You send an existing drawing for quotation, only to discover that a few tolerance callouts dramatically increase cost or trigger supplier concerns.
Yes, many production drawings contain tolerances that no longer protect product function but continue because they were inherited from prototypes, previous suppliers, or older manufacturing methods. During RFQ reviews, it is common to find tight tolerances that increase machining and inspection cost without improving assembly, performance, or reliability.
The challenge is distinguishing between tolerances that protect assembly, sealing, alignment, or reliability and those that only increase machining and inspection cost. Understanding that difference can reduce cost, expand supplier options, and avoid production risks caused by changing the wrong feature.
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Are You Paying for Manufacturing Precision That Doesn't Improve Part Performance?
Yes. During RFQ reviews, manufacturers frequently find production drawings where a small number of tight tolerances drive a large share of machining and inspection cost, even though those features do not affect assembly, performance, or product reliability.
It is common to see tight tolerances on non-mating surfaces, cosmetic features, or dimensions that do not control fit or function. Many of these tolerances were created during prototyping, copied from legacy drawings, or inherited from previous suppliers. Once released into production, they often remain unchanged because changing them requires engineering review and ownership.
The manufacturing impact is not always obvious. A single tight tolerance may require slower machining, additional setups, tighter process control, or more inspection time. Suppliers often quote to the most demanding requirement on the drawing because one difficult tolerance can influence process selection, yield, and inspection strategy for the entire part.
Tight tolerances should earn their place on the drawing. Ask suppliers which dimensions drive machining difficulty, inspection time, or scrap risk, and request quotations using both the current and proposed tolerances. If relaxing a tolerance reduces cost without affecting assembly or function, the precision may have been inherited rather than required.
Why Do Some Expensive Tolerances Survive for Years Without Being Challenged?
Expensive tolerances often survive not because they are still necessary, but because nobody owns the decision to change them. Once a drawing enters production, changing tolerances may require engineering review, requalification, customer approval, or validation work that teams prefer to avoid.
In RFQ reviews, it is common to find tolerances that originated from prototypes, early development stages, or previous suppliers. A tolerance that made sense years ago may no longer match current manufacturing methods, production volumes, or product requirements, yet it remains on the drawing because it has never caused an obvious failure.
Many organizations treat existing drawings as proven designs. As long as parts ship and customers do not complain, few teams revisit whether every tolerance still creates value. Over time, inherited tolerances become part of the drawing’s history rather than part of the product’s functional needs.
Legacy tolerances should not be changed simply because they are expensive, but they should periodically justify their existence. When suppliers identify costly tolerances, ask which machining operations, inspection steps, or setups those tolerances drive. If multiple suppliers repeatedly point to the same dimensions during quoting, those features usually deserve closer engineering review before production begins.
Are You Paying for Precision You Don't Need?
Determine whether expensive tolerances protect product performance or simply increase manufacturing cost.
When Does Relaxing a Tolerance Create Real Production Risk?
Relaxing a tolerance creates real production risk when that dimension controls how parts fit, align, seal, move, or repeat in assembly. The cost of a tolerance is visible during quoting, but the consequences of changing it often appear much later.
Some tolerances quietly protect downstream processes that are not obvious from the drawing alone. Hole position may determine assembly alignment, bearing fits affect motion and wear, and sealing features influence leakage performance. A dimension that appears unnecessarily tight in manufacturing may actually protect product performance in use.
This is why experienced manufacturers investigate function before cost. A tolerance on a cosmetic surface may be negotiable, while a tolerance controlling fit, motion, or alignment often deserves much greater scrutiny.
Before relaxing a tolerance, ask whether the feature controls assembly, sealing, alignment, or motion and whether similar parts have been successfully produced with wider tolerances. If suppliers cannot clearly explain the impact of the change, maintaining the original requirement until validation or production testing is completed is usually the safer decision.
Which Tolerances Should Never Be Changed Just to Reduce Cost?
