Robust Design: Interface Design
The foundation of RD8 Software, Robust Design, and tolerance stack-up analysis is the principles of interface- and system-design of mechanical systems.
This page will elaborate on the principles.
One overconstraint in a system may drive 50% additional dimensions in a system.
One overconstraint in a part-part interface may drive +100% additional dimensions.
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INTERFACE DESIGN: System level
One of the arguments for avoiding overconstraints are that they add unwanted complexity - often invisible to engineering teams. Typically just a single overconstraint on system level can add more than 50% dimensions to a drawing.
We often see more than 10 overconstraints in moving assemblies with more than 15 moving bodies.
In the video below we explain how a single extra constraint in a simple assembly drives more than 50% additional dimensional requirements.
The principles are based on theory from moving mechanics - the mobility equation; also known as the Kutzbach equation, the Kutzbach criterion, or the Chebychev–Grübler–Kutzbach criterion.
Interface Design: Part-part LEVEL
- Axiomatic design in practice
The foundation for Robust Design ties to Axiomatic Design - the key elements is that if you can reduce the amount of information in a design - then do it!
Axiom 1 | The Independence Axiom
Maintain the independence of the functional requirements.
Axiom 2 | The Information Axiom
Minimize the information content in a design.
When dealing with mechanical models indented for mass production - it is paramount to understand the logic behind cause and effect. You want to be in control. You want the exact amount of information to be in control. No more, no less.
A crucial element for archiving high quality and reliability in mass production is to master tolerance stacks. When doing and reviewing tolerance stacks - you want to trust the calculations. The first thing you want when setting up a tolerance stack is predictability - a clear tolerance path (tolerance chain).
What is often overlooked are potential overconstraints in an interfaces between the mating parts.
So in essence you:
- Don't know which features that defines your tolerance stack
- Over specify drawings to counteract
- Missing to specify important dimensions
If you in example have 4 positioning features between two parts like shown below - the result is 5 overconstraints - which results in a staggering +700% increase in needed dimensions.
Time is lost for firefighting. Quality is lost. Too many parameters needs to be controlled to guarantee quality.
The usual 'go to' strategy is to improve manufacturing and processes instead of making better designs.
The breakdown
Let's examine an interface between two parts. For simplicity - let's only focus on the constraints in the xy-plane.
Zero Overconstraints
- A grey bottom part "Btm Part" - with two pins.
- A yellow top part "Top Part" - with a hole and an oblong hole.
This would be characterized as a Robust Design - if reviewed by the 'Interface X-ray Engine' in RD8 - you would see a clear datum scheme and zero overconstraints.
Let's say we want to be able to ensure predictability in the positioning scheme between the two parts.
- For the "Btm Part" we would need to dimension the two pin diameters. 2 dimensions.
- For the "Top Part" we would need to dimension the diameter of the hole on the left hole and the sloth width of the oblong hole. 2 dimensions.
- 4 dimensions in total.
One Overconstraint (The Lost Detail)
If just a little detailed is missed - let's imagine that the yellow part does not feature an oblong hole - the result is one overconstraint in the interface (could automatically be identified by RD8 Software) - then suddenly:
- For the "Btm Part" we need to consider the distances (x-y coordinates) between the pins. 2 additional dimensions.
- For the "Top Part" we need to consider the distances between the holes. 2 additional dimensions.
- 4 additional dimensions in total. 8 in total.
A 100% increase of dimensions.
Three Overconstraints (Three Pins)
Imagine that the designer wants extra strength of just even more precision in the interface - and decides to add an extra pin. An extra pin will result in two additional overconstraints. 3 overconstraints in total. Again this is automatically reveal by the RD8 Interface X-ray function.
Now it's starts to get complicated.
- For the "Btm Part" - the extra pin diameter. Ideally we need to consider the x-y coordinates between the 1st pin to the 2nd pin, 1st pin to 3rd pin and 2nd pin to 3rd pin. 5 additional dimensions.
- The same for the counter part - the "Top Part". 5 additional dimensions.
- 10 additional dimensions in total. 18 in total.
