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Figuring a 6-in f/8

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#101 dogbiscuit

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Posted 15 September 2020 - 11:22 AM

The lap looks nice.  I don't see anything there that should cause trouble.

 

Something you can try is to work about 20 minutes, press 15 and work another 20, press 15 work another 20.

 

Depends somewhat on the temperature and peculiarities of the lap and strokes.  Sometimes better to just work a full hour, sometimes better to work a few short periods with short press in between.  Sometimes doesn't make a difference smile.gif



#102 kcoles

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Posted 15 September 2020 - 06:40 PM

Will give it a try, and find something that needs doing around the house in 15-minute increments!



#103 dogbiscuit

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Posted 15 September 2020 - 06:42 PM

Of course there is still a long press before the work session.



#104 kcoles

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Posted 16 September 2020 - 07:05 PM

Here we are after the next round. It is only when I put the pictures together that I can look for differences, and this time I don't see much change from the last round. This is after two rounds of ABABABAB. Since the details may be helpful, let me give those. I pressed overnight, TOT, with the edge taped. Checked the channels and did AB followed by a 15-minute press (and the usual rotation of the lap and fresh CeOx) and the next AB. After four of those I trimmed the lap and looked at the mirror on the tester. Pressed for 2 hours, and back to A,B,15 minute press, repeat. The lap feels like it is in good contact - clearly more drag or resistance, so I'm learning to push it smoothly while giving weight, but it now feels pretty even and consistent.

 

Is it time to texture the lap surface a bit? Or maybe try something else? Or just keep going? Thank you for the patience and ongoing suggestions!

 

f8_fourteenth_test.jpg



#105 dogbiscuit

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Posted 16 September 2020 - 07:50 PM

Do only the A strokes. 

Take a picture of the lap before starting and at the end of the next work session.

 

Can you work TOT?

Have to work on a flat surface, some sort of pad under the mirror to buffer between irregularities in table surface and back of the mirror, and frequent partial rotations of the mirror on the table (every 1 or 2 trips around the mirror).



#106 chantepierre

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Posted 17 September 2020 - 12:49 AM

Regarding the lap, do you brush it ?
I’ve been stuck with no progress on my mirror for days until lockdown was reopened a bit and I could find a brass brush (found at shoe maker shops) and brush the lap with it before each work session.
The pitch had a « glazed » appearance and nothing got done..

#107 dogbiscuit

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Posted 17 September 2020 - 07:15 AM

We haven't said much about how much slurry to use, how long to work before adding water or cerium oxide.

That is near impossible for me to know without watching an entire work session.

I can tell you what I do, but details of quantities are not so easy to communicate.

 

MOT gravity pulls water into the channels and away from between mirror and pitch, and work dries faster.

TOT gravity pulls water in the channels to the mirror surface and work dries slower.

If you keep the lap too wet, the lap can slide on a relatively thick layer of water, sort of like hydroplaning tires on a wet road.



As the water in the slurry evaporates or spills away from the lap the layer of water between mirror and lap gets thinner, friction increases and you must work harder to push the lap, more work is done to the mirror's surface.

Try to make a wet last without adding water until you can no longer stroke smoothly.

Sometimes that will only be 5 minutes or so,  sometimes 20 minutes.  I rarely ever get much more than 20 minutes. It depends on humidity, the stroke and stroke speed being used, working only MOT, only TOT, or alternating MOT TOT, and other factors.

 

It is good to have a misting spray bottle to add water when needed.  When you have a good wet going and it has dried too much, just adding a very light mist can add enough dampness to work several more minutes.  There is probably some cerium hiding in the channels or clinging to the edge of the facets that the mist will bring to work, enough to continue working. It's not necessary to separate the disks to mist the surface, just offset the top disk to one side and mist, then to the other side and mist.

 

For working by hand I find using a bush more convenient to paint thicker slurry and add additional water if needed from the misting bottle.  Slurry is kept in a jar and allowed to settle to a sediment at the bottom of the jar. During a work session if cerium oxide is needed I will dip a corner of the brush in the sediment and paint a thin line on the mirror or lap. For pressing I paint the entire lap surface with the broad end of the brush.  There seems to be a relatively wide range of

 

The best result is somewhere between too much water and too dry.  Of course it would be best if it was just right through the entire work session. It's not likely to happen that way but you can do things to keep it near just right most of the time. 

