2INTRODUCTIONThe ISO System of Limits and Fits (referred to as the ISO system) is covered in national standards throughout the world, as shown by the following list:Global ISO 286USA ANSI B4.2Japan JIS B0401Germany DIN 7160//61France NF EUK BSI 4500Italy UNI 6388Australia AS 1654
3HISTORY OF THE ISO SYSTEM The present ISO system is based on the ISA System of Limits and Fits published in ISA Bulletin 25 (1940), and on comments included in the Draft Final Report of ISA Committee 3, December The unification of the various national systems of limits and fits was one of the essential tasks discussed at the initial conference of the ISA in New York, in April, The same year the Secretariat of ISA Committee 3, Limits and Fits, was entrusted to the Germany Standardizing Association, and needless to say, the system was all metric from the start.
4Tolerancing ∙ Tolerances are used to control the variation that exists on all manufactured parts.∙ Toleranced dimensions control the amount of variationon each part of an assembly.∙ The amount each part is allowed to vary depends on thefunction of the part and of the assembly. For example:the tolerances placed on a swing set is not as stringentas those placed on jet engine parts.∙ The more accuracy needed in the machined part –the higher the manufacturing cost.
5Representing Tolerance Values ∙ Tolerance is the total amount adimension may vary and is thedifference between the maximumand minimum limits.(A) Tolerance = .04(B) Tolerance = .006∙ Tolerances are representedas Direct Limits (A) or asTolerance Values (B).Which part costs more to manufacture?
6Tolerances can also be expressed as: 1. Geometric Tolerances.“GDT”2. Notes Referring to Specific Conditions.3. A General Tolerance Note in the Title Block.Example: ALL DECIMAL DIMENSIONS TO BE HELD TO ± .002”
7Plus and Minus Dimensions With this approach, the basic size is given,followed by a plus/minus sign and the tolerance value.Notice that a Unilateral Tolerance varies in only one direction,while Bilateral Tolerances varies in both directions from the basic size.
8Important Terms of Toleranced Parts A System is two or more mating parts.Nominal Size is used to describe the general size (usually in fractions).The parts above have a nominal size of 1/2”Basic Size – theoretical size used as a starting point for the application ofTolerances. The parts above have a basic size of .500”
9Important Terms of Toleranced Parts Limits – the maximum and minimum sizes shown by the tolerance dimension.The slot has limits of .502 & .498, and the mating part has limits of .495 & .497.The large value on each part is the Upper Limit, the small value = Lower Limit.Actual Size is the measured size of the finished part after machining.The Actual Size of the machined part above is .501”
10Important Terms of Toleranced Parts Allowance – the tightest fitbetween two mating parts.(The minimum clearance or maximum interference).For these two parts, the allowance is .001,meaning that the tightest fit occurs when theslot is machined to it’s smallest allowable sizeof .498 and the mating part is machined to itslargest allowable size of The differencebetween .498 and .497, or .001, is the allowance.
11Important Terms of Toleranced Parts Tolerance – the total allowable variance in a dimension;the difference between the upper and lower limits.The tolerance of the mating part is .002”( = .002)The tolerance of the slot is .004”( = .004)
12Important Terms of Toleranced Parts Maximum Material Condition (MMC)The condition of a part when it containsthe greatest amount of material. The MMCof an external feature, such as a shaft,is the upper limit. The MMC of an internalfeature, such as a hole, is the lower limit.Least Material Condition (LMC)The condition of a part when it containsthe least amount of material possible.The LMC of an external feature is the lower limit. The LMC of an internal feature is the upper limit.
13Important Terms of Toleranced Parts Piece toleranceThe difference between the upper and lower limits of a single part(.002 on the insert in this example, .004 on the slot.).System toleranceThe sum of all the piece tolerances.For this example (.006)
14Fit Types: Clearance & Interference fits between two shafts and a hole Shaft A is a Clearance fit, shaft B is an Interference fit
15Fit Types: Transition Fit A Clearance Fit occurs when two toleranced mating parts willalways leave a space or clearance when assembled.An Interference Fit occurs when two toleranced mating parts willalways interfere when assembled.A Transition Fit occurs when two toleranced mating parts are sometimesan interference fit and sometimes a clearance fit when assembled.
16Functional Dimensioning Functional Dimensioning begins with tolerancing the most important features.Then, the material around the holes isdimensioned (at a much looser tolerance).Functional features are those that come in contact with other parts,especially moving parts. Holes are usually functional features.
17Tolerance Stack-up AVOID THIS!!! Occurs when dimensions are taken from opposite directions of separateparts to the same point of an assembly.Tolerance Stack-upDimensionedfrom theleft.Dimensionedfrom theright.AVOID THIS!!!
18Avoiding Tolerance Stack-up Tolerance stack-up can Better still, relate the twoholes directly to each other,not to either side of the part.The result will be the besttolerance possible of ±0.005.Tolerance stack-up canbe eliminated by carefulconsideration andplacement of dimensions.(Dimension from same side).
19Basic Hole System The basic hole system is used to apply tolerances to a hole and shaftassembly.The smallest hole is assigned thebasic diameter from which thetolerance and allowance is applied.
