Re: [ATM] Mirror Cells - Springs vs. Push/Pull

["Donald Good" <donald.good@comcast.net>]

WARNING!  Simply making hachsaw cross-cuts in your collimation bolts will
leave scratches in the bolts at the thinnest part (at the bottom of the cut)
creating stress risers.  Stress risers are locations in materials that lead
to concentrations of stress forces leading to fatigue failure, something
that you do not want in your collimation bolts.  The most common cause in
the fabrication process is sharp corners or scratches while making load
bearing parts.

For such modifications in critical load bearing parts like collimation
bolts, it is strongly recommended that scratches and tool marks left at the
high stress parts be polished out.  In this particular case, simply round
off the back edge of the hacksaw blade and use the finest grade grinding
grit and then polishing compounds (from your mirror making supplies)in the
cut with the hacksaw back edge as the polishing tool.  Take special care
that the polishing strokes are linear (don't rock the hacksaw blade) and
that the thickness of the material remaining at the bottom of the cut is
constant along the full length of the cut.  Use a strong magnifying glass
(e.g. jeweler's loop) to examing the cut.  The bottom of the cut should be
an even high-gloss or shiny (not necessicarily mirror shiny) for the full
length.  Also make sure that the bolts have a non-threaded shank portion and
that these cuts are made there.  The threaded portion have stress risers
already (the threads themselves) that may compromise the strength if the
cuts are made there.

If you want proof, make a test (round, not threaded) rod with a set of cuts
as described by Arjan with just the hacksaw and right next to those, an
iddentical set of cuts (same final thickness) that are polished out.  Then
repetedly flex the rod until it breaks and see where the break occurs.  If
made equally, the polished out cuts will survive.  Then continue to flex the
polished out cuts to see how much longer til they break.

Don

> -----Original Message-----
> From: atm-bounces@atmlist.net 
> [mailto:atm-bounces@atmlist.net] On Behalf Of Tom Krajci
> Sent: Sunday, November 27, 2005 3:39 PM
> To: atm@atmlist.net
> Subject: Re: [ATM] Mirror Cells - Springs vs. Push/Pull
> 
> >From: "Arjan te Marvelde" <arjan.te.marvelde@hetnet.nl>
> 
> >> However, I still don't know what Don's flexural U joint 
> looks like in 
> >> his collimation cell....
> 
> >I guess that you could make those for example by making cuts with a 
> >hacksaw on opposite sides of the screw, almost meeting 
> eachother at the center.
> >Repeat the process 1mm along the screw, but at 90deg angle. 
> The result 
> >is a cross-coupling which gives some flexing but keeps axial 
> position.....
> 
> Thank you!  This sounds reasonable, and fairly easy to accomplish!
> 
> Tom Krajci
> Cloudcroft, New Mexico
> 

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