12 inch countertop overhang how much

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    Hello glass athletes,

    since the end of December I have been grinding a 12 “borosilicate blank with a thickness of 25mm. Now the good piece is polished and would like to come into a spherical shape first. The starting point today was as follows:


    With a 6 “star tool with circular lines with a slight overhang, I tried to make the too steep ascent between zone 2 and 5 flatter again, and to remove the mountain with lines across the middle. In addition, I made tangential and circular strokes with a slight overhang, and again and again neat chaos strokes. The 2nd measurement then gave the following picture:
    "Only" the central mountain had worn away a bit!
    Then I tried to use the thumb method to “lift” the sunken edge, which also led to a reduction in the difference in cut lengths. Then again lines with a slight overhang, circular lines, and the last ¼ hour always with chaos.

    Today I was able to reduce the sunken edge and shorten the differences in the cutting width. Once the mirror was spherical up to zone 3.5. I think it would be best to first consistently lift the sloping edge with a small tool or with the already tried "thumb method" by Stathis. Or is there a way to put both together to remove the rise in zone 3-5 and the sloping edge?

    Thank you for your tips and suggestions!

    Hans Georg

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    Hi Hans Georg,
    As I see you have set desdired deformation = 0, i.e. spheres. In any case, I would go easy on my thumbs for now. You have a sunken edge but with a subsequent ring wall. The quickest way to cure something like this was with a ring-shaped tool, full diameter, short strokes and a tool on top. After that, a flat, wide mountain will likely remain. But you can cope well with that.

    I wish you success!
    Greetings Kurt

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    Hi,
    I had almost the same picture on my last 12, only the edge wasn't sloping off quite as extreme as yours.

    In your case, the thumb method led to success in zone 5 the fastest. Turn ten laps (depends on the pressure and execution). That will probably not be enough, but it shouldn't be anymore for the beginning. Then you will see a picture with several rings on the outside. You can get them away in no time with a third stroke of the same size tool mirror.

    The elevation in the middle will probably be even higher, which you can get with a 6 "tool with W strokes without overhanging directly to the parabola [:)]

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    Hello Hans Georg,

    You made beautiful Foucault pictures there, the course of the curve from the Figure XP graphic can be seen very nicely on the contrast course.
    The method that bumbum describes is very effective, but carries the risk of negotiating zones (especially with thin mirrors).
    Kurt's method is not quite as radical and is more gentle on the surface because it works over a large area.

    Greetings Ulli

  • Hello Hans Georg,
    very beautiful pictures!
    My opinion: Both methods work. I'm favosing the thumb because I'm lazy. I always have my thumb as a pitch skin wide (no, it's not worn through yet), the ring-shaped pitch skin has to be done first (which of course isn't that wild either, e.g. by pressing a central sandwich paper star into it).

    From the development of the pictures you can see that you are already on the right track. I also know that the edge drop initially remains the same in absolute terms, but is always thinner - that is, figuratively speaking, is pushed outwards over the pane.

    The mountain hanging in the middle is not a problem at all, it goes away immediately when parabolized.

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    Hello Kurt, Michael, Ulli and Stathis,

    First of all, thank you very much for your opinions and tips on my questions! Of course, I am also very happy about the praise for my pictures!

    On the thumb method: Well, I've already tried it, around 10 laps, and I ran around the mirror to make it as even as possible. Since the fallen edge has become narrower, it shows me that the method has at least led in the right direction for me too. I had wrongly expected that a round zone would form immediately and that the difference in cut lengths would have to decrease. Now I have learned that the edge waste is initially thinner, but unchanged in its back focal length. (Thanks Stathis!)

    Well, as Kurt wrote correctly, I not only have a sunken edge, but also a ring wall. Does it change with the removal of the sloping edge?
    I think he is at least influenced by that?

  • <blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">Zitat:<hr height="1" noshade id="quote"><i>Original erstellt von: hgstein</i>

    Does it change with the removal of the sloping edge?
    I think it is at least influenced by this?



    The ring wall may not get any better as a result. But I would always tackle the biggest problem first, and that's the edge here.

  • Hello Hans Georg -
    I am amazed at your clear Foucault pictures. I also see a very light mountain in my mirror no. 2 (Pollux), but so weak that you can hardly see it. Nevertheless I have the feeling (unfortunately not anymore!) That something is wrong. On the other hand, I can still see streaks of air even in an unheated basement ...
    But because the mountain in the middle only lengthens the parabolic process a little, but does not hinder it, it has already started.

    What does your Foucault tester look like? How big is the hole? Which light source do you use? And - last but not least - how do you photograph your Foucault pictures?

    Questioning

    ullrich

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    Hello Ulrich,

    as a light source i use a super bright wise LDE. It is in a small aluminum tube, which I insert through a hole in an aluminum square tube:

    As a light gap 2 carpet knives which are mounted against each other on the aluminum square tube over a 5mm hole.
    I can open the knife to adjust the mirror to the tester, then I have full light intensity and can easily find the reflection of the seal even in daylight.
    I use an Olympus 3030 as a camera. It then sits on the bench in front of or above the crossbar table. [;)] The 3030 is fully adjustable manually. This is very important! An automatic exposure and auto focus would not work so well!