Tolerances controlling assembly, sealing, motion, alignment, or safety are rarely good candidates for cost reduction because small dimensional changes can create disproportionate production risks.
Features such as bearing fits, sealing diameters, hole locations, datum relationships, optical alignment features, and mating interfaces are often changed cautiously because problems frequently appear during assembly or field use rather than during machining. A part may pass inspection and still fail when assembled into the final product.
A tolerance may appear expensive because it requires additional machining or inspection, but removing it can shift cost downstream into scrap, rework, customer complaints, or field failures. The cheapest dimension to machine is not always the cheapest dimension to own.
Before changing a costly tolerance, determine whether the feature affects validated assemblies, mating parts, qualification status, or long-term performance. If changing the tolerance requires requalification or creates uncertainty in product function, the savings in machining cost may be smaller than the downstream risk.
Before Relaxing a Tolerance, Verify the Risk
A small tolerance change can create assembly or performance issues that appear only after production begins.
Is Your Supplier Reducing Cost or Shifting Risk to You?
Suppliers may recommend relaxing tolerances to reduce genuine manufacturing cost, but they may also be reducing their own scrap, inspection burden, or production risk. The key question is not whether the change lowers cost, but who carries the risk after the change.
Not every tolerance reduction is unreasonable. Some drawings contain legacy tolerances that no longer serve a functional purpose, and adjusting them can improve yield, shorten lead times, and reduce cost without affecting performance. In these cases, both the supplier and customer benefit from the change.
However, reducing manufacturing difficulty and transferring manufacturing risk are not the same thing. A supplier benefits from easier machining, wider process windows, and lower inspection costs, while the customer may absorb the consequences if assembly issues, performance drift, or reliability problems appear later in production.
Supplier behavior often reveals where risk is moving. Requests to revise the drawing, widen tolerances through customer approval, or remove responsibility for functional performance after the change may indicate that manufacturing risk is shifting downstream. Statements such as “other customers accept this” or “this should be sufficient” are less valuable than demonstrated production data or proven application history.
Ask whether the supplier will continue inspecting to the revised tolerance, guarantee part performance, and accept responsibility for related quality issues. If the recommendation relies primarily on customer approval rather than proven production experience, the proposed savings may come with hidden manufacturing risk.
Why Do Different Suppliers Recommend Different Tolerance Changes?
Different suppliers recommend different tolerance changes because manufacturing capability, process planning, and inspection methods vary from supplier to supplier. A tolerance that is routine for one manufacturer may be expensive, unstable, or difficult for another.
Machine accuracy is only part of the equation. Two suppliers with similar equipment may still quote differently because fixturing, tooling strategy, operator experience, and inspection capability often influence tolerance achievement more than machine specifications alone. As a result, suppliers may use entirely different manufacturing routes for the same part.
Business models also affect recommendations. A supplier optimized for high-volume production may avoid tolerances that reduce throughput, while a precision-focused supplier may achieve them routinely. In many RFQ reviews, supplier recommendations reflect internal capabilities as much as engineering requirements.
When suppliers recommend different tolerance changes, ask which machining operations, setups, or inspection steps drive the cost and whether similar parts have been produced successfully under those conditions. If multiple capable suppliers identify the same tolerance as costly or difficult, the requirement itself may deserve review. If only one supplier raises concerns while others quote it routinely, the issue may be supplier-specific rather than design-related.
What Happens After a Tolerance Is Relaxed on a Production Part?
Relaxing a tolerance may reduce cost immediately, but its consequences often appear later in assembly, qualification, or field use rather than during inspection.
Tolerance changes rarely fail immediately. More often, parts continue passing inspection while assembly variation, fit issues, or performance drift gradually appear as production volume increases or parts from different lots are mixed. A change that looks successful during sampling may behave differently in full-scale production.
Production changes that succeed in prototypes do not always succeed at scale. Higher volumes, additional suppliers, lot-to-lot variation, and changing production conditions can expose issues that were not visible during early validation.
