Tolerancing and capability

Results 1 to 7 of 7

Thread: Tolerancing and capability

  1. #1
    Member
    Join Date
    Jun 2010
    Location
    uk
    Posts
    23
    Downloads
    1
    Uploads
    0

    Default Tolerancing and capability

    Hi All,

    When faced with a diameter tolerance of h9 do you machine towards top limit ie nominal size or + 0.000 or do you aim for a capable process and aim for mid limit?

    I just want to know what peoples views are as my own seem to be at odds with many of our suppliers!

    Thanks for your views

    Similar Threads:


  2. #2
    Member deadlykitten's Avatar
    Join Date
    Jun 2015
    Location
    Antarctica
    Posts
    4131
    Downloads
    0
    Uploads
    0

    Default Re: Tolerancing and capability

    hy depends, for example :

    1) if i have to setup an OD part for mass production, i would calibrate the machine towards the minimal diameter, so not to lose stability when insert wear appears
    2) if i should deliver an asamble, i would try to find out what it is for, and i would decide about how to create the play : towards minimal, towards maximal, etc
    3) experimented persons that craft tools knows that h7/H7 may mean 0-0, so they craft the shank somewhere between the lower 1/3-1/2, in order to ensure a minimal play
    4) there are cases when you have to shift the dimension, for example, instead of 70h9, is better to consider 70.01h9

    so, it depends, but in the end, is all about tolerances and shape deviations

    suppliers are not always also manufacturers, they only buy and sell, and may have no clue about what'so-ever ( or how should i spell it ? )

    if i may, can you be more specific ? kindly

    we are merely at the start of " Internet of Things / Industrial Revolution 4.0 " era : a mix of AI, plastics, human estrangement, powerful non-state actors ...


  3. #3
    Member
    Join Date
    Jun 2010
    Location
    uk
    Posts
    23
    Downloads
    1
    Uploads
    0

    Default Re: Tolerancing and capability

    Hi,

    I was trying not to be specific as my intention was to get an overview of what people generaly do when faced with such a tolerance, if I specify a part to h9 I would like to know how most people interpret that?

    Best regards



  4. #4
    Member peteeng's Avatar
    Join Date
    Jul 2018
    Location
    dum dum
    Posts
    6248
    Downloads
    0
    Uploads
    0

    Default Re: Tolerancing and capability

    Hi Brit - The hole and shaft system tolerance is a statistical system. Therefore you aim at the middle of the range so your process variables keep the part to the spec limits (assuming the process is capable of doing that) . If the result is within the range then it is satisfactory from a machinists point of view. It is up to the designer or specifier to consider the functional intent and decide if the part if at top size or bottom size via the tolerance, that it will function correctly.

    If then by in-service experience the process wonders or the part in service needs to be bigger or smaller then the tolerance (or process) needs to be reviewed. Cheers Peter S

    eg a 100mm dia shaft with a h9 tolerance is 100mm -0.000 to -0.0087 so middle is -0.004mm (99.996mm so your machine has to be able to hold say +/-0.003mm if it can do that, then your aim is to machine at 99.996 and the process will go 99.999 to 99.993 which is always within the h9....

    If your making one of, you just mic it and if its in the range your cool...

    Attached Thumbnails Attached Thumbnails Tolerancing and capability-selected-fits-shaft-basis-jpg  
    Last edited by peteeng; 04-07-2020 at 04:42 AM.


  5. #5
    Member deadlykitten's Avatar
    Join Date
    Jun 2015
    Location
    Antarctica
    Posts
    4131
    Downloads
    0
    Uploads
    0

    Default Re: Tolerancing and capability

    I was trying not to be specific as my intention was to get an overview
    hy brit ... the decision to go to top, middle, bottom, or derogate, are all based on specifics also, in some cases, you have to consider not only the tolerance, but also the shape deviation and the control method ( for example a snake has a constant diameter, but is not coaxial )

    If the result is within the range then it is satisfactory from a machinists point of view.
    hy peteeng reading what you posted, i remembered talking to a guy that was used to work with a machine with 4 levels of corections, like offset, wear1, wear2, and wear 3 ... he was operating with at least 3 levels, and one level was the tolerance indeed : for example, if he should deliver +0.3..+0.5, then all corections inside last level were only between 0.3 and 0.5

    it is a complex approach, definetly beyond common behaviour, that makes it easy to corelate the real dimension with the tool corection, and also to inspect periodically the cnc for calibration loss / kindly

    we are merely at the start of " Internet of Things / Industrial Revolution 4.0 " era : a mix of AI, plastics, human estrangement, powerful non-state actors ...


