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How often and how long do you cold press when...

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#1 KerryR

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Posted 10 January 2013 - 03:14 PM

How often and how long do some of you folks cold press when getting close to the final polish and sphere? And, while we're at it, parabolizing? I'm using Willmann-Bell's tempered burgundy pitch.

Thanks

#2 MKV

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Posted 10 January 2013 - 03:56 PM

Depending how close you are. The closer you are the more often you press.

At the very last stage, I press every time I figure the mirror, and the figuring is usually no more than once around the barrel. I press for about 5 minutes, with weights, then figure using no pressure other than to push gently, using very "thin" polishing suspension (usually rouge as much as I hate it), wash the mirror, let the mirror cool for 15 or so minutes (depending on size), sometimes longer, test, then press again, and so on. Sometimes it takes a whole day to do the final few strokes before you say "done".

Mladen

#3 df_2112

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Posted 10 January 2013 - 04:08 PM

I agree totally with the above post. You really cannot overdo this. In fact aside from cleanliness this is the next best area to be neurotic about.

The two major improvements from frequent and thorough pressing are 1) improved predictability of the figuring session and 2) the ability to more closely follow subtle changes and "finishing" touches on the figure.

I take it one step further and am pretty much neurotic about pressing any time I am working toward a figure. To me is easily worth the effort and saves on unpredictable and unexpected changes...thus saving time and frustration.

#4 KerryR

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Posted 10 January 2013 - 04:34 PM

Thanks, guys. That was exactly the kind of information I needed.

I just finished another polishing session, and inspected very closely for pits. I'm not as close as I thought I was near the edge, so it looks like I've got a few more hours to go... at least.

#5 mark cowan

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Posted 10 January 2013 - 10:43 PM

How often and how long do some of you folks cold press when getting close to the final polish and sphere? And, while we're at it, parabolizing?


I never cold press - it's a waste of time. I warm press as required using heavy pressure for a short period of time.

The only time this can be a problem is working plate or similar glass, where it can raise the area you're using for pressing, which would result in planing that off if you set to work too soon - in which case you let it return to working (room) temp, for which room temp water works fine.

The polisher needs to conform to the work, but there's no magic involved in making it do so, nor in maintaining this contact through any needed working period.

Best,
Mark

#6 kfrederick

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Posted 10 January 2013 - 10:54 PM

I like to hot press screen in then use cold water to set it then polish for as long as the pattern is still there . Cold pressing just removes the screen marks .

#7 MKV

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Posted 10 January 2013 - 11:49 PM

I take it one step further and am pretty much neurotic about pressing any time I am working toward a figure.

Yup, it's an obsessive-compulsive thing, but for very good reasons. Couldn't agree more. Sometimes the last figuring steps take longer than it took to polish out the mirror. The shortest period of time spent in those final steps is actually figuring! Cooling, pressing, testing, etc. take a lot longer.

Mladen

#8 JohnH

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Posted 11 January 2013 - 10:04 AM

I find I have to adapt to my shops conditions. The shop is dry but the temperature is less than 10C overnight. I will run a heater inside for a few hours before starting polishing spells.

For my 12 1/2", I did spells where I warmed the mirror with warm water and warmed the lap by suspending it over hot water, then after a while, pressing the two together for 30 minutes with a piece of mesh screen between the two.

If the action is not very smooth, I'll press them cold for up to 60 minutes, and try again.

If I find the pitch too hard to get good contact, I would lightly dampen a piece of paper towel with turpentine and leave that on top of the pitch lap overnight, then let the pitch sit for a day or two more.

More recently, I had to work on an 10" plate glass mirror and the pitch is much harder a year later, so I had to remake the pitch with some turpentine to make is softer.

#9 ed_turco

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Posted 11 January 2013 - 11:36 AM

I do all my cold pressing by putting tool and mirror in a plastic tub of water with a little soap added, making sure that the mirror won't slide off the tool, and having some Cerium in-between, after stroking a bit to make sure the tool and mirror don't stick together!

As I tend to polish for an hour or two, with a dinner and sleep break, there is plenty of time for contact to be maintained, if you have contact to begin with -- and if you don't have it after an hour of polishing, you really need to study your technique.

In my way, I let Isaac Newton do all the work while I sleep, and only have to do lap trimming in a minimal way.

Hot pressing is a legitimate way to get started to get contact but every time? I hate trimming my laps!

With my method, the lap may be cranky for under a minute, but it settles down to "that sweet pull of polishing" on a good lap and good contact.