Most likely it's wetter than necessary when you start a session  and as  you work it gets better and you will feel more drag.  Drag is not bad.  If you add water to reduce drag too soon you don't get to the best part of the wet.  Keep stroking as long as you can keep smooth motion. Grabbing and jerky motion is not good. Speed change is ok. Sometimes slowing the stroke or speeding it up can maintain a smooth stroke.

 

I press all the time and frequently the lap will stick to the mirror.  I put water in the channels and let it soak in TOT.  When it has soaked between the lap and mirror I pour the excess water out of the channels, then slide the disk on top to free it  and go right into stroking.  If they weren't stuck I don't add water or cerium oxide (that was applied for pressing) I just start stroking and if too dry I will add just a little mist and things will be working good right off the bat.

There is a feel when things are working right.  When things don't feel right water (mist) or cerium oxide might be added in and attempt to make things right, or if it's too wet either pour some water out of the channels and work or wait until it's dried out some.  It takes a while to see the correlation between feel and results seen on the mirror surface.  Then you know what you want to feel when you are stroking.

 

You mentioned that you had 100 pounds in you work table.  I like to work around a 55 gal drum barrel (drum).  I fill it about half way with water, in the neighborhood of 200 pounds.  Not that I'm tipping it working on a 6" mirror, but if I only had 100 pounds holding it down I think I might sometimes tip it a little. 


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#108 kcoles

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Posted 17 September 2020 - 09:53 AM

One of the many interesting aspects of this project is getting all the tools together. I couldn't find a barrel anywhere in the vicinity except those that looked like they had had nasty chemicals in them. So I built a table out of new plywood, varnished. I leveled it with a bubble level, and put a plastic shelf liner and then a non-slip pad (visible in photos of the lap). I'm fairly confident I have that much properly set, but it is always good to ask.

 

Weight on the table is another matter...I'm probably closer to 80 lbs and would like more on there - it does move once in a while when the friction is way up. I can't find anything more around the house to add to the assortment of counterweights, garden blocks, and dumbells under there. I may just have to go to the garden store (they sure are busy these days) and look for more landscaping blocks (I can fit something up to about 10 inches across under there). A spray bottle is an example of the sort of item I looked for in half a dozen stores in the small town I live in and came up empty this summer. These times have led to odd abundances and shortages here and I'm sure for many of you also. I'll confess after 6 or 8 online orders for this project I was a bit too used up (or lazy) to order small items like that, but of course I could.

 

I appreciate dogbiscuit taking the time to detail wet vs. dry and easy vs. sticky. It is hard to guess when you haven't done it. I do notice that MOT the slurry drains off into the channels quickly and then you reach a sort of medium friction which I was guessing is productive. I may be going too long beyond that without rewetting, but sometimes I stop and do that too.

 

I am going to brush the lap - "glazed" is a fair description of how it looks and feels now. Will try the A strokes MOT and do the lap photos. If there are strokes TOT that will help I'm willing to try, though I recall wisdom here that one must do it carefully to avoid astigmatism (which I really do want to avoid!) and I would need to change the mat relative to the cleats (which I know to keep not terribly tight) and so on. Given all that I'm not giving up on MOT yet. Thanks.



#109 dogbiscuit

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Posted 17 September 2020 - 01:27 PM

You might have an extra bottle around the house.  Windex bottle has a nice mist sprayer.  It's what I use. smile.gif

 

When I said "just adding a very light mist can add enough dampness to work several more minutes" I mean very light mist.  I slide the top disk to one side and spray the mist toward a point several inches away from the mirror or lap and let the margins of the mist fall on the work, most of it doesn't land on the work.  Then I slide the top disk to the other side and do the same.  That is usually enough water to continue working for at least a few minutes.


Edited by dogbiscuit, 17 September 2020 - 03:38 PM.


#110 dogbiscuit

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Posted 18 September 2020 - 11:19 AM

Depth of channels is something you change to find what works best for you.

Narrow shallow channels works good for me but maybe for you deeper might be better.

You tried shallower channels. You could try deeper channels, maybe 1 1/2 or 2 times deeper than what you have now.

Angle of channel walls like this V.



#111 kcoles

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Posted 19 September 2020 - 10:48 AM

Next report: I pressed overnight, brushed the lap, and did another press. Did AA pressed 15 minutes, AA. Then stoppped and checked on the tester. Pressed an hour, AA, press 15, AA. That's eight cycles of A, about 80 minutes of figuring. It clearly feels better, the friction is not nothing but not extreme, so maybe the lap was glazed. Following as many of the other CN hints as I can, which I won't belabor here.