  6. #6
    Member
    Join Date
    Jun 2010
    Location
    uk
    Posts
    23
    Downloads
    1
    Uploads
    0

    Default Re: Tolerancing and capability

    Quote Originally Posted by deadlykitten View Post
    hy brit ... the decision to go to top, middle, bottom, or derogate, are all based on specifics also, in some cases, you have to consider not only the tolerance, but also the shape deviation and the control method ( for example a snake has a constant diameter, but is not coaxial )



    hy peteeng reading what you posted, i remembered talking to a guy that was used to work with a machine with 4 levels of corections, like offset, wear1, wear2, and wear 3 ... he was operating with at least 3 levels, and one level was the tolerance indeed : for example, if he should deliver +0.3..+0.5, then all corections inside last level were only between 0.3 and 0.5

    it is a complex approach, definetly beyond common behaviour, that makes it easy to corelate the real dimension with the tool corection, and also to inspect periodically the cnc for calibration loss / kindly
    Hi Deadlykitten,

    My whole reason for asking this question was to ascertain peoples views on where in the tolerance band to machine to given the absence of any other specifics!

    Best regards



  7. #7
    Member deadlykitten's Avatar
    Join Date
    Jun 2015
    Location
    Antarctica
    Posts
    4131
    Downloads
    0
    Uploads
    0

    Default Re: Tolerancing and capability

    hy brit, yes, i got it i only think of too many variants ...

    i just read your last reply, and i remembered that i use a trick to achieve a stable setup, thus to maintain a consistent part dimension, when required tolerance is close to machine precision

    trick is that i use udp ( uni directional aproach ) and in position width parameters : for example, for an od part, i don't use this
    G00 home position & turet index
    G00 X100
    G01 cut F_mm/o


    but this
    G00 home position & turet index
    G00 X100-10
    G01 X100 F_mm/min + codes for increased positioning accuracy
    G01 cut F_mm/o


    this approach delivers a more consistent turret positioning, and also keeps the backlash in oposite direction, in respect to cutting forces

    i have codes for grinding, initial touching, and dressing, for cnc lathes

    i hope this is usefull; just a piece of my thoughts / kindly

    100mm dia shaft with a h9 tolerance is 100mm -0.000 to -0.0087
    petteng, h9 means -87um, thus -0.087; for 0.0087 you would need a 0.0001 control accuracy; try an okuma machine with 0.1um cnc control and abso scales

    we are merely at the start of " Internet of Things / Industrial Revolution 4.0 " era : a mix of AI, plastics, human estrangement, powerful non-state actors ...


  8. #8
    Member peteeng's Avatar
    Join Date
    Jul 2018
    Location
    dum dum
    Posts
    6248
    Downloads
    0
    Uploads
    0

    Default Re: Tolerancing and capability

    Hi DK - Yes you are correct my typo, typing too fast......... Peter



  9. #9
    Member
    Join Date
    Jun 2010
    Location
    uk
    Posts
    23
    Downloads
    1
    Uploads
    0

    Default Re: Tolerancing and capability

    Hi to all sorry not to have replied sooner.
    Thankyou for your thoughts.

    I am with peteeng on this in as much as you should always strive for mid limit and it is up to the designer to decide what that limit should be, we are base in the UK and coincidentally often order Ø100.000mm h9 material however our two main suppliers both choose to manufacture towards top limit which struck me as odd so I was just trying to to get some other views on the subject to see if I was correct in thinking that you should always aim for mid limit in order to achieve better process capability.

    It would have been nice to have a few more replies but there we are!

    Thank's



  10. #10
    Member deadlykitten's Avatar
    Join Date
    Jun 2015
    Location
    Antarctica
    Posts
    4131
    Downloads
    0
    Uploads
    0

    Default Re: Tolerancing and capability

    coincidentally often order Ø100.000mm h9 material however our two main suppliers both choose to manufacture towards top limit
    hy brit, i was thinking that your original post was about tool shanks, but it seems it was about materials

    calibrated materials may be used for turning as long as od tir, after clamping it, is lower than the tir allowed between od & id ( there may be also other conditions )

    when i replied about shape deviations, or snakes, was a reference to the fact that (at least) calibrated materials are not always straight, but if it work, it works

    i once needed material with "minus", so i ask the suplier to send only "minus" ... in the end, is all about how strong is the conection with the suplier, and the method use to craft those calibrated materials : it may be extruding, it may be under pressure casting in looong molds, etc, and there are variations along the day ( chance for smaller values in the morning, on mondays, at least until molds heat up )

    if I was correct in thinking that you should always aim for mid limit in order to achieve better process capability.
    if you are working at tolerance H, and sudenly a client requires you to deliver at H/2, in that moment, quality control and a few production costs will increase a looooot! this is why tools with shank delivered at h7/3 are more expensive then regular h7 tools : such shops are always targeting h7/3, and when a tool is not within that range, is sold at a lower price

