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Figuring a 6-in f/8

mirror making
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#126 kcoles

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Posted 22 September 2020 - 01:09 PM

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

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

I don't see it visually. Weeks of not terribly productive effort (see previous four make that five pages of this thread!) have taught me that the edge of the lap is always getting away from the mirror somehow, like some animals shying away from water. The tape experiment makes a very slight outward flow seem the culprit, but bear in mind I'm still learning other good habits (but I think I'm getting many of them). The lap seems reasonably stiff, though the channels moved outward over several weeks of pressing and use.

 

Gugolz 64, working in an environment that was around 70 F until the Autumn weather hit us a week ago, now a steady 66 F (this is in my basement).


Edited by kcoles, 22 September 2020 - 01:16 PM.


#127 kcoles

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Posted 22 September 2020 - 02:43 PM

While working with the mirror and lap today to set up a long, taped press, some chips jumped out of one side of the lap. They are evident in the oblique view, which shows the right edge of the vertical view. Maybe 1/8" max. After all the talk of bevels making edges impossible, I'm curious if this is a case where I should halt the proceedings? It's OK the rest of the way around, even a little proud of the edge.

 

Lap_chipped_oblq.JPG Lap_chipped_vert.JPG



#128 kcoles

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Posted 22 September 2020 - 02:46 PM

Here is what I've found that could make a pad for TOT work. One is the checkerboard non-slip drawer and shelf lining I'm already using as a non-skid surface on the work table. The other is some open-cell but quite dense foam that came as packing with a telescope case. It is 3/4" thick. Would either of these be adaptable for my purposes? Thanks as always for the unending and patient advice!

 

potential_pads.JPG



#129 dogbiscuit

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Posted 22 September 2020 - 03:07 PM

I don't think those chips out of the edge are too much.  The lap grows back with pressing and work.

you could take that pitch and melt it in a spoon and pour it in the channel on the other side of the lap, left side in the bottom picture.  Pour it in press it down with to surface level.  press a few hours and if not in good contact some work and pressing will get it in contact soon.  Don't know if it has anything to do with your rolled edge but it's location coincides perfectly.

 

The foam and drawer liner will probably work ok.  Cover it with some plastic so it doesn't soak up slurry.  Cerium oxide drying in the foam might make harder areas in the foam.

 

With plastic under the mirror and cleats not pinching the mirror, the mirror could rotate a little as you go around. That's ok but keep up with where you started and where you will end relative to the mirror, not the work table.  I put a small piece of tape at the  6 o' clock position on the mirror.   At least with what I use I don't get much slippage as I go around.

.



#130 LarsMalmgren

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Posted 23 September 2020 - 07:01 AM

I don't see it visually. Weeks of not terribly productive effort (see previous four make that five pages of this thread!) have taught me that the edge of the lap is always getting away from the mirror somehow, like some animals shying away from water. The tape experiment makes a very slight outward flow seem the culprit, but bear in mind I'm still learning other good habits (but I think I'm getting many of them). The lap seems reasonably stiff, though the channels moved outward over several weeks of pressing and use.

 

Gugolz 64, working in an environment that was around 70 F until the Autumn weather hit us a week ago, now a steady 66 F (this is in my basement).

Ok, fair enough, i just put too much in your description :)

 

I've never heard anyone use tape to dam up the lap on the edge, but it kinda makes sence.

I'm looking forward to hear about your results with it. If it works it's another trick in the book waytogo.gif



#131 dogbiscuit

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Posted 23 September 2020 - 09:27 AM

Yes, the mirror rocks a little with the stroke but it does the same all the way around.

For 6" or 8" mirrors of 1/6 D thickness might not have much trouble working TOT without a pad between table and mirror, depending on how flat the table is and how flat the back of the mirror is.

 

Larger or thinner mirrors will have more flexure troubles.

 

I've used ~6 to  9 mm thick closed cell foam with good results (and they last a long time).  Don't know about 3/4", haven't used something that thick but it might work ok. It shouldn't take long to find out.

 

If the 3/4" thick foam doesn't work well, you can try just the shelf liner. I've used that once before and it worked ok.

