Robust Design: Definition, Examples, Principles, and Process
Robust Design is sometimes best explained by it's antomy: 'when something is very sensitive' - Robust Design is just the opposite of sensitive.
Taguchi defines the term robustness and describes it as a design with a functional output with minimal sensitivity to its input variations.
This is of great matter when talking high volume production and the scope is quality. In a high volume production - you will see all possible combinations of variation - and in sensitive designs - this will cause malfunction - in example on lifetime of the product or function. Imagine a medical device that is supposed to give you an accurate dosis of a certain drug - but due to an unlucky combination of parts - you end up getting too much or too little dosage. In cases like this Robust Design matters to always guarantee the desired functionality, product after product, day and night, hot or cold, ...
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Robust Design Breakdown
RD8 apply the Robust Design mindset into 8 disciplines - all in the context of being insensitive to variation.
RD8 Disciplines
Motivation
Context
Interface Design: Making sure that geometry defined in CAD is robust in the sense of predictability. Making a predictble model is step 1. When predictability is ensured - the CAD model and placement of features can be optimized.
When talking about robustness of the CAD model - the key is to focus on how interfaces between parts are designed - they are paramount for a design to be robust.
Interface Rule Examples
Interface Design - Rule Number 1: Making sure that there are no overconstriants is paramount for predictability and to ensure clarity in tolerance stacks. In the example with with two pins and two holes - the two pins are fighting to position in the x-direction - you have no chance to figure out which one that will position. If you leave clearance to one of the pins - you are always in control of which one that will position.
Interface Design - Rule Number 5: In this example - to make a nice fit - you would need to keep the whole surface intact - not allowing for the shaft to be convace at any point.
By making clever cutouts and well defined interfaces you can allow the 'non important' geometry to vary without having any effect on the function.
Another example: a flat planar surface. Looks simple. In reality - only 3 points (small surfaces) are needed. In one case you would need to keep a whole surface in control - typically done with GD&T modifiers such as flatness, planarity or form tolerance - instead of just making sure 3 plateaus are in control.
Interface Design - Rule Number 5: If you are working with moulded parts - make sure to account for flashes.
The full list of interface rules can be found in the RD8 Help Center embedded in RD8.Software. The rules and principles is built-in to the RD8 Software and is a part of the Interface Analysis tool and the Automated Interface Screening.
Functions and Sensitivity
The classic example is the wall bracket for a TV - if you wish to make the best horisontal alignment you are best of with a wide bracket instead of a narrow bracket.
First step is to ensure that the interface design is optimized - predictability is ensured.
When done, you can start to calculate and set up the transfer function/tolerance stacks that you can trust. It is also typically much easier to dissect a problem or design when the design is predictable.
This can be proven by determining a transfer function and analysing it - by sensitivity analysis. Sensitivity analysis is embedded in RD8.Software. In this simple example you will figure out that the distance, x, between the bracket holes is determining the angle, alpha.
If you double the distance from x to 2x, the angle error, alpha will to from alpha to alpha/2.
Design Drivers
Refers to the basic understanding of how to best achieving a given function. Let's say you want to make the best drawer/slider. The basic understanding for how to optimize this comes from understanding the physics behind it.
In this 'Guide Ratio Calculator' the math between a slider function is explained.
The design drivers for making the best slider possible is to:
- Minimize A (place the attack point - e.g. the drive of a motor or a belt - in the center of the slider)
- Maximize B (maximize the distance between the sliding surfaces) (and with respect to interface design - ensure that this is always a pair of sliders - instead of a full profile that in theory can be concave)
RD8.Software can be used to explore this by sensitivity analysis.
Also see this example with a syringe.
The Robust Design Lifecycle Model
The RD8 Robust Design Lifecycle model orchestras the principles in order/sequence in 5 steps - from project start to start of production.
Understand the system - define design drivers. Do cost planning. Decide on main functionality and requirements.
Design product architecture. Define ideal interfaces based on kinematic theory.
