Re: Fwd: [ATM] Open Question Concerning Pitch Laps

[Mike Lockwood <melockwo@uiuc.edu>]

Bryan,

Bryan Stiles wrote:
> 1.Blank is Pyrex. Full thickness (~1.33" I think)
> 2. Lap is Lab Stone from Modern Materials. ~1.5.-1.75" thick
> 3. Pitch is now about 1/4 - 5/16" at edges
> 4. Facets ~1" sq.  Channels 1/4" wide 3/16" deep
> 5. 2-3 hrs. Seems to vary with type of pressing (cold or warm) (weight or not)

All good.  That's medium softness pitch on my scale, and it's just fine.

> 6. ~ 8-10 lbs.

Aha!  This may not be enough to get full contact, especially for a 
cold press.  I'd use 20 lbs or so, for at least 1/2 hr, preferably 1 
hr.  Then you can be fairly sure you've got contact.  You should 
immediately feel heavy drag when polishing.

Another trick is to lightly scratch the top of the facets with a 
utility knife a few times.  This will help you get pressed out more 
quickly.  This is easier than using netting.

> 7. Yes I typically do remove the lap prior to beginning. To add CeOxide

Try to avoid separating lap and mirror - the lap will start to flow 
immediately.  Not a huge deal if you're just polishing out, but in 
figuring it can't possibly help.  Just decenter the lap/mirror and 
squirt on a little water/cerium and rotate the mirror/tool to 
distribute it.  Then go to work immediately.

> 8. To start a new session, I'll press about 20-30 minutes. Work glass
> about 15-20 minutes. Rinse off lap and glass. Check progress. I'll do
> this about 3 or 4 times before pressing again. Sometimes however I'll
> press sooner if I get skipping, jerking, etc.

You may need to press before EVERY 15-20 minute work session if you're 
trying to get spherical or if you're figuring.  Every time you 
separate the mirror and lap and rinse them, they change temperature 
and shape.  Pressing more frequently keeps them in better contact and 
  may be necessary to remove a central hole or other depression. 
(Press, work, press, work.)  During final figuring, you will press for 
a while and then work for only a minute or two.

> 9. The hole was 4.25" in diameter. Depth I have no idea as I couldn't
> figure out how to measure that. The rest of the mirror is spherical.
> Except now I have a wicked narrow turned edge. (Another reason to go
> back to fine grinding)

The turned edge makes me feel better about you going back to grinding. 
  Turned edge is also an indicator of lap contact issues.  Make sure 
to be gentle in fine grinding so you don't turn the edge there.

> 10. Cerium Oxide in a mix of water with a couple of drops of Dawn
> dishwashing liquid.

Use just enough soap to get some bubbling - this helps keep everything 
moist - but no more.  A couple drops sounds about right.

> 11. Well I figured I'd want to make a new lap because the one I have
> seems to have some performance issues. However If I pressed and got
> good contact then maybe I could continue with this one. I actually
> ordered more pitch today in preparation of making a new one.

Re-use the old pitch.  You can add a little unboiled natural linseed 
oil to soften it if it's been over-heated.

Based on these answers, I suspect your problem was not pressing often 
enough, or possibly not using enough weight.

	Mike Lockwood

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