Re: [ATM] First Pitch Lap - any tips?

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

This is turning into a really good discussion on lap making!

The parchment paper suggestion sounds like a great idea.  I used 
aluminum foil because it was thin and easy to get to conform to the 
curve of the mirror, resulting in a lap that needed very little 
pressing.  Parchment may have many other applications too (using it 
around the tool instead of masking tape, pressing areas of the lap 
with it for a formatted area polisher, etc).

I've made 9 mirrors with Gugolz.  I've only bought 55 and 64, since 73 
always seems hard.  I like Gugulz because it can be softened with 
natural (raw, not boiled, not synthetic) linseed oil or hardened by 
heating.  It seems to stay softer longer than using turpentine as a 
solvent.  I can always modify the hardness of the Gugolz I have on 
hand to what I need, whether it's adjusting to a softer pitch for 
figuring or hardening the pitch for higher working temperatures.  I've 
used it at temperatures between 62 and 80 degrees, with good results 
(smooth mirrors, controlable figuring action).

For me, the key to this is MEASURING the hardness in a QUANTIFIABLE 
manner.  I built a pitch tester, and since then my optical 
frustrations have been greatly reduced!  I can get a good idea of the 
hardness with a thumnail (measured after the lap has hardened for a 
day), but the hardness tester does not lie.  Mine is similar to 
Texereau's but I added an old dial indicator to make it easy to read. 
  I've measured the hardness of many laps at two or three 
temperatures, and I also recorded what they worked well for and what 
they failed at.  (I will share that data when I can get it in a form 
that's useful to others.)

I think many mirror makers underestimate the difference temperature 
has on pitch.  Yes, Gugolz may be inconsistent in hardness, but if 
your house varies in temperature with seasons like mine does, you then 
have two variables to deal with.  If you measure the hardness and 
record the temperature, then you quickly learn what works and what 
doesn't (trying to parabolize an F/4.5 mirror with a hard subdiameter 
lap) and can immediately correct the problem.  My work area has three 
things that I rely on - a TV to fight boredom, a clock, and a good 
thermometer.

If you don't want to build a tester, then the best indicator I have 
found of the hardness is how fast the lap channels close once you 
achieve good contact during polishing.  If I don't have to rechannel 
after a few hours of polishing, then I declare the lap too hard and 
remake it.  You can also make pitch ACT softer by increasing the 
polishing pressure, but this is unpredictable and physically tiring.

Mark Holm wrote:
> My brief experience with Gugloz is not good.  I used 64 for a 8 inch 
> f/6.  Supposed to be medium, but acted more like really hard.  Was nasty 
> to trim too.  Little shards broke off and flew in all directions.  Some 
> larger pieces would chip out.  Had trouble getting smooth action, it was 
> very grabby unless I slathered it in thick CeO.  The action improved to 
> fair after I microfaceted it, but still wasn't the silky smooth type of 
> motion one would really like during final polish and figuring.

Mark, what temperature were you working at?  You may have gotten a 
really hard batch of Gugolz 64, too.

> Next time, I am going to try that synthetic stuff.

If I could adjust it's hardness, I'd use it.

	Mike Lockwood

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