Tolerancing Interchangeability of manufactured parts is a critical element of present day production. The production of closely mating parts, although theoretically possible, is economically unfeasible. For this reason, the engineer, designer or drafter specifies an allowable deviation (tolerance) between decimal limits. The definition of a Tolerance, per ASME Y14.5.5M-1994, is the total amount a specific dimension is permitted to vary. For instance, a dimension shown as 1.498” to 1.502” means that it may be 1.498” or 1.502” or anywhere between these dimensions. Since greater accuracy costs money, you would not callout the tightest possible tolerance, but instead would callout as generous a tolerance as possible. Definition of Terms Example 1 Maximum Material Condition (MMC) – Is the condition where a feature of a finished part contains the maximum amount of material. That is, the largest shaft or smallest hole. See Example 1. Least Material Condition (LMC) - Is the condition where a feature of a finished part contains the least amount of material. That is, the smallest shaft or the largest hole. See Example 1. Nominal Size – Approximate size used for the purpose of identification such as stock material.
Transcript
Tolerancing
Interchangeability of manufactured parts is a critical element of present day production. The production of closely mating parts, although theoretically possible, is economically unfeasible. For this reason, the engineer, designer or drafter specifies an allowable deviation (tolerance) between decimal limits. The definition of a Tolerance, per ASME Y14.5.5M-1994, is the total amount a specific dimension is permitted to vary. For instance, a dimension shown as 1.498” to 1.502” means that it may be 1.498” or 1.502” or anywhere between these dimensions. Since greater accuracy costs money, you would not callout the tightest possible tolerance, but instead would callout as generous a tolerance as possible. Definition of Terms
Example 1
Maximum Material Condition (MMC) – Is the condition where a feature of a finished part contains the maximum amount of material. That is, the largest shaft or smallest hole. See Example 1. Least Material Condition (LMC) - Is the condition where a feature of a finished part contains the least amount of material. That is, the smallest shaft or the largest hole. See Example 1. Nominal Size – Approximate size used for the purpose of identification such as stock material. Basic Size – Is the theoretical exact size from which limits of size are determined by the application of allowances and tolerances. Tolerance – The total amount by which a given dimension may vary or the difference between the limits. Limits – The extreme maximum and minimum sizes specified by a toleranced dimension.
Allowance – An allowance is the intentional difference between the maximum material limits (minimum clearance or maximum interference) of mating parts.Refer to Example 1 above: MMC of the hole – MMC of the shaft = Allowance. MMC Hole = 1.250 - MMC Shaft = 1.248 Allowance = .002 Fits Clearance fit – A clearance fit results in limits of size that assure clearance between assembled mating parts.Refer to Example 1 above: LMC of the hole – LMC of the shaft = Clearance. LMC Hole = 1.251 - LMC Shaft = 1.247 Clearance = .004 Interference fit (also referred to as Force fit or Shrink fit)– interference fit has limits of size that always result in interference between mating parts. For example, a hole and shaft, the shaft will always be larger than the hole, to give an interference of metal that will result in either a force or press fit. The effect would be an almost permanent assembly for two assembled parts.
Example 2
Least amount of Interference is: LMC Shaft = 1.2513 - LMC Hole = 1.2506 Min Interference = .0007 Greatest amount of Interference: MMC Shaft = 1.2519 - MMC Hole = 1.2500 Max Interference = .0019 Transition fit – A transition fit might be either a clearance or interference fit. That is, a shaft may be either larger or smaller than the hole in a mating part.
Example 3
LMC Hole = 1.2506
- LMC Shaft = 1.2503 Positive Clearance = .0003
MMC Shaft = 1.2509 - MMC Hole = 1.2500 Negative Allowance (Interference) = .0003 Basic Hole System – The basic hole system is used to apply tolerances to holes and shafts assemblies. The minimum hole is assigned the basic diameter (basic size) from which the tolerance and allowance are applied. This system is widely used in industry due to standard reamers being used to produce holes, and standard size plugs used to check hole sizes accurately.
Computed Clearance Fit using Basic Hole System.500 = hole basic size .500 basic hole.002 = Allowance (decided) - .002 allowance .498 Maximum shaft
Step 1 Step 2If tolerance of part is = .003 then: .498 maximum shaft .500 basic hole -. 003 tolerance +.003 tolerance .495 minimum shaft .503 maximum hole Step 3 Step 4 Calculate clearances:
.500 smallest hole (MMC) .503 largest hole (LMC) -. 498 largest shaft (MMC) -.495 smallest shaft (LMC) .002 minimum clearance .008 maximum clearance Step 5 Step 6
Drawing annotation of toleranceExample 3
Basic Shaft System – The basic shaft system can be used for shafts that are produced in standard sizes. When applying this system, the largest shaft is assigned the basic size diameter from which the allowance for the mating part is assigned. Then, tolerances are applied on both sides and away from the assigned allowance. One situation for using the basic shaft system is when a purchased motor, with an attached shaft, from which a mating hole must be calculated.
