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ISO FITS - Explained

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  • ISO FITS - Explained

    I found this in an old book I had from trade school. I put some translations on it. Hope it will help somebody to understand how the system works.
    One can find all tolerance fields on many tables - but it helps to understand just how everything fits together. Juergen.

    Sorry the print is not very good. The two systems are: STANDARD SHAFT - Fits are derived by changing the hole size, and STANDARD BORE - Fits are derived by changing the size of the shaft. Most use the standard bore system since it is cheaper to work with just one go-no go plug gage and measure the shaft with a micrometer or a Sheffield Gage to achieve various fits.
    PS.: The fit shown in the drawing is H on h and h o H.
    Last edited by juergenwt; 06-23-2008, 09:40 PM.

  • #2
    Juergen, I wish I could read it but it's just to fuzzy probably my monitor. Posted a question about class fits quite some time ago but I'm sure this would help. Any chance it could be posted in a different format that may help resolution?
    Cheers, Bob


    • #3
      It seems like you have scanned the page with too high jpeg compression.
      Try again, with no more than 70% compression.


      Problems worthy of attack prove their worth by hitting back
      - Piet Hein


      • #4
        Thanks - I will try to get a better picture. Juergen
        Last edited by juergenwt; 06-23-2008, 09:36 PM.


        • #5
          Well i can read it but is is not English so i have no idea what it says.
          Every Mans Work Is A Portrait of Him Self


          • #6
            Lane - Just look at the picture. I have put some translations on the drawing but the picture explains it all. This is very basic but I learned from it. Long time ago.


            • #7

              Thanks juergenwt.

              I will posts some pics later in the day - or tomorrow - to explain these metric matters in English.

              Pretty well all standards relating to limit and fits, tolerances are just about the same now.

              At least they are all in decimal - which is a good start.


              • #8
                Here is a more up to date diagram

                Precision takes time.


                • #9
                  Limits and fits

                  First of all, thanks to juergenwt for starting the thread and to Ringer for posting that very good page from OZ (I will use more of it in a later post in this thread).

                  This is in line with my commitment at post #8 to continue with this topic - in metric.

                  I have posted copies of two double pages from a pocket-sized book that I bought in OZ - it is tops.

                  Please do have a close look at the tables as even though it is metric the principle is universal - OZ, UK, US, Europe etc.

                  0.01mm ~ 0.0004" - say 0.01mm = "4 tenths" (almost "half a thou").

                  This pic/page has tables of useful tolerances and preferred fits that should be adequate for/in most shops for most applications.

                  The left side shows the "Useful Tolerances" (mm). It uses the "basic hole" approach (explained later).

                  The right side of the pic shows and defines the "preferred Fits" (which most of us are familiar with) and is in fact a representation of the "Class of Fits" for general shop work.

                  The next pic shows a graphic and tables to show the limits and tolerances defined in the previous pic.

                  It - like the previous pic - uses the "basic hole" approach where the lower size of the hole is the datum - ie the lower limit for the hole is zero (0.000mm) ie the lower limit for a say 30mm hole is 3.000mm ("spot on" the "nominal size"). It can in fact be converted to "Basic Shaft" by simply pushing up both the "hole" and "shaft" graphics so that the top of the "shaft" graphic just touches the "zero" point. In this case all shafts will have an upper limit of the "nominal size".

                  So lets see an example of how it all works and comes together in terms and ways we can use.

                  Let's take a "free running fit" for a 30mm nominal diameter shaft.

                  Pic 1 right side shows a Free running Fit (Shaft basis) is F8/h7 and for a Hole Basis is H9/e9

                  Using the "Hole Basis" go to pic 2 and the left side/panel shows a Free Running Fit as H9/e9 which on the line for "Nominal Size - Above 18 and up to & including 30" that the:
                  H9 tolerance for the 30.00mm hole is +0.052/0.000 - that is to say that the limits for the hole are 30.000 +0.052/0.000 = 30.052/30.000mm ie a difference of 0.052mm

                  and the:
                  e9 tolerance for the 30.00mm nominal shaft is -0.040/ -0.092mm - that is to say that the limits for the shaft are 30.000 - 0.040/ -0.092mm = 29.960/29.908mm ie a difference of 0.052mm

                  Now those separate tolerances/limits of 0.052mm (= 0.002") are reasonably "tight" if taken separately - as they would be if every one of several/many parts had to be inter-changeable and to achieve a "Free-running fit" (within its limits).

                  But as many items - particularly in a "jobbing" shop of a HSM shop have one part made to fit an existing other part and will stay that way, then advantage may be taken of these limits/tolerances to "widen" the tolerances for the part being made and still maintain the limits required of the class of fit for that nominal size.


                  The largest/widest "fit" limit will be between the largest hole (30.052mm) and the smallest shaft (29.908mm) = (30.052 - 29.908) = 0.144mm

                  The smallest/least "fit" limit will be between the smallest hole (30.000mm) and the largest shaft (29.960mm) = (30.000 - 29.960mm) = 0.040mm

                  The tolerance for the "free running fit" for a 30.00mm nominal diameter shaft are: 0.144/0.040mm - a difference of (0.144 - 0.040) = (0.104mm) ~ 0.0041" - say "4 thou" (signs are positive therefore limits are "clearance" (and not "interference" as would be the case if one or both were negative).

                  (It helps to have sketched this out by this stage).

                  Now, if our "real" (in the shop) hole (which exists) is actually 30.030mm and our fit tolerance on our shaft (which we have to make to suit for the required "Free Running" fit is -0.040/-0.144mm then the shaft limits are: 30.030 -0.040/-0.144 = 29.990/29.886mm which is a difference (machining range) of (29.990 = 29.886) = 0.104mm (= 0.0041" - say "4 thou").

                  I have discussed "clearance" fits here. You mist be very careful with the "signage" in "Transitional" and "Interference" fits as there will be negative quantities.

                  So, as previously, be very careful and sketch the process.

                  It seems difficult at first but once you "get the hang" of it, it is relatively easy and should be within the competency of any machinist.


                  • #10
                    This is super stuff ! When do we get the next lesson?
                    cheers, Bob


                    • #11
                      Thanks oldtiffie. Juergen


                      • #12
                        More to follow

                        Originally posted by Quetico Bob
                        This is super stuff ! When do we get the next lesson?
                        cheers, Bob
                        In the next several days Bob. I will post pics of parts of a book published here in OZ (one page posted at post #8 by "Ringer"). It will include tolerancing as well as "Class of Finish" which fit "hand in glove" with "Class of Fit".

                        Originally posted by juergenwt
                        Thanks oldtiffie. Juergen
                        Thanks juergenwt.

                        I was really pleased and relieved to get that post of yours as I was concerned that I might have both hi-jacked your thread and got your intent wrong.