(ATM) Fergie's mirror 3

[kferguson@aquilagroup.com]

Like a stale joke, there's good news, and there's bad news:

First the good news: 
I turned a 3" dia. cup type spherometer friday evening. This seems to work 
great.  Thanks for the e-mail on this to all. One advantage I have not heard 
mentioned for cup vs ball type is that I can zero the cup with a ground 
parallel, but would require a flat for the ball type.

I made one modification to the discriptions I have read on cup-spherometers.  
I beveled the inside and outside at 45, so that there is only one edge (I 
left about .002" flat, to improve durability) this way I don't need to 
correct concave and convex (tool) surfaces for the thickness of the rim. The 
thin edge is much less sensitive to surface contamination, with less area to 
keep clean.  This edge would probably not stand up to industrial duty, but is 
fine for the use I will give it.                                      

This was a pretty simple lathe project, but used a good variety of turning 
techniques.  I would recommend it to anyone just getting started with lathe 
work (like me).  Many thanks to my friend Bob for the use of his machine, and 
his valuable & patient advice. (not to mention the slab of round stock).  If 
I didn't have access to a lathe, I would probably stick with the 3 ball type.

With this spherometer, I was able to predict the ROC to within 1.5".  I 
maeasured this by standing a sheet of typing paper next to a flash-light, and 
focusing a "best image" (mirror is still very rough) of the bulb on the 
paper.  I marked off the distance before unrolling my measuring tape so I am 
sure I wasn't fooling myself.  This was much easier after I taped a black 
paper tube on the flashlight to cut the glare.

I think I read where someone was having trouble with the "move the light , 
and see if reflecton moves same or opposite" methode of determining radius. I 
also couldn't make this work very well, there was about a ten inch range 
where everything pretty much looked the same. I would recommend the focusing 
screen methode to other tyros like me.    

Now for the bad news:

The ceramic from which I made my tool is going to pot :-(  It seems to be 
absorbing water.  When I started, it was stone hard, like you expect ceramic 
to be.  But it is now only a little harder than chalk.  I can cut it with a 
knife without dulling the edge..the ceramic kind of crumbles before the 
blade.  I still think the clay tool idea has merit, but I obviously got the 
wrong clay, or the wrong firing methode. The clay I used was refered to as 
"low firing" so if anyone wants to try this, I would recommend another type.
also, I would reccomend soaking the tool overnight, and see if it still seems 
hard, before starting grinding.  

This greatly prolonged (I think) the forming process (9 hrs so far).  I 
finally have the proper curve out to about ~1/16" from edge, but very poor 
contact (LOTS of suction over center).  I now have only 1/8" of tool left at 
the edge...and my arms are tired (whine whine whine)

The soft tool seems to be an unstable situation.  Even with very short 
strokes (1/4 dia) it wants to dig a hole in the center.  By moving the mirror 
off center, and pressing over the center of the tool, I can mow down the hill 
that builds up on the tool, and thereby make some progress on the curve, but 
this is obviously never going to give me a nice sphere.  The effort is not a 
complete loss, as I think I now understand a lot about one way TDE happens, 
as this is what the soft tool kept trying to do. When I read "to fix TDE, you 
have to work the edge" it made no sense to me..now it seems obvious what they 
were getting at. I think with an hour or so on a good tool, I would have a 
nice curve. 

So I will be making another tool. I still have no source for unglazed tile, 
but maybe some more effort will locate one.  I have heard 3 negitive, to 1 
positive experience with glazed tile, so think I'll skip that route.  If I 
can't find unglazed tile, perhaps I will try cutting squares from plate 
glass.(yech!)      

There will now be waiting for plaster, or whatever, to dry. (Has anybody used 
Bondo,for this purpose??) (Bondo = popular U.S. brand of polyester based 
automotive body filler putty) 
The delay will force me to take time to build a knife edge tester, and I 
couldn't grind Teusday night anyway, for all the trick-or-treatsters 
interupting me.(I enjoyed as a kid, so feel obliged to reciprocate) 

Next installment should be Tool II and how it works.

ciao,
Kevin