    calibrated material does not enter in this area, simply because there are few requests for tolerance changing ... for example, you say that "I was correct in thinking that you should always aim for mid limit in order to achieve better process capability." and this is not specific, there is no reason, but more it seems to be general subjective ... about tools, a toolholder producers which sells worldwide, said that shrink-fit does not work well with h7 shanks, because grip decreases once tool shank diameter is towards minus, and his clients are in the upper range of shops ( thus well organized shops ), so imediatly tool producers started to update their quality control; this hapened because there was a reason, and also enough demands, in order to create a superior quality class ( thus there was a demand on the market, while your case is just a spot on the radar )

    a tolerance class is changed based on a reason, on a specific demand, and that may be satisfied by going " - / + / mid / or out of range ", thus there are 4 classes .. "mid" may not be the most frequent class; less frequent class is "out of range"

    costs for changing quality class for calibrated materials are greater than costs for changing quality class for tool shanks; even if you are right, this does not mean that a production sector should change their product ... if you need an increase amount of calibrated material, ask your suplier to quote it

    Hi DK - Yes you are correct my typo, typing too fast......... Peter
    off course, you were typing to fast

    eg a 100mm dia shaft with a h9 tolerance is 100mm -0.000 to -0.0087[here] so middle is -0.004[here]mm (99.996[here]mm so your machine has to be able to hold say +/-0.003[here]mm if it can do that, then your aim is to machine at 99.996[here] and the process will go 99.999[here] to 99.993[here] which is always within the h9....
    a grinder, which spents most of it's time with tolerances, would know these even during sleeping

    duryng typing, you had at least 7 ocazions to see that tolerances are not right ... i guess you were thinking of cars / wine / rock-n-roll kindly

    we are merely at the start of " Internet of Things / Industrial Revolution 4.0 " era : a mix of AI, plastics, human estrangement, powerful non-state actors ...


  11. #11
    Member
    Join Date
    Jun 2010
    Location
    uk
    Posts
    23
    Downloads
    1
    Uploads
    0

    Default Re: Tolerancing and capability

    Hi DK - just wondered in what part of the world is h7/3 a thing as I have never heard of it in the uk?

    Also I think your remarks to peteeng were a bit unkind given that it probably was a typo!

    Best regards



  12. #12
    Member deadlykitten's Avatar
    Join Date
    Jun 2015
    Location
    Antarctica
    Posts
    4131
    Downloads
    0
    Uploads
    0

    Default Re: Tolerancing and capability

    in what part of the world is h7/3 a thing as I have never heard of it
    hy brit, there are requests for h7_arbors and H7_bores, as parts of an assamble; an experienced person will imediatly understand that such an assamble may not work

    i will share 3 examples :
    ... id toolholders for lathes are described within H7 tolerance, while sleeves/bushings are h7
    ... hydraulic toolholders for mills are described within H7 tolerance, while sleeve/bushings are h7
    ... toolshanks are described as h7, while er collets are described as H7
    ... many tools are described as h7, but this does not mean that they will deliver an H7 bore
    * h7/H7 is the most common play, but it may mean 0-0 ... because of this, the trick to make a h7/H7 work, is by crafting at least one dimension away from 0

    in most cases, this is done by using an "epsilon"<>0 and a "modified" h7/H7, but this trick is not be shared inside a toolholder catalogue

    for example, if you buy a h7 sleeve that does not fit inside your toolholder, is just a customer choice, and not a bad product ... the vendor will replace that product, by charging the shopping costs to the client, and he may not accept unsealed products ... so if you buy it, and you open it, and it does not fit, you can not blame anyone

    but, if you buy 10 toolholders, and 10 sleeves, that may be combined as you wish and will work, then you may not be aware about the fact that those were crafted using an "
    epsilon"

    the "epsilon" designed to assure that a h7/H7play will always work, is specified inside internal quality control documents, but is not know by all employs : for example, is possible to use a less qualified operator on a machine with close loop corection, that always targets the upper/lower zone of a h7/H7

    if an
    assembly does not work, maybe someone will explain what happened, while someone else will avoid explaining the situation, but may decide to steal your client

    this "epsilon" is known as a minimal guaranteed play, so to ensure that, for example, a h7/H7 will never hit into the 0-0 case

    factory documentation

    think your remarks to peteeng were a bit unkind given that it probably was a typo
    i dont know, maybe he was thinking of something else, that's why i talked about rock'n'roll ... but if i was to decide to hire a grinder, that just missplaced a h7 tolerance with a .0, i would ask him if he is 100% sure ... imagine that you are working as a grinder from a life time, and your new teammate earns as much as you, but he does not give importance to an extra .0; i have seen such cases / kindly




    we are merely at the start of " Internet of Things / Industrial Revolution 4.0 " era : a mix of AI, plastics, human estrangement, powerful non-state actors ...


Tags for this Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  


About CNCzone.com

    We are the largest and most active discussion forum for manufacturing industry. The site is 100% free to join and use, so join today!

Follow us on


Our Brands

Tolerancing and capability

Tolerancing and capability