#10 KerryR

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Posted 11 January 2013 - 11:53 AM

How often and how long do some of you folks cold press when getting close to the final polish and sphere? And, while we're at it, parabolizing?


I never cold press - it's a waste of time. I warm press as required using heavy pressure for a short period of time.

The only time this can be a problem is working plate or similar glass, where it can raise the area you're using for pressing, which would result in planing that off if you set to work too soon - in which case you let it return to working (room) temp, for which room temp water works fine.

The polisher needs to conform to the work, but there's no magic involved in making it do so, nor in maintaining this contact through any needed working period.

Best,
Mark


If I understand this correctly, you work with a warm lap, that's not allowed to cool to room temp? Is the mirror warmed at the same time?

Would you say that this applies (if I have it correct) to 'beginners' methods of polishing, i.e. same size lap, MOT (most of the time)? In short, is this something you'd say a beginner could do and get away with it? (Pyrex assumed) If so, that could save a lot of time spent pressing during correction to sphere and paraboloid... Sounds a bit scary considering the degree to which most texts admonish against it. But then, most texts are not aimed at professionals or super experience amateurs.

Should I try working warm, or is that asking for trouble, considering this is only my second mirror and the first was not parabolized (f11 sphere).

#11 ccaissie

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Posted 11 January 2013 - 09:03 PM

On a sphere, towards the end, I polish a rotation/ turn or two, press cold for 5 minutes, and back to polish. This allows temps in the glass to equalize (from evaporation, hands,etc.) lap to conform, brain to think, and ability to alter strokes for randomness.

With an asphere, I'm using sub-diameter, or using an accented stroke more than relying on the contact factor, so I use more quick pressing in a warmer room. Or softer pitch.

#12 ed_turco

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Posted 13 January 2013 - 05:48 PM

I have an allergy to WB pitch. Ordered some tempered stuff and got taffy instead. Attitude at W-B was arrogant, and tried to accuse me of inexperience. :rofl5: W-B tempered pitch was always in the 15-20 second range until that last time.

Dick Parker and I disagree about this; he likes the softer pitches while I like the 15-20 second stuff. It takes all kinds, and beautiful mirrors result either way.

#13 Ed Jones

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Posted 13 January 2013 - 07:38 PM

I only hot press to get the pitch to the right curve, after that I only cold press. For safety I usually cold press dry with a sheet of thin plastic (5 micron) between the mirror and lap to prevent sticking.

#14 StarDusty

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Posted 14 January 2013 - 09:27 PM

I cold press whenever I feel bad contact. Or when my arms are tired. On average about every 7 minutes. I cold press during my break, 5 minutes.

Sometimes I just can't get good contact cold pressing. Then I hot press.

I work in one hour sessions. I trim and bush my lap each session.

I have found that if I maintain the lap each session it only takes a few moments. I use a single edged razor to trim.

During figuring, I work in shorter sessions and again I stop if it does not feel right. I am cold pressing more often, maybe every 4 minutes on average. Of course, when I stop and check my figuring, I restart by cold pressing.


#15 Mike Lockwood

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Posted 14 January 2013 - 09:35 PM

I only hot press to get the pitch to the right curve, after that I only cold press. For safety I usually cold press dry with a sheet of thin plastic (5 micron) between the mirror and lap to prevent sticking.

Agreed, minus the plastic.

I use nothing between the lap except polishing compound - less of a chance of contamination that way.

Warm pressing is for moving pitch around, IMO.

#16 Ed Jones

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Posted 15 January 2013 - 08:09 AM

Mike,
Have you ever got one of your big laps stuck? I've never got a scratch from plastic but I won't get my lap stuck.

#17 KerryR

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Posted 15 January 2013 - 02:08 PM

It seems like variation in plastic thickness would cause problems; one would think the variations in the material would be much greater the wavelength of light.

But, I know ED knows his way around glass, so it must work fine.

#18 ed_turco

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Posted 15 January 2013 - 02:16 PM

Personally, I don't think the plastic is such a hot idea. But there will be others, far more knowledgeable than me, who like the plastic.

But I still wouldn't do it on my optics. :)

#19 KerryR

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Posted 15 January 2013 - 02:21 PM

Luckily, my small mirrors haven't stuck while pressing, hot or cold. I'll keep it in mind when I start my next mirror (10" f6)

#20 scopemankit

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Posted 15 January 2013 - 02:32 PM

A sometimes overlooked aspect of cold pressing is that before working, you have wet the mirror, thereby cooling it somewhat. The cold press allows the mirror temp to stabilise before the final figuring strokes.






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