Describe transfer functions - set up tolerance stacks - do tolerance analysis - do sensitivity analysis - optimize for maximum robustness. Align with manufacturing processes (tolerance capabilities), DFM, DFA, material selection, and structural integrity.
Close the verification triangle. Establish coherence between calculations, CAD model, and prototypes. Progress from prototypes to real manufacturing processes.
Define inspections measures for CtQ. Monitor production. Check that measurements are within tolerances. Ensure feedback loop from production to designers.
Application of Robust Design
Robust Design theory can in practice be applied in all design cases - but may be overkill for a lot of one-off designs. RD8 specializes in application for high volume production of must-not-fail products in 3 categories:
- Automotive
- Industry (consumer products, B2B products, robots, ...)
- Life Science (auto injectors and medical devices)
Benefits of Robust Design
Robust Design has a direct impact on: Quality, Delivery and Cost. And is typically the most effective lever to make radical changes.
The logic is explained by:
Reduction of complexity start with elimination of overconstraints → less dimensions and tolerances on drawings → faster tooling/machining, less iterations, less quality control → better quality, faster speed, less cost.
Challenges and Limitations of Robust Design
The challenges of Robust Design is to master and orchestra the know-how and apply it in system context to get the full yield.
Often the hero is the production worker fixing an issue rather than the designer that prevented it day 1.
Care to making the ideal foundation and gather know-how early in the design process is key for success and easier said than done.
RD8 Software is a helpful tools when the foundation is made - to ensure predictability and robustness of the function. But the software does not dictate if you should design a gear or a four-bar mechanism to achieve the desired movement (as an example) or use a belt or a chain in your application.
What is Robust Design?
Robust Design is an engineering methodology focused on making designs insentive to variation.
Robust Design differs from traditional design approaches by focusing on design rather than manufacturing.
95% of profit is defined in the design stage*. That means that ultimatively the main power is at the design team. Reflect on this. Toyota have been open about their production system and has gained massive traction and reliability recognition both at customers and production specialists. But their product development strategy is very secret.
(Source: *The Secret Behind the Success of Toyota: How the Original Chief EngineerSystem Works to Generate Most of the Product Value and Profit, TakaoSakai, Independently Published, 3 Mar 2018)
Robust Design aims towards addressing the 'Design Quality' by making top performning mechanical design that serves the intented function at the lowest cost to offer the user most value for money and maximum profit to the company.
The primary goal is to meet specifications. But the goal of Robust Design methods is not just to meet specifications. But rather to be closer to the target value than being within the limits. And by staying within the limits with the minimum requirements to production/tolerances - hence being robust to any noise factors (part tolerances, assembly tolerances, use cases, temperature, ...) and to be robust to any variability in the product use case or production process.
One can buy the most expensive manufacturing equipment, buy the most expensive and experienced production worker, the best Quality Control systems and achieve a good product.
Robust Deisgn is about achieving a good product by being smart rather than by being extra careful and accurate in all dimensions.
Why Robust Design Matters in Engineering and Manufacturing?
Robust Design matters in engineering and manufacturing because being smart is the key to be competitive - incresing quality, increasing speed and lowering cost.
Increased Quality
Why it matters
The reason
Robust Design Methods ensure the minimum amount of dimensions to be kept in control together with optimal placement for robustness of functions
Fewer dimensions to keep in control with less strict requirements.
Better performance = better perceived quailty of customers.
Better reputation.
The logic
Increased Development Speed
Robust Design Methods ensures predictability from day 1. Ensures a guided approach for ideal placement of features to get it 'right the first time'.
Fewer iterations due to higher understanding and predictability of the concept, system, parts.
Lower Cost
Robust Design Methods ensure the minimum amount of dimensions to be kept in control with less strict tolerances
Parts are easier to source, easier/faster/simpler to manufacture, easier to inspect.
Assemblies are easier to test and verify.
Fewer warrently cost/claims/recalls.
1D Tolerance Stack-Up Analysis
2D Tolerance Stack-Up Analysis
3D Tolerance Stack-Up Analysis
What are the types of tolerance stack-up analysis?
1D: The width of the boxes and cradles are in scope.
