Re: Pride Goeth Before a Fall

[Tom Droege <droege@wwa.com>]

David,

Good point.  Actually I am putting a fair force on it to try to spread out
the wear point.  Steel screw, brass thread also for that purpose.
Precision micrometers used to be made with a leather thread section for the
purpose you describe below.  I planned to do that from the start, but it
just got too complicated to implement.  I may yet have to try that method.
Drill out the brass threaded section and insert something like leather.
But not an easy fix.  

Tom Droege

At 10:23 PM 3/18/99 -0000, you wrote:
>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.
>>
>>
>
>
>