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  1. #1
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    Default Finest finish achievable flat surface

    Hey guys.

    Started the surface grinding portion of my apprenticeship about 6 months ago. I quite enjoy it and my work has been good.

    I haven't asked the guys in the shop yet, but I was wondering how fine can you truly get a finish on a surface grinder? (6x18 Okamoto auto feed w/ coolant)

    I have a test indicator base/surface gauge that's been pretty beat up. I need to either have the bottom surface lapped flat, which I'd have to send out, or I can try grinding it. I've already flattened the bottom of the base with a 46 grit wheel and the finish is quite good but not as good as some lapped surfaces look.

    Ultimately, now that I have it flattened, how fine a finish can I get on this thing while maintaining flatness? I've never used anything higher than a 60 grit for general surface finishing, only 80+ when trying to hold a corner. If I used an 80, 100 or even higher grit wheel - very light passes, maximum spark out - is a mirror-like finish possible? If not, how close can I get? The surface is about 3x4. Will there simply be too much heat if I try to grind with one of these wheels? Will it still cut okay if I take small cuts, say. 00005-.0002 per pass? Moderate to slow continuous crossfeed I would assume.

    Was just wondering if anyone had any opinions/advice. Haven't tried grinding any surface > 1x1 with a fine wheel. Hope to hear what you fellas think!

    Thanks.

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    I don't thik you will get a mirror finish with a grinding wheel, but I have not been a "grinder hand' in quite a few years. Will probably need to *lap*.... You can do this with varying grits of wet-dry paper to a certain degree. Preferrablt, IMO, you want the center to be slightly concave by a thou, or a few tenths at least only giving contact to the outer corners, but that is pushing it. I think dusting with an 80 grit wheel, then *lapping* with some 600-1000 grit paper, then diamond paste (if you want to go that far) will be more than sufficient.

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    Trying not to start a religious war, but lapping with sandpaper will almost always give a convex shape to the part (more removed from the outer areas) due to compliance of the paper and inevitable rocking induced by hand movement.

    If your surface plate is granite then a very smooth surface is OK, but certainly if ground cast iron having a scraped bottom is probably preferable to prevent slip/stick. If the CI plate is scraped, then you can have a lapped or ground smooth bottom on your base.

    Frankly, if the base moves smoothly as-ground and doesn't rock, I'd leave it be.
    Last edited by Milland; 05-14-2021 at 08:39 AM.

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    Ultimately, now that I have it flattened,
    How do you know it is flat?
    How flat?

    how fine a finish can I get on this thing while maintaining flatness?
    I thought "fine finish" (mirror?) was antithetical to the surface of gages that are used on surface plates?

    Or, consider, you don't want a fine finish on a surface grinder chuck.

    Scrape it differentially if you want the ideal. Shouldn't take more than an hour or so if the surface is that small and already flat.

    Alternately, make yourself a lap by grinding a chunk of CI about the same size as the base or a little larger.
    "Lap" it on a piece of sandpaper which will make it convex. Clean it. Charge it with diamond grit well rolled in. Wash/clean it.
    Use that with a little olive oil to lap your gage base until it is faintly concave in the center. Keep checking with faint blue, or evaporated alcohol (watch the temperature gradient)

    smt

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    Quote Originally Posted by stephen thomas View Post
    How do you know it is flat?
    How flat?




    I thought "fine finish" (mirror?) was antithetical to the surface of gages that are used on surface plates?

    Or, consider, you don't want a fine finish on a surface grinder chuck.

    Scrape it differentially if you want the ideal. Shouldn't take more than an hour or so if the surface is that small and already flat.

    Alternately, make yourself a lap by grinding a chunk of CI about the same size as the base or a little larger.
    "Lap" it on a piece of sandpaper which will make it convex. Clean it. Charge it with diamond grit well rolled in. Wash/clean it.
    Use that with a little olive oil to lap your gage base until it is faintly concave in the center. Keep checking with faint blue, or evaporated alcohol (watch the temperature gradient)

    smt
    I ran it under my .0001 test indicator. I get less than .0001 variance across the whole surface, maybe .00004-.00005 total.

    I suppose I didn't stop to consider if this was necessary/desirable. In any case I may well give lapping on sandpaper a shot.

    Out of curiosity, have any of you guys actually ground > 3x3 surfaces with a fine grit wheel? 80 or higher say? Is the surface finish significantly better than that from a 46 grit? A 46 grit wheel can produce a damn good finish if dressed & run properly so I'm just wondering if the same techniques were applied with a finer grit wheel if the quality of the finish would be markedly better.

    Thanks for the suggestions guys!

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    In any case I may well give lapping on sandpaper a shot
    No.
    As several of us have noted, that will make it convex. Certainly at the edges.

    My aprox 1/2 tongue in cheek suggestion was lap your lap on sandpaper until it is convex (before charging it.)
    Then charge the lap with diamond, roll it in good, and wash and clean the lap. Now you have a convex lap, that can be used to lap your gage flat or slightly concave; if you pay attention and spot it to a surface plate for reference. Hi-spot blue is too thick, but the haze from wiping it on and rubbing it off, then smearing it with your fingers and EtOH until the EtOH evaporates can give useful indication.
    Same as final scraping. I also noted that when going for levels of flatness like that, you have to pay strict attention to heat, and alcohol evaporating is a cooling agent.

    Not sure why you want to do this?
    Or are you just trying to make something shiny?

