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    Default Accuracy question, scraping vs CNC

    This is purely theoretical question; just trying to learn new things / skills...

    How does accuracy achieved by hand methods like scraping compare to hand Lapping techniques and how do those two compare to accuracy achieved by modern CNC machining?


    I am considering to learn hand scraping and just trying to gauge the usefulness of this skill in terms of "metrology level accuracy".

    The project I am considering is making a gauge block; i will be testing it with 50 millionth gauge (i realize its not a real "metrology grade" block comparator, but that is what i have at the moment) vs grade B gauge block set. What is the practical difference that I should expect to see between lapping and scraping for the final finish? I have a variety of lapping compounds, used to be into knife sharpening big time...

    Other than in a low budget situation when there is no modern machining available for lapping / CNC machining; is there still a place in modern high dollar industry for hand scraping? I am thinking in terms of accuracy.

    Thanks ahead. Throw all info you have at me, trying to learn from "the old folks" what modern day text books won't teach ya...

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    What are you actually asking here? Your question seems so vague and stupid I suggest you head over to Home Sop Machinist and run it by the experts there.

    Your thread title is "scraping vs CNC". WTF? That makes no sense.

    Hopefully The Boss will delete this type of crap.

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    As an engineer working for aerospace industry I know that a CNC machine can finish parts and how to feed it the model... but what I do not know is if the "best attainable" finish from CNC is better or worse than a "properly scraped surface"... No one is scraping in out shop any more... I am interested in the type of stuff they don't teach you in today's engineering schools... Is scraping as a skill worth learning for someone who can just feed a model into a CNC? Has automatic machining matured to the point where it can completely replace a skill like scraping or hand lapping or are these skills still relevant in today's world, simply because even the best machines are not yet up to snuff?

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    Scraping is what allows your CNC machine to be so accurate. Unless it's a Haas...

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    You are talking about three different things. And you left out one.

    CNC..... as good as any machined surface.

    Scraping.... as flat as you want, essentially, but in a usual practical sense, very good for alignment and ways. NOT a gage block. Generally has a perceptible surface variation, but the tops of the variation are all in a plane to whatever you can measure and have time for.

    Lapping..... As good as you have time for and can measure.

    Grinding..... 10x as good as machining, at least. Can be done much better, but starts to turn into lapping. Way flatter than common scraping. Works on steel, scraping can be done, but IMO sucks to do on steel. This is what you left out.

    Not 100% perfectly true, and folks will quibble with good reason, but you can check into it for yourself.

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    Quote Originally Posted by Pete F View Post
    What are you actually asking here? Your question seems so vague and stupid I suggest you head over to Home Sop Machinist and run it by the experts there.

    Your thread title is "scraping vs CNC". WTF? That makes no sense.

    Hopefully The Boss will delete this type of crap.
    It's not a stupid question.
    He says it in the very obvious underlined section:
    How does accuracy achieved by hand methods like scraping compare to hand Lapping techniques and how do those two compare to accuracy achieved by modern CNC machining?

    He even used a question mark.

    Yes, there is still a place for scraping in the industry, but its mostly geared towards the machine building end of things, rather than part making side.

    Using three surfaces, and comparing and scraping them together you can get incredible flatness and straightness.
    To get something flat and parallel or square to another edge like you want with a gauge block will require more expensive testing.

    A lot of mating surfaces for the frames, and where spindles mount and such are still hand scraped.

    I have a feeling Cameraman could chime in on the accuracy of a scraped surface vs a machined/ground surface.
    He's done a lot of in depth comparison with different machine brands and might even know who grinds/scrapes and where they do it.

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    Quote Originally Posted by ManualEd View Post
    It's not a stupid question.
    He says it in the very obvious underlined section:
    How does accuracy achieved by hand methods like scraping compare to hand Lapping techniques and how do those two compare to accuracy achieved by modern CNC machining?

    He even used a question mark.
    Except you can not compare scraping and lapping as they are different processes to achieve different outcomes.

    Scraping the surface you end up with lots of tiny divots.

    Lapping is to create a smooth surface.

    scraping needs a master to compare it against.

    Lapping you can use electronic or light based equipment to determine your plane.

    Neither process is just for flatness. Lenses and mirrors are lapped and round plain bearings are scraped.

    The original question is as Pete said, very unclear and the OP does not know what he wants to ask.

    The question should be can a machine achieve the same outcome as hand work.

    I would say yes it can for most levels of work. Some grinder manufacturers in Japan claim to be able to grind large areas to the single digit micron level. Of course whether anyone could afford such a machine is a different thing altogether. You are not going to see them at Mike Kandu.

