From: "John Murray" 
To: ,
        "Amateur Telescope Makers" 
Reply-To: "John Murray" 


G'day All,
        I've had quite a few suggestions as to my problem so I'll
condense them into one email to both lists.

1> By the way, what size mirror are you grinding ?
8"

2>What strikes me about your tools is that they seem to have an
excessive height. This could be causing a very lopsided pressure
distribution on the face of the tool and preventing a smooth operation.
This had occurred to me which is why my last 5" tool was made up of 2
x 3/4" ply disks and a layer of tiles laid horizontally. This made no
difference.

3>I just had another thought (a new record for me !) I've found that the
fore/aft movement of the sliding box that carries the swing arm assm. can
influence the rotation of the tool. When I went to the taller 2"
tiles, I had to move the whole assm. out about an inch to get better
spinning. I know that it shouldn't make a differance, but in my case it
certainly did...
I would have thought that in this case with the quill not centred over the
centre of the mirror that the centre would not have been worked as much. Is
this assumption of mine wrong?

4>Make the tool heavy. Set the pivot point very low. Make sure the
machine is tight. Expect more problems with smaller mirrors. I'll be
setting the overarm much lower than it was originally. Currently the length
of the quill from the base of the toggle to the underside of the overarm is
5 3/4". I'll reduce this to a bit over 1" and see what effect it
has.


Cheers

John Murray
Whyalla
South Australia
Our ATM page is at
http://astronomy.sa86net.com

> -----Original Message-----
> From: Daryl P. Dacko [mailto:mycrump{at}usol.com]
> Sent: Wednesday, 19 March 2003 12:45 AM
> To: Mirror-O-Matic{at}yahoogroups.com
> Subject: Re: [M-O-M] Still refusing to spin properly............
>
>
>


>
> The mirror/tool size seems to be really important.
>
> I just finished haveing a LOT of trouble with my tools, and
> I'll list what worked for me.
>
> On my 12.25", I couldn't get good spinning, or good hogging
> with an 80% tool. I ended up casting a 6" tool, useing large
> .5" quartz rocks in pourstone. The placement was totally random.
>
> I finally found that I needed to start the tool with some dry
> grit underneath it, to give a gap to allow the new grit to
> get into the gap.
>
> I used the grit mixed with water and a drop of detergent, and
> dripped onto the mirror surface at the rate of a drop every
> second or two. I used some cheap mustard/catsup squeeze
> bottles to hold the grit.
>
> Once the mirror was ground down past the sag I needed, I
> switched back to the 8.75" tool, and ground until the sag was
> correct, and the surface was mostly spherical.
>
> Importent lessons: Smaller tools hog faster, but don't leave
> a nice sphere. Larger tools don't hog fast, but leave a nice
> sphere. Start out with dry grit under the tool, at the start
> of each grinding sesson. Use detergent to help suspend mud
> and wash it away. Use a thin mix of grit and water, renewed
> much more often than you'd guess that you need, since machine
> grinding is so fast. Weight is importent, you can use too
> much. I use about .25 Lb per sqare inch of surface (about
> half of what I've heard recomended)
>
> When I finally got the hang of it, I found that I could get
> .001" change per ten min. of grinding, but it took several
> weeks to figure out how to get these results ;')
>
> >     I'm considering adding a drive to the overarm to drive
> the tool. Any
> >thoughts or suggestions?
>
> Not to rain on your party, but you'll likely be adding a lot
> of new varables to the equation, when we already don't
> totally understand what's going on.
>
> I just did a bunch of reading on machine grinding, and one of the
> constants is the comment that "no detailed instructions can
> be made, due to the complex nature of the process"...
>
> Don't lose faith, just keep trying things that you didn't try
> the first few times.
>
> Daryl the Moderator

--- BBBS/NT v4.00 MP