How high is a "high spot"? vague...
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    Default How high is a "high spot"? vague...

    Hi all,
    I've run into a dilemma at work. I run a CNC cylindrical grinder, I grind OD's and ID's. Most ID's I grind are what we call "cones", they vary in size, but are about 10 degrees included angle for the most part with a profile tolerance of .0002" per side. We are currently required to use what they call a "smear gage" to test the form of the cone, and must have 75% contact on the gage to the ground standard. I feel this is fairly subjective to who's using it, how much dye is put on, and on top of ALL that, it does not give you a quantitative number for form, which they are apparently ok with... for now...
    I'm raising a stink about it because although I have zero experience in scraping or using hi spot dye to check scraped surfaces, my impression from reading about the scraping process, is the high spot dye could likely not show up over .0002" profile tolerance per side. What are your thoughts on this? Thanks

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    It really depends on how think the high spot is applied. I use a ink roller and if I want it really thin I might wipe most off with a clean rag. If done in this way it is possibly less than .0002 thick, but some more knowledgeable in this subject might say.

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    Off your topic but it is important to consider that if the customer has agreed to this method of dimensional confirmation, YOU can only make a problem for everyone by exposing details or weakness.

    Remember, regardless of the print dimensions, the part must work! Form, fit, and function. If the smear test is the agreed on procedure, that puts all the parts that pass the test into the books as good and billable. If the parts don't work, it is the customer who will lead any change of the confirmation procedure.



    Any idiot can put a number on a piece of paper. Even a very small number like .0002

    OT I know, but such are lessons learned.

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    I would also use a small hard foam roller and I use Canode Inkyou can buy them at a hardware store and are used for painting. They are white and cost about $3.00. I would also put inside a gallon size baggy when using it to keep it clean. Most shops I go to use Dykem High Spot. If you use it be careful as it will stain your fingers. The Canode won't. The rule of thumb is to just roll on enough to get a super thin layer and can see thru it. I tried to measure the ink film once and it's rough. I would guess its .00001 thick. I would suggest using a piece of Plex-i-Glass and squirt in small spots and spread it with the roller evenly on the glass before spreading it in the standard.

    I am assuming the customer supplied the standard. Did they send a long a instruction sheet how they want it done? Have to be care full when inserting it in the hole and once seated rotate the standard, then pull it out and look on both the standard and the hole. The ink will wipe clean or good contact will transfer the ink and wipe off the ink on the standard. So shinny or same color as standard is with no ink is highest and if the ink stays on there and turns a grey or almost black, that area is slightly lower then the shinny wiped off ink. IT will take a few tries to figure it out. Be sure to wipe everythig with your fingers and not gloved fingers and not a rag as you need to feel for the dirt and a clean hand of fingers will feel. A rag or glove will leave lint or transfer dirt. After you get the prints. I would take a photo and document the part with a number written on a paper laying to the part. Good luck. Rich

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    Quote Originally Posted by pianoman8t8 View Post
    Hi all,
    I've run into a dilemma at work. I run a CNC cylindrical grinder, I grind OD's and ID's. Most ID's I grind are what we call "cones", they vary in size, but are about 10 degrees included angle for the most part with a profile tolerance of .0002" per side. We are currently required to use what they call a "smear gage" to test the form of the cone, and must have 75% contact on the gage to the ground standard. I feel this is fairly subjective to who's using it, how much dye is put on, and on top of ALL that, it does not give you a quantitative number for form, which they are apparently ok with... for now...
    I'm raising a stink about it because although I have zero experience in scraping or using hi spot dye to check scraped surfaces, my impression from reading about the scraping process, is the high spot dye could likely not show up over .0002" profile tolerance per side. What are your thoughts on this? Thanks
    .
    have you tried a .0001" indicator and tried to measure it. test indicators can come with points that have a fairly small ball on the end. i often test surfaces in the machine to see is a flat surface is wavy or a round bore is not round. usually a indicator will show .00005" or 1/2 a "tenth"........ that way you are not guessing. you got a actual number. by the way when you got a .0001" indicator setup up give it time or at least 10 seconds, you might see your machine servos moving the slide axises back and forth .0002" it is not unusually on many machines

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    The high spot blue seems to feather out into a haze that can't even be a tenth thick, although I have no clue how to measure it, I don't have the equipment. I'd think it has the capability to show 2 tenths, but possibly significantly below that the "alcohol haze" might be needed.

    The best I have checked is exploring the surface with a tenths indicator, and my judgement is that Dykem Hi-Spot blue easily shows 2 tenths.

    The canode always seems thicker, and doesn't seem to ever become a "haze"..... Maybe with the windex spray method it will.... But it doesn't turn me into a Smurf, so I prefer it for everything but the last bit on something very tight.

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    I just took Richard's scraping class and I have a lifetime of practice left for that skill ahead. I just came back to this thread and realize I have access to a machine that will read a surface finish with nanometer precision. It won't be in the next month but I'll work on getting some samples together and testing them for fun.

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    The theoretical capability is some small mulitple of what ever the pigment size is.
    I think Forrest once explained it down in the few millionths. I'm personally confident that hi-spot blue is well under a tenth.
    (<.0001")

    But yeah, you can slather it on and make perhaps as big as .0005 or maybe even .001 look ok.

    smt

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    Here's some examples from scraping a straight edge.

    When you are just starting out, it is common to put a lot of dye on the surface plate (which transfers to the straigth edge shown, being scraped) both to protect the marking gage from the rough condition of the work, and because you need to knock of as much material in a pass as possible to make good time. Heavy marks will facilitate this. I can knock off .001 in 2 passes before re-marking at this stage. Though on the tools shown would probably be a little more conservative so as not to dig a hole.



    Now at this point you can see in my ambition I did sightly excavate a hole but the main idea is to not pivot on the center. Now we can settle down with dye about as thick as shown, and work off a tenth or 2 at a pass.



    Here you probably could not measure the change in a couple passes with a tenths indicator.



    Here the high spots are the twinkly dots shining through evaporated dye. The whole surface will barely and mostly not tickle a .0001 indicator.



    So you are correct. It is relatively easy to manipulate the results between .001 or .0005 and "millionths" But as has wisely been pointed out, do you really want to stir the hornets nest? My approach would be to do "good work" and mark "conscientiously" to satisfy myself, and just live with the standard. If you retire or move on, it will become a shop legend how well you did when the new guy get sloppy and nothing fits or the parts fail.

    smt

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    Check this video out. It was made by Nick Müller in Germany and has got to be one of the best renderings of what varying amounts of contact look like when spotting blue is used.

    https://www.youtube.com/watch?v=y2AUew5Evxc

    Mauro


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