 

It's going a bit oblate in the center; I can add some B strokes back in, as those prevented it pretty well. I had trouble taking Foucault photos (did it twice) but with my eye I can see the oblate bump at the center. The edge looks narrower to me. If the TDE ever disappears the neighbors will wonder why I'm dancing around outside.

 

f8_fifteenth_test.jpg



#112 kcoles

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Posted 19 September 2020 - 10:51 AM

Here is the lap after trim and the overnight press and before starting this last round, and then after the 80 minutes of figuring.

 

Lap_before15th.JPG Lap_after_15th.JPG

 

It went down to 40 F/5 C here last night, and my other half is lobbying to turn on the heat. That will raise the temp in the basement a bit (the boiler is near where I'm working) and I'll check the temperature to see how different it gets. Hoping it doesn't make my lap too soft.



#113 dogbiscuit

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Posted 19 September 2020 - 11:22 AM

I agree there turn down has narrowed.

Oblate is not a problem.  When the edge is good parabolizing from an oblate hardly any more difficult than from a sphere.

A strokes gave you some improvement, so I recommend staying with them.

 

Warmer might work better.  See what happens.

Remind me what temperature you have been working at, and let me know what the temp is with the heat on.



#114 kcoles

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Posted 20 September 2020 - 06:21 PM

Another 120 minutes of figuring. 20 minutes (two A cycles) and a 15 minute press. That three times is an hour of figuring, before/after which I press TOT at least three hours. So twelve A cycles allogether. Rotation of lap after 20 minutes, check channels and trim when/where needed after each hour.

 

It is going more oblate in the middle; I think I see progress at the edge. My current plan is to continue.

 

 

f8_sixteenth_test.jpg

 

Weather report: basement was between 70 and 72 in late summer (at all hours). It has turned cold here this past week, and when I checked basement was about 65, going up to about 67 after the furnace turned on today. So less variation than I expected.



#115 dogbiscuit

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Posted 21 September 2020 - 06:52 AM

The oblate has the turn up inside the roll down so that makes the edge near the same height as the intermediate zones.  That's good.

The width of the roll down is the same.

 

Do only the two strokes on the left side of the A strokes. Ws.    The

1/4 and 1/3 D stroke length. Both only 1/4 D wide.

I think that will get the outer zone turning up more and roll off narrowing.

Don't worry about more oblate... looking at the last 1/3" at the edge.

45 minutes to an hour of work should be enough.

 

I know you planned on working only MOT but if the roll off/turn down at the edge doesn't go away with the 1/4D width Ws TOT might be the thing to do.  TOT can get more work on the outer zone.

 

Differences between MOT and TOT

Working  with short strokes near center there is little overhang and there is not much difference between MOT and TOT pressures between mirror and lap, so results are similar.

When using strokes with more overhang the locations of pressure between lap and mirror are different for MOT and TOT. MOT overhang pressure is more on the lap's edge and some distance from the mirror's edge toward center, depending on degree of overhang.  MOT shape changes are made mostly by overhang pressure on the edge of the lap.

 

TOT overhang pressure is more near the edge of the mirror, and on the lap more away from the edge toward center.

 

With a 1/3 D or less stroke, overhang about 1/6 D at each end of the stroke there is not much overhang pressure and there is not a heck of a lot of difference between MOT and TOT results.

Some where around 1/2 D strokes, overhang about 1/4D, differences between MOT and TOT results become more noticeable, and differences grow with increased overhang.



#116 dogbiscuit

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Posted 21 September 2020 - 07:30 AM

There is something about your lap that is making that turn down as fast as you remove it.

 

Frequently the cause of a roll off at the edge like what you have is the lap is not in good contact all the way to the edge.

I've mentioned the layout of the channel grid to not unnecessarily reduce pitch at the lap's edge, and importance of plenty of pressing.

You've worked on these and maybe you've done as good as you can do. 

 

Here is something that is worth a try.

It could be the pitch at the edge of the lap is flowing out the edge and reducing contact near the edge.

A solution would be to wrap the edge of the lap with non stretch tape.  I use clear packaging tape.

I carefully wrap so the edge of the tape is a couple of mm below the top of the lap, same all the way around.

This stops the flow of pitch out the edge of the lap and makes the outer zone pitch flow much more like the rest of the lap.

You mentioned using tape when you press for a day or two.  This time don't remove the tape.

 

Don't use a stretchable tape such as vinyl "electrical" tape.  It will pull pitch inward from the edge.  Don't want that.


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#117 kcoles

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Posted 21 September 2020 - 07:53 AM

I use the clear packaging tape for the long presses, yes. Will try this and the strokes you mention - thanks.



