The making of 30 in telescope - 12, #320 fine grinding over

[Chanlunlun]

Dear ATMs,

We have been delayed by 10 days – due to the Open Day of our school. The optical shop had to be cleaned up thoroughly to change into a reception area. The mirror and the tool has to be wrapped up and sent to my staff room for safe keeping ( two 30 inch glass disks on my desk, can you imagine how strange I was in the eyes of my colleague ?)

It was not until today that the work on the 30 inch project resumed back to normal. And it was not until today that we discovered a disaster – the grit from the supplier has been wrongly labeled – what supposed to be #80 grit is actually #160 grit. It was not observed until the first 2 hours of #320 fine grinding was finished – a lot of rather big pits left on the mirror surface. These pits were obvious not until the mirror surface became fine enough.

So, instead of 2, we had spend 5.5 hours to make the mirror surface smooth enough to accept #600 fine grinding. It was a long #320 session.

Anyway we have moved one step closer to the finishing of the 30 inch scope.

Best regards
Chan Yuk Lun
26-4-2005

[simonii]

it's quite a serious mistake!
what is the shop name? that's a terrible job to work in double expected time

[Chanlunlun]

it's quite a serious mistake!
what is the shop name? that's a terrible job to work in double expected time

Dear Mr. Simonii,

The name of the supplier is "Sun Chun Ming" it's the same supplier of the grit for the current mirror making class.

My suggestion is that you should check the current #80 grit and compare with the #160 grit to see whether they are the same or of different grit size. A digital camera in macro mode will do the job well.

It will not be a big problem in grinding a 4.5 inch or 5 inch mirror with mistaken grit, since the volume of glass to be removed is so small that you probably won't notice the difference.

But for a 30 inch mirror, where we are speaking of huge volume of glass to be removed, the difference is very pronounced.

Best regards
Chan Yuk Lun

[simonii]

Hi Chan Yuk Lun,
Cannot imagine sun chung ming will make such serious mistake.
It's no point for them to get 2 different grit mixed.
Next time really need to pay more attention.
Simon

[豆豸苗]

the grit from the supplier has been wrongly labeled – what supposed to be #80 grit is actually #160 grit.

so the grit is finer than marked?

[Chanlunlun]

the grit from the supplier has been wrongly labeled – what supposed to be #80 grit is actually #160 grit.

so the grit is finer than marked?

Dear BeanKitty,

Yes exactly. We had been using #160 grit ( supposed to be #80 )to fine grinded a #40 surface. A lot, lot of pits left behind after #320 fine grinding.

Best regards
Chan Yuk Lun