CADFamily.com: GD&T Tolerance Stack-up Analysis PartA

Chapter 1

Tolerance Stack-up Introduction

TSU Introduction

What’s TSU?

What issues TSU can solve?

Function of TSU

Categories of TSU

What’s TSU?

Definition of TSU

A tolerance stack-up is a calculation which determines the theoretical maximum or minimum distance between two features on a part or in an assembly.

Tolerance allocation—the process of determining the allowable manufacturing tolerance on a piece part of assembly dimension to achieve the given function.

Tolerance analysis—A mathematical technique to assess the tolerance variation across several dimensions or parts. To confirm that form, fit and function have been achieved.

What issues TSU can solve?

Typical issues including:

Will two surfaces interfere?

Will the pin fit the hole?

If reduce the size of the clearance holes, will the parts still assemble well?

How to control tolerance

Function of TSU

Optimize the tolerance of parts and assemblies in a new design.

To achieve the most cost effective design which will meet all functional requirements

Determine the part tolerances required to satisfy a final assembly condition. required to satisfy a final assembly condition.

Determine the allowable part tolerance s if assembly tolerance is known.

Troubleshoot malfunctioning existing parts of assemblies.

Explore design alternatives using different or modified parts

Categories of TSU

Two TSU categories are part TSU and assembly TSU.

Chapter 2

Linear Tolerance Stack-up

Linear TSU

TSU calculation steps

TSU Summary

TSU Exercises

TSU calculation steps

Determine the distance to be analyzed

2. Determine the positive and negative direction

a) Dimensions spanning the distance A-B:“+”

b) Dimensions decreasing the distance A-B :“﹣”

3. Convert all dimensions and tolerance to equal bilateral format

a) Convert dimensions and tolerance to limit dimensions

b) The maximum limit subtracts the minimum limit to get total tolerance

c) The total tolerance is divided by 2 to get the±bilateral tolerance )

d) The maximum limit subtracts half of tolerance or the minimum limit adds halfof tolerance to get the median dimension)

For example: 8.5 +0.25 -0.10

（Upper limit)=8.5+0.25=8.75

（Lower limit)=8.50-0.10=8.40

Total tolerance=8.75-8.40=0.35

Divide the total tolerance by 2 to obtain the equal bilateral tolerance value.

Equal bilateral tolerance value=0.175

The median dimension is equal to either:

8.75-0.175=8.575 or 8.40+0.175=8.575

The median dimension with its equal bilateral tolerance:

8.575±0.175

4.Put all dimensions and tolerances into a chart. Place each positive dimension value in the “+”column, and each negative one in the “－” column. Then Add the entries in each column, entering the results at the bottom of the chart

5.Subtracting the negative total from the positive total gives the nominal dimension of distance. Adding or subtracting the total tolerance from the nominal dimension gives the maximum and minimum distance values.

TSU Summary

Linear TSU Calculation Steps:

1. All dimensions and tolerances must be converted to equal bilateral format.

2. Draw a vector diagram indicating the path that each dimension must be included in TSU

3. In a vector diagram, the direction of each successive dimension from the starting surface is indicated by an arrow and is given a positive or negative sign

4. After drawing the complete vector diagram, enter the dimensions and tolerances in the chart

5. Total the positive and negative columns as well as the tolerance column. Subtracting the negative total from the positive total gives the nominal dimension of distance

6. Adding or subtracting the total tolerance from the nominal dimension gives the maximum and minimum distance values.

TSU Exercises

TSU Exercise 1

Calculate the max and min distance of A-B