#118 kcoles

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Posted 21 September 2020 - 07:09 PM

I'll post the images...Pressed for several hours. 60 minutes of the narrower strokes (1/4 long and 1/3 long) net, with same breaks every 15 minutes to rotate tool and 10 minutes presses TOT. Lap and tool taped except right at top as advised, throughout. Maybe that is what made a difference, as it looks different to me - I could swear there is less edge now. I assume the reversal of the approx ROC of each zone (Zone 1 farthest from mirror, Zone 3 closest) is consistent with the oblate appearance. Nice to have something new to look at, at least!

 

f8_seventeenth_test.jpg



#119 ckh

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Posted 21 September 2020 - 07:30 PM

 

Here is something that is worth a try.

It could be the pitch at the edge of the lap is flowing out the edge and reducing contact near the edge.

A solution would be to wrap the edge of the lap with non stretch tape.  I use clear packaging tape.

I carefully wrap so the edge of the tape is a couple of mm below the top of the lap, same all the way around.

This stops the flow of pitch out the edge of the lap and makes the outer zone pitch flow much more like the rest of the lap.

You mentioned using tape when you press for a day or two.  This time don't remove the tape.

 

Don't use a stretchable tape such as vinyl "electrical" tape.  It will pull pitch inward from the edge.  Don't want that.

dogbiscuit,

 

That sounds like a good idea. I had an experimental lap made by pressing a coarse mesh into the pitch. The problem was that the edge facets never got good contact because the pitch was not contained at the edge.

 

Does some of the pitch creep out to the mirror bevel with the tape on? If so, what do you do about it?

 

Carl

 

Carl



#120 dogbiscuit

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Posted 21 September 2020 - 11:44 PM

I think that did make it a little better.

Probably wouldn't hurt to do the same thing one more time before parabolizing, see if the edge gets any better.

I think the edge will get a little better as you start parabolizing and figure gets near spherical.

 

Have you made a Couder mask for Foucault testing?



#121 dogbiscuit

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Posted 22 September 2020 - 04:55 AM

Does some of the pitch creep out to the mirror bevel with the tape on? If so, what do you do about it?

Some does come out at the edge and curls around the mirror's bevel edge.  At the beginning of a work session I stroke the mirror short strokes fore aft left and right and the curl chips off or is shaved by the edge of the mirror, usually a little of both shaving and chipping.  Without separating them I wipe the chips off the edge of the mirror and lap with my finger and begin the work session.  I can see where the pitch has been shaved, there is a fine (~1 mm) ring on the top surface around the edge that is bare black pitch with no cerium oxide yet embedded.  The lap is then actually a slightly larger diameter than the mirror.  There is no doubt contact is good at the edge.  lol.gif

 

Contrary to what I have read from some sources saying to bevel the edge of the lap, that the pitch curl would turn the edge, I haven't had that problem and actually find that it makes the edge better.  I've been doing this with Gugolz pitch and haven't done it with other types of pitch so don't know, maybe it wouldn't work so well with other types of pitch,

With or without a taped edge, the chips fracture mostly vertically at the edge, almost a cleanly as if cut with a razor.


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#122 dogbiscuit

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Posted 22 September 2020 - 06:20 AM

I had my struggles with tde, for a long time.  It didn't take long to notice that the width of tde was very nearly the same as the with of the bevel on the pitch lap.  But all the books I had advised the lap must be beveled and don't let the pitch curl around the edge of the mirror or I'd get a tde.  Never made a really good mirror following that advice.  The writers in those books were experts and I was a beginner, so the tde couldn't really be related to the beveled thus slightly under sized lap.  I could force a fairly good edge with various "tde fixing strokes" but those were never really good parabolizing strokes for me.  They messed up a parabola if I had one, or if done before parabolizing, parabolizing made the edge not so good.

 

After a long long long time, watching width of tde very near the same as width of bevel on the lap I decided to use a steep bevel, almost but not quite vertical.  The lap diameter was 1 or 2 mm smaller than the mirror.  Turn down edge was very narrow.  An improvement, not perfect but better than ever before.

Still stuck on not letting pitch curl around the edge of the mirror.

 

After a not so long while, I'm polishing a mirror, long sessions near two hours and the lap expands with the work and by the end of the session the lap diameter is the same or maybe just a little larger than the mirror.  When I test the edge is super.