It wears out in a few hours of work, but it's cheap so can be replaced easily, so you could just use that.


Edited by dogbiscuit, 23 September 2020 - 11:25 AM.


#132 kcoles

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Posted 25 September 2020 - 10:31 AM

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?

Over the past several days I did soften some pitch and filled in the channel that is too near the edge of the lap. Shaved it level with a knife and pressed over night. Most of it has filled in; my photo of the lap in contact with the mirror doesn't show it very well so I'll take another in the next figuring session. I trimmed all the channels and did another 6 hour press (with tape). Then got to a session of the two narrower strokes from A. This was pretty much like the one in post 118: an hour of figuring net, with breaks every 15 minutes to press and rotate the tool.

 

It's gotten more oblate and the edge is less. (the lower left Ronchi image is vignetted at its lower right - that was my camera placement, not the figure itself).

 

As for being ready for other strokes, I found some 9 mm closed cell foam, so TOT is an option if it makes sense, or continuing MOT. I do have a three zone Couder mask made on the Berry book model - it helps me find where to take the Foucault images, even though I'm still learning to do that (knife edge readings I have to do at night when there is no stray light in my basement; the windows are covered).

 

Thanks for the continuing interest - I'm amazed at how many folks are checking this thread. wink.png

 

f8_eighteenth_test.jpg



#133 dogbiscuit

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Posted 25 September 2020 - 01:55 PM

9 mm closed cell foam... perfect

When your wife asks what happened to her yoga mat, don't blame it on your dog.  lol.gif

 

 

channels and pressing

 

TOT 2 turns.  See what it does.

Parabolize3fourthsD7stroke.png


Edited by dogbiscuit, 25 September 2020 - 02:19 PM.


#134 kcoles

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Posted 25 September 2020 - 02:27 PM

OK, I will take some time to set up for this, and press (probably overnight) before starting, and I will check it on the test stand after one time (oh, I see, 2 times) around. The last time I "overshot" a target it was a very long recovery!

 

[Actually it is a scrap of an old camping mat; the other part with straps was turned into a dew shield for an 8-inch SCT. My wife got fancier exercise mat last year. smile.png ]


Edited by kcoles, 25 September 2020 - 03:28 PM.


#135 dogbiscuit

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Posted 25 September 2020 - 08:15 PM

Ok, the dog is safe.  We take a lot of blame, yoga mats and homework.confused1.gif



#136 kcoles

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Posted 26 September 2020 - 03:51 PM

I did all of the above I believe: made a foam disk and adjusted the cleats and put plastic over all (it turned out to have one pinhole opening in it, got the foam wet in one spot but I mopped it up). Trimmed channels and pressed two hours. Two cycles of the 7-stroke parabolizing dance. It helped to be forewarned of the tiny shifts by the mirror and its possible rotation (about 15 degrees in the first cycle, essentially none in the second), so those were as expected. I rotated the mirror 90 degrees after the first cycle. After all that it looks like this:

 

f8_nineteenth_test.jpg



#137 dogbiscuit

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Posted 27 September 2020 - 07:32 AM

I put a couple heaping table spoons of cerium oxide in a pint jar, fill with water, shake it up vigorously for a few minutes to be sure all cerium oxide is wet, let it settle for several hours, then pour off most of the water leaving only about 1" of water on top of the sediment at the bottom of the jar.  I can tilt the jar to expose the sediment and dip a brush in the sediment to pick up thick slurry without much water.  This way I can apply mostly cerium oxide without excess water.  If I want more water I use a spray bottle to mist water onto the work.

 

At the end of the next work session, without separating the mirror and lap turn it MOT and take a picture of the lap through the mirror from straight overhead.  Don't add cerium oxide or water for the picture.

 

All TOT 1 turn each. Clock the mirror on the table after each turn a little less than 90°, something near  80° give or take a little.

 

The A strokes from post 51

kcolesStrokes.png

 

And then these

kcolesStrokes137.png

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 



#138 dogbiscuit

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Posted 27 September 2020 - 12:57 PM

I don't know much about how you are applying cerium oxide and water so I don't know how long you can go without adding water. And it depends on humidity.