2D: Imagine that the parts are not perfect along the y-axis. Any variation here should be included.
3D: Imagine that possible variation in the depth (z-axis) is included.
What are the methods of tolerance stack-up analysis
Worst-Case Tolerance Stack-Up Analysis
Root Sum Square (RSS)
Statistical
Monte Carlo
How to do tolerance stack-up analysis
Tolerance stack-up analysis is done by defining the functional requirement, modelling how dimensional variation accumulates, and calculating whether the assembly still meets its limits under real life variation.
Overview Of How To Do Tolerance Stack-Up Analysis
- Define the critical output you want to control (e.g., clearance, alignment, seal compression, stroke, force transfer, gap, contact pressure) - and accompany it by an illustration and clearly mark the point of interest (POI).
- Set limits: specify the acceptable min/max output range, not just nominal targets.
- Clarify operating conditions: what matters at room temperature, at end-of-life wear, under load, after assembly, or after environmental exposure?
- Decide the quality target: is it 100% of all samples that may pass or only a fraction? Sigma-levels is often used to define an acceptance rate. The term Six Sigma is popular and refers to that 3.4 defects per million opportunities (DPMO) is accepted.
- Trace the path from start to end.
You need to connect "one side of the POI" to the "other side of the POI". - Define a positive direction of the stack and if it is vertical, horizontal or slantered.
- Clearly mark start- and end-points of dimensions by leader lines.
- Mark each dimension on the illustration and give it an ID for reference.
This is typically refered to as the main part of tolerance stacking, tolerance chain, tolerance stack-up analyis.
- Link the parameter to the illustration - match the notation.
- Link the parameter with the given part or subassembly.
- Add a description for each parameter.
- Add/link the ID of the parameter with technical drawing or 3D model.
- Add the nominal dimenion.
- Add the assumed tolerances or lookup the tolerance from a tolerance class.
- For GD&T/GPS: translate geometric modifiers to linear tolerances.
- Specify distribution type or add production data (Cp, Cc, Cpk).
All four are standard approaches in tolerance analysis, each offering a different balance between simplicity, accuracy, and how realistic the predicted assembly variation. See full details in the section above.
1. Worst Case (WC)
Maximum safety margin
- Typically used for fits (scope of two mating parts)
2. Root Sum Square (RSS)
- Typically used for assemblies with more than 2 parts.
- Typically a bit too optimistic.
3. Statistical
- RSS approach - but taking process capability parameters into account (Cp, Cc, Cpk)
- Typically used for more precise estimates than a simple RSS if Monte Carlo functionality is not available (in an excel sheet or similar).
4. Monte Carlo
- Best for assemblies with more than 2 components.
- Best for most realistic estimates.
The calculation is often refered to as the 'trasfer function', 'the calculation model', 'total tolerance', 'tolerance accumulation', or 'tolerance equation'.
The calculation can be of different types:
- 1D stack: refered to as a plus/minus calculation based on geometrical dimensions.
- 2D and 3D stacks: more complex geometrical stacks that includes e.g. angles.
- Unlinear stacks: where a geometrical stack is enriched with other parameters, e.g. to calculate pressure, force or momentum.
Evaluate if the result is within the acceptable limits.
Also check if the mean matches the mean-target.
If the mean is far from mean-target the process could be more prone to failure when production starts as it is often more likely that the parameter will drift out of its limits due to wear and tear.
Write a conclusion.
Check for sensitive- and high impact parameters. Strive to mitigate these if possible by design.
Check for tight tolerances. Update the design to reduce the need for strict tolerances.
Check if it possible to relax tolerances.
Check and verify the tolerances with 2D documentation and part mesurement reports.
What the Typical Tolerance Analysis Guide Does Not Tell You:
The Basics - Tolerance Stack Up Example
How To Handle Clearances
Finding Your Tolerance Stack Path
Parameter Sensitivity and Optimization
Multidimensional Stacks - What is the Difference Between 1D, 2D, and a 3D Stack?
