The difference between machining accuracy and positioning accuracy
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    Default The difference between machining accuracy and positioning accuracy

    Dear all,

    Some of vertical milling center suppliers write the acuuracy in thier machin’s catalog as positioning accuracy while other write as machining accuracy!

    My inquiry is that is there any difference between machining accuracy and positioning accuracy? Or they are same?

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    This will be an interesting read. I will tell you this. We just purchased a Hermle, C42 5x machine. The utmost in positioning accuracy. When I check the kenematics with the ball over the entire 800mm range of travel, I get an error or around .003-.005 microns, very impressive to say the least. BUT, trying to do a simultanious cut and hold tight tolerances, that is another story. Machining accuracy consists of, programming software, toolpath tolerance, tool tolerance, holder accuracy, shop temperature, tool runout, the list goes on and on. Take a ball endmill with a .0004 tolerance, then add a .0002 toolpath tolerance, then add holder runout of .0001 or so, now the tool is hanging out a way and you have some deflection, the material is heat treated so add some more deflection, now warm up the spindle of the machine a while, now the shop heats up.. you are out .0005-.0006 before you even hit a button. IF I am very, very careful, I can hit a .0005 range, +-.00025. But that is taking everything into account. To just set up a tool, run it thru the laser and hit the button, maybe .0007 if your lucky. And this is with premium tolerance tooling.

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    We have 2 Okuma MU4000's that position "least increment of .00002",I believe. With all the defections and runout the 5 axis Fidia just wrote of, what's the point?

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    Quote Originally Posted by Med View Post
    Dear all,

    Some of vertical milling center suppliers write the acuuracy in thier machin’s catalog as positioning accuracy while other write as machining accuracy!

    My inquiry is that is there any difference between machining accuracy and positioning accuracy? Or they are same?
    In the case of Machine Tool advertisements I think they are the same thing.

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    Positioning accuracy may be what the machine is capable of but machining accuracy is as 5axis points out, Machining Accuracy is including all the variables in actual machining... much out of control of the machine builder and more in control of the skills of the machinist..IMHO

    But that is just advertising words so not as important as the machine's reputation.

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    Quote Originally Posted by 5 axis Fidia guy View Post
    This will be an interesting read. I will tell you this. We just purchased a Hermle, C42 5x machine. The utmost in positioning accuracy. When I check the kenematics with the ball over the entire 800mm range of travel, I get an error or around .003-.005 microns, very impressive to say the least. BUT, trying to do a simultanious cut and hold tight tolerances, that is another story. Machining accuracy consists of, programming software, toolpath tolerance, tool tolerance, holder accuracy, shop temperature, tool runout, the list goes on and on. Take a ball endmill with a .0004 tolerance, then add a .0002 toolpath tolerance, then add holder runout of .0001 or so, now the tool is hanging out a way and you have some deflection, the material is heat treated so add some more deflection, now warm up the spindle of the machine a while, now the shop heats up.. you are out .0005-.0006 before you even hit a button. IF I am very, very careful, I can hit a .0005 range, +-.00025. But that is taking everything into account. To just set up a tool, run it thru the laser and hit the button, maybe .0007 if your lucky. And this is with premium tolerance tooling.

    Is there an error in the bolded part above or did you really mean to say 3 thousandth of a MICRON?

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    Quote Originally Posted by SRT Mike View Post
    Is there an error in the bolded part above or did you really mean to say 3 thousandth of a MICRON?
    No that is indeed microns. The first time I run the program it will give an error of about .01-.012 microns, then it will ask if you want to correct this or continue as is. When you run it again it will correct it and I am always seeing less than .005 microns with both A and C axis.

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    Quote Originally Posted by michiganbuck View Post
    Positioning accuracy may be what the machine is capable of but machining accuracy is as 5axis points out, Machining Accuracy is including all the variables in actual machining... much out of control of the machine builder and more in control of the skills of the machinist..IMHO

    But that is just advertising words so not as important as the machine's reputation.
    What is it with you drunken old dribblers? Its not 1960 anymore. Fairly sure when he programs his HERMLE C42. It actually does it.
    Quote Originally Posted by 5 axis Fidia guy View Post
    No that is indeed microns. The first time I run the program it will give an error of about .01-.012 microns, then it will ask if you want to correct this or continue as is. When you run it again it will correct it and I am always seeing less than .005 microns with both A and C axis.
    You wouldn't be related to the George Molvitov the Cocktail Gmotov by any chance?

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    Quote Originally Posted by machtool View Post
    What is it with you drunken old dribblers? Its not 1960 anymore. Fairly sure when he programs his HERMLE C42. It actually does it.

    You wouldn't be related to the George Molvitov the Cocktail Gmotov by any chance?
    No idea who that is. But I will take a picture of it next time I do it.

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    Quote Originally Posted by machtool View Post
    What is it with you drunken old dribblers? Its not 1960 anymore. Fairly sure when he programs his HERMLE C42. It actually does it.

