During quotation or drawing review, a supplier may ask whether a tolerance on your custom part can be relaxed. The request may reduce cost or improve manufacturability—but it also raises an important question: is the supplier protecting your project, or protecting their own limitations?
You should not automatically agree—or reject—the request. A supplier asking to relax a tolerance may be identifying unnecessary cost, a genuine manufacturing challenge, or a limitation in their own capability. If the part has already been produced successfully in the past, an equally important question is why the previous supplier could achieve the requirement while the new supplier cannot. Once a drawing is changed, the risk of that decision often shifts to the buyer.
Before revising the drawing, it is worth confirming whether the tolerance still protects a critical function, whether other suppliers see the same issue, and whether there are better ways to manufacture the part without changing the requirement.
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What Should You Verify Before Relaxing a Tolerance on a Part Drawing?
Before relaxing a tolerance, verify four things: whether the tolerance protects a critical function, whether it has been successfully manufactured before, whether other capable suppliers raise the same concern, and what risks the project accepts if the requirement changes.
The first question is function. Some tolerances directly control assembly, sealing, alignment, motion, or product performance. Relaxing these tolerances may reduce manufacturing cost but also introduce functional risk. If the tolerance protects a critical feature, any change should be reviewed and approved by engineering rather than decided during sourcing.
The second question is production history. If the drawing is for a new design or prototype, the supplier may be identifying an over-specified requirement that has never been validated in production. However, if the part has been manufactured successfully for years, ask why the previous supplier could achieve the requirement while the new supplier cannot. The issue may be supplier capability rather than drawing design.
The third question is supplier validation. If multiple capable suppliers independently raise the same concern, the tolerance may represent a genuine manufacturability issue. If only one supplier requests a change while others can meet the requirement, the limitation may belong to the supplier rather than the drawing.
Before approving a tolerance change, ask the supplier to provide measurement capability, sample data, or historical production evidence showing why the original requirement is difficult to achieve. For prototype parts, many teams keep the original tolerance for initial validation and only relax it after testing confirms the feature is non-critical. For existing production parts, if previous suppliers have consistently met the requirement, it is often worth comparing with another capable supplier before revising the drawing. Experienced manufacturers usually treat tolerance changes as an engineering decision supported by production evidence—not simply a quoting decision.
Does This Tolerance Still Protect a Critical Part Function?
If relaxing the tolerance changes assembly, sealing, alignment, motion, load transfer, or product performance, then the tolerance is still protecting a critical function and should not be changed without engineering validation.
Some tolerances exist because a product simply will not work reliably without them. Bearing fits, sealing surfaces, optical alignment features, mating interfaces, and precision locating features are common examples. In these cases, even small dimensional changes may create assembly problems, leakage, wear, vibration, or accuracy issues that only become visible after the product enters use.
However, not every tight tolerance is functionally critical. Some dimensions are inherited from older drawings, copied from previous projects, or tightened conservatively during early development. This is especially common for prototype drawings that have never been validated in production.
Before changing the drawing, ask engineering a simple question: “What failure could occur if this tolerance is relaxed?” If no clear failure mode can be identified, many teams keep the original tolerance on prototype builds, complete functional testing first, and only relax non-critical dimensions after validation. Experienced manufacturers usually reduce cost by relaxing non-functional tolerances—not by changing dimensions that protect product performance.
Should You Change the Drawing—or Change the Supplier?
Validate the request before a tolerance change creates years of quality issues.
Is Your Supplier Identifying Unnecessary Cost—or Challenging a Critical Requirement?
If relaxing the tolerance does not affect function, the supplier may be identifying unnecessary manufacturing cost. If function, fit, or performance could change, the supplier may be challenging a critical requirement.
Tight tolerances increase machining time, inspection effort, fixturing complexity, and scrap risk. When a tolerance does not contribute to product function, relaxing it can sometimes reduce cost significantly without affecting performance. In these situations, supplier feedback may provide valuable DFM insight.
The situation becomes more complicated when the part has already been manufactured successfully in the past. If previous suppliers consistently met the requirement, the buyer should ask why the new supplier cannot. The request may reflect different equipment, fixturing methods, process capability, or supplier experience rather than a problem with the drawing itself.
Ask the supplier to explain why the feature is difficult to manufacture and what improvement they expect if the tolerance is relaxed. Experienced manufacturers can usually describe the specific challenge—such as tool deflection, fixturing stability, yield loss, or inspection difficulty—and quantify the expected impact on cost, lead time, or production stability. If the explanation remains vague, the issue may be supplier capability rather than unnecessary cost.
Did the Supplier Identify a Feature That Is Difficult to Manufacture Consistently?
If the supplier struggles with yield, tool wear, fixturing stability, or holding dimensions consistently across production, they may have identified a genuine manufacturability challenge rather than a simple capability limitation.
In custom-part manufacturing, making one good part is often much easier than making hundreds or thousands of good parts consistently. Features such as thin walls, deep pockets, long unsupported geometries, tight positional tolerances, cosmetic anodized surfaces, and precision mating features frequently become more difficult as production volume increases.
This is why some suppliers can successfully machine prototypes but later struggle during production. A tolerance that appears achievable on a few parts may create low yield, unstable processes, or excessive scrap during larger runs. In these situations, the supplier’s concern may be less about making the part and more about making it consistently and economically.
