Tolerance stack-up analysis is a powerful method to estimate the final variation on the key characteristic (KC) of an assembly before manufacturing.

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In the manufacturing industry, various methods bring products to life, each suited to different projects. Understanding these techniques is crucial to determining the best fit for your applications. Today, we delve into two transformative machining techniques: CNC machining vs conventional machining. Each method has distinct advantages and drawbacks, influencing modern production.

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The distance between the pulleys and the base on the tensioner should be maintained. Because, the pulley should not touch the base when the tensioner is in operation.

Different names refer to the same tolerance stack-up analysis, such as: tolerance analysis, tolerance-chain analysis, variation stack-up analysis and assembly-chain analysis.

Figure 7 shows the tolerance chain of the two parts’ assembly. The KC is the vertical distance between point A and point B and also shown in figure 7. In figure 7, we can observe there are many variation sources related to the centre of the pins and holes that are the assembly features.

The experience of design and manufacturing engineers play an important role on the first determination of tolerance values. Because, the value of tolerances are directly related to how difficult a part can be made and how much the cost to make the part.

It is important to note that datum A on both part 1 and 2 has the smallest (tightest) tolerance values compare to other tolerances. The reason is that datum A is the main reference of other datums and tolerances and should be manufactured with the highest accuracy compared to other features on part 1 and 2.

The nominal value of the KC (the clearance between the pulley and base) can be calculated by summing all the nominal values ($X_{n}$) for every feature (points) in the tolerance chain. The nominal value of the KC is:

Based on this method, the total variation is calculated by summing all the absolute values of $T_{x}$. Based on this method, all manufactured parts (base, support, pulley and rotor) should be inspected to assure that all parts are in tolerance.

Conventionalmachining process pdf

Designers use Computer-Aided Design (CAD) software to create digital 3D models of desired parts, which are then translated into instructions (G-code) that CNC machines understand. These machines precisely remove material from a workpiece, guided by computer-controlled motors, resulting in high-quality parts with precise dimensions and intricate details.

From figure 11, the tolerance chain is A—B—C—D—E—F—G—H—I—J—K—L—M—N—O. B,C,E,H are nominal dimensions so that their variations are zero. A,D,F,G,I,J,K,L,M,N,O are due to tolerances both dimensional and geometrical tolerances so that the mean value is zero.

The tolerance chain of the belt-tensioner is shown in figure 11. In figure 11, the KC is clearance or gap between the pulley and base, as being mentioned above, and is shown as green arrow in figure 11.

The main goal of tolerance analysis is to check that the dimensions and tolerances of components are correct so that after the components are assembled, the assembled product can function as desired.

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Statistical-based analysis is a tolerance analysis method that sum-of-squares all values of allocated tolerances. This method assumes some degree of confidences on the estimated sum-of-squares total variations ($2\sigma$ is 95% confidence).

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In this guide, we delve into both CNC and conventional machining, exploring their mechanisms, strengths, and limitations. Our goal is to empower you with the knowledge needed to make informed decisions for your project. But before we delve into their differences, let's understand their basic principles.

ECM machining

Ability to Produce Complex Shapes: CNC machines offer high manufacturing complexity capacity, excelling in creating intricate parts.

Where $ Tol_{i}$ is the $i$-the tolerance in equal-bilateralformat ($\pm Tol_{i}$) and $k$ is a safety factor to take into account variances from components supplied from other companies. In general, $k=1.5$ if components are supplied from other companies. If all components are made in-house, $k$ can be set to 1 (meaning the variations of components can be controlled because the components are made in-house).

For this analysis, the total variation is calculated by root-sum-squared all the $T_{x}$. The safety factor in this analysis is 1.5 considering some parts are made from other manufacturers.

Higher Potential for Human Error: Manual control introduces the potential for errors, increasing the risk of inconsistencies.

CNC machining

Types ofconventional machine

The other geometric tolerances on base are profile tolerance and position tolerance. The position tolerance controls the accuracy of the holes as the features to assemble other parts to the base.