Another often-overlooked consequence is supplier lock-in. Once a drawing is modified to fit one supplier’s capability, future suppliers inherit the revised specification even though they may have been able to manufacture the original requirement. Over time, design intent can gradually disappear from the drawing.
After approving a tolerance change, monitor first-article inspection results, assembly yield, customer complaints, and early production performance across initial lots. Before updating related drawings or assemblies permanently, confirm that the change remains stable under actual production conditions. Restoring a tolerance later is often more expensive than maintaining it in the first place.
Is It the Drawing or the Supplier?
Find out whether the tolerance challenge is truly design-related or specific to your supplier.
Which Features on a Custom Part Justify Tight Tolerances?
Features controlling assembly, sealing, motion, alignment, and validated product performance usually justify tight tolerances because failures in these areas are often far more expensive than additional machining cost.
In custom-part manufacturing, bearing fits, sealing diameters, hole locations, datum relationships, optical alignment features, and mating interfaces are often prioritized because small dimensional changes can significantly affect performance. A part may pass inspection individually yet fail when assembled into the final product.
Experienced manufacturers protect these features first because assembly failures, field returns, and qualification issues are usually more expensive than tighter machining. The lowest machining cost does not always produce the lowest total production cost.
When evaluating cost reduction opportunities, relax non-functional features first and preserve tolerances that affect fit, sealing, motion, or validated performance. If a feature controls mating parts or qualified assemblies, maintain the tolerance unless testing demonstrates that wider limits perform equally well. Tight tolerances create the most value when the cost of failure exceeds the cost of precision.
Conclusion
Not every tight tolerance creates value, and not every tolerance reduction creates risk. The challenge is distinguishing between precision that protects product function and precision that only increases manufacturing cost. Before changing a proven drawing, understand what each tolerance controls and whether the limitation is truly design-related or supplier-specific.
If you’re evaluating tolerance changes or comparing supplier recommendations, feel free to contact us for a drawing review or second manufacturing opinion.
Frequently Asked Questions
No. This is the most expensive mistake in product development. That “safety factor” typically doubles or triples cost with zero functional benefit. Instead, identify truly critical features (mating, sealing, aligning) and tolerance only those tightly. We analyzed 500 parts where clients loosened “safety” tolerances — 100% worked perfectly at 40-60% lower cost.
Specifying ±0.01mm on deep holes over 5xD. This single callout can triple your part cost because it requires specialized tooling, pecking cycles, and sometimes EDM. Unless it’s a bearing bore or precision alignment feature, ±0.05mm works perfectly fine and costs 60-70% less. We see this mistake on nearly every drawing we review.
The ±0.02mm threshold is where standard machining ends and specialty work begins. Below this, you’re paying for climate-controlled environments, premium tooling, and 100% CMM inspection. Each step tighter doubles cost: ±0.05mm to ±0.02mm adds 50-80%, but ±0.02mm to ±0.01mm multiplies by 2-4x. Only specify below ±0.02mm for proven functional requirements.
Divide wall thickness by 100 — that’s your practical tolerance limit. A 2mm wall can realistically hold ±0.02mm; a 1mm wall struggles at ±0.01mm even with special fixturing. Below this ratio, expect quotes 3-5x higher or shops declining to quote. If you need ±0.01mm, design walls at least 3mm thick in aluminum or 5mm in stainless.
Yes. ISO 2768-m provides ±0.1-0.3mm based on feature size — perfectly adequate for 70-80% of part features. Boeing, Mercedes, and medical device companies use it standard. Parts made to ISO 2768-m look identical to tighter-tolerance parts. The savings come from reduced inspection, faster machining, and standard processes. Add “Unless specified: ISO 2768-mK” to immediately cut costs.
Rarely. Most shops quote exactly what you specify to avoid liability. They won’t suggest loosening tolerances even when they know it’s unnecessary. That’s why design review is critical — you need to optimize before quoting. At Okdor, we’ll suggest cost-saving relaxations, but only after confirming they won’t affect function.