Including Forces and Deflections in your Stacks
How to Optimize a Tolerance Stacks by Design
What Are the Common Challenges in Tolerance Stack-Up Analysis
Avoiding common tolerance stack-up analysis mistakes improves product reliability and reduces costly rework.
Wrong tolerance path
Leads to - incorrect results and unpredicatable product behavior.
Avoid it - by identifying the correct tolerance path. The RD8 path finder feature automatically detects and set up the path for the user.
Conflicting tolerance path
Leads to - unpredictiable results (only right in some samples).
Avoid it - by checking for design clarity and overconstraints in the design prior to set up of the stack. The RD8 path finder feature automatically checks for possible paths to ensure that tolerance stack is unambigious.
Too tight and too many tolerances:
Leads to - high part cost, difficult sourcing and quality control.
Avoid it - simplify the tolerance stack by design to loosen tolerances. The RD8 Optimization feature helps with just that.
Wrong tolerance allocation:
Leads to - unnessary high part cost or assembly malfunction.
Avoid it - know you process capabilities and assign tolerances accordingly. The RD8 tolerance allocation function that is based on "ISO 286" and automatically looks up the suited tolerance based on the given process capability (IT grade).
Tolerances that is out of date
Leads to - assembly and functional product errors.
Avoid it - by keeping the 3D model, tolerance stack-up calcualtions and 2D drawing documentation in sync. RD8's parameter list gives an up-to-date overview of dimensions and tolerances that should go onto the 2D drawing.
Last minute changes
Leads to - functional failures if details are overlooked.
Avoid it - check all calculations after a change. The RD8 system is based on a global parameter list and the user can simulate effect of changes to all calculations before a change is rolled out.
Poor specifications
Leads to - that product design does not meet the user needs.
Avoid it - by (systematically) breaking down the user needs to a functinoal requirement. See the RD8 Critical-to-Quality approach.
Stacks are not done
Leads to - random 2D drawings without any reasoning for specifications resulting in failing product functions.
Avoid it - by setting up calculations/stacks for all critical functions. This is often skipped if it too complex to break down the function to a calculation or if it is too time consuming to set up an analysis. The RD8 system is made for quick analysis and easy set up.
Too late stack-up calculations
Leads to - need for unesssary strict toelrances (and the downstream effects hereof).
Avoid it - by making pre-CAD estimates to layout the product functions and tolerances stacks to define the most robust concept design possible (before it is too late to change anything). The RD8 system can be used up-front -before a CAD model is present - to estimate and layout optimal stacks.
Using too many GD&T modifiers
Leads to - ultra high part cost.
Avoid it - by being in control of the design. The RD8 Optimization feature augments to break down complex 3D-problems to simple 1D- or 2D-stacks.
Important dimensions are not marked on the 2D drawings
Leads to - unpredictable product functionality.
Avoid it - by taking all the elements from the tolerance stack analysis and mark them on the 2D drawings as inspection measures. Depending on criticality of the inspection measures the IPC (In Process Control) strategy can be defined.
What are the practical applications of tolerance Stack-up analysis
Automotive
To ensure performance for X years or XXX.XXX km in various conditions.
To ensure minimal part cost in a competitive landscape.
To maximize the user need with the lowest cost - to maximize profit.
Life Science
To ensure functionality - always.
Ensuring that close to 100% of devices always work (in harsh conditions) with intended performance (e.g. dosing accuracy)
Consumer Products
To balance functionality, durability and cost.
Maximizing value to the customer at the lowest production cost.
Bring new innovation to the market, frequently, with no delays.
What Are the Best Practices to Optimize Tolerance Stack-Up Analysis?
The best practices are simple - but hard to master in practice. The RD8 Academy offer courses to master tolerance optimization by design.
- Optimize the constraint-set (typically the tricks is to move positioning features as far away from each other as possible)
- Eliminate play (by incorporating local compliant features)
- Shorten the stack (bypass parts to make the stack shorter)
What Are The Benefits of Doing Tolerance Stack-Up Analysis Early
The benefits doing tolerance analysis early in the development phase is that it requires you to make a lot of important decisions that could often have been postposed with the result of unpleasent surprises.
