Re: Pride Goeth Before a Fall

["david garnett" <fj73@dial.pipex.com>]

If the nut is made of metal, it will only contact in one place at a time,
however long
you make it ! In fact, the contact point will probably move around as
everything turns
leading to all sorts of interesting anomalies !

When they made the original diffraction grating ruling engines they used a
soft
nut - made of something like cork if I remember correctly - to average out
the
errors. Mind you, they were starting with a pretty good screw thread anyway.

Still, the possibility exists that a PTFE 'nut' might improve things with a
bit of
averaging.

Are you sure that the shaft is straight enough ?

regards
Dave

-----Original Message-----
From: Tom Droege <droege@wwa.com>
To: tass@wwa.com <tass@wwa.com>
Date: 18 March 1999 17:11
Subject: Pride Goeth Before a Fall


>
>What to do?  Well, I picked the screw for certain properties.  It is made
>by cold forming which makes a very smooth and cheap thread.  Possibly
>though, it also allows the rod to go through the die at an angle which
>would introduce a periodic error.  I note that the cork screw star trails
>were very regular.  They have a 60 second period.  Lots of time to do
>something in software to correct it if we know where the screw rotation is
>in time.  All that is needed to measure that is to add a limit switch with
>a once around flag on the screw.  Now one will know where it is.  There is
>already a DAC and a VCO to allow small changes in the stepping rate.  There
>should be enough range, I will check that there is.  I think I will leave
>this project for one of you to figure out how to code.
>
>Meanwhile, I will investigate other screws.  There is a chance that I just
>made an unfortunate choice, and that some other screw will be better.  Yes,
>I know you can buy precision lead screws.  When I was trying to build a
>cheap (I actually did it - the only sale was to a Japanese company)
>photoplotter I investigated precision lead screws.  One for the RA drive
>would run about $5000.00 and there would be other expensive parts.  I still
>think I have a good scheme, I will just have to make it a little better.
>
>