The wedge
only moves when the Bar is perpendicular -- while the Bar is tilting back
and forth, the wedge does not move. This backlash gets worse as the
wedge/tripod bolts are tightened. The problem seems to have three causes: 1)
The run of threads in the bar is short and the thread tolerance is loose, 2)
The pin in the Bar does not fit tightly into the slot in the Tangent Arm,
and 3) the Tangent Arm does not press tightly enough against the Thrust Bar
(any gap between the two makes the tilt even worse.)

My first approach to the problem was to try to get the pin to fit more
tightly into the Tangent Arm. I tried slipping a compression fitting over it
in order to wedge it into the slot, but that didn't work (the compression
fitting couldn't hold the pin.) Next, I tried tapping screw threads onto the
pin. The idea was to tap identical threads into a cylinder and screw the
cylinder onto the pin, pulling the pin and Bar tightly against the Tangent
Arm. The problem is that the pin is not a standard size. It is slightly
larger then 1/4", too big for a 1/4" thread die. I couldn't find a die that
was large enough and would cut deep enough threads (neither 5/16" nor 7mm
worked.)

I returned to the MAPUG posts and Jim Leonard's newsletter, where I had read
notes on four modifications that fixed the problem. One, posted to this list
a long time ago, referred to replacing the Thrust Bar with a "T", custom-
made out of aluminum. Without details, I had a hard time visualizing what
had been done (I kept thinking that the horizontal part of the "T" must fit
in the slot of the Tangent Arm.) Jim Leonard, in his newsletter, showed a
diagram of a custom "T" that would solve the problem -- this cleared up my
confusion. The horizontal part of the "T" threads on to the screw rod, and
the vertical part of the T hangs down from it just like the original Thrust
Bar (presumably a pin would be fitted into this part of the "T" to mate with
the Tangent Arm.) The idea is that the horizontal part of the "T" lengthens
the run of threads enough to prevent tilting. Jim didn't have a lathe to
make a "T" (neither do I), so he solved the problem by screwing two
oversized 1/2" nuts against the Thrust Bar and welding two metal plates on
either side of them. I didn't like this because I'm not a welder. A reader
of the newsletter implemented a variation on this idea by turning the plates
90 degrees and putting holes in them just big enough to fit over the top and
bottom of the Thrust Bar. The plates were held against the nuts by four
bolt/nut sets at the corners of the plates. This configuration eliminated
the welding, but required punching big holes into two metal plates. I tried
it, but the hole made by my 1" chassis punch wasn't quite big enough to fit
the Thrust Bar (another non-standard sized part?) Another reader wrote to
describe an elaborate setup using nuts welded to iron straps -- I couldn't
figure out what he did (no diagrams) and there was the welding again.

Finally, I got the idea that a standard "T"-shaped pipe coupler used for
plumbing might do the trick. After some exploration at the hardware store, I
came up with a rectangular "T" coupler with the following dimensions:
Horizontal part: 1 9/16" W x 11/16" H x 11/16" D; Vertical part: 11/16" W x
7/16" H x 11/16" D. The part number on the side was 162-1262. There were
screw threads in all three ends, but they did not go all the way through the
horizontal part. Also, they were pipe threads, which I gather are not the
same as bolt threads (a 1/2" bolt screwed in about 1/8" and stopped.) Using
a 1/2-13 tap, I tapped a set of bolt threads all the way through the
horizontal portion of the "T" coupler. Surprisingly, this worked rather well
. The new set of threads went over the old set cleanly enough so that the
1/2" azimuth adjustment screw rod threaded through smoothly without binding.
The vertical part of the "T" was not long enough to reach the Tangent Arm,
so I threaded a 1/2"-13 x 1" steel hex head bolt into it. I didn't retap
this set of threads because I wanted the bolt to wedge tightly into the hole
(which it did.) The bolt forms the vertical part of the assembly, taking up
the remaining distance between the screw rod and the Tangent Arm. Then,
using a no. 7 bit, I drilled a 1/2" deep hole in the center of the bolt head
. I threaded the hole with a 1/4-20 tap, and screwed in a 1/4" x 1" hex head
bolt. Here's a crude diagram (view in a fixed-spaced font):
______________________
| |
Screw Rod -> | "T" coupler | <- Screw Rod
|______________________|
| |
| |
|________|
| |
|____| <- 1/2" x 1" bolt
|______|
Washer goes here -> | |
|__| <- 1/4" x 1" bolt
|____|

The resulting "T" matches the vertical dimensions of the original Bar pretty
well. I ended up placing a thick steel washer (about 1/16") between the head
of the 1/2" bolt and the Tangent Arm to keep them snugly pressed against
each other. The new "T" provides about twice the thread length as the old
Thrust Bar. As long as the 1/2" bolt is pressed tightly against the Tangent
Arm, there is no tilt at all.

6) Lubricate Both Controls

I used a Teflon-based spray lubricant on both the altitude and azimuth screw
rods and the threads with which they mate. I wiped off the excess so it
wouldn't get on any scope optics or electrical connections.

RESULTS

With the central knob and seven thumbscrews moderately tight, the movement
in altitude and azimuth is as smooth as glass and there is absolutely no
backlash. The time it takes to make the adjustments to center Polaris on
each alignment pass is a fraction of what it used to be. On the last pass,
as I tighten the knob and thumbscrews as tightly as I can by hand, there is
very little movement of Polaris away from the center. The controls are stiff
, but can still be used to make the final adjustment smoothly and without
backlash.

I tried it out tonight and, while it may have been just one of those rare
nights where all mechanical things worked well, a star centered in the 9mm
reticle stayed in the box for a full 30 minutes. This time, I completed
alignment in about 1/4 the time it has previously taken.

While my solution is kind of kludgy, it's easy to do and the parts are
readily available. I'll probably refine it a bit -- maybe find an
appropriate end-cap to use instead of the 1/2" bolt and press-fit a steel
pin into it instead of using a 1/4" bolt. I imagine that those who are handy
with metal cutting tools and have access to aluminum stock can fabricate a
nice replacement for the Thrust Bar. But for now I feel like a long quest
has finally ended in success (and it only cost me $395 to have all this fun.
.. ;-)

Dick Green