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

["Richard Schwartz" <richas@earthlink.net>]

It is good to see that someone else has discovered the problems of using
turpentine to soften pitch.    For me the problem is that the turpentine
evaporated from the surface and left a hard crust over a soft interior.  
Here are three more ways to get it soft::

(1) add bee's wax to the pitch.   The problem is that when you go to the
artsy store, you will not be able to find bee's wax.  If you ask store
personnel, they will sense that you don't belong there and they will
pretend not to notice you and then run away.   And if you do locate the
bee's wax, be prepared for sticker shock!

(2) add just a drop or two of castor oil.   It is commonly available in
drug stores, and after you soften the pitch you can just drink the rest of
the bottle for an evening of entertainment.   I do not know if ricin can be
made froim castor oil; all I know is that ricin is a water soluble protein.

(3) soften the lap without softening the pitch.    That is, make the facets
of the lap taller and deeper so that the pitch flows more easily.  This
will make hard pitch flow more easily, and the lap will handle like one of
softer pitch.   Just add a second set of channels bisecting the existing
channels.

As for ways to make it hard... I never needed any of those.    I have a
supply of Gugholz 73, and it is always hard enough, even in a hot Los
Angeles summer when polishing can only be done at night.

. . . Richard

> [Original Message]
> From: Mike Lockwood <melockwo@uiuc.edu>
> To: <atm@atmlist.net>
> Date: 6/9/2004 3:49:58 PM
> Subject: Re: [ATM] First Pitch Lap - any tips?
>
> 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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