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TOLERANCE ANALYSIS

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• 1.0 What is tolerance analysis ?

• 2.0 What is Tolerance Stackup ?

• 3.0 Generally, the Tolerance Stackup Process

• 4.0 Method and Types of Tolerance Analysis

• 5.0 Worst-case Tolerance Stackup

• 6.0 Assembly shift

• 7.0 Worst-Case Tolerance Stackup Example

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• Is a global term that includes two subcategories.

1.0 It describes the methods used to determine the meaning of individual tolerancingspecification

2.0 It is the process of determining the cumulative variation possible between two or more feature ( Tolerance Stackup )

First Step :

• Before a Tolerance Stackup can be performed, the dimensioning and tolerancing specification applied to a drawing must be clearly understood.

• Tolerancings specifications are complex, it takes training and practice to be able to fully understand tolerancing specification.

Second Step:

• By using tolerance stackup techniques, it allows the Tolerance Analyst to study the cumulative effects of multiple tolerances.

• A distance or displacement is chosen as the subject of the study (usually represents a nominal gap or interference)

• Typically the distance or gap between the features to be studied is not directly dimensioned or tolerance ( the distance between two parts that must not enough)

• Is a study of individual tolerances an their meaning, and it is the study of the cumulative variation between part feature.

• Are the means of analyzing and predicting that variation, regardless of whether the features only exist on paper or if parts have already been manufactured

Tolerance Analysis

Tolerance Stackup

• The most common application;

# to verify needed clearance or it may be to verify a needed interference condition.

# to verify fit

# determined the size location, and orientation of every clearance hole and tapped hole that receives a fastener

Some of this tolerance analysis are so simple that the engineer doesn’t even realize

that he or she is analyzing tolerances!!

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• Quite simple : Is a decision making tool

• By performing tolerance stackup, information is obtained that helps to answer one or more questions about a particular design.

• The result of tolerance stackup is almost always a minimum and maximum distance.

• The information obtained can be used to determined if a change must be made to the dimensions and /or tolerances of the parts being studied.

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• The distance to be studied is identified and labeled

• The positive and negative directions of the Tolerance Stackup are identified

• A Tolerance Stackup sketch is create

• The dimensions in the positive direction are added together

• The dimensions in the negative direction are added together.

• The negative direction total is subtracted from the positive direction total to find the “nominal”distance.

• All applicable tolerances are added together. This is the total possible variation.

• Half of the total possible variation is added to the nominal distance to find the Upper Limit for the distance

• Half of the total possible variation is subtracted from the nominal distance to find the lower limit for the distance.

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Two method

1.0 Manually modeled

Analyses done by hand, using pen and paper or

spreadsheet programs.

2.0 Computer Modeled

Analyses performed by computer statistical

simulation programs. Three-dimensional analyses

are best suited to computer-modeling tools.

• Tolerance stackups may be done on any toleranced part, or any assembly of tolerancedparts.

• A tolerance stackup cannot be done on a part or assembly that is not toleranced.

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Worst-case Tolerance Stackup determine the absolute maximum variation possible for a selected distance or gap. This distance is usually not dimensioned (it may have a reference dimension) and is not directly toleranced

This method assumes that all dimensions in the Tolerance Stackup may be at their worst-case maximum or minimum

• Tolerance Stackups as defined in this chapter follow a chain of Dimensions and Tolerances.

• The dimensions and tolerances in a Tolerance Stackup are called a chain of Dimensions and Tolerances.

• It is because the Dimensions and Tolerances that make up the Tolerance Stackup are arranged like the links in a chain, and followed head-to-tail from one end of the distance being studied (call it point A ) to the other ( call it point B)

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Step 1:Indicate the distance (gap or interference) want to analyze (asindicate A and B in figure 1.1).

Step 2: Change the dimension on the drawing to mean dimensions and mean tolerances (refer figure 1.2)· Mean Dimension= [(Upper Limit + Lower Limit) / 2]· Mean Tolerance= [(Upper Limit - Lower Limit)/2]

Step 3: Set the positive (+ve) and negative (– ve) direction. For this analysis, the rightdirection is +ve and left direction is – ve. (no rule to choose +ve and –vedirection but both of its must be opposite direction). (See figure 1.3).