Computed Interference fit using Basic Shaft System.500 = shaft basic size .500 basic shaft.002 = Allowance (decided) - .002 allowance .498 Maximum hole
.498 largest hole (LMC) .495 smallest hole (MMC) -. 500 smallest shaft (LMC) -.503 largest shaft (MMC) - .002 minimum interference -.008 maximum interference Step 5 Step 6
Drawing annotation of toleranceExample 4
Preferred precision fits – The American National Standards Institute publishes the “Preferred Limits and Fits for Cylindrical Parts” (ANSI B4.1-1967) to define terms and recommending standard allowances, tolerances, and fits for mating parts. The chart data is provided in thousandths (.001) of an inch. For example: -1.2 and -2.2 (See Example 5) for calculation purposes would be -.0012 and -.0022. Running and Sliding fits (RC1-RC9)Loosest of the class fits, used when a shaft is must move freely inside a hole or bearing, and the positioning of the shaft is not critical. This fit would always allow a clearance between shaft and hole. Clearance locational fits (LC1-LC11)Tighter than RC fits, but the shaft and hole may be the same size. LC fits allow the shaft to be located more accurately than the RC fits but may still be loose. With this fit, a shaft would move less freely inside a hole. Transition locational fits (LT1-LT6)These fits are a compromise between LC and LN (interference/force) fits. These fits would allow either a small amount of clearance or interference. Interference locational fits (LN1-LN3)Used where accuracy of location is the prime importance such as alignment of dowel pins and other devices where location relative to another part is of prime importance. Force and shrinks fits (FN1-FN5)With this fit, the shaft is always considered larger than the hole. These fits are used to transmit torque such as a motor shaft to a bearing.
Limits Calculations Using ANSI B4.1 Standard TablesClass RC6 Clearance Fit
Partial Table from ANSI B4.1Example 5
A nominal hole size of .8750 Diameter and a RC6 Class Fit has been selected.Hole nominal size range = .71 – 1.19Minimum clearance = .0016Maximum clearance = .0048Tolerance of hole = +.0020, -.0000Tolerance of shaft = -.0016, -.0028Calculations:Hole: Basic size .8750 .8750 Tolerance +.0020 -.0000 Maximum hole .8770 Minimum hole .8750 Shaft: Basic size .8750 .8750 Tolerance -.0016 -.0028 Maximum shaft .8734 Minimum shaft .8722
Limits of size for Hole and Shaft
Example 5 Limit Calculations when one Design Feature ExistsWhen calculating the limit tolerances for features that mate with purchased parts, the purchased part size must be known. This may be obtained be requesting a drawing from a vendor or, a caliper or micrometer can be used to obtain an accurate size.Example:A shaft diameter of .2500 is to be pressed into a part using a FN4 interference (force) fit.Limits of size for the shaft diameter are .2500 and .2495.
The table shows a minimum acceptable interference of .0006 and maximum interference of .0016.Calculations:Maximum shaft: . 2500Maximum interference: -. 0016Minimum hole: . 2484 Minimum shaft: . 2495Minimum interference: -. 0006Maximum hole: .2489 References:Dimensioning and Tolerancing, ASME Y 14.5M-1994, The American Society of Mechanical Engineers.Technical Drawing Tenth Edition, Frederick E. Giesecke, Prentice Hall, Upper Saddle River, NJ 07458.Geometric Dimensioning and Tolerancing, 2003, David A. Matson, Goodheart-Wilcox Co. Inc., Tinley Park, Illinois.
Dimensioning FundamentalsBy William Bussard, March 2004
TerminologyDimension lines: Lines located between extension lines ending with an arrow and to include a numerical value. They should be spaced uniformly approximately .375 to .500 inches apart. Extension lines: Extend away from a view to indicate a size or location constraint origin. When extension lines cross object or extension lines, no gap in either line should be made. Leader Lines: Lines drawn at an angle (never horizontal or vertical) extending from a note to a feature to which the note applies.
Dimension and Extension line placement1. Always place shorter dimensions nearest to the object lines. Dimension lines should never cross. However, extension lines may cross each other.
2. Extension lines should never be shortened.
3. Dimension lines should never coincide with any object line or center line of the drawing. Therefore no line should be used as dimension line nor coincide with a dimension line.
4. Dimensions should line up in chain fashion or be grouped together as much as possible.
5. Do not repeat a dimension. By repeating a dimension will only confuse the reader and could cause errors in the manufacturing process. In Figure 5A below, there are many cases where dimensions are duplicated. Figure 5B correctly shows only those dimensions needed to describe the part.
6. Dimensions should be given in views where the shapes are presented in profile and/or contour. Figure 6A below shows each dimension given in the correct profile view. Figure 6B on the other hand, shows each dimension given in the wrong view.
7. Dimensions should always be placed off or outside of a view where possible.
8. Hole features must be located and given size in the view where they appear as a circle not as a rectangle or hidden lines. Never dimension to hidden lines.
9. Avoid a complete chain of dimensions. Either omit one or use reference notation. Reference notation indicates that a dimension is used for information purposes only. Indicate a reference dimension by placing a parenthesis around the dimension. For example, (1.51) would indicate a reference dimension. See example below.
10. Cylinder location and size constraint. Locate cylinders in the circular view but give its diameter and length in the rectangular view. See illustration below.
