When sourcing custom machined parts, it’s common to encounter dramatically different quotations from various suppliers — sometimes varying by more than 300%. To the untrained eye, this may seem arbitrary or even suspicious. But in reality, these price gaps often reflect fundamental differences in how each supplier interprets the drawing, plans the process, and accounts for hidden risks.
Here are five key reasons behind these discrepancies, illustrated with a real case we encountered, where a supplier underestimated the process complexity — resulting in a revised quote more than four times higher than the original.
1. Different Interpretations of Dimensional Complexity
Not all suppliers interpret technical drawings in the same way. Some will recognize geometric dimensioning & tolerancing (GD&T) features and flag them as high-risk; others may assume the features are straightforward or within their capability.
In the case we experienced, the supplier initially quoted based on the assumption that a high-speed spindle (accelerator) could handle all the critical features — including hole locations and profile contours — in a single setup. However, they overlooked the fact that this method could not deliver the required surface finish for the datum face, nor maintain the profile integrity of radii and holes. These shortcomings made it impossible to achieve the tight center-to-center tolerance of ±0.001″, and ultimately required a shift to a more robust process involving both milling and turning to meet the specification.
2. Differences in Machining Capability and Workflow Planning
Suppliers vary widely in their ability to plan detailed workflows. Some assume all features can be handled in one setup, while others anticipate the need for multi-step, precisely sequenced operations.
In this case, the supplier’s original plan overlooked the need to maintain alignment between multiple features with tight positional tolerances. It became clear that the component required a milled datum surface for accurate referencing, followed by precision hole-making operations on a different machine, increasing both time and tooling complexity.
3. Material Quality and Tooling Assumptions
Not all suppliers quote using the same quality of materials or tooling strategies. One supplier may assume the need for special tools and cutting parameters to maintain tight tolerances, while another may simplify the setup — risking inconsistency.
In this instance, incorrect assumptions were made regarding tool stability at high speeds and the expected tool wear rate, both of which significantly impact long-term dimensional consistency and overall production reliability.
4. Inspection and Tolerance Verification Not Included
Low initial quotes often do not reflect the cost of inspection required for tight-tolerance components. If critical GD&T features are present — especially with ±0.001″ positional tolerances — precision fixturing and verification processes are essential.
The supplier in this case had not considered the time and cost of in-process inspection, fixture design, and post-machining verification. Once these elements were factored into the workflow, the total production cost increased substantially.
5. Strategic Underquoting to Win the Order
Some suppliers quote aggressively low at first, hoping to revise prices later or to simplify the process after winning the order. While this strategy might secure short-term customer interest, it often leads to re-quotes, delays, or disputes when production realities surface.
This was precisely what happened in our case — the supplier underestimated the part’s complexity and had to retract their quote and issue a much higher one once they revisited the process planning with more technical scrutiny.
Takeaway: Recognize the Cost of Precision Early
When requesting quotations, it’s important to evaluate not just the price, but also to understand how the supplier plans to meet your dimensional and tolerance requirements.
In our experience, the most dependable suppliers:
– Ask the right technical questions early in the quoting process,
– Review drawings from a manufacturing perspective (not just a commercial one), and
– Plan their processes to ensure fixture stability, process capability, and quality inspection are all aligned from the start.
By proactively identifying critical dimensions — especially those with tight tolerances relative to datum features — you can help suppliers avoid underestimation and ensure your project stays on time and within budget.