Step 4: Draw the first vector. The first vector started at the one of the point to be analyzed. The second vector must start from the end of the first vector. The third vector will starts from the end of the second vector. The next vector will continue draw until the last vector which will finished at the Point B. (see figure 1.3). Figure 1.4 shows the chainof the vector starts from Point A and finished at Point B. The distance (gap or interference) between the starting point and finishing point will be analyzed.

Step 5: From the chain of the vector shows in figure 1.4, the dimensions and tolerances are filled into the Table 1.1. The totals of dimensions in positive and negative direction are calculated.The cumulative of the tolerance also calculated.

Step 6: Based on data on Table 1.1, the maximum and minimum distance gapare calculated by equation (1) and (2). From tolerance analysis done for this part, themaximum distance is 15 and the minimum distance is 8. This calculation can easy applied by using Microsoft Excel.

So, if the maximum and minimum distance (15 and 8) is not fulfilled the Product Designer, the changes can be make easily by changing any tolerances and dimension.The minimum distance = [(Total +ve direction) – (Total –vedirection)]-(Total Tolerance)= [(56.5) – (45)] - (3.5)= 8 ------------------------------------------------------------------------------ (1)The maximum distance = [(Total +ve direction) – (Total –vedirection)]+ (Total Tolerance)= [(56.5) – (45)] + (3.5)= 15 ---------------------------------------------------------------------------- (2)

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• The amount that part can move during assembly due to the clearance between a hole and a fastener, a hole and a shaft, a width and a slot or between any external feature within an internal feature.

• Assembly shift accounts for the freedom parts have to move from their nominal locations due to the clearance between mating internal and external features at assembly.

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• Consider an 8-mm fastener passing through a 10 mm hole, there is 2mm clearance and the part can shift 2 mm total, or ± 1 mm in any direction normal to its axis.

• Given the hole and fastener combination, it is apparent that the maximum assembly shift is possible when the hole is manufactured at its largest (LMC) size of 10.6 mm. The worst-case assembly shift is determined by subtracting the smallest possible fastener diameter from the largest possible hole diameter.

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The worst-case assembly shift applies to each part. Each part may shift +/-1.3mm relative to the fastener, leading to Total Assembly Shift of 2 *±1.3 = ±2.6

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• The tolerance Stackup example that follow increase in complexity from finding a minimum and maximum distance on a very basic part and to finding a minimum and maximum distance on a complex welded assembly.

• All of the examples are based on parts dimensioned and toleranced using the plus/minus (±) system.

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• A pin is the subject of study

• To determine the minimum and maximum width of the groove in the pin.

• The groove was not directly dimensioned and toleranced on the drawing.

• Unfortunately, drawings are not always dimensioned and toleranced functionally, and this example shows how tolerances may accumulate when the dimensioning and tolerancing scheme is not optimized.

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Pin with groove

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• A part like the one presented at the beginning of this slide

• To determine the minimum and maximum distance between two parallel surfaces on the part.

• The distance was not directly dimensioned and toleranced on the drawing.

• If this distance had been directly dimensioned and toleranced a Tolerance Stackup would not be required.

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• A simple assembly is studied.

• To determine the minimum and maximum distance between opposing surfaces on two parts in the assembly.

• The distance was not directly dimensioned and toleranced on the drawing.

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• An assembly with parts assembled in the vertical direction is studied.

• The assembly will be greatly affected by the force of gravity, which will most likely pull the bracket ( part number 3) down against the fasteners.

• The fasteners will in turn be pulled down against the holes in the hanger ( part number 2). This will add assembly shift to the chain of Dimensions and tolerances twice, one for the hole in the hanger and once for the hole in the bracket.

• It is assumed that the frame ( part number 1) and the hanger are fixed in space.

• To determine the maximum distance between the upper surface of the frame and the lower surface of the bracket

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• An inseparable assembly ( or weldment) is studied.

• To determine the maximum distance between part 5 and 6 in the assembly.

• The distance was not directly dimensioned and toleranced on the drawing.

• If this distance had been directly dimensioned and toleranced a Tolerance Stackup would not be required, as the minimum and maximum value could be easily calculated right from the drawing.

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• The end