RE: Pride Goeth Before a Fall

["Gamble, David" <DGamble@Acacia.com.au>]

How about a plug of Teflon inserted into a hole in the side of the lead
screws? They could be held in place with a grub screw and replaced or
adjusted relatively easily

> ----------
> From: 	david garnett[SMTP:fj73@dial.pipex.com]
> Sent: 	Friday, 19 March 1999 9:23
> To: 	tass@wwa.com; Tom Droege
> Subject: 	Re: Pride Goeth Before a Fall
> 
> 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.
> >
> >
>