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This smaller total variation implies that with statistical-based method, we can allocate higher values for the tolerance than with worst-case method. This higher tolerance values mean that the part can be manufactured and inspected at lower cost than the one with tighter tolerance.

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Tolerance stack-up analysis is unique because this analysis is half-science and half-art (depends on how we determine a tolerance chain)

Datum B and C are assigned to the side surfaces of datum A and are given perpendicularity tolerances with respect to datum A. All geometrical tolerances on this part will refer to these datum A,B and/or C.

Worst-case analysis is a tolerance analysis method that adds all maximum values of allocated tolerances. Worst-case analysis is formulated as:

Precision and Complexity of Parts: CNC machining offers higher precision and complexity compared to conventional machining.

Cost-Effectiveness: While CNC machining has a higher initial cost, it offers long-term cost advantages for large-volume production.

In this example, both tolerance analysis and allocation will be presented. The 2D tolerance analyses of the parts constituting the belt-tensioner assembly use both worst-case and statistical-based methods.

As explained before, with statistical-based method, the total variation is smaller than that calculated from worst-case method. In this case, the total variation calculated by using statistical-based method is 47% lower than that of worst-case method.

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We sell tutorials (containing PDF files, MATLAB scripts and CAD files) about 3D tolerance stack-up analysis based on statistical method (Monte-Carlo/MC Simulation).

The tolerance chain is shown as red arrows and is flowing through the features affecting the KC. The determination of this tolerance chain is half-science and half-art. Because, very often, there are several ways in defining the tolerance chain of an assembly.

These bonus tolerances are obtained because the hole and pin features deviates from the maximum material condition (MMC) of the holes and pins.

Finally, the dimension and tolerance for the pulley (figure 10) are similar to the rotor part as both the pulley and rotor are cylindrical parts. Datum A is the internal (middle) cylinder of the pulley. Position tolerances are applied to control the axis of the big cylinder to be coaxes with the middle cylinder (datum A).

Lower Initial Investment: Conventional machining tools are less expensive upfront, making them suitable for small workshops and startups.

CNC machining automates processes traditionally done manually, using computerized controls to shape materials with unparalleled precision. Conversely, conventional machining relies on manual guidance from skilled operators.

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Belt-tensioners are commonly found on various applications, such as timing-belt systems in car engines, chain tensioners in bicycles and conveyor belts in factories. The main components of a belt-tensioner are pulleys to bear the belts so that tensions can be applied to the belts.

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Table 3 shows the results of the 2D tolerance stack-up analysis based on worst-case method and table 3 shows the results of the analysis based on statistical method. As can be observed from table 3 and 4, beside assembly shift, other variations occur and are caused by bonus tolerances.

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Table 5 shows the detailed calculation of the mean ($X_{n}$) and variation ($T_{x}$) for each point on the tolerance chain in figure 11. In table 5, the mean and variation value for each point on the chain are presented. Note that the tolerance format is in equal-bilateral format.

Production Volume and Efficiency: CNC machines excel in high-volume production, offering automated and efficient processes.

Note that with this statistical-based method, we do not need to inspect all parts to be within their tolerances. Instead, we can sample parts from a batch production and inspect them.

In figure 6, datum A (again, the most stable and easy to access surface on a part) on part 2 is the bottom surface of the part and is also given a flatness tolerance. And, similar to part 1, the other datum B and C are assigned to the side surfaces of datum A.

Figure 4 shows the two parts used in this example. The assembly features are the two pins and holes of the parts. These pins and holes are the features that join the two parts.

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In figure 5, datum A (the most stable and easy to access surface on a part) on part 1 is the top surface of the part and is given a flatness tolerance, that is an unrelated tolerance (the family of form tolerance).

The design and assembly of the belt tensioner are shown in figure 8. Meanwhile, figure 9 shows the different 2D projection views of the belt-tensioner assembly.

