Re: ATM Re: string spiders

[Daniel Fundo <fundo@bellsouth.net>]

Since I received several requests, I'll go ahead and post it now.  I
originally had this written up for an ATMJ article, but I lost the
drawings I had when I left my last job (apologies to Bill Cook!)

I started making wire spiders shortly after the article in S&T back in
the late  60's or 70's.  I've made about a dozen for myself and others;
each hsas been slightly different.  This will be what I feel are the
best features.

WIRES
I make 4/8 spiders, that's 4 vanes made up of 2 wires each.  The wires
are arranged like the spokes of a bicycle wheel, i.e. two wires start at
the approximately the same point on the hub and go to different points
on the rim.  This creates a triange, the most rigid structural form.  I
make individual wires, just in case one should break, it is easier to
replace one wire than an entire vane.  Also, I had tried with one wire
looped through the "hub", but didn't like the results.  I use 0.013"
stainless steel wire from Small Parts, Inc in Florida (add standard
disclaimer).  You can also use music wire from hobby shops, spring wire,
or old guit-fiddle strings from your gee-tar!  On smaller scopes you
could use thinner wire.  This stuff has a listed breaking strength of
about 40 pounds.

First calculate the length of the wire.  I make all of mine the same
length and adjust for secondary offset at the mirror holder.  An angle
between 20 and 30 degrees will be very rigid.  The hub will have some
diameter, don't forget to take that into account.  Basically: inside UTA
diameter minus hub diameter, divide the result by the cosine of the
whatever interior angle you choose.  Make a jig out of a piece of wood
and accurately drill two small holes this distance apart.  If you error,
make it on the short side.  Find some small brads, usually listed as 16
x 1" (16 gage wire, 1 inch long).  Carfully wrap the wire around the
brad twice and then several times around itself (the free end).  Have a
soldering gun or pencil ready with some electronics solder (rosin
core).  Hold the wrapped end with tweezers or needle nose pliers and
keep it tight.  Solder the wrap and the brad.  Place this brad in one of
the jig holes (oh.....wait till it cools a bit! <G>), and place a second
brad in the other hole  Wrap twice around the brad, keeping a bit of
tension on the wire by pulling with pliers, sold this, then wrap several
turns around the wire and solder this.  All this wrapping around the
brad and the wire will end up looking like a hangman's noose around the
brad, nicely covered with solder.  Trim the excess wire and the brad a
short distance on either side of the wire.  Repeat this 7 more times.  I
can make all 8 wires in about 30 minutes.

OUTER ENDS
You will need 4 sets of bolts, fender washers (the big ones with the
relatively small holes) and acorn nuts to fit the bolt.  I actually find
fender washers one size smaller and hand ream the hole to fit the bolt
better.  File or cut two slots on the inside of the washer, 180 degrees
apart.  Cut these slots long and wide enough the accept the wrapping you
did around the wire, but not the brad, and so the bolt will pass through
the hole.  Once you put the wires in the slots and the bolt through the
hole, the wires should be "locked in".  Repeat 4 times.  The fender
washers are needed for the extra material so they don't break after the
slots are cut, but they are also lange enough to grip on the inside of
the UTA so that the wires can be alligned to the optical axis.  Don't
worry is they bend when tension is applied.

HUB
I have used various diameters of  aluminum tubing for the center hub. 
It must be large enough to accept the stalk of the secondary support,
and have a bit of extra room for the ends of the wires.  Calculate the
length of tubing you need for the interior angle you have chosen.  Allow
an extra 1/2 inch or more.  This is the hardest part:  cut two slots 180
degrees apart into each end of the tube, THEY MUST LINE UP END TO END. 
The slots have to be wide enough to accept the wrapping around the wire
and their inner ends must be the distance you calculated above.   I have
cut the slots using a Dremal tool (disclaimer) and a cut-off disk.  I
hold the hub tube down on my workbench with a clamp.  The cut-off disk
is held on with a screw.  I place shims on my work bench until the disk
is at the center line of the tube.  The screw rides on these shims while
I cut the slots.  Move the shim stack to the other end and repeat. 
Perfect alignment every time.

You will need two end caps for the hub. They will hold the wire inplace
during assembly.  These can go inside the tube, but for added strength,
use at least 1/4 inch thick aluminum or hard plastic and drill it so
that it goes around the outside. In plastic (Delrin) this can be done
with a spade bit or Forstner bit.  Only drill halfway through the
material.  If you use wood, use maple at least 1/2 inch thick, and 1
inch larger daimeter than the tube.  Drill a center hole through both
for the secondary stalk.  I used to drill my secondary offset distance
into these caps (the stalk hols were not through the centers of the
caps, but off center by the off set distance for the secondary), but a
better idea is to center them.  That way, you can remove the secondary
during alignment and use the centered holes through the hub to square
the primary (using the center dot method).

ASSEMBLY
Final assembly goes like this: place two wires in each hub slot and
place the cap over the end.  Get the two wires closest to the center of
the length of the hub tube and run them through the slotted fender
washer, place a bolt through the hole and insert it into the UTA hole,
add the cap nut.  Get the remaining two wire from those slots and put
them through another slotted fender washer and through one of the UTA
hols next to the first one.  Repeat for the remaining four wires.  By
tighten the acorn nuts you increase the tension on the wires. 
tightening one nut will increase the tension in the set of wires
opposite it.  Use washers or shims under these nuts if the tension isn't
great enough to generate some tone around Middle "C".  You may have to
adjust the length of the bolt.  The "vanes" can be aligned by looking
down the OTA and turning the fender washers (maybe with pliers!)

NOTE
Clive Milne had some great ideas for the center hub.  He used just a
piece of flat stock.  He had photos on his web site, but I lost the
URL.  There is no need for using guitar tuning pegs.  I feel these are
too heavy, and if only 4 are used, it becomes difficult to adjust the
lengths.  Centering also becomes hard.  To PERFECTLY center my spiders,
I have a dowel that fits through the center hole of the hub.  Attached
to the dowel, at a right angle, is an old telescoping antenna off of a
radio.  Extend the antenna until it touches or just clears the UTA at
each of the vane ends (this works for all spider types).

That's it.....clear as mud ain't it!!!???  Sorry, I don't have a web
site or a digital camera or a scanner!  Fire away!!!  Questions and
comments will be accepted, but will be ignored or answered in a
condescending manner, worthy of my former PHB!  <G>

*****-----
Dan F.
Secretary/MAS
Memphis, TN