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Bubble in pitch

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#1 MAGNETARZ

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Posted 11 February 2013 - 07:29 AM

Friend i am at my 2nd mirror polishing 8" F/8. After pouring pitch in mould it has formed 4 small bubble. After cooling those bubble became sharp.

Can i use carborundum stone to level all those 4 bubble to make it flat surface?

Thanks in advance

#2 siriusandthepup

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Posted 11 February 2013 - 07:57 AM

I would not use that stone on your lap. You might drop some grit from the stone and that could scratch your mirror during polishing.

Trim the bubbles with a knife or razor blade, then press the lap with your mirror as normal.

#3 ed_turco

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Posted 11 February 2013 - 03:54 PM

You can actually fill these bubbles if they are of appreciable size. Small bubbles, below 1/4" should be left alone; they don't factor into getting a good figure at all.

For those BIG ones, you can slightly heat up some pitch until it is like warm taffy and won't hurt your hands and stuff a wad into your lap so it is slightly higher than the general curve of the lap.

Gently hot press, (I like to think of it as "warm" pressing), and the bubbles will fill in to the level of your lap. There may be a round seam, but after an hour of polishing, you might have a very hard time trying to see it.

Try it and see! :D

#4 ed_turco

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Posted 11 February 2013 - 04:58 PM

Now that I am thinking of it, you might have the opposite problem of bubbles rising above the surface of the pitch. In all cases these are air bubbles. Gently "pop" them with a razor blade and then press to see if any pockets need filling in, as mentioned above.

If these bubbles are solid, you've got something in your pitch that doesn't belong there, but this is very unlikely to happen.


Ed

#5 MAGNETARZ

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Posted 11 February 2013 - 11:51 PM

Thank you, let me try with razor blade and see if it goes or not. Bubbles are looking very hard.

#6 MAGNETARZ

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Posted 12 February 2013 - 07:40 AM

I have cut the bubble using razor but their it forms a small hole. Do i need to pour pitch to fill those holes?

#7 neo

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Posted 12 February 2013 - 12:42 PM

I was going to say the same thing as Ed did :).
How big are the holes? A picture would help.
Not a big problem, fill them up with a tiny bit of pitch and do a worm press untill the surface is even and you're ready to polish. Anyway you'll discover as you polish that the lap will get thinner after so much pressing and channel recutting so the small holes will disappear.

#8 StarDusty

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Posted 12 February 2013 - 10:41 PM

If the bubble is into the pitch forming a trapped air pocket, I would just connect the pocketed area to the nearest pitch groove by cutting a channel to the groove with a razor blade. If you haven't cut your grooves yet, see if you can locate one of your grooves right through the bubbled area. I have found that small pockets will eventually pressed out by the end of polishing.

Here is a link to a pitch lap making tutorial

http://www.clearskyo...irror-making...

#9 MAGNETARZ

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Posted 12 February 2013 - 11:37 PM

Thanks it solved during hot pressing all the holes dissapear. Now its very smooth






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