Three overconstraints = 350% increase of dimensions.
Five Overconstraints (Four Pins)
Let's say that the designer adds another pin for symmetry. Another pin results in 2 additional overconstraints. 5 overconstraints in total.
- For the "Btm Part" - an extra pin diameter. Ideally we need to consider all the possible relations between the pins. If just one dimension is off - either it will be impossible/difficult to assembly or the positioning scheme between the parts is gone - and hereby the predictability.
- The same for the counter part - the "Top Part". Ideally we need to consider all the possible relations between the holes.
- 14 additional dimensions in total. 32 in total.
A staggering 700% increase of needed dimensions compared to the starting point - the ideal constrained design.
It might seem like an extreme example. But in reality 5 overcontraints in an interface is more a commonality than an ideal interface design.
The Takeaway - Why? Clean Interface = Easy Tolerance Analysis.
Imagine a tolerance stack with 8 parts - resulting in 7 interfaces - with each 2-5 overconstraints.
Predictability is gone.
Control is gone.
Complexity has exploded.
In reality only a few of the dimensions have been marked on the drawings.
Test results are not reliable.
Typically the countermeasure is to improve manufacturing, tightening tolerances, increased QA, ..., endless testing.
Finally - it works - don't touch it.
Not to reflect on any cost related manners.
The Robust Design way would have been to ensure no overconstraints and optimize the tolerance stack to gain max control and cater for Axiom 2.
The lesson. A few overconstraints may seem like not noteworthy details but actually drives the need for 700% more dimensions.
The last centuries European and American companies have been focusing on improved manufacturing processes rather than better mechanical designs. Meanwhile there is a giant potential to improve!
RD8 Software has automated the detection of overconstriants directly from a CAD model to simplify designs before doing tolerance stacks on a poor basis. Cleaning interfaces before tole to gain quality and speed.
Stop guessing - start engineering with tolerance clarity
Tolerances shape everything - from product quality to manufacturing cost. Yet in most teams, variation analysis is disconnected, slow, and often skipped. RD8 wants to change that. With automated tolerance chain analysis and data-driven risk detection, engineers can now pinpoint which dimensions really matter, reduce over-specification, and build more robust products - faster.
Avoid Instability. Eliminate Overconstraints
Beyond basic positioning, the true power of robust mechanical design lies in the mastery of constraints in interfaces. This principle dictates that for every degree of freedom a body possesses, a precise constraint must be applied to fully define its position and orientation in space. An under-constrained system is prone to unwanted movement and instability, while an overconstrained system can introduce internal stresses, assembly difficulties, and reduced performance.
RD8’s cutting-edge tools allow engineers to manage the 6 degrees of freedom (3 translational, 3 rotational) for each component, ensuring every interface and connection is intentionally defined. By guiding you through the application of the right number and type of constraints – from fixed and revolute joints to advanced planar and spherical constraints – we eliminate ambiguity, guarantee predictable behavior, and enable the creation of truly robust and reliable products.
Control constraints, Prevent Failures
In the complex world of mechanical product development, the success of any design whether static or dynamic relies on a profound understanding of kinematics and constraints to master the interfaces in your designs. It is not always about movement, it’s about predictable, reliable positioning of your parts within defined boundaries. This is where constraints become critical.
From assembly processes to complex mechanical movement, every part of a mechanical system interacts in a precise manner, governed by these constraints. The constraints decide the design of interfaces and can control, size, location, orientation and form.
Creating robust interfaces and controlling the interactions between parts, can lead to costly late-stage failures, unexpected behaviors, and significant delays in market entry. RD8 provides the robust tools necessary to precisely define, analyze, and optimize these fundamental aspects, ensuring your designs perform exactly as intended.
Master tolerances with RD8 software
Instead of relying on gut feeling or Excel-based stack-ups, RD8 gives your team a modern, visual, and collaborative piece of software to manage variation. Whether you're designing precision mechanisms or high-volume components, we help you brings clarity to complex assemblies - and avoid costly rework, ensure functionality, and speed up development cycles.
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