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    Thank you all for informative replies and kind words.
    I have been sharpening knives by hand on water stones for a long time; now that i got a belt sander I still do the knives by hand, simply because the belt wobble (as little as there is) is still less accurate than hand sharpening. This is kind of where my CNC vs Lapping vs hand scraping question originated from. I can see how a larger degree of control could be achieved by hand scraping than for a powered machine like CNC... I did not include grinding in my question, because its just a rougher version of lapping (although some might argue that true lapping requires a large flat that part is gliding on). I also did not include machining in my question, because it's basically covered by CNC, just that in CNC it's the computer that is controlling the drive/feed....

    I guess it will boil down to how little can a scraper remove in 1 cut, since that is what will ultimately determine the accuracy attainable by the scraper when the part is properly checked against true flat...

    Is the reason why scraping is geared towards machine building end of things, because it is cheap and restoring old machines is something people do not want to pay allot of money for, or are the machines being scraped today rather than polished or CNC cut, is because scraping is still more accurate?

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    Quote Originally Posted by pcm81 View Post
    This is purely theoretical question; just trying to learn new things / skills...

    How does accuracy achieved by hand methods like scraping compare to hand Lapping techniques and how do those two compare to accuracy achieved by modern CNC machining?


    I am considering to learn hand scraping and just trying to gauge the usefulness of this skill in terms of "metrology level accuracy".

    The project I am considering is making a gauge block; i will be testing it with 50 millionth gauge (i realize its not a real "metrology grade" block comparator, but that is what i have at the moment) vs grade B gauge block set. What is the practical difference that I should expect to see between lapping and scraping for the final finish? I have a variety of lapping compounds, used to be into knife sharpening big time...

    Other than in a low budget situation when there is no modern machining available for lapping / CNC machining; is there still a place in modern high dollar industry for hand scraping? I am thinking in terms of accuracy.

    Thanks ahead. Throw all info you have at me, trying to learn from "the old folks" what modern day text books won't teach ya...
    Making a gage block? Why?

    OTH There is a lot of info on scraping on this forum, there is also a lot of stuff on youtube. Fire up the google, it can be your friend.

    dee
    ;-D

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    Quote Originally Posted by RC99 View Post
    Except you can not compare scraping and lapping as they are different processes to achieve different outcomes.

    Scraping the surface you end up with lots of tiny divots.

    Lapping is to create a smooth surface.
    Not trying to be a smartass but this is kind of what i am interested in. If the amount of material removed in 1 pass by a scraper is small enouth, then it would seem that it can compare to effect of lapping. Although, then one can always take smaller micron grit ( i personally own down to 25 nano-meter mnocrystaline diamond spray) to remove metal that much slower.

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    Quote Originally Posted by dcsipo View Post
    Making a gage block? Why?

    OTH There is a lot of info on scraping on this forum, there is also a lot of stuff on youtube. Fire up the google, it can be your friend.

    dee
    ;-D
    To learn... A good gauge block has to be flat, parallel and accurate... many things to learn in 1 project...

    Been looking at youtube all day, good stuff on the technique for scraping, but no one is mentioning the accuracy that can be attained vs other methods.

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    Quote Originally Posted by pcm81 View Post
    To learn... A good gauge block has to be flat, parallel and accurate... many things to learn in 1 project...

    Been looking at youtube all day, good stuff on the technique for scraping, but no one is mentioning the accuracy that can be attained vs other methods.

    Why do you want to learn gauge block making skills? What projects do want to make which would require those skills?

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    Quote Originally Posted by ManualEd View Post
    It's not a stupid question.
    He says it in the very obvious underlined section:
    How does accuracy achieved by hand methods like scraping compare to hand Lapping techniques and how do those two compare to accuracy achieved by modern CNC machining?

    He even used a question mark.
    It IS a stupid question brainiac, and putting a question mark at the end of it doesn't change that fact.

    Firstly "CNC" doesn't mean shit, it's a method of controlling a machine and not a level of precision. A plasma cutter is CNC, how many gauge blocks do you reckon you're going to get off one of them Einstein?

    He then went on to say he wanted to make a gauge block. So WTF has that got to do with CNC?

    That this sort of crap actually comes from a professional engineer is just plain scary!

    Despite what you were told all the way through school, there IS such a thing as a stupid question. Case in point above.

    A gauge block is a precision LENGTH that just happens to be lapped so it can be wrung to another.

    A scraped surface is a scraped surface, just because something is scraped doesn't mean it's precise. I can go get my Biax and go to town on a hunk of CI and it won't mean squat.

    A lapped surface is a method of finishing, that will give a particular surface finish. It also doesn't mean it's accurate in any particular dimension.

    CNC is a method of controlling a machine, and it could be anything from a plasma cutter to a lapping machine. It's often used by punters (and apparently professional engineers working in the aerospace industry) to refer to a typical VMC. in which case it's a milling operation.

    All 4 of the above are completely unrelated to each other and to start mixing them up is stupid beyond words.

