The "Decimal Point Tax": How Smarter Tolerancing and Onshape MBD Can Slash Your CNC Costs by 35%

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In the world of hardware startups, there is a silent killer lurking within your CAD files. It isn’t a flaw in the physics of your design, nor is it a lack of market fit. It is much more subtle: It is the "Decimal Point Tax."

Recently, I stepped in to assist a small engineering startup that was on the verge of a financial crisis. Their initial CNC quotes for a relatively simple batch of aluminium parts had come back nearly double their projected budget. Their first instinct was that the machine shops were price-gouging.

My instinct? The drawing was the problem.

The High Cost of Fear-Based Design

When I opened their manufacturing package, the issue was immediately apparent. Because their CAD software was set to a "standard" precision default, every non-critical feature on the part carried a tolerance of $\pm 0.05 \text{ mm}$.

To a designer sitting behind a screen, $\pm 0.05 \text{ mm}$ feels like a "safe" number. To a CNC machinist, that number is a mandate for high-precision setups, slower feed rates, and specialised inspection tools. They were asking for aerospace-level precision on a simple cover plate that functionally only needed to keep dust out of a gearbox.

They were paying for precision they didn't use.

The Axis and Datum's DFM Process

At Axis and Datum's, we don't just "draw parts." We engineer outcomes. My goal wasn't to change the geometry; it was to re-engineer the documentation for reality. We followed a three-step Design for Manufacture (DFM) audit:

1. The Functional Review

We mapped every single feature to its mechanical purpose. We asked the hard questions:

  • Does this surface touch a moving part?

  • Is this a press-fit for a bearing?

  • Or is this simply "air" where a tolerance of $\pm 0.2 \text{ mm}$ would be more than sufficient?

2. Implementing Smart Defaults (ISO 2768-m)

We stripped away the "Decimal Point Tax" by applying ISO 2768-m (medium) general tolerances to 90% of the part. This tells the machinist that for non-critical features, standard machine shop tolerances apply. This immediately signals to the shop that the part can be run on standard equipment at high speeds.

3. Targeted Precision

We reserved the "expensive" tolerances for the features that actually mattered—a bearing journal and a key alignment hole. By focusing the precision where it was functionally required, we maintained the part's integrity while making it exponentially easier to produce.

The Outcome: The revised package went back to the same machine shops. The new quotes came back 35% lower. The project was back on budget, and the "Decimal Point Tax" was abolished.

The Future: Moving Beyond the 2D Drawing with Onshape MBD

While fixing 2D drawings is a vital service we provide, the industry is moving toward a more efficient "Single Source of Truth."

At Axis and Datum's, we’ve recently undergone a major technical evolution. I’ve migrated our entire workstation to Pop!_OS Linux. Why? Because engineering requires a lean, stable, and high-performance environment—free from the "bloatware" and forced updates of traditional operating systems. This same philosophy of "lean efficiency" is why we have fully integrated Onshape Model-Based Definition (MBD) into our workflow.

What is MBD and why should you care?

Traditionally, the 3D model was for design, and the 2D drawing was for the shop floor. This "gap" is where mistakes happen. With Onshape MBD, we embed Product Manufacturing Information (PMI) directly into the 3D model.

  • Zero Ambiguity: Design intent is baked into the geometry. When you click a hole in the 3D model, the tolerance and GD&T data are right there.

  • Intersectionality by Design: MBD forces the designer to think about manufacturing while they are modelling, rather than as an afterthought on a drawing sheet.

  • Browser-Based Collaboration: Using Onshape’s cloud-native platform, I can share a live 3D model with your machinist. They can see the critical tolerances in their browser—no PDFs to download, no "missing font" errors, and no outdated versions.

Stop Overpaying for Your Designs

Whether you are a startup trying to get your first MVP off the ground or a firm looking to optimise a legacy product for the supply chain, the documentation is just as important as the design.

At Axis and Datum's, we bridge the gap between the CAD screen and the shop floor. We ensure your parts aren't just "theoretically possible," but economically manufacturer.

Are you ready to audit your current designs for the "Decimal Point Tax"?




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