Manufacturability
Terminology of Limit Systems
Learning Objectives:
- Understand importance of limits and fits for manufacturing operations.
- Interpret and explain terminology of limits system.
- Classify types of limits.
- Calculate sizes of shafts and holes for a specified fit or given basic size and grade.
- Limits of size: The two extreme permissible sizes of a component between which the actual size should lie including the maximum and minimum sizes of the component.
- Nominal size: It is the size of the component by which it is referred to as a matter of convenience.
- Basic size: It is the size of a part in relation to which all limits of variation are determined.
- Zero Line: It is the line w.r.t which the positions of tolerance zones are shown.
- Deviation: It is the algebraic difference between a limit of size and the corresponding basic size.
- Upper Deviation: It is the algebraic difference between the maximum limit of size and the corresponding basic size. It is denoted by letters ES for a hole and es for a shaft.
- Lower Deviation: It is the algebraic difference between the minimum limit of size and the corresponding basic size. It is denoted by letters EI for a hole and ei for a shaft.
- Fundamental Deviation: It is the deviation, either upper or lower deviation, which is nearest to the zero line for either a hole or a shaft. It fixes the position of the tolerance zone in relation to the zero line.
- Allowance: It is the intentional difference between the hole dimensions and shaft dimension for any type of fit.
- Unilateral Tolerance: When the two limit dimensions are only on one side of the nominal size, (either above or below) the tolerances are said to be unilateral tolerance.
- Bilateral Tolerance: When the two limit dimensions are above and below nominal size, (i.e. on either side of the nominal size) the tolerances are said to be bilateral tolerance.
Unilateral tolerances, are preferred over bilateral because the operator can machine to the upper limit of the shaft (or lower limit of a hole) still having the whole tolerance left for machining to avoid rejection of parts. -
Interchangeability: Interchangeability occurs when one part in an assembly can be substituted for a similar part which has been made to the same drawing. Interchangeability is possible only when certain standards are strictly followed.
Universal interchangeability means the parts to be assembled are from two different manufacturing sources.
Local interchangeability means all the parts to be assembled are made in the same manufacturing unit. -
Selective Assembly: In selective assembly, the parts are graded according to the size and only matched grades of mating parts are assembled. This technique is most suitable where close fit of two components assembled is required.
Selective assembly provides complete protection against non-conforming assemblies and reduces machining costs as close tolerances can be maintained. Suppose some parts (shafts and holes) are manufactured to a tolerance of 0.01 mm, then an automatic gauge can separate them into ten different groups of 0.001 mm limit for selective assembly of the individual parts. Thus high quality and low cost can be achieved.
Selective assembly is used in aircraft, automobile industries where tolerances are very narrow and not possible to manufacture at reasonable costs.
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Solved Example: 98-1-01
Basic shaft is one:
A. Whose upper deviation is zero
B. Whose lower deviation is zero
C. Whose lower as well as upper deviations are zero
D. Does not exist
Correct Answer: A
Solved Example: 98-1-02
Basic hole is one:
A. Whose upper deviation is zero
B. Whose lower deviation is zero
C. Whose lower as well as upper deviations are zero
D. Does not exist
Basic Hole: If the lower limit size or minimum size of hole is equal to the basic size then that type of hole is known as basic hole for a given assembly of hole and shaft.
For the basic hole the zero line will be lying on minimum size of the hole. For the Basic Hole the lower deviation is zero because the difference between the Lower Limit Size and Basic Size is zero.
Correct Answer: B
Solved Example: 98-1-04
In the specification of dimensions and fits:
A. Allowance is equal to bilateral tolerance
B. Allowance is equal to unilateral tolerance
C. Allowance is independent of tolerance
D. Allowance is equal to the difference between maximum and minimum dimensions specified by the tolerance.
Correct Answer: C
Types of Fits
Learning Objectives:
- Understand the basic principles of fits and tolerances.
- Explain various types of fits and their applications.
- Analyze the various types of tolerances and applications.
- Know the fundamental of the systems of fits.
- Clearance fit: In this type of fit, the largest permitted shaft diameter is less than the smallest hole diameter so that the shaft can rotate or slide according to the purpose of the assembly.
e.g. are:
- Free running fit (H9/d9)
- Sliding fit (H7/g6)
- Locational clearance fit (H7/h6)
- Loose running fit (H11/c11)
- Close running fit (H8/f7)
- Interference Fit: It is defined as the fit established when a negative clearance exists between the sizes of holes and the shaft. In this type of fit, the minimum permitted diameter of the shaft is larger than the maximum allowable diameter of the hole. In case of this type of fit, the members are intended to be permanently attached.
examples are:
- Locational transition fit (H7/k6)
- Locational transition fit (with more accuracy) (H7/n6)
- Transition fit: Transition fits are a compromise between clearance and interference fits. They are used for applications where accurate location is important but either a small amount of clearance or interference is permissible.
Examples are:
- Location interference fit (H7/p6)
- Medium drive fit (H7/s6)
- Force fit (H7/u6)
Ex: Bearing bushes, Keys and key ways.
Holes are designated by capital letters.
Shafts are designated by small letters.
Solved Example: 98-2-01
A shaft (Diameter $\displaystyle 20^{+0.05}_{-0.15}$ mm) and a hole (Diameter $\displaystyle 20^{+0.20}_{+0.1}$ mm) when assembled would yield:
A. Transition Fit
B. Interference Fit
C. Clearance Fit
D. None of these.
Best Case Scenario = biggest hole - smallest shaft
= 20.20 -19.85 = +0.35 mm
Worst Case Scenario = smallest hole - biggest shaft
= 20.10 -20.05 = +0.05 mm
In both cases, the assembly produces positive clearance, hence it is a clearance fit.