    You wouldn't be related to the George Molvitov the Cocktail Gmotov by any chance?
    "The name "Molotov cocktail" was coined by the Finns during the Winter War,[1] called in Finnish: Polttopullo or Molotovin koktaili. The name was an insulting reference to Soviet foreign minister Vyacheslav Molotov, who was one of the architects of the Molotov–Ribbentrop Pact signed in late August 1939. The pact with Nazi Germany was widely mocked by the Finns, as was much of the propaganda Molotov produced to accompany the pact, including his declaration on Soviet state radio that bombing missions over Finland were actually airborne humanitarian food deliveries for their starving neighbours. The Finns sarcastically dubbed the Soviet cluster bombs "Molotov bread baskets" in reference to Molotov's propaganda broadcasts.[2] When the hand-held bottle firebomb was developed to attack Soviet tanks, the Finns called it the "Molotov cocktail", as "a drink to go with the food".[3]".

    Molotov cocktail - Wikipedia

    __________________________________________________ __________________________________________________ ________

    ISO test parts, and German equivalents [VDN] "Indicate" the gap between stated "Positional accuracies" and what a machine can produce under ideal circumstances with minimal intervention. There are special tests and tests parts specific to 5 axis machines.

    Random side note, my impression of the quality of parts a Hermle can generate is not so much about positional accuracy but the ability to absorb and dampen vibration quite considerably with their "Mineral" based castings. Almost like half the machine is made from a synthetic granite... + everything really well tweaked and tuned in.

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    Quote Originally Posted by 5 axis Fidia guy View Post
    No that is indeed microns. The first time I run the program it will give an error of about .01-.012 microns, then it will ask if you want to correct this or continue as is. When you run it again it will correct it and I am always seeing less than .005 microns with both A and C axis.
    On an 800 mm trunion to 10 nanometer spatial accuracy... That is impossible.

    Maybe the numbers refer to a statistical iterative "Fit" where the residual errors for the adjustment resolve to 0.01 micron.

    As I would expect for a good mechanical trunion of the size of 800 mm to have volumetric errors of the order of 12 micron and linear and positional errors of the order of 3 to 5 micron.

    Would be interested to learn what the "errors" of the order of 10 nano-meters actually refer to.

    I wonder if the control has been annotated in an odd way to stress 'Metric" versus "Imperial/English" units?

    Interesting never the less.

    Ta.

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    I'm a little confused too. A micron is 0.001mm or 0.000039". So 0.005 microns is about 0.000000197". A fraction of a millionth of an inch.

    I'm pretty sure even a Hermle can't achieve that kind of accuracy, and I'm pretty sure you would not be able to measure it if it did. Hermle only claims 8 microns accuracy for the machine and its positioning measurement system resolves only to 0.1 micron.

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    Apples and oranges. One is how accurately the machine can follow a path repeatedly and the other is how accurately the machine can go to and hold a point repeatedly.

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    Even within machine positioning accuracy, there is wiggle room. The German VDE standard for example specifies bi-directional tests and the 3 sigma point, Japanese JIT is unidirectional and 2 sigma. Also repeatability is different than accuracy. So while machining accuracy is more difficult to achieve than positioning accuracy, machining repeatability within the positioning accuracy of the machine is not nearly as difficult to achieve. That's the difference between the first part - with many unknowns - and the 10th part with many unknowns now known.

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    Gotta love this stuff. I recall back in the late 80's a salesman struggling with trying to answer my question about the three dimensional volumetric positional accuracy of his machine vs the technical document's two dimensional accuracy statement.

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    Quote Originally Posted by cameraman View Post
    Quote Originally Posted by machtool View Post
    You wouldn't be related to the George Molvitov the Cocktail Gmotov by any chance?
    "The name "Molotov cocktail" ...
    I rather think he was asking about George "gmatov", a moderately explosive PM Forums personality.

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    Quote Originally Posted by sfriedberg View Post
    I rather think he was asking about George "gmatov", a moderately explosive PM Forums personality.
    Thanks for the translation... I thought there had to be more to it than that :-)

    Ta.

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    The Ballbar Trace from RENISHAW seems to be the most accurate tool to show the overall 5 axis machine accuracy. Machining accuracy will differ from that by cutting parameters.

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    Here is a photo of the kenemetics correction. The first numbers are the check, the second set of numbers is the corrected values. Hermle guarantees .008mm accuracy, they say many machines will be much better. What do these values mean, the probe checks a ball in space at theoretical values, these numbers are the error from that value. How does anyone know if this is truely the case? beats me, all I know is what the machine is capeable of, for those who doubt the numbers, feel free to bring it to the attention of Hermle and Heidenhain engeneers, I am sure they would love to hear from you.
    Attached Thumbnails Attached Thumbnails x.jpg  

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    Quote Originally Posted by 5 axis Fidia guy View Post
    Here is a photo of the kenemetics correction. The first numbers are the check, the second set of numbers is the corrected values. Hermle guarantees .008mm accuracy, they say many machines will be much better. What do these values mean, the probe checks a ball in space at theoretical values, these numbers are the error from that value. How does anyone know if this is truely the case? beats me, all I know is what the machine is capeable of, for those who doubt the numbers, feel free to bring it to the attention of Hermle and Heidenhain engeneers, I am sure they would love to hear from you.
    That's .0036mm

    3.6 Micron

    Not .0036 Micron

    .001mm = 1 Micron


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