 

The short stroke don't expose as much water to the air so evaporation is not so much.

Longer strokes expose more water to the air and there is more evaporation.

 

In most cases I would get through all 6 strokes without adding cerium oxide or water.

Sometimes maybe just a very light mist of water on the last two.

 

Let me know how it goes for you.



#139 kcoles

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Posted 27 September 2020 - 02:01 PM

Will do, thanks. It's a busy day (they're taking down a big tree outside my house) so I'll press overnight and start in the morning. It was typical when I did pattern A with MOT to go all the way through without more Cerium or water.



#140 kcoles

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Posted 28 September 2020 - 02:55 PM

Did all as suggested. I need to try another plastic bag; this one (rather thin) stuck to the mirror which made "clocking it" which made pulling any wrinkles of the bag between the mirror and foam a complex operation. Here is the result.

 

f8_twentieth_test.jpg

 

And here are shots (looking opposite directions) of the mirror and lap right at the end of figuring. The filled in and partially annealed former channel too near the edge is at the left of the first photo and the right of the second.

 

Lap_post20th_a.JPG Lap_post20th_b.JPG

 

edited to add PS: the whole set of strokes went without needing cerium oxide or water (it is on the damp side here today).


Edited by kcoles, 28 September 2020 - 02:56 PM.


#141 dogbiscuit

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Posted 28 September 2020 - 04:47 PM

For the most part the turn down has not changed much.

There is something about the lap that is turning the edge as you work

The lap shows undersized around much of the edge.

There are some signs of poor contact in a lot of places.  Also a lot of bubbles on the surface of the pitch facets.

Those things should have worked out of the surface long ago.

The cause of these bubbles could be (I can only guess) the top surface being made irregular and out of shape by brushing, followed by not enough pressing to get it back into shape.  This is one reason that in figuring I don't pressing weaves or brush the lap unless there is a good reason.  Usually figuring can be completed long before there is a good reason.

 

You have a near perfect sphere out to about 80% radius as seen in the center Foucault image.  Then a turn up across the next 10% and then a turn down.  The edge is probably not lower than the central sphere so maybe it can be made  a little better with some TOT parabolizing. Just a little TOT to wear some of the turn up off, most parabolizing done MOT.  Can work with a good lap if the overhang is just right, enough to wear the turn up without adding to the turn down.

 

It seems my suggestions aren't going to make your edge better than it is now.  I've pretty much run out of idea for making a good edge.  I don't think there is anything I can add.  Everything I know about it I've already told you.

 

I think now I am only holding you back from finishing your mirror.

 

You might try Pinbout's suggestion about scrubbing the edge with short fast strokes.  I haven't done that stroke so I don't know how well it works.  It probably can't work worse than the way things have been going.

 

Maybe someone else has any ideas.

 

Or you could parabolize without making the edge good first.


Edited by dogbiscuit, 29 September 2020 - 01:02 AM.


#142 dogbiscuit

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Posted 28 September 2020 - 05:35 PM

To parabolize, I would start by doing each of these once.

 

TOT one turn

Parabolize3fourthsD7stroke.png

 

MOT one turn

Parabolize4fifthsD7stroke.png

 

 



#143 kcoles

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Posted 29 September 2020 - 08:53 AM

Thank you - I will keep all this in mind. But I guess I'm stubborn and not ready to call a mirror with TDE finished. Many reading this may have silently suggested what I am now going to plan out loud. It is time to discard this lap. I have blood, sweat, and tears in it and thus feel attached to it, but I can keep the pieces in a little keepsake box. smirk.gif  I'm going to pour a new one when time allows later this week. (I have lots of pitch, though I've used nearly all of a 1/4 lb jar of cerium oxide!) I am now aware of half a dozen things to do differently to ensure good contact all the way to the edge of the lap from the first minute of day one. This one never had good edge contact the first two weeks (it is six weeks old), and in the last week or two too much of the pitch is missing near the edge. I'm also planning to do pressing with a weight and the edge of the pitch confined until I get, and retain, good contact all over. Dogbiscuit has patiently taught what good contact looks like, and ensuring that it is there will be at the top of my checklist every day. Ditto for what several of you (pinbout et al.) have advised about channels as well. Wish me luck - I may be just ornery enough to get this one to the finish line.



