Precision & Tolerance Standards for CNC Parts

Specify CNC machining tolerances around fit, location and final-use requirements. Separate general drawing tolerances from individually controlled features, choose suitable datums and GD&T where needed, and confirm manufacturing and inspection requirements during DFM.

Start with the controlled drawing and its cited standards. Identify individual feature requirements, then the scope of any general tolerance note. Where CAD, a drawing or a PO disagrees, resolve the conflict and record the agreed revision before production. A company planning value does not replace a stated drawing requirement.

Where no individual or general drawing tolerance applies, ±0.10 mm is a common delivery basis, to be confirmed in the quotation. The sections below distinguish that basis from conditional feature capability and the limits agreed for your order.

Start with the relationship the assembly needs: a bearing fit, mounting distance, locating-hole pattern or sealing interface. Identify which dimensions and geometric relationships contribute before tightening individual limits.

Teaching example: two chained lengths

For two same-direction lengths of 20 ±0.10 mm and 30 ±0.10 mm, the nominal total is 50 mm. The shortest combination is 19.90 + 29.90 = 49.80 mm; the longest is 20.10 + 30.10 = 50.20 mm. The worst-case total is therefore 50 ±0.20 mm.

If the total distance is the functional requirement, discuss controlling that distance directly and revising the dimension scheme. Avoid retaining conflicting closed-chain requirements. This is a one-dimensional teaching calculation, not a customer result or a statistical analysis; it excludes geometric error, thermal effects and material movement.

For a mounting bracket, distinguish the face that supports the assembly, the face that orients it and the feature that locates it. Choose controls around those functions, with an inspection setup that can reproduce them.

GD&T communicates requirements for form, orientation, location and runout beyond a size tolerance. State the applicable standard and edition, such as ISO 1101:2017 or ASME Y14.5, and use its conventions consistently. Inch dimensions, UNC/UNF threads and ASME Y14.5 requirements need project-specific DFM and inspection-plan confirmation.

• Form: flatness, circularity and cylindricity control the feature itself without a datum reference. A flatness requirement is different from how that face is oriented to another face.
• Orientation and location: controls such as perpendicularity and position use the applicable datum references to define the required relationship. Choose datum features from actual support, orientation and location in the assembly.
• Profile: its datum references and freedom depend on the intended control and the drawing specification. Agree what the profile must relate to.
• Runout: relates a rotating surface to a specified datum axis. A two-point diameter reading alone does not establish circularity, cylindricity or runout.

Manufacturing and inspection need access to the specified datum features and a suitable support or restraint condition. Agree the method and alignment for each characteristic with inspection methods and coverage.

Material grade and condition affect the route used to hold a requirement. For plastics, review temperature and moisture sensitivity, wall stiffness, clamping and the supported or free state during measurement. About ±0.05 mm remains a planning reference that needs feature-specific review.

Heat treatment, coating and material removal can change size and shape after the initial machining check. State whether each critical feature is accepted before or after those operations, with any masking and final inspection responsibility.

Teaching example: a coated fit

If a precision bore must meet its fit requirement after the specified finish, mark that acceptance state on the drawing. Agree the finish specification, masking or allowance, and final bore check before machining. Coating thickness, outward growth and the resulting bore or outside diameter change are different quantities; a fixed 0.05 mm adjustment cannot be applied to every surface. Any model change needs customer agreement.

Specify surface texture where function requires it, with its unit, affected area and acceptance state. Ra alone does not define form, lay or cosmetic appearance, and no cutting process guarantees the same Ra on every material and feature. See surface finishes and dimensional allowances.

• Feature and purpose: drawing reference or feature ID, mating condition and the functional relationship to protect.
• Requirement source: controlled drawing revision, units, applicable standards and editions, individual limits and general-note scope.
• Material and state: grade, temper or condition, finish and masking, and the stage at which each critical feature is accepted.
• Datum references where applicable: identify the features and support or restraint condition needed to reproduce the requirement.
• Inspection and records: request the proposed method, every-part or sampled coverage, and the dimensional report, customer FAI form, MTR or CoC that must be quoted. These documents serve different purposes.
• Order context: quantity, prototype or repeat-batch intent, target delivery and any agreed DFM changes returned to the controlled drawing package.

Use CAD and drawing preparation for the complete submission checklist and inspection scope and required reports to agree how requirements will be evaluated. Named characteristics receive every-part checks when specified in the agreed order / inspection plan; other characteristics may follow the agreed sampling plan. Customer-specified FAI forms and drawing-characteristic coverage are available when specified and quoted.

For the RFQ form, use STEP (.step / .stp) and a marked PDF drawing where possible. Direct uploads accept .stp, .step, .igs, .x_t, .prt, .sldprt, .pdf, .zip and .rar. For .iges, DWG, DXF or .sldasm files, export suitable PDF/STEP or contact Jaquent to confirm delivery and dependencies. Archives count toward the limits; acceptance does not confirm their contents can be read. Attach up to 5 files, totaling 25 MiB. Attachments, material and quantity are optional form inputs; the checklist helps us review your request.

Upload CAD for DFM Review. With a complete RFQ we typically respond within one business day; complex multi-process work may take 1–2 business days.