 

I give some thought to letting the pitch curl around the edge while pressing and carefully cutting the curl away from the mirror bevel with a razor but decided that would be risky for the mirror edge and maybe my fingers.  I just let the pitch curl and I'm thinking it is a pretty thin cross section to the curl so why not see if it breaks off with a stroke.  It did and work proceeded with no trouble and the edge was exceptionally good.


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#123 dogbiscuit

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Posted 22 September 2020 - 06:39 AM

I think you will be able to get a better edge doing some TOT work.

If you decide to try some TOT work here as some suggestions about ways to avoid trouble with warped shapes on the mirror.

 

 

Put some padding under the mirror.  The pad is a buffer between the table top and back of the mirror to absorb small irregularities of the surfaces.

Something like this 3/8" thick closed-cell foam padding is perfect
https://www.amazon.c...XNY0&th=1&psc=1

or a yoga mat or anti fatigue floor mat.  You might have something similar around the house.  I have a couple of extra mouse pads that could work well for a 6" mirror, although they are thinner than I would like I could use two thicknesses.

In times not quite gone by a piece of carpet, or several layers of wet news paper have been used for padding under the mirror for TOT work.  Even an old bath towel could make a suitable pad.

I've used carpet and it worked ok. Of course the carpet should be a style that has a uniform pile height, not textured or shag.  I've used news paper but didn't like the mess or smell.  I like foam padding better than carpet or newspaper.

I cut the foam circular the same diameter as the mirror so it won't bunch up at the cleats that hold the mirror or lap in position on the table. I cover the pad, cleats, and table with plastic to keep the table dry.  A single layer of household garbage bag is inexpensive.  Take care there are no wrinkles in the plastic under the mirror
A little bit of wiggle space is left between the mirror and cleats so that the mirror is not pinched during work.  One method is to put business cards between mirror and cleats when fastening the cleats, then removing the cards will leave a small amount wiggle room.

 

The mirror is frequently "clocked" on the work table, partial turns so that if the table has a slight warp that flexes the mirror, the mirror will be flexed the same way across many diameters as work progresses.  I like to do this about every 5 minutes or every 1 or two trips around the mirror.  If you alternate MOT/TOT be sure the next time you work TOT the mirror is clocked on the work table relative to the previous TOT work.  



#124 kcoles

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Posted 22 September 2020 - 10:38 AM

I had my struggles with tde, for a long time.  It didn't take long to notice that the width of tde was very nearly the same as the with of the bevel on the pitch lap.  But all the books I had advised the lap must be beveled and don't let the pitch curl around the edge of the mirror or I'd get a tde.  Never made a really good mirror following that advice.  The writers in those books were experts and I was a beginner, so the tde couldn't really be related to the beveled thus slightly under sized lap.  I could force a fairly good edge with various "tde fixing strokes" but those were never really good parabolizing strokes for me.  They messed up a parabola if I had one, or if done before parabolizing, parabolizing made the edge not so good.

 

After a long long long time, watching width of tde very near the same as width of bevel on the lap I decided to use a steep bevel, almost but not quite vertical.  The lap diameter was 1 or 2 mm smaller than the mirror.  Turn down edge was very narrow.  An improvement, not perfect but better than ever before.

Still stuck on not letting pitch curl around the edge of the mirror.

 

After a not so long while, I'm polishing a mirror, long sessions near two hours and the lap expands with the work and by the end of the session the lap diameter is the same or maybe just a little larger than the mirror.  When I test the edge is super.

 

I give some thought to letting the pitch curl around the edge while pressing and carefully cutting the curl away from the mirror bevel with a razor but decided that would be risky for the mirror edge and maybe my fingers.  I just let the pitch curl and I'm thinking it is a pretty thin cross section to the curl so why not see if it breaks off with a stroke.  It did and work proceeded with no trouble and the edge was exceptionally good.

This makes sense to me. I've also concluded that my lap flows out any time I have the tool sitting upright (pitch on top) by itself. The pitch seems to flow away around the edges. Even with a TOT press later before figuring, it is clear I haven't restored it to full contact. And when I break off chips with the mirror they are nearly always very thin. From now on I'm keeping the lap taped, and it will stored on top of the mirror when feasible. Will be getting more tape as I'm running through what I have.



#125 LarsMalmgren

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Posted 22 September 2020 - 10:57 AM

This makes sense to me. I've also concluded that my lap flows out any time I have the tool sitting upright (pitch on top) by itself. The pitch seems to flow away around the edges.

Oh, you see this visually?   - this sounds like your lap is _very_ soft.

Remind me, what type pitch did you use for the lap?
 




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