#144 dogbiscuit

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Posted 29 September 2020 - 10:12 AM

Lap same diameter as the mirror

 

Take care in laying out the channel grid and facet size so as to have uniform facet squares size, minimize small partial facets, center of lap on a facet but not centered on it. 

I thought I sent you this lap layout but looking back through the thread I don't see it, so there is one more thing I can tell you.

LapGrid-B.png

A uniform grid square size on a round disk will have some small and very small partial squares. The size of squares and position on the circle can be done in a way minimizes the number of those small partial squares and meets all of my requirements (uniform facet squares size, minimize small partial facets, center of lap on a facet but not centered on it).

 

On that grid the center most row and center most column of squares has 5 full squares and one partial square that is about 1/3 the width of a full square (squares at 9 an 12 clock positions).  The way I have placed the grid on the tool circle is about the best I can do to minimize small partial pitch squares,  I have other grids with more and less full + ~1/3 squares.

 

At the upper left is the dimension between channel centers, 28mm for a 6" lap. The size of the pitch square is 28 mm minus the width of a channel.  If channels are 7 mm wide pitch squares will be 21mm across.

 

Sure, other lap layouts can work fine but there are some that don't work so well.  This one I know works fine for me.

 

Here is another size grid with 4 full squares and a partial across.

LapGrid-C.png

This one could work ok for a 6" f/8 too. Square size is a little bigger, and of particular interest the partial squares are larger.

I think I like this one better for a 6" f/8.



#145 dogbiscuit

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Posted 29 September 2020 - 10:35 AM

Glad to see you have the patience to continue, and get the edge right before parabolizing.

 

I'm can continue to offer advice.  Things have been going in small circles for a while and I don't want you to feel like if you go in a different direction than I suggest that I would be offended. If someone have ideas or you have your own, if you think they'll work try 'em.

 

I want to see you finish a fine mirror.  I won't be happy about your mirror until you are happy with it.

 

If I'm not much help, maybe you'd still like sharing your progress and discussing mirror making.

 

And once more... The secret sauce is the lap. When you overcome your trouble with the edge, it will be that you changed your ways with the lap.


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#146 kcoles

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Posted 29 September 2020 - 12:07 PM

Don't worry, I get it! This is an art that takes a while to learn, and that is with someone standing beside you who knows it. I get a sense here that most folks agree there are multiple roads to the finish line. I do have an idea what I'll try and in what order, but it differs very little from all the techniques I've gotten help with already. As for the lap, a lesson learned the slow way is a lesson remembered.

 

I did have the lap patterns from the first thread started by astrokhan. Another lesson there -- making a new lap got that first mirror finished.



#147 kcoles

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Posted Yesterday, 03:41 PM

For those of you wondering how the mirror making went after my recess, a brief update. A local who made a mirror years ago suggested the Texerau method. What the heck, I thought. On Oct. 3-4 I made a mold and poured pitch to make squares. Over the next days I cut these and attached them to the tool. Repeated bouts of reattaching ones that came off, releveling by pressing, and so on consumed the next week. Then I was away on family duties for a week, and more days of finding contact was not good all over or having to reattach squares (using a candle or mineral spirits). Yesterday it was finally good enough to try some simple strokes (MOT 1/4 narrow W) for a few minutes and test. Looked OK, so today I tried some TOT and kept it short (about 12 minutes total with frequent rotation of mirror) and simple - I won't add the details here, though I'll confess the strokes may have had too little side-to-side.

 

If anyone does the casting-pitch-squares method, do use all the suggested tricks to attach them; I didn't use beeswax or mineral spirits at first. Me, I will go back to pouring the lap the next time, which I may be doing soon, as you'll see in a minute.

 

While I saw the tiny improvement in the edge I hoped for, I also see the mirror getting lumpy in the center. (I'm still doing a prime number of steps around the barrel.) Oh boy, a road I do not want to go down. I'll just attach one pair of Foucault images, from the last test with the old lap and from today. You'll see what I mean.

