The failure of the making of the 24 inch full size lap

[Chanlunlun]

Dear ATMs,

We have tried to make the full size 24 inch pitch lap for polishing of the 24 inch mirror on the date 28th Feb, but the work had been proved to be a failure.

Originally we planned to make several pieces of rectangular pitch blocks of 300 mm x 200 mm with thickness 10 mm. However the thickness, after casting, was observed to be so uneven that it could not be employed to be glued onto the tool surface. And it was our reluctant decision that the operation had to be announced to be a failure.

So what』s wrong with our process? After some thoughts we had drawn our conclusions :-

1. We had collected different shallow boxes for use as the moulds for pitch casting. However those boxes had different depths and hence the pitch castings are of different thickness. The thickness, in the worst case, ranged from 3 to 12 mm, making it impossible to be glued onto the tool surface. If we insisted to finish the lap by fixing onto such pitch squares it was undoubtedly out of question to fit the lap to the mirror』s curve by later cold or hot press.

2. We place the moulds onto floor directly for pouring the molten pitch. Afterwards we realized that the levelness of the floor is poor, which consequently alter the thickness of the pitch cast on alternate sides of the mould.

Hence a second trial will be carried out today wit the following improvements :-

1. The pitch mould will be precisely made by cardboard by ourselves. We will close monitor the dimensions, especially the depth, of the mould to within 0.5 mm tolerance.

2. Instead of placing onto the floor directly, the moulds will be placed onto a plywood base plate with levelness adjustment. All pouring of molten pitch will be carried out only AFTER a very careful levelness adjustment of the base plate.

3. Increased amount of turpentine will be added into the molten pitch to soften it as much as possible so that a perfect match of the mirror』s curve to the pitch lap is going to be accomplished more easily.


Hopefully with such improvements we are going to be fabricate the full size pitch lap successfully this time.

If we still get fail, we are forced to use a sub-diameter lap ( 16 inch ) to polish the mirror, no matter how reluctantly we are. Mirror making can be sometimes as tough as the bitterness of life.

Let us hope for the best and prepare for the worst.

Best regards

Chan Yuk Lun
29-2-2004

[alyf]

dear mirror god,

i m quite curious is it feasible to use a 24" wooden board or other subsitute that can't be crack easily for the tool......please advise. thanks.

[Chanlunlun]

dear mirror god,

i m quite curious is it feasible to use a 24" wooden board or other subsitute that can't be crack easily for the tool......please advise. thanks.

a
Dear Mr. alyf,

Thanks for your advice. We can use wood subtrate, provided than the wooden base is curved to a f/3.5 convex. Needless to say it is a very big job to do so.

If we use a flat base, the thickness of pitch varies a great deal which means that they will undergo different expansion when temperature changes. You end up with a pitch lap that will never match the mirror's curve.

So we are left with no option but to use the old poor glasss tool.

Best regards
Chan Yuk Lun
1-3-2004

[sodium]

dear mirror god,

i m quite curious is it feasible to use a 24" wooden board or other subsitute that can't be crack easily for the tool......please advise. thanks.

a
Dear Mr. alyf,

........
Best regards
Chan Yuk Lun
1-3-2004

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[wilsonclass1d]

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