A quote prices the whole requirement
The same shape may need different fixtures, separated roughing and finishing, controlled measurement and more complete inspection when hole size, spacing, flatness or position is tightened. Cost changes when the requirement becomes harder to make repeatedly and harder to prove.
Requirements also interact. A hole size can be straightforward until its position is tightly controlled from a datum machined in another setup. Quoting one isolated dimension can miss the process constraint created by the complete drawing.
Start with function: why does the tolerance exist?
A tolerance should protect clearance, sealing, location, motion, load transfer or appearance. Tight limits without a functional purpose convert directly into extra finishing, inspection and scrap risk. Every critical characteristic should have an answer to the question: what fails if this moves out of control?
Use process-appropriate general tolerances for noncritical dimensions and reserve explicit fits, datums and acceptance methods for features that control function. This is not lower quality; it is focused quality.
Size tolerances change tools and operation sequence
Bores, shafts and fits may move the process from drilling to boring, reaming, grinding or dedicated gauging. Thin walls, deep cavities, high aspect ratios and stress-sensitive material make the same numeric tolerance harder to repeat.
A supplier may rough the part, allow stress to redistribute, rebalance stock through another setup, and then finish. The added cost includes queue time, re-indication, tool control and in-process measurement—not just a slower final pass.
GD&T cost often comes from the datum chain
ISO 1101 defines the symbolic language for form, orientation, location and run-out. Flatness controls one surface, while parallelism, perpendicularity, position, profile and run-out connect features to datums or to one another. Difficulty cannot be ranked by the tolerance-zone number alone.
When a requirement spans several machined faces or setups, the supplier must establish, transfer and inspect one datum relationship. Datums that do not match assembly, loading or practical inspection can make the part both harder to produce and harder to accept.
- Align datums with the real locating and assembly logic.
- Explain how cross-setup relationships will be transferred and verified.
- Do not use GD&T symbols to hide an unclear functional requirement.
Surface texture and edge requirements are easy to underprice
A finer surface may require a smaller stepover, stable tooling, lower feed or secondary grinding and polishing. State where and in which direction it applies. Requiring one finish everywhere is very different from controlling only a seal or sliding surface.
Notes such as remove all burrs, no sharp edges or no visible tool marks also create manual work. Without an agreed edge range, appearance boundary or reference sample, suppliers have to include uncertainty in the quote.
Inspection can cost more than the added cutting
Calipers, micrometers, plug gauges, vision systems, roundness instruments, roughness testers and CMMs fit different features. Planning, fixturing, programming, calibration status and environment consume real capacity.
Full dimensional reports, 100% inspection, specified equipment, material certificates, first-article reports and traceable raw results all add quality work. Near a specification boundary, measurement uncertainty and the decision rule should be agreed before production.
- Separate first-article, in-process, sampling and 100% inspection.
- State whether reports require actual values or pass/fail only.
- Match the method to the tolerance and physical access of the feature.
Why one mounting plate can produce two very different quotes
Version A controls bore size, envelope and ordinary surfaces. Version B adds a positional requirement for the hole pattern, flatness on the mounting face, a specified surface texture and a full report for every part. The CAD shape can be identical while the route is not.
Version B may require datum tooling, separated rough and finish operations, CMM programming, inspection time and report control. If function permits, identify locating holes, seal faces and fits as critical and apply sensible general tolerances elsewhere.
Clean the tolerance scheme before issuing the RFQ
Make fits, critical characteristics, datums, surface requirements and acceptance clear. The model, drawing and notes must carry one revision and unit system, with an explicit precedence rule if they conflict. Clarity does not guarantee a low price, but it removes guesswork and change risk.
- Give every tight tolerance a functional reason and flag deformation-prone geometry.
- Check whether datums match assembly, machining and inspection.
- Define surface-texture zones, edge treatment and appearance boundaries.
- State equipment, sampling, report and first-article requirements.
- Ask for both the manufacturing and inspection approach, not only a total price.