Table 1 shows the analysis based on worst-case method. The direction of tolerance propagations that are relevant is only in horisontal direction. From table 1, the results of the total variation based on worst-case method is that the distance variation between point A and B is $5.325mm$ and the nominal distance is $50.33mm$.

There are four main parts constituting the assembly: base, support, rotor and pulley. The KC or the assembly key characteristic that should be controlled is the clearance between the pulley and the base so that there is no friction between the pulley and base during operation.

Skill Requirements for Operation: CNC machining requires less operator skill due to computerized control, ensuring consistent quality.

Precision and Repeatability: CNC machines replicate CAD drawings with near-perfect accuracy, ensuring precise results consistently.

What is Conventional Machining?Conventional machining involves shaping materials with manual tools and machines like mills, grinders, and lathes. Operators manually control these machines, adjusting cutting parameters based on experience.

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As can be seen in table 5, the nominal values ($X_{n}$) for all geometrical tolerances (at point A, D,F,G, I,J,K,L,M,N) are always zero. Because, in perfect condition, all geometrical variations should be zero.

Multilateration is a position determination method that is locating an object by accurately computing the distance between the object and three or more other objects (e.g., transmitter or laser tracker) with known positions. This method is also known as hyperbolic position determination.

In this example, an assembly consisting of two parts is used. The assembly use not only “+/-” tolerance, but also geometric tolerance (GD&T).

Figure 1 shows an illustration of the effect of mean-shift of the production process of a component. In this illustration, even if the process is under control (the process variations on both red and green situations are within $3\sigma$ tolerances), the produced component will be out of its tolerance.

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In figure 5 and 6, to tolerance pin and hole features, position tolerances are commonly used. The reason is that position tolerance also controls cylindricity of the pins and holes.

If you want more details and interesting 3D tolerance stack up analysis, we sell tutorials (containing PDF files, MATLAB scripts and CAD files) about 3D tolerance stack-up analysis based on statistical method (Monte-Carlo/MC Simulation). You can purchase from the given link.

The nominal dimension and tolerances (both “+/- ” and GD&T) of the parts are shown in figure 10. In figure 10, for the base, there are three datums: A, B and C. Datum A (selected due to the surface is the most stable and easy to access) is the main reference and has a flatness tolerance of $0.01 mm$, that is the tightest tolerance due to datum A is the main reference for all geometric tolerances on the base.

To perform the tolerance stack-up analysis, we need to define the tolerance chain of the part. Figure 3 shows the KC and tolerance chain of the part in figure 2. Tolerance chain describes the propagation of accumulated tolerances from point A to point B.

Tolerance stack-up analysis is a method used to evaluate the cumulative effect of tolerances allocated on the features of components and to assure that the cumulative effect is acceptable to guarantee the functionality of a product after assembly processes.

The detailed dimensions and tolerances of the two parts are shown in figure 5 and 6. It is worth to note that in figure 5 and 6 there are datums. These datums are the requirements of using GD&T, especially to tolerance related features.

By performing tolerance stack-up analysis, important questions regarding the assembly process and the final KC of a product can be answered before manufacturing, for examples:

However, with GD&T, the tolerancing of the part is better than only using “+/-“ tolerances. Because, with GD&T, the representation of the parts and assembly considers the real conditions of the manufacturing process of the parts so that the results of the tolerance analysis will be significantly more accurate than the analysis that only considers “+/-” tolerance (you may read this post for more explanations).

The smaller the tolerance value, the tighter the tolerance, and then the higher the production and inspection cost, and otherwise.

For the support (figure 10), the most stable surface for datum A is the bottom surface of the support with flatness tolerance of $0.01 mm$. The other datums are datum B and C that have perpendicularity tolerances with respect to datum A.

Efficient for High-volume Production: CNC machining minimizes manual intervention, ensuring quicker and more consistent production.

