Primary processing 300 mm f/6

Home Forums Machining Parabolic Mirror Primary processing 300 mm f/6

This topic contains 150 replies, has 5 voices, and was last updated by Giulio TiberinI 1 month, 3 weeks needle.

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  • #10209
    Stefanosky
    Stefanosky
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    We start with the creation of a leading 300 mm in diameter with focal length f/6.
    I'll use this thread for progress, recommendations, solution errors etc so feel free to criticize, recommend, help and intervene when and how should.
    The blank is a borofloat33 manufactured by Schott and has a thickness of 25 mm. As the name suggests this is a commercial that compared to borosilicate glass sodium silicate alters in an inferior in size when the temperature changes and this should allow me, during the parabolizzazione, You can take measurements and fixes without having to wait for the biblical times of sodium silicate.
    IMG_20170720_110804

    For a focal length of 1800 mm I need to get a 3, 125 mm.
    According to an interesting article by Dave Grennan, Giulio gave me many, I tried to create a tool consisting of a diamond blade from 115 mm to try and get a blocking faster. However this way to me, at least at the moment, didn't work out just producing orphans on the surface of the blank, so I decided to set it aside temporarily and go to classic ceramic tool full diameter.
    I put the pictures of the tool created by me:
    IMG_20170729_110445
    IMG_20170729_120502_1
    IMG_20170729_122521

    The full diameter tool that I created consists of dental plaster type IV and from squares of glazed ceramic drowned in casting. I forgot to attach the pieces with glue stick and then they kinda move but I think might go well anyway. I then made a sort of funnel with the Dremel.
    IMG_20170730_152020

    At this point I then magnets on the edge of the mirror and the tool, smoothing out sharp corners of ceramics to avoid sbeccare the blank.
    Now I'll spend the grana chordal 80 but I want to first check the back of the tool is decently plan.

    #10223
    Stefanosky
    Stefanosky
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    Chord passes begin to bear fruit, the tool works well and the center of the mirror slowly starts to fall. If I can find fault with the tool made in this way affects the highest consumption of carborundum which, working face up, inevitably tends to disperse partly drains.
    IMG_20170731_124558
    Pick a few more hours and then I will start with the measurements of the arrow to assess progress.

    #10224
    Massimo Marconi
    Massimo Marconi
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    Bravo Stephen, a great start ! In my opinion for this work the tool at full diameter was the best choice. :good: To limit the dispersion ( and increase the efficiency ) the abrasive you can reduce the amount of water, that however would put very little. If you already use the abrasive almost “dry”, Don't be afraid to overdo the pressure. When you hear a noise “terrible” scraping then means you're digging well !

    #10228

    GIM
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    Hello Stephen
    In my view the tool you could leave him without the grooves at least until use of grain 400.
    In fact, with the coarser grit you just dig without worrying much about create scratches on the surface. I used a tool like drowning in chalk pieces of glass with a thickness of 20 mm and for the digging went very well. Starting from grain 400 I think it is good that the plaster part should not be in direct contact with the mirror as though in principle more tender than glass can cause micro-scratches (perhaps due to the fact that if cleanup is not perfect when changing abrasive some grain may remain stuck in the plaster?) boring enough that you need to go back to the previous grain. Grind the edges of each tile perfectly agrees to prevent fall of micro scales rifts.
    However overall the tool you've done looks well built, in my opinion, arguably better diamond wheel that even if it worked quickly to reach the desired arrow, you would still have to build a tool adapted to the excavation done in order to continue with the finer grits.
    In any case, know that I follow with interest your work, also because I am currently prevented from further processing of my 300 mm. In my garage currently you are not going too well and to give you an idea of why I attach a picture ... .... B-)

    https://postimg.org/image/6ueikkhu1/

    #10229
    Stefanosky
    Stefanosky
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    Thanks Massimo, I started to get a little’ his hand and saw that just wait that the channels are a little’ saturated and use a little water to get the best performance. GIM, the grooves I did right away because in a couple of articles said to ask you to avoid that mirror and tool they glued or rubbing it became too “psstoso”. Frankly I don't have the experience to evaluate the thing from a technical point of view. The plaster at the moment looks nice sealed, in the sense that when I clean the tool with my toothbrush no trace remains of the grains. Most likely you just have to be careful that there are no holes or cracks on the surface and seal it, but it is a problem that I'll discuss later… One of many that I will meet.
    At the time I worked a few hours and as arrow proceeded to 1, 09mm so I'm at -2, 04mm from target.
    However in no article I had read how taxing the work of grattavetro! In summer then is devastating :wacko: I'm working on 30 and passing grades, bare-chested and with a headband across forehead, I look like karate kid B-)
    In this heat I guess very complicated pitch work without an air conditioned space. But since I'll be at that point will already be cooler :whistle:

    #10233

    Giulio TiberinI
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    :yahoo: I am really very happy to read your exchanges of comments on your own experiences and thoughts, Why is this the real utility of an informative Forum set in a specific blog !!
    Are exposed and subject to comment TRUE THINGS, the result of his knowledge.

    I agree with the thought of GIM and with your Stefanosky, and I stress that the real struggle is known to be one of the excavation of the curve.

    Once you reach the desired arrow, continue the work by applying the much less athletic races 1/3 Diameter c.o. c, you do not need to exert pressure(*), and they are very efficient in the excavation, but that lead to generate and maintain the desired depth of the desired arrow ball, extending it to the edge of the mirror during all stages of refining surface, with increasingly fine abrasive grits.