Examples of benefits:
- Aligning on production capabilites to make a design that is fit for production (and not the other way around)
- Optimizing and simplifying designs before it is too hard to clean up
- Knowing what you are doing. Tolerance analysis forces you to describe product behaior by math and physics. Truly understanding these will save a lot of iterations (time and money) and yield a better product functionality - all in all - better quality to the customer
What is a Tolerance Stack-Up Analysis Software?
Software is used to go beyond excel spreadsheets to:
- Reduce manual errors
- Manage large data sets
- Run Monte Carlo simulations
- Keep track of versioning
- Collaborate seamless
- Look-up tolerances
- Streamline workflows and quality of analysis
- Make documentation of results and assumptions
- Speed up the design process
RD8 stands out as a tool that:
- Provides full overview of multiples stacks and relations.
- Can be used from the early design state
- Can be used through the design process and easily syncs with the 3D CAD model
- Work with 1D-, 2D-, 3D-, unlinear-calculations (can do Worst Case, RSS, Statistical and fast Monte Carlo Simulations).
- Does automatic tolerance stack-up detection and setup.
- Has a unique optimization feature - automatic identification and highlight of overconstraints. Checks DOFs in 3D.
- Is fast and easy to use
Can You Do Tolerance Stack-Up Analysis in Excel?
Yes, tolerance stack-up analysis can be performed in Excel. Thus Excel has it's limitations.
Excel is commonly used for simple or basic stack-up calculations, especially for linear dimension stacks and worst-case calculations.
Each country seems to have their 'excel template' that has been circulated and improved, tweaked, altered by each company.
Some excel templates can do RSS, some has a built in parameter list, some has more advanced macros.
When macros are present - you cannot work simultaneously in the sheets. You can in RD8.
When worst case and RSS is not enough - you can work with statisical and Monte Carlo simulations in RD8.
Typically you cannot apply assymetric tolerance in Excel - you can in RD8.
Filehistory is typically based on One-Drive - often random - in RD8 you work with iterations and user logs.
Excel is prone to errors - if you delete a row or delete a formula in the wrong cell - hell is loose - not in RD8.
Excel gives a lot of freedom to different style for annotation, tolerance setting and assumptions - in RD8 everything is uniform and streamlined.
In Excel you manually have to lookup suiting tolerances - in RD8 suiting tolerances can be looked up automatically.
In Excel - the workflow is scattered between different applications. CAD for screenshots. Power Point or Paint for making annotaions. Excel for the calculus. In RD8 everything is in one application.
And the list goes on...
How Can Engineers Learn Tolerance Stack-Up Analysis
Engineers can learn tolerance stack-up analysis through formal engineering education, hands-on design experience, specialized training programs, and the use of tolerance analysis tools.
RD8 offers:
- RD8 Academy: An engineering academy teaching the theory behind and techniques for how to optimize designs.
- RD8 Software: For working structured and efficient with tolerance analysis.
- RD8 Software Onboarding: Dedicated training courses in how to use all features in the RD8 Software - supported by skilled RD8 tolerance experts.
- RD8 Help Center: support center with online course material, user guides, examples and in depth explanations.
- RD8 YouTube Channel: features tutorials, cases, tips and tricks and much more.
- RD8 Cheat Sheets: quick reference guides to master tolerance design.
- RD8 Consulting: learning by doing. Working alongside skilled consultans gives a lot of hands on experience and saves a lot of costly learnings
What Is Included In RD8's Tolerance Stack-Up Analysis Training?
RD8’s training provides structured instruction on tolerance stack-up analysis, including theoretical concepts, practical methods, and real engineering applications.
- Understand Fundamental Tolerance Concepts
- Apply Geometric Dimensioning and Tolerancing (GD&T) Principles
- Perform Tolerance Stack-Up Analysis
- Design for Manufacturability (DFM) with Precision Tolerances
- Understand ISO and GPS Standards
- Interpret and Modify Technical Drawings Based on Standards
- Collaborate Effectively in Tolerance Design
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