From wpk@saao.ac.zaWed Nov  4 13:30:47 1998
Date: Mon, 7 Sep 1998 10:35:00 +0200 (GMT+0200)
From: Willie Koorts <wpk@saao.ac.za>
To: John Ewing <ewing@easynet.fr>
Subject: Re: rubber bottom truss


On Fri, 4 Sep 1998, John Ewing wrote:

> I'm casting around for designs just now. I like your scope page, though 
> I'm interested to see how you resolve the photography problem.

Hi John

Thanks for the nice comments.  

To solve my photography problem, I thought of two possible solutions.

1. Add another 2:1 or better gear ratio somewhere in the drive chain.
   This might not be too easy since I cannot see where it would fit 
   easily.

2. Change the stepper motor driver to micro-stepping - even just an 8 
   micro steps/step driver will improve it 4 times (it is half stepped 
   at present which is equivalent to 2 micro steps/step).  This would  
   require some design but would probably be easier to implement in the  
   end.

> Re your (ingenious!) rubber truss clamps, a stack of rubber door-stops 
> would probably fit the bill if other rubber sources fail. Interspersing 
> plywood discs would cut down the expense (I'm half Scots) and make them 
> spread more. And they already have the holes drilled.

I like the idea, particularly as a good source for rubber.  When shopping for
rubber door-stops one should probably avoid the ones that are tapered to the
top - this might prevent them from expanding tightly into the inside of the
pipe since there is dead space to expand into, unless one could try and turn
them in a lathe (and to the desired diameter) - which might not be so easy
for rubber.  Too hard a type of rubber might also be a problem to expand
easily. 

> Did you roughen the contact area inside the tubes?

Yes, I "scratched" the tubes' insides by cutting a very fast "thread" - more
like a spiral with about 3mm pitch and 0,5mm deep.  This was just to give me
a bit more confedence and was probably not necessary.  These expanded rubber
plugs can produce a massive amount of grip - I got the idea when I saw a
hand-tightened one withstand 10bar water pressure inside a perfectly smooth
hole. 

> Cheers,
> John Ewing
> Ewing Hopwood Sarl
> Preuschdorf,
> France 
> tel: +33 3 88 80 73 71 
> fax: +33 3 88 80 73 89 
> http://www.ewingh.com

Best Regards
Willie
                        Willie Koorts   wpk@saao.ac.za   

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