Double Plus or Minus Tolerances

[jjonas]

This may or may not be the correct place to ask tnis question but here goes. The company I work for is manufacturing rplacement and repair parts for aircraft made in Israel. Some of the drawings are in metric and some are in SAE and some have a combination of both. I am a machinist not an engineer so please forgive me if I use the incorrect terms for my descriptions. Some of the metric drawings use a double minus (-.027mm,-.034mm) tolerance or a double plus (+.027mm,+.034mm) tolerance. Based on my experience this would be interpreted as the nominal diemenision (theoretical desired size) with a unilateral tolerance of -.027mm, -.034mm. I have been told this is not correct and they have no evidence to back this up except there past experience. I was told this is a european tolerancing method. but can not fine the needed standard for this method. this is the way it was explained to me I just need some hard evidence to back this up.

8.00mm nominal size
tolerance on drawing -.027mm,-.034mm
8.00mm-.027mm = 7.973 actual desired size
8.00mm-.034mm = 7.966 minum actual desired size
total unilateral tolerance = -.007mm
Is this correct? What drawing standard is this covered by?
Thank you in advance for any and all help. If I need to provide a better example please let me know.

[Kelly_Bramble]

Welcome to Engineers Edge...

First, the drawings are either ISO, ANSI or ASME not SAE...

ISO - International Standards Organization
ANSI = American National Standards Institute
ASME = American Society of Mechanical Engineers

Expressing dimensional data with double minus or positive is not wrong (different maybe), this practice is very ISO btw. These tolerances may have been derived from ANSI B4.1 or ANSI B4.2 or ISO 286 (part I & II).

8.00mm (size) -.027mm,-.034mm means that the feature (with or without size) should be as-manufactured between 7.973 and 7.966. There is not a "desired size or distance" simply a range of acceptable sizes or distances.

If you have a ~need to rewrite the size and tolerance expression for a bi-lateral equal tolerance - have at it, however the end item should be manufactured within the same dimensional range or limits.

[jjonas]

Kelly, thank you for the reply. I understand that the drawings are ISO,ANSI or ASME. The SAE was refering to the type of measurement metric or english, American etc. I also did not mean to imply that it was wrong as well. I am trying to find the correct way to properly interpret this particular method or style of diemenisonong a drawing tolerance so the part can be manufactured correctly.If I understand your statement correctly with this particular style you are to subtract smallest tolerance from the nominal given size 8.00mm -.027mm will give the starting range for the diameter of the part to be manufactured. Is this correct? The reason I am asking this is I am a quality control inspector and I need some information to give to the machinst on the floor so that they can under stand how to correctly read the drawings because most of these people have been trained on ANSI Y14. The part we are manufactureing is a bolt that has a sleeve that is suposed to be a shrink on the bolt. The print has 8.00mm being both the O.D. an I.D. of the bolt and sleeve based on the way we have been trained if you have 1 part on the high side of the tolerance and the other on the low side of the tolerance you will end up with .025mm clearence.

Welcome to Engineers Edge...

First, the drawings are either ISO, ANSI or ASME not SAE...

ISO - International Standards Organization
ANSI = American National Standards Institute
ASME = American Society of Mechanical Engineers

Expressing dimensional data with double minus or positive is not wrong (different maybe), this practice is very ISO btw. These tolerances may have been derived from ANSI B4.1 or ANSI B4.2 or ISO 286 (part I & II).

8.00mm (size) -.027mm,-.034mm means that the feature (with or without size) should be as-manufactured between 7.973 and 7.966. There is not a "desired size or distance" simply a range of acceptable sizes or distances.

If you have a ~need to rewrite the size and tolerance expression for a bi-lateral equal tolerance - have at it, however the end item should be manufactured within the same dimensional range or limits.

[jboggs]

jjonas,
Can you get your hands on a Machinery's Handbook? If not I would STRONGLY recommend you buy one. It would be well worth the investment. Look up the chapter on "Allowances and Tolerances". You will see that there are two standard ANSI systems of fits and tolerances (inch and metric). The metric ANSI system is based on the ISO system. Both of them describe a range of fits for mating shafts and holes.

In the inch system, each standard fit is given a name like RC (for Running or Sliding Clearance Fit) or LN (for Locational Interference Fit), based on the "tightness" of the fit to serve a specific function. Pages of data show you the specific tolerances in each of those fits for certain size ranges of holes or shafts. You will notice that some of them do indeed show a double plus or a double minus for the actual tolerance based on some standard dimension.

In the metric pages you will see a similar approach. But you will also see something I first saw on German drawings done to DIN standards. Individual tolerance ranges are referenced by number and letter, like "H8" or "f7". (Cap letters for holes; small letters for shafts.) There are charts that describe very clearly what the intended fit is for each of these ranges. This allows the designer to simply select the shaft/hole size he wants and the type fit he wants. Then he just calls out a shaft diameter as "45 g6" for example. The machinist can then go to the tables to identify exactly what the tolerance is for a 45mm shaft with a g6 tolerance. Frankly I really like that system but it is so unknown here in the US it is pointless to try to use it. It will also give you double pluses and double minuses.

So, bottom line: whoever told you that double pluses or minuses are wrong was "misinformed". Unless the designer in misusing them, they are in fact based on ANSI and ISO standards for fits and tolerances.

[jjonas]

Thank you for your reply. I have 2 coppies, a great book. I have read the section on classes of fits and some what understand the alpha numeric , upper case lower case designations and this would be useful if the class of fit had been called out on the drawing but they were not. The drawings that I have seen for these parts also does not state what standard the drawing is drawn to. I am sorry I did not provide enough details. As you have stated this system is not a commonly used system in the U.S. That is why I am asking these questions. I always say If you don't know ask. In my job I have also learned not to take any thing for granted or to assume things as 9 times out of 10 it will come back and bite you.

jjonas,
Can you get your hands on a Machinery's Handbook? If not I would STRONGLY recommend you buy one. It would be well worth the investment. Look up the chapter on "Allowances and Tolerances". You will see that there are two standard ANSI systems of fits and tolerances (inch and metric). The metric ANSI system is based on the ISO system. Both of them describe a range of fits for mating shafts and holes.

In the inch system, each standard fit is given a name like RC (for Running or Sliding Clearance Fit) or LN (for Locational Interference Fit), based on the "tightness" of the fit to serve a specific function. Pages of data show you the specific tolerances in each of those fits for certain size ranges of holes or shafts. You will notice that some of them do indeed show a double plus or a double minus for the actual tolerance based on some standard dimension.

In the metric pages you will see a similar approach. But you will also see something I first saw on German drawings done to DIN standards. Individual tolerance ranges are referenced by number and letter, like "H8" or "f7". (Cap letters for holes; small letters for shafts.) There are charts that describe very clearly what the intended fit is for each of these ranges. This allows the designer to simply select the shaft/hole size he wants and the type fit he wants. Then he just calls out a shaft diameter as "45 g6" for example. The machinist can then go to the tables to identify exactly what the tolerance is for a 45mm shaft with a g6 tolerance. Frankly I really like that system but it is so unknown here in the US it is pointless to try to use it. It will also give you double pluses and double minuses.

So, bottom line: whoever told you that double pluses or minuses are wrong was "misinformed". Unless the designer in misusing them, they are in fact based on ANSI and ISO standards for fits and tolerances.

[jjonas]

Sorry to ask the same question over and over again. It finally sunk in and I understand what is going on. Some times we get a little to deep into the forrest. Thank you all for the help.