    Best wishes

    Hans Georg
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  • Hello Hans Georg -
    thanks for the great description of your Foucault tester. In the absence of a decent digicam, I will either have to borrow one or use the webcam. The latter is associated with a lot of work because the PC always has to be carried down first.
    But the gap is very good, I should have figured that out too. My hole is <0.2mm in diameter, but gives very little light (even with an ultra-bright white LED) or - if the hole is larger - poor contrast.
    I'm going to experiment with a gap.

    greeting

    ullrich

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    <font size="2">Nach 20 Runden "Daumendrehen" sieht mein Spiegel nun wie folgt aus:</font id="size2">




    The measured values ​​seem to have gotten a little worse. But that's because I covered the outermost edge of the earlier measurements in order to be able to measure anything at all. Since today's action, I get almost the entire zone 6 darkened. I was amazed at what you can do with a thumb as a tool!
    The question for me now is [?] Continue "turning your fingers" until the edge is completely "flat" or first align the edge to the middle.

    I'm curious what you think about it!

    Best wishes!
    Hans Georg

  • Send pictures, I'm thrilled.

    I wouldn't care too much about the measured values ​​as such, they make little sense with a zonal mirror. Only tendency is important to see if you are walking in the right direction.

    In this case I would personally insert another session with my thumb, because the outermost edge has still sunk a bit. In order to better integrate the thumb-width edge into the rest of the curve, I make so-called V-shaped thumb lines: alternately one line parallel to the edge (as you obviously did) and then one at an angle inward. This takes the adjoining surface with it and avoids the sharp transition that begins to show up on you. Then even out with a large, well-adjusted pitch skin.

    There are of course other methods (maybe the others will write something about it).

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    Hello stathis
    Hello everyone,

    here is the result of another round of intense thumb exercise.

    3 times 20 rounds! [: p] This time with the V dashes suggested by Stathis. Thanks for the tip! However, the transition became quite sharp. But you can also see a little bit the effect of the lines inwards. I probably started too late. [?] Although the sunken edge has still not completely disappeared, the next thing I will try to level out the zones.

    If then I still need to do another thumb round.
    I'll sleep on it first. [xx (]

    Hopefully everything is still OK: When I look at it this morning, I have doubts [:(]

  • Hello Hans Georg -
    do not worry. My mirror Nr2 (Castor) looked similar after using the thumb or mini tool. And only one hour of polishing (in total!) Later it is perfect - ready to be aluminized.
    By the way, thanks again for the great idea with the light source - now I can see how the shape looks wonderful.

    After my initial euphoria, the mirror No. 2 then turned out to be not 100% optimal (only lambda / 6), even with a slightly submerged knowledge. I just got my thumb out and worked on the transition for 5 minutes. Then smoothed for 10 minutes with a deformed full-size tool - now the troubles with the mirror are over. According to multiple measurements, it is now better than 1/12 lambda, that's enough. It looks as smooth as a child's bottom - I don't even like to touch it anymore [: I]

    The next adventure is still waiting for me: No. 1 (Pollux) should be that good too ...

    Oh - I should perhaps mention that I * only * dared to approach 8 "... [8)] - but twice ... it's effectively almost 12" [: o)]

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    Hello everybody!

    With my 12 “I struggled with the problem of adjusting the pitch skin of the full size tool over the entire surface after casting! The problem is the middle, which always only comes into contact after a long press. The result is that the surface structure of the pitch skin in the middle remains as it is, the surfaces towards the edge are evenly larger and larger, and the grooves become ever flatter! The pitch skin has less area in the middle and therefore works the middle less. That's why I've always had a more or less large central mountain from the beginning until today. After pouring a pitch skin with a pouring mat, I had the mirror almost spherical (0.5mm zone difference) within a short time, except for the sunken edge. After every polishing session with thorough hot pressing, the mountain then got bigger again. Which I couldn't really explain at first. Due to the clear structure resulting from the cast mat, I noticed that the outer facets have become wider than the inner ones because they have sunk together.

    I've tried a lot to get the matter under control, but slowly I don't know what to do. [Xx (] Maybe you have one or the other tip. Maybe I just think of it instead of using a silicone casting mat To work with a mat or form with which one can also press warm in order to obtain a certain structure of the surface over a longer period of time.

    Thanks!
    And many greetings!
    Hans Georg

  • Hello Hans Georg,

    You can also use the casting mat during hot pressing to restore the facets. Simply soften the pitch skin a little so that you can press it in with your finger and then press it together with the casting mat. More information about this on my website (see below) under 14 "grinding. I always sat on the tool with my whole body weight, then the grooves were back in half a minute and the tool was like new.

    greetings
    Purer