    For gage flatness, scraping would be the benchmark.

    smt

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    I have ground up to 220, and yeah it produces a very fine finish but you absolutely can not remove much material between wheel dressings. It also will burn the crap out of your part if you try to push depth of cut at all. Basically in steel or iron it's only good for tickling a finish out at the last couple tenths or three depth. I agree with Stephen that you'd be better off scraping at least for flatness. Then lightly lap to a finer finish if you like.

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    In terms of "shiny", I know there was a specific wheel that Levin/Tsugami used to test their OD grinder way back when. I think they were rubber bonded and gave a mirror finish. They probably would have worked on a SG, but the problem today is your wheel choices are pretty limited. Even if it's in a catalog, you might not be able to actually buy it. You also have to consider material. Hardened tool steel grinds way better than low carbon stuff. Don't be fooled by the expensive blue seeded wheels either. They don't work well on small grinders and don't give as good a finish as the right traditional aluminum oxide wheels. As a practical matter, you should probably limit yourself to maybe 60 grit at most. Finer grits are more for holding corners than improving the finish. 46 is more practical. Balance it, give it an open dress, avoid burning and you'll be far ahead.

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    Emo expo, machine tools, germany, where I was in 2012, with Haas.

    They had a grinder that ground flat steel ..
    to a perfect mirror finish, better than a bathroom mirror, and possibly close to an optical finish.

    The samples were kept under glass, so we could not touch them.

    I don´t know if You can do that, but grinding will definitely produce a mirror finish.
    That can even be flat and straight, if your machine and technique allows.

    Typical modern T&C cutter grinders, 5 axis, will likely make mirror finishes on anything, given the right tooling.
    They cost 500k, so that might be a problem.

    My clients used them to grind stamping dies, in carbide, to make tuna fish cans.

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    check out what mr. Renzetti can do with his "diy" Harig grinder Robin Renzetti • Instagram

    scroll back in his IG feed to see what it took to get there, the results speak for themselves...

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    No doubt this is expensive, and made for OD grinding, but the claims are impressive-
    Mirror-finish CBN Wheel | NORITAKE CO.,LIMITED

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    The Struers Abraplan machine really puts the shine on metal, prepares samples for the electron microscope. A common item over at the nuke site.

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    Quote Originally Posted by Cphockey16 View Post
    Hey guys.

    Started the surface grinding portion of my apprenticeship about 6 months ago. I quite enjoy it and my work has been good.

    I haven't asked the guys in the shop yet, but I was wondering how fine can you truly get a finish on a surface grinder? (6x18 Okamoto auto feed w/ coolant)

    I have a test indicator base/surface gauge that's been pretty beat up. I need to either have the bottom surface lapped flat, which I'd have to send out, or I can try grinding it. I've already flattened the bottom of the base with a 46 grit wheel and the finish is quite good but not as good as some lapped surfaces look.

    Ultimately, now that I have it flattened, how fine a finish can I get on this thing while maintaining flatness? I've never used anything higher than a 60 grit for general surface finishing, only 80+ when trying to hold a corner. If I used an 80, 100 or even higher grit wheel - very light passes, maximum spark out - is a mirror-like finish possible? If not, how close can I get? The surface is about 3x4. Will there simply be too much heat if I try to grind with one of these wheels? Will it still cut okay if I take small cuts, say. 00005-.0002 per pass? Moderate to slow continuous crossfeed I would assume.

    Was just wondering if anyone had any opinions/advice. Haven't tried grinding any surface > 1x1 with a fine wheel. Hope to hear what you fellas think!

    Thanks.
    You need a rubber wheel of desired grit. It'll produce a perfect mirror finish (limited by your machine). Alternatively, you can hand lap with a lead lap and adjust the stroke length to end up with a flat surface. For that you'll have to compare with an optical flat or run a test using a special spherical mirror. Both would bring you into 1/4 of a tenth flatness if you watch temps. With a mint condition spindle a good milling machine or better, a jig borer will fly cut to mirror finish if you sharpen the tool properly. Which is not trivial.

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    For a super fine (mirror-like) finish a rubber bond wheel is what I have heard should be used also. Robin has a pretty fine finish there but as can be seen in the video that's not a mirror when viewed dead on, only at a high angle of incidence - and almost anything with a decent finish will reflect like a mirror that way.

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    Mirror finish does not mean flat by any stretch.
    I can make bowed mirror finishes all day long on Blanchards and Sgs and is problem in my world.

    In fact the very shiny may be and often is less flat.
    Do not get sucked into "pretty" or nice looking for flatness or true to form.

    If shiny one thing I like to do is look at the reflection of the overhead tube lights and rotate the part. Are the tubes staying straight all across the mirror you just made or do they bend?
    Bob

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    I was going to mention that as well. Robin's finish for example is not a mirror when viewed dead on, but his optical flat check shows that it's pretty damned flat, though still not perfect.

    A rubberized wheel might tend to roll off the edges a bit of not used carefully, would have to try it out and see.

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    Swap the grind wheel for a cotton disc, buff with jeweler’s red.

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    Adding a rotary grinding table to a surface grinder can get you to 0.0001

    Compact Rotary Grinding Table - Roto-Tech Inc.
    Last edited by WesY; 06-16-2021 at 08:45 PM.

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    Spam Spam Spam Spam. Reported.

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    Quote Originally Posted by Mechanola View Post
    Swap the grind wheel for a cotton disc, buff with jeweler’s red.
    no need to swap. put the rouge (or similar) on the wheel.

    question: how flat is a regular mirror?


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