Correct Answer: C
Solved Example: 98-2-02
The fit on a hole shaft system is specified as 117-s6, the type of fit is:
A. Clearance Fit
B. Running Fit (Sliding Fit)
C. Push Fit (Transition Fit)
D. Force Fit (Interference Fit).
Correct Answer: D
Solved Example: 98-2-03
The diameters of the hole and shaft are specified respectively as 50$^{\pm0.05}$ mm and 50$^{0}_{-0.04}$ mm. The fit is a:
A. Interference fit
B. Transition fit
C. Clearance fit
D. Data insufficient to decide
Largest clearance = biggest hole - smallest shaft
Largest clearance = 50.05 - 49.96 = 0.09 (clearance)
Smallest clearance = smallest hole - biggest shaft
Smallest clearance = 49.95 - 50.00 = -0.05 (interference)
Since there is clearance in one case and interference in the other, this is a transition fit.
Correct Answer: B
Feature Control Frame
Learning Objectives:
- Understand the basic principles of fits and tolerances.
- Explain various types of fits and their applications.
- Analyze the various types of tolerances and applications.
- Know the fundamental of the systems of fits.
A geometric tolerance is prescribed using a feature control frame. It has three components:
- The tolerance symbol,
- The tolerance value,
- The datum labels for the reference frame.
Maximum Material Condition (MMC): The condition in which a feature contains the maximum amount of material within the stated limits. e.g. minimum hole diameter, maximum shaft diameter.
Least Material Condition (LMC): The condition in which a feature contains the least amount of material within the stated limits. e.g. maximum hole diameter, minimum shaft diameter.
Basic shaft: It is a shaft whose upper deviation is zero. i.e. the maximum limit of shaft coincides with the nominal size.(zero line). Eg: shaft ‘h’
Basic hole: It is a hole whose lower deviation is zero. i.e. the minimum limit of hole coincides with the nominal size.(zero line). Eg: shaft ‘H’
Solved Example: 98-3-01
What is a Basic Dimension?
A. An exact dimension with no tolerance associated with it.
B. A dimension in a box that must be strictly adhered to by manufacturing
C. The nominal size of a feature.
D. A numerical value that describes a theoretically exact size, profile, orientation or location of a feature or a datum target. It is the basis from which permissible variations (tolerances) are establi
Correct Answer: A
Solved Example: 98-3-02
What is a Feature Control Frame?
A. A symbolic means of expressing the type of control on a feature, and defining a tolerance zone for that control.
B. A box on the drawing that tells the manufacturer how to make the part.
C. A box on the drawing that tells the inspector what measurements have to be made for every part.
D. A symbol that dictates the tolerance of position for a datum target.
Correct Answer: A
Solved Example: 98-3-03
What is a Material Condition Modifier?
A. The maximum dimension possible according to the tolerance specified.
B. The minimum dimension possible according to the tolerance specified.
C. A symbol placed on a feature of size that states how much tolerance is available for that feature.
D. A symbol (or lack thereof) placed in a feature control frame that indicates how much, or if, any extra geometric tolerance is available for the feature as the actual mating size of the feature changes
Correct Answer: D
Basis of Fits
Learning Objectives:
- Understand the basic principles of fits and tolerances.
- Explain various types of fits and their applications.
- Analyze the various types of tolerances and applications.
- Know the fundamental of the systems of fits.
- Hole Basis: In this system, the basic diameter of the hole is constant while the shaft size is varied according to the type of fit.
Significance of Hole basis system: The selection depends on the production methods. Generally, holes are produced by drilling, boring, reaming, broaching, etc. whereas shafts are either turned or ground. If the hole basis system is used, there will be reduction in production costs as only one tool is required to produce the ole and the shaft can be easily machined to any desired size. Hence hole basis system is preferred over shaft basis system. - Shaft Basis system: In this system, the basic diameter of the shaft is constant while the hole size is varied according to the type of fit.
If the shaft basis system is used to specify the limit dimensions to obtain various types of fits, number of holes of different sizes are required, which in turn requires tools of different sizes.
Solved Example: 98-4-01
In a hole basis system, the lower deviation for the hole 'H' is:
A. 0 mm
B. 0.1 mm
C. 1 mm
D. 0.01 mm
Correct Answer: A
Solved Example: 98-4-02
In a shaft basis system, the upper deviation of the size of shaft is:
A. 0
B. 1
C. Less than zero
D. More than 1
Correct Answer: A
Limit Gauges
Learning Objectives:
- Understand the fundamental of the gauges and their classifications.
- Explain the working principles of various types of gauges and their applications.
A Go-No GO gauge refers to an inspection tool used to check a workpiece against its allowed tolerances. It derives its name from its use: the gauge has two tests; the check involves the workpiece having to pass one test (Go) and fail the other (No Go).
It is an integral part of the quality process that is used in the manufacturing industry to ensure interchangeability of parts between processes, or even between different manufacturers.
A Go - No Go gauge is a measuring tool that does not return a size in the conventional sense, but instead returns a state. The state is either acceptable (the part is within tolerance and may be used) or it is unacceptable (and must be rejected).
Solved Example: 98-5-01
Wear allowance is provided on:
A. Go gauge
B. No GO gauge
C. Both Go and No Go gauge
D. When both are combined in one gauge
Correct Answer: A
Solved Example: 98-5-02
The main purpose of using Go and No Go gauge is to use it as:
A. Limit gauge
B. Radius gauge
C. Pitch gauge
D. Slip gauge
Correct Answer: A