 

Fou_Zone2maybe.JPG Fou_Zone1.JPG

 

After all my work the lap doesn't look too impressive either, though I make good and sure I see contact around the edge. I'm already tempted to trash it and make another. The channels and huge and deep (6 mm or 1/4 inch of pitch) but they, and the squares, have gotten irregular and I can easily believe they cause the problem. (Really, they all started out square!) Your thoughts are welcome, though if your true opinion is that this lap won't work I'd rather hear you say "Redo it," than try a lot of fixes that may be futile. Thanks as always!

 

Lap_post21st.JPG



#148 Pinbout

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Posted Yesterday, 05:59 PM

You want the old perfect pitch lap matte I got from Steve? I have to clean it up first.

 

your first lap looked much better than this thing. 


Edited by Pinbout, Yesterday, 05:59 PM.


#149 dogbiscuit

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Posted Yesterday, 06:16 PM

Is the lap substrate a ground surface?

Does it have a convex curve that matches the mirror?

 

The lumpiness is not so unusual.

It happens sometimes with a new lap, and after a little work to break the lap in goes away.  Probable reason being the new lap has not yet been completely pressed to full contact and polishing agent not yet uniformly embedded in the pitch surface.  Just my opinion, don't know for sure.

 

I have seen this occur when working 8 rotation positions of the mirror and or lap.  I Don't think you are doing that.

 

Another cause is excessively wide channels, and this is probably the major contributor in this case.

 

I have never needed to use mineral spirits or turpentine to get pitch to stick to a glass tool, but I don't cast the individual squares, I pour the lap either on the tool, or on the mirror and set the tool on top of the pitch.  

 

After making and wrapping a collar around the mirror I rub a few drops of vegetable oil over all the mirror surface, place a piece of aluminum foil on the mirror and rub it into contact with the mirror and rub the wrinkles out. Neatly fold foil up an the collar to make a dish to pour the pitch into.

 

The collar is made to precise width to extend above the mirror surface the desired thickness of the lap.  I like to use strips accurately cut to width from paper poster board to make the collar because it's strong enough to hold up to pressures likely to occur when placing the tool on top of the pitch.

Width of the strip is substrate thickness + desired lap thickness.

 

With collar taped tightly in position around the mirror, level before pouring.  

Melt enough pitch to fill to the top of the collar.

Pour pitch to top of collar.

Carefully place tool on top of the pitch, making contact with the center of tool at the center of the pitch. Carefully lower the tool until the the edge of the mirror is even all the way around with the top edge of the collar. Don't press the mirror past the collar.  When the edge of the mirror is at the collar all the way around there will not be much room for the pitch to come out at the edge. A little, very little might come out before getting the mirror edge all the way to the collar because the tool is convex and extends past the level of the collar, very little for a 6" f/8.

I recommend thickness between 3/8" and 1/2", let's say 7/16" thick.  This is so you have enough thickness so the lap will last long enough to make a few mistakes on the way to the parabola, or maybe your finished lap is thinner than planned.  Some people like 'em but at abut 1/4" thickness the lap is very close to being to thin for me.

 

Another method I've used similar collar on the tool (no oil or foil).  Pour thick slow pouring pitch on the center of the tool,  cooling pitch piles up and domes in the center.  Mirror face coated with thick slurry is pressed down on the dome, contacting center of mirror to center of pitch dome.  Pressing mirror down spreads contact across the mirror face center to the edge.

 

Need to pour the right amount of pitch so the dome fills the collar when the mirror's edge gets to the collar.

 

If you want to try one of these methods let me know, I can give some more detail.

Both of these form a curved surface on the pitch and will quickly press to full contact.

I cut channels with a single edge razor blade.



#150 dogbiscuit

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Posted Yesterday, 06:29 PM

I forgot to mention...

I guess the splotches around the edge are just some slurry residue and it is only the lumpiness (primary ripple) you are concerned about.

 

The mirror looks pretty good other than the ripple.  Still a little rise and fall at the edge but better than before.

 

Don't know if you can do it with your current lap but removing the ripple doesn't need to take very long.

Lap time between these two photos was about 1 hour and 15 minutes.

6f10RipParab.jpg

 


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