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Switching to Centered Tolerances (±) – Practical Experiences in Onshape?
Domechanic
Member, pcbaevp Posts: 13 PRO
in General
Hi everyone,
We’re currently evaluating a shift in our tolerance specifications: moving from one-sided tolerances (e.g., 45 mm 0/-0.01) to centered tolerances (e.g., 45.005 ±0.005). Our main motivations are:
- Clarity in Design Intent:
Centered tolerances make it explicit that the nominal dimension (45.005) is the target, with equal allowance for variation in both directions. This reduces ambiguity and aligns with modern best practices for dimensional specifications. - Manufacturing and Inspection Simplicity:
Symmetrical tolerances (±) are often easier to implement in CNC programming and quality control. They simplify toolpath calculations and reduce the need for offset adjustments during machining. - Consistency Across Drawings:
Using centered tolerances as a standard can improve consistency, especially when working with external suppliers or across different teams.
Our questions to the community:
- Has anyone here switched to centered tolerances (±) in Onshape? What was your experience?
- Did you encounter any challenges with legacy drawings, suppliers, or specific part types (e.g., press fits, seals)?
- How did your manufacturing partners react to this change? Did it simplify communication or require additional explanation?
- Are there best practices or pitfalls to avoid when implementing centered tolerances in Onshape?
Background:
- We primarily use Onshape for both in-house and externally manufactured parts.
- Our goal is to make tolerancing more intuitive for both designers and machinists, without sacrificing functional requirements.
We’d love to hear your real-world experiences, tips, or even examples of how you’ve handled similar transitition.
1
Comments
I think in most industries/practices (that I have been exposed to anyway…) the "default" is to use centered tolerances.
As you say in today's world where a lot of work is done starting directly from the 3CAD files, it makes sense to draw the part in it's "nominal" size, i.e. in the middle of the tolerance range.
In most cases (i.e. if the tolerance is reasonable for the process used), then these is nothing more to do for the fabricator than program the part at "nominal" and inspect to make sure it's good. If you design the 3D file at the edge of the tolerance then some manual adjustment is always going to be required to fall within range.
If your process results in a normal distribution, it makes more sense for your tolerances to be symmetric. 45 mm 0/-0.01 is really just wishful thinking in my mind.
I'm not a stats person, but my understanding is that it makes a lot of the calculations easier to manage if everything is symmetric.
Simon Gatrall | Product Development, Engineering, Design, Onshape | Ex- IDEO, PCH, Unagi, Carbon | LinkedIn
Thanks a lot for the valuable input so far — especially the points about statistical distribution and programming at nominal. That aligns well with our thinking.
However, this raises a practical question in Onshape, especially when modeling multiple interacting parts in a single Part Studio — for example, a shaft and a sleeve.
Currently, many of our legacy models are built “0 on 0” (e.g., shaft Ø45.000 and bore Ø45.000), and the fit is only defined later via drawing tolerances. That works, but it doesn’t reflect the actual design intent in the 3D model.
If we want to model press fits or clearance fits more explicitly and still use centered tolerances, what would be the best approach?
You can still model at "size on", and then use a "move face" or "boolean with offset" to create the clearance (or interference). This does a good job highlighting design intent (especially if you name the feature that produces the clearance).
I would also recommend using a variable for that offset dimension to make it easier to edit (or multiple if you have different values you are going to use in a few places.)
See also this about using variables to relate clearances and tolerances.