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    Quote Originally Posted by pcm81 View Post
    Is the reason why scraping is geared towards machine building end of things, because it is cheap and restoring old machines is something people do not want to pay allot of money for, or are the machines being scraped today rather than polished or CNC cut, is because scraping is still more accurate?
    OMG it just gets worse!

    I hope Milacron will delete this bullshit.

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    Quote Originally Posted by John Welden View Post
    Why do you want to learn gauge block making skills? What projects do want to make which would require those skills?
    I want to learn the skills like polishing and scraping to the point that I can make a gauge block that is accurate enough (flatness, parallelism) to be indistinguishable from my grade 00 mitutoyo using a 50 millionths federal depth gauge.

    Why?
    1. Because learning new skills is fun.
    2. Often I have to take part in scheduling and planing meetings where I have to guistimate how much time it would take to install part X or machine part Y in an aerospace industry (read accuracy matters). So, if I have the same skills as the mechanics and machinists that will ultimately be doing the job, I can better estimate it than to just pull a number out of my ass. I am not; and do not intend to become, one of those pencil pusher engineers that ultimately find their way into managerial roles because they suck at all things technical.

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    Quote Originally Posted by Pete F View Post
    OMG it just gets worse!

    I hope Milacron will delete this bullshit.
    Is there a good reason why you keep replying to this thread rather than just going away?

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    Quote Originally Posted by pcm81 View Post
    To learn... A good gauge block has to be flat, parallel and accurate... many things to learn in 1 project...

    Been looking at youtube all day, good stuff on the technique for scraping, but no one is mentioning the accuracy that can be attained vs other methods.
    In order to qualify a gage block, you will need tools that are way beyond your stated budget. You can get a set of blocks for far less. Make something that is useful, and less readily available. You want to scrape? Get a surface plate and a casting.
    You want to lap? Make some part for a machine that needs to be lapped. etc. Don't pick a project for a skill, pick a useful project and develop the skills needed for that project.

    dee
    ;-D

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    Quote Originally Posted by pcm81 View Post
    Is there a good reason why you keep replying to this thread rather than just going away?
    Yes because I don't want to see a credible forum contaminated with bullshit like this. At least you came up with one good question

    I find it difficult to believe even the most stupid of engineers would ask "questions" like you have, and not know the difference between polishing and lapping. Is it school holidays over there already?

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    The basic problem here is that the OP is using a gage block as an example, he does not want t MAKE one, he wants to know why one process is selected over another to make one.

    Someone said it above... and I did also. Scraping makes a surface full of small depressions. The TOPS of the spaces between depressions are all in one very flat surface. But the entire surface is not flat. It has small pits in it a few tenths of a thou deep. If magnified, it looks like a field where shallow short trenches have been dug all over in random directions, but then the tops of the dirt piles have all been smoothed off to the same level. The scale of the "trenches" is actually pretty big, on the order of 2 or 3 mm wide, and maybe the same long for very precise work.

    Lapping uses very fins abrasive. each grain of abrasive has cutting edges on it from being fractured (crushed) to that fine-ness. The result of using millions of these tiny grains to lap a surface would look like a plowed field if magnified enough, but the scale of the "furrows" is so tiny that it looks and acts like a polished flat surface that can be accurate to a couple millionths of an inch. Smaller than the wavelength of red light.

    For a gage block you want lapped. Depressions a couple mm wide are not wanted on a gage block. Lapping provides the surface finish, and is fairly easily done to finish the block to a very precise size. Scraping would actually be way too coarse to get into millionths of an inch

    For a machine way, you really want the scraped surface, because it carries oil in the scraping marks. A lapped surface would squeeze out the oil, and might even gall.

    Nowadays, the ways would be ground, but the sliding part, carriage or whatever, is often scraped, or at least "flaked", to provide oil pockets. It may also have an applied layer of a low friction plastic like Turcite, etc, which is scraped or flaked.

    Scraping is still around, and is used because you can get a VERY good result for machine ways with hand tools. All you need that is special is an appropriate reference surface, a surface plate, granite flat, so-called "straightedge", or the like. And some form of "marking compound" like Prussian blue, to show up the high spots.

    Way grinding requires a large very accurate grinder. Which is probably scraped to the precision required.....

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    Quote Originally Posted by Pete F View Post
    Yes because I don't want to see a credible forum contaminated with bullshit like this. At least you came up with one good question

    I find it difficult to believe even the most stupid of engineers would ask "questions" like you have, and not know the difference between polishing and lapping. Is it school holidays over there already?
    Hate to break it to you, but engineering school includes calculus, loads of differential equations, statics, dynamics, control system analysis and feedback loops, physics, chemistry, reliability analysis, thermodynamics, thermohydraulics, but no scraping...


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