For the rotor (figure 10), the most stable feature for datum A is the main axis of the cylinder. Because, the rotor is cylindrical and does not have a large stable or flat surface. Geometric tolerances applied to the rotor are cylindricity to control the cylinder shape including the axis and position tolerances to control the axis of the middle cylinder.

This post will explain the method used to analyse tolerance/variation stack-ups in 2D. Examples are given to give a clear understanding on how to perform 2D tolerance stack-up analysis on parts.

The results of the tolerance analyses with the two methods are $(70\pm 2.485) mm$ for the calculation based on worst-case method and $(70\pm 1.26) mm$ for the calculation based on statistical method.

In this case we consider tolerance stack-up analysis in 2D, that is variation is considered in a planar plane and rotational variations are not considered.

But remember, there will be a small probability that the KC will be not met after assembly because there is a change that some defective parts pass the inspection.

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What is CNC Machining?CNC (Computer Numerical Control) machining is a subtractive manufacturing process that employs computerized controls and machine tools to shape raw materials into custom-designed parts. It integrates traditional machining with CNC technology, computer-aided design, and auxiliary manufacturing processes.

Difference between CNC andconventional machine

A belt-tensioner assembly is presented in this example. The belt-tensioner has a function to provide force so that the tension level of a belt can be maintained.

Since the analysis is 2D, we only consider variation in horizontal and vertical directions. Specifically for this case, only variation chains in vertical direction are relevant since the clearance (KC) is in vertical direction.

Every part constituting an assembled product has tolerances assigned to the part’s features. These assigned tolerances cause variations on the key characteristics (KCs) of the assembled part. How can we know how much the variations on the KCs before manufacturing the parts?

With the capability of estimating the final variation, design correction and improvement can be done at early design stages and can significantly save product development costs.

In figure 2, the tolerance is shown in two formats: unequal bilateral (figure 2 left) and equal bilateral (figure 2 right) formats. Both worst-case and statistical-based analyses require tolerances to be in equal bilateral format.

Tolerance allocation and tolerance stack-up analysis always come “hand-in-hand” and cannot be separated each other. Tolerance stack-up analysis and tolerance allocation are iterative processes that are carried out until the desired KC of an assembly is satisfied based on given geometrical and dimensional tolerances (GD&T) on parts.

Less Cost-effective for Very Small Batches: CNC machining may not be efficient for small-scale production due to high setup costs.

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Meanwhile, table 2 shows the analysis based on statistical method. From table 2, the total variation calculated by statistical-based method is $2.82mm$ with the same nominal dimension of $50.33mm$.

It is worth to note that, in general, with additional geometric tolerances, the total variation will be higher than in the case where only “+/-” tolerancing is used. The reason is that with geometric tolerance, the source of tolerance values is more than only use “+/-” tolerances.

A systematic approach on tolerance stack-up analysis is necessary to reduce the number of iteration processes to determine correct tolerance values on part’s features.

Higher Initial Investment: CNC machines require significant upfront investment, making them less suitable for small-scale businesses.

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Lower Precision and Repeatability: Conventional machining offers less precision and consistency compared to CNC machining.

In this example, the slot feature of a component made by a milling process is analysed. Figure 2 shows the dimensions and tolerances of the features on the component. The KC is the distance between the two slots.

Tolerance stack-up analysis cannot be separated with tolerance allocation. Because both tolerance analysis and allocation are an iterative process that is performed until the variation on the final KC below a threshold.

The datum B and C on the support is used as the references of the hole position to insert the rotor part. Other geometrical tolerances on the support are profile tolerances to control the surface profile of the surfaces.

Every part constituting an assembled product has tolerances assigned to the part’s features. These assigned tolerances cause variations on the key characteristics (KCs) of the assembled part.

Conclusion:The choice between CNC and conventional machining depends on specific needs and production objectives. While conventional machining may suit unique projects with lower upfront costs, CNC machining is preferred for high-volume production requiring precision and efficiency. Ultimately, understanding the capabilities of each method is essential for making the right choice for your project.