    (*) Recalling the well-known fact that racing 1/3 D c c.o., you have three possible actions:
    1) With the mirror in his hand and the tool on tour We will concentrate on the Centre of the mirror leaving untouched the surface from 0,7 D to the edge; While vise versa:
    2) With the tool in his hand and a mirror on the counter you work only border leaving intact the Center.
    3) Alternating between the two locations you drive, Yes “pilot”, corrects, and maintains the desired ball and arrow.

    Some pressure may be exercised only in cases 1) and 2) to retrieve each other the ball a little arrow (Cone action 1), or an arrow too (with the action 2). while maintaining an arrow reached does not require pressure but just dragging (and patience) to generate the desired ball… the more free and accurate pressure dragging applied through the edge of the glass moved, fine grit progresses…(In fact by applying pressure, you run the risk of making “game over”, countersinking, too much or too early in the sphere in the parable or worse in Hyperbola, and having to work long 1/3D racing c.o. c without pressure to resume the initial ball).

    In summer the pitch black #55 and #64 ..He leaves alone “for their own business”, as seen in the image tool posted by GIM.

    #10239
    Stefanosky
    Stefanosky
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    Scratch scratch cards I realized that, in my case, the most profitable is to wet slightly the tool and deploy the abrasive on its outermost range. On the glass, nothing, talking water abrasive… In this way the carborundum used goes to work exclusively in the area of interest along the entire circumference of work while handing out the abrasive on glass that was mostly lost in the first quarter making it less effective the rest of the job.
    If in the meantime the abrasive around the circumference of the tool it dries a little too just a sprinkling of light water and leave.
    The arrow now measures 1, 84mm missing then 1, 28 mm from the target theory, actually I will descend a little something more to work mainly with the tool above to use less abrasive… We continue. :good:

    IMG_20170803_133012

    #10240

    Giulio TiberinI
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    I take note of the interesting suggestion :good: ..Congratulations!!

    #10244

    GIM
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    Sorry Stephen I have a question to ask.
    In your last posting I read that you still lack more than 1 mm excavation par reach the desired arrow, so I suppose you're using carborundum grit 60 or more 120.
    Now what I don't understand is that in the picture you've posted, unless I have hallucinations, You can clearly see the reflection of the ruler that you place on the glass processing, and the depth of the excavation. Now since I arrived to use the grain 1200 that gave me a fine glaze but without even the shadow of a reflection, explain to me how you did it? What am I missing? :wacko:

    #10246
    Stefanosky
    Stefanosky
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    Well the reflection you see because the mirror was slightly wet, otherwise the non-reflective satin finish would have seen that I am using 80-grit… So everything okay :good:

    #10249

    Bartolomei Mirco
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    Hello Stephen, :bye:
    very nice work you are pursuing…I am following with interest… :good:
    Good luck with your work…

    #10283
    Stefanosky
    Stefanosky
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    With chord passes I reached the desired arrow and by tomorrow I'll start the ball using regularization phase switch 1/3D CoC, passes that will accompany me even in subsequent processing steps with increasingly fine abrasives.
    With the Dremel Meanwhile I redid the raceways along the outermost range of the tool since they had gradually diminished chord passes the depth up to delete them at several points. This fact caused a worse performance of the abrasive and less driving “cranky” that the work, “kneading dough” much earlier and forced to clean glass and increasingly tool. Reopened the channels all came back to go smoothly :good:

    #10284

    Bartolomei Mirco
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    Hello Stephen… :bye:
    Congratulations on the progress… :good:
    We see that you got your hand and you're very sure when handling the tool…I see in this video… :heart:
    Seguiró with interest to future developments…Good luck with your work…Hello

    #10286
    Stefanosky
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    I'm using the passes 1/3D but I doubt there is a… I read that some depart with the chord with an offset side of 40% until 25% of the arrow, then the 30% until 50% of the arrow, then the 20% offset alignment until 75% Once you have reached the desired arrow and arrow start with 1/3D coc. I, like you, I kept the percentage of deviation always around 40% until the arrow, only after I started with 1/3D. Now I find myself clearly with an edge of about 15 mm unworked which should gradually thin with 1/3D. What are the differences between these two techniques? Someone has tried both?
    I take this opportunity to expose you also another doubt… As you can see from the picture the two areas highlighted from the beginning are not practically never been touched unlike the other border areas that, Although minimally, were affected by processing just enough to become Matt. Sure a homogeneous glass working, in what may have caused this difference? Mirror not perfectly paved originally? Continuing to work it is something that will fall into place if? Trying the arrow by placing the bar on the parts marked this turns out to be less than about 1 Tenth than the same amount done on two other areas. Measure in fact 3.05 mm instead of 3,15 the other measurement.
    IMG_20170808_181652

    #10289

    GIM
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    Hello Stephen.
    What you get with a chordal passed as what looks from the video explores very quickly the Center. Unfortunately that kind of passed does not provide a good ball. If you own a spherometer measure the radius of curvature in the Center and near the edges. You will notice immediately that the RADIUS measured is shorter than at the edges. Move from the periphery toward the Center as they reach the desired arrow would have a smoother ball right away. In my opinion anyway to bring everything back in track, simply continue to "scratch" with canonical passes by 1/3 COC reminding you to swap position often mirror tool so as not to exceed the desired arrow. Great deal., also according to the way I see it, move to finer grits before the spherometer indicates a good regularity of surface, allungheresti only the processing times. This should allow you even to eliminate the irregularities at the edges.
    Greetings to all

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