20Creating a Clearance Fit Using The Basic Hole System Check the work by determining the piece tolerances for the shaft and the hole. To do so, first find the difference between the upper and lower limits for the hole. Subtract .500” from .503” to get .003” as a piece tolerance. This value matches the tolerance applied in Step 4. For the shaft, subtract .493” from .496 to get .003” as the piece tolerance. The value matches the tolerance applied in Step 3.The difference between the largest hole (.503” upper limit) and the smallest shaft (.493” lower limit) equals a positive .010”. Because both the tightest and loosest fits are positive, there will always be clearance between the shaft and the hole, no matter which manufactured parts are assembled.Using the basic hole system,assign a value of .500” to thesmallest diameter of the hole,which is the lower limit.The allowance of .004” is subtracted from the diameter of the smallest hole to determine the diameter of the largest shaft, .496”, which is the upper limit.The lower limit for the shaft is determined by subtracting the part tolerance from .496”. If the tolerance of the part is .003”, the lower limit of the shaft is .493”Using the assigned values results in a clearance fit between the shaft and the hole. This is determined by finding the difference between the smallest hole (.500” lower limit) and the largest shaft (.496” upper limit), which is a positive .004”. As a check, this value should equal the allowance used in step 2The system tolerance is the sum of all the piece tolerances. T o determine the system tolerances for the shaft and the hole, add the piece tolerances of .003” and .003” to get .006”The upper limit of the hole is determined by adding the tolerance of the part to .500”. If the tolerance of the part is .003”, the upper limit of the hole is .503”The parts are dimensioned on the drawing.
21Creating an Interference Fit Using The Basic Hole System ADD hereFollow the same sequence of steps as you did for a Clearance Fit,except that you ADD the allowance in Step 2, instead of subtract.
22Cylindrical Fits – Metric Units ANSI B4.2 standardbasic size – the diameter from which limits are calculatedupper and lower deviation – the difference between the hole or shaft size and the basic sizetolerance - the difference between the maximum and minimum sizes
24Cylindrical Fits – Metric Units fundamental deviation – a letter grade that describes the deviation closest to the basic sizeInternational Tolerance (IT) grade – a series of tolerances that vary with the basic size to provide a uniform level of accuracy within a given gradethere are 18 IT grades: IT01, IT0, IT1, …, IT16
25Cylindrical Fits – Metric Units Hole basisa system of fits based on the minimum hole size as the basic diameterthe fundamental deviation for a hole-basis system is “H”Appendices 35 and 36 give hole-basis data for tolerancesShaft basisa system of fits based on the maximum shaft size as the basic diameterthe fundamental deviation for a hole-basis system is “h”Appendices 37 and 38 give shaft-basis data for tolerances
46Dimensions are used to show an object’s: 1. Overall: WidthDepthHeight2. The actual size of features (rounds, fillets, holes, arcs, etc.)
473. And where features are located such as centers, angles, etc.
48The proper placement of dimensions is critical to ensure that the part can be read and manufactured to specifications.Here are some practices that need to be followed when applying dimensions to an object:
491.Dimensions should be stacked in a “broken chain” format to aid in the readability of the plate.“Breaking the Chain” refers to leaving out one dimension as shown above so that manufacturing tolerances are maintained.
512.Do not place dimensions directly on the object unless it is unavoidable.
52As a general rule…Stay off the object as much as possible.
533.Extension lines can be shared and even broken to clarify crowded dimensions.
544. Some features are dimensioned from their center lines. The center line may also be used as an extension line.
555.Dimensions may be laid out in different configurations. Unidirectional dimensioning is the current standard in most industrial applications today.
566.Leaders with dimensions are used to show negative cylinders (holes).The leader should always be placed to penetrate the center of all round features.
57Features such as counterbores, countersinks and spot faces are all dimensioned using a leader. Note: Each of these features has a special dimensioning symbol that can be used to show: a. Diameter b. Shape c. Depth
58Here we see several examples where notes are used instead of symbols to dimension features. Notes that describe a specific feature are known as “local notes”.“General notes” are used to describe a characteristic that effects the entire part, i. e. materials, production instructions, etc.
597.Arcs are always dimensioned as a radius. Full circles are dimensioned showing their diameter value.
608.When dimensioning a part, always start with the inner-most dimensions and work to the outer-most values. Remember: Dimensions are used to show both the size and location of features.
619.Always dimension features and not lines…..and remember…. NEVER, NEVER, NEVER dimension to hidden lines!
62Here are some rules (See Table. 15.1) on how to dimension properly: 1. The overall Width, Height, and Depth must be shown on the object.2. Each feature should be dimensioned in the view where it appears true shape and size. Never dimension a feature where it appears only as a line.3. NEVER dimension to hidden lines!!!
634. The measurement standards must be 4. The measurement standards must be maintained and clearly noted on the print.5. Dimension lines should never cross.6. Leaders must point at an angle that allows them to penetrate the centre of the object which they are describing.7. Chains MUST be broken to allow for tolerance variation in the part.
648. Dimensions should be placed to allow 8. Dimensions should be placed to allow order to the print and promote ease of reading.9. Negative cylinders (holes) must always be measured by their diameter.10. Arcs are always shown by specifying a radius value.11. When laying out centres, specify one common reference point for the X and Y axes.
6512. Dual dimensions may be used, but they must 12. Dual dimensions may be used, but they must be consistent and clearly noted.13. Angles may be dimensioned by showing their degree(s), or their 3 point location.14. Only dimension each feature once!!!