Dimensioning Symbols1. Symbols represent specific information which could be difficult to duplicate in note form. They aid in clarity, ease of CAD drawing presentation, and above all, save time.Seven such symbols are show below.
2. Local Notes: Local notes apply to specific features only and are connected to a feature by a leader. Notes should always be lettered horizontally. Leader arrows for notes should always point toward the circular view of the hole feature and if extended, would pass through the center. Local notes showing hole features, using the above symbols, are presented below reflecting Current and Previous ANSI practices.
Direction of Dimension Figures1. Two systems of placing dimensions are used—Unidirectional (Mechanical) or Aligned (Architectural). Unidirectional: Dimensioned numeral values and text are placed so that they can be read only from the bottom of the drawing. Aligned: Dimensioned numeral values and text are placed so that they can be read either from the bottom or right side of the drawing.
Ordinate DimensioningOrdinate dimensioning is also known as Datum dimensioning or Baseline dimensioning. Dimensions may be applied either using dimension lines or arrowheads (Figure 1) or without dimension lines or arrowheads (Figure 2). A much clearer method of presenting a part consisting of numerous holes is Tabular Ordinate dimensioning. This method involves labeling each hole feature with a letter and then providing a Hole Chart indicating the X location constraint and Y location constraint with a complete size description of each hole feature (Figure 3). This eliminates the need for both dimension and extension lines to locate hole features.
Using Dimension lines and Arrowheads Figure 1
Omitting Dimension lines and Arrowheads Figure 2
Tabular Ordinate Dimensioning Figure 3
What is the Material composition in IBR tubes?
Carbon max………………… 0.10%
Manganese max……………. 0.60%
Nickel max…………………. 0.25%
Sulphur max………………… 0.050%
Phosphorus max…………….. 0.050%
What is the difference between P11 & P22 pipes?
P11 the chromium molybdenum composition that is 1% ofchromium and 1/4% of
molybdenum
P12 the chromium molybdenum composition that is 1% ofchromium and 2% of
molybdenum
1st law of Thermodynamics
a. HEAT AND MECHANICAL WORK ARE MUTUALLY CONVERTABLE.
b. ENERGY CAN BE CREATED NOR BE DISTROYED BUT IT CAN BETRANSFERED
FROM ONE FORM TO ANOTHER FORM
Difference between Unilateral tolerance and Bilateral tolerance
A unilateral tolerance is tolerance in which variation is permitted only in one
direction from the specified direction.e.g. 1800 +0.000/-0.060
Bilateral tolerance is tolerance in which variation is permitted in both direction
from the specified direction.e.g. 1800 +0.060/-0.060
Why we need two angle of projections ( 1st angle and 3rd angle)? Cant we standardize to one.
Generally 1 St. angle followed in India and 3rd angle projection is used in
European countries. In 1st and 3rd angle projections there is no overlapping of
principal planes while drawing side and top views. but in 2nd and 4th angle
projections the principal planes are overlapped while drawing top and side views.
It results confusion in drawing. For this reason 1st and 3rd angle projections are
used commonly.
What is the difference between Corrective actions and preventive actions?
Corrective actions are taken on discrepancies noticed duringinspection of
products/documents/process whereas preventiveactions are taken to eliminate
the possibility ofdiscrepancy in future.
What is the Shaft coupling and its types
It is used to join two shafts for transmission motion from one shaft to another
coupling)Flexible coupling;(Flexible bush pin coupling)
Oldham’s Coupling is used when axis of shaft are not aligned
Other types are:
loveys joy coupling
Spider coupling
Tyre coupling
Pin and bush coupling
Fluid coupling
Vulcan coupling
Flexible coupling
Universal coupling
Articulate joints
What is meant by One ton Air conditioner, how fast it cool a room.
1 ton refrigeration means 210 kJ/min extracts heat from thesystem.
Boyle’s law
Boyle’s law describes the inversely proportional relationship between the
absolute pressure and volume of a gas.
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Types of dimensioning
Parallel Dimensioning
Parallel dimensioning consists of several dimensions originating from one projection line.
Superimposed running dimensioning simplifies parallel dimensions in order to reduce the space used on a drawing. The common origin for the dimension lines is indicated by a small circle at the intersection of the first dimension and the projection line. In general all other dimension lines are broken.
The dimension note can appear above the dimension line or in-line with the projection line
Chain Dimensioning
Chains of dimension should only be used if the function of the object won't be affected by the accumulation of the tolerances. (A tolerance is an indication of the accuracy the product has to be made to. Tolerance will be covered later in this chapter).
Combined Dimensions
A combined dimension uses both chain and parallel dimensioning.
Dimensioning by Co-ordinates
Two sets of superimposed running dimensions running at right angles can be used with any features which need their centre points defined, such as holes.
Simplified dimensioning by co-ordinates
It is also possible to simplify co-ordinate dimensions by using a table to identify features and positions.
Dimensioning Small Features
When dimensioning small features, placing the dimension arrow between projection lines may create a drawing which is difficult to read. In order to clarify dimensions on small features any of the above methods can be used.
