--- title: "CNC Machining Tolerances & GD&T Guide: ISO 2768, Cost, and Design Rules" description: "Engineering guide to CNC machining tolerances: ISO 2768 m/f/c/v grades, GD&T datums and true position, and how tolerance selection drives machining cost." lang: en json-ld: | [ { "@context": "https://schema.org", "@graph": [ { "@type": [ "LocalBusiness", "ManufacturingBusiness" ], "@id": "https://www.olympusmachining.com/#organization", "name": "Olympus Machining LLC", "alternateName": "Olympus Machining", "url": "https://www