Re: ATM Re: string spiders

["rguinamard" <rguinamard@infonie.fr>]

Some informations about the diameter of the wire:
In order to test the minimum diameter of the guitar string for my 3"
secondary (rebuilding of my 13" f/4.5) , I've tried strings of 0.008" ,
0.010" and 0.012".
Both the 0.008" , 0.010" broke. It seems the 0.012" is the limit to avoid
breaking of the wires for my 550 g mirror, but I'm pretty sure the 0.010"
could be use for a 10" or less telescope

Raphaël Guinamard
----- Original Message -----
From: Daniel Fundo <fundo@bellsouth.net>
To: <atm@shore.net>
Cc: <oeli@comesso.de>; Rocky <wb5ftr@zianet.com>
Sent: Tuesday, October 23, 2001 9:03 AM
Subject: Re: ATM Re: string spiders


>
> 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
>
>