Before changing the drawing, ask whether the challenge affects only cost or also affects yield and production stability. Experienced manufacturers often solve these problems through improved fixturing, process optimization, additional inspection, or different machining strategies before recommending a drawing change. A tolerance relaxation should usually be considered after manufacturing solutions have been evaluated—not before.
Why Could the Previous Supplier Make It?
Find out whether the problem is your drawing—or your supplier.
Will Another Supplier Raise the Same Concern About This Drawing?
If multiple capable suppliers independently raise the same concern, the drawing may contain a genuine manufacturability issue. If only one supplier requests a tolerance change while others can meet the requirement, the limitation may belong to the supplier rather than the drawing.
Not all supplier feedback carries the same weight. A supplier with limited equipment or experience may struggle with a feature that another supplier manufactures routinely. At the same time, if several experienced suppliers identify the same tolerance as costly, unstable, or difficult to achieve consistently, the issue deserves investigation.
Production history matters as well. If the part has been manufactured successfully for years, ask whether the previous supplier used special fixtures, dedicated processes, or tighter process controls that are not immediately visible on the drawing. In some cases, the drawing is manufacturable, but hidden manufacturing know-how is missing during supplier transfer.
Before revising the drawing, compare feedback from multiple capable suppliers whenever possible. If several suppliers independently recommend similar changes, the drawing may truly benefit from optimization. If concerns come from only one supplier, it is often worth qualifying another manufacturer before changing a proven requirement.
What Happens if You Relax a Critical Tolerance on a Functional Feature?
Relaxing a critical tolerance can reduce manufacturing cost immediately, but it may create assembly failures, performance variation, shorter product life, or field failures later.
Critical tolerances often exist because they control how a product functions rather than how it is manufactured. A looser bearing fit may introduce vibration. A relaxed sealing dimension may create leakage. A positional tolerance change may affect alignment, accuracy, or assembly yield. The impact is not always visible during incoming inspection and may only appear during product testing or customer use.
This is why tolerance changes on functional features carry more risk than changes on cosmetic or non-critical dimensions. Once the drawing is revised and approved, future quality issues become much harder to trace back to the original decision.
Before approving a change, ask engineering what failure mode the tolerance was originally intended to prevent and how that risk will be validated after modification. Many teams verify changes through prototype builds, functional testing, or limited production runs before releasing the new requirement to full production. Experienced manufacturers usually validate critical tolerance changes with evidence rather than assumptions.
Are There Better Ways to Manufacture the Part Without Relaxing the Tolerance?
Yes. In many cases, experienced manufacturers improve process capability before recommending a drawing change, especially when the tolerance protects an important function or has already been proven in production.
A difficult tolerance does not automatically mean the drawing is wrong. Manufacturing challenges can sometimes be solved through better fixturing, different machining sequences, process optimization, alternative cutting strategies, improved inspection methods, or dedicated tooling. These improvements may increase process stability while keeping the original requirement unchanged.
This is especially common for long-running production parts. Previous suppliers may have developed fixtures, machining know-how, or process controls over years of production that are not obvious from the drawing alone. A new supplier may initially struggle until similar methods are developed.
Before changing the drawing, ask the supplier what manufacturing solutions were evaluated first. Experienced manufacturers usually explore process improvements before recommending tolerance relaxation, particularly for critical features. If the supplier immediately requests looser tolerances without discussing manufacturing alternatives, it may be worth seeking a second opinion.
What Risk Are You Accepting?
Confirm the trade-offs before approving a tolerance change.
When Should You Keep the Requirement and Look for Another Supplier?
Keep the original requirement when the tolerance protects product function, has been successfully manufactured before, or multiple capable suppliers can achieve it consistently.
A supplier’s inability to meet a requirement does not automatically mean the drawing is unreasonable. This is particularly true for mature products where previous suppliers have already demonstrated stable production over many years. In these situations, changing the drawing may create unnecessary engineering risk while solving only a supplier-specific limitation.
However, supplier replacement is not always simple. Different manufacturers use different machines, fixtures, inspection methods, and production strategies. Before changing suppliers, confirm that the requirement is genuinely achievable and not dependent on undocumented know-how from the previous supplier.
If function depends on the tolerance and other capable suppliers can meet it, keeping the requirement is often the safer decision. Experienced buyers frequently validate supplier concerns with additional quotations before changing proven drawings. Sometimes replacing the supplier carries less risk than changing a requirement that protects the product’s performance.
Conclusion
A supplier requesting a tolerance change is not automatically a warning sign—or a reason to revise the drawing. The key is understanding whether the request reflects unnecessary cost, a genuine manufacturability issue, or a supplier capability limitation. Once a requirement changes, the functional risk often shifts to the buyer. If you need a second manufacturing opinion on a drawing, tolerance, or supplier feedback, feel free to contact us to discuss your project.
Frequently Asked Questions
If a feature doesn’t affect fit, sealing, or alignment, standard tolerances usually perform the same — at lower cost and lower risk.
Repeated requests to relax specs, unclear answers to control questions, or variation showing up late in the batch are strong indicators that capability is not real.
Material stress changes shape when restraints are removed. If the supplier doesn’t re-establish datums afterwards, drift appears.
No. One part can be controlled manually. Only repeatable process control keeps every part within tolerance.
Ask for previous CMM reports on similar features, details on fixture strategy, and how they monitor tool wear during the run.
Many shops quote assuming standard capability, then discover too late that advanced control is needed. Without process planning, accuracy becomes reactive instead of managed.
