Re: ATM 8" refractor project

["Ken Hunter" <kb7h@onemain.com>]

Anyone that needs a Spherometer spreadsheet contact me and I'll forward 
the file to you or you can find it on the ATM_FREE Group files section.

Ken Hunter

----- Original Message ----- 
From: "Bob May" <bobmay@nethere.com>
To: "atmlist" <atm@shore.net>
Sent: Friday, January 24, 2003 6:31 PM
Subject: Re: ATM 8" refractor project


> 
> You will want to build several spherometers as Dave indicates.  Difference
> in them is the radius you measure to.  Also several dial indicators are
> helpful.  A good place to get dial indicators is at a used machinery shop.
> I've got a place called Grandpa's shop near me that is a little treasure
> trove of odds and ends of machine tools, etc. and he's got a cabinet which
> had several different dial indicators.  A working .0001" indicator usually
> goes for $30 and one that needed a little oil in it went for $10.  Since I
> can take a watch apart and actually get it back together and working right,
> opening a dial indicator is no problem and the thing just needed to be
> cleaned of crud and lubricated.  I did get one that had a bent shaft but
> that wasn't any problem either as I just bent it back until it started
> moving easily and lubed it and put it back together.
> As to the spherometers, you generally know the radii that you are going to
> need to use so sit down and calculate (use a spreadsheet to make things easy
> on you and you might as well print the results out so you don't have to keep
> on calculating for that spherometer!) and see what the step size is in the
> areas of interest.
> FWIW, knowing how thick a piece of material is (preferably metal) exactly
> can help getting the right offset if the dial indicator is of a low travel
> type to get the right zone for doing a measurement (you have one that goes
> .050" (they usually go a little past that) max) get a shim that is .040"
> thick and that will give you the ability to measure almost .090" for a sag.
> when you set things up right.  You will probably prefer to have 3 of the
> spacers so that you can go either way with the measurement.  I will note
> that .050" on a 2" radius spherometer is really a pretty short radius tho.
> I will note that the longer the travel on the spherometer and thus, closer
> to the limits of the dial indicator, the more accurate the instrument will
> be.
> I'll also note that if you have a lathe, making a 3 ball spherometer is
> quite simple but the simpler 2 ball one is quite as accurate and can be made
> with just simple hand tools in short order.  I have one at the class that is
> just a dial indicator pushed into a 1/4" (standard size for dial indicators)
> hole in a stick that is fairly straight.  It works fine and tastes great for
> doing quick and dirty measurements across a concave surface but it won't
> work on a convex surface for the obvious reason that the ends won't be in
> contact with the diameter of the circle of measurement!
> Bob May
> http://nav.to/bobmay
> bobmay@nethere.com
> NEW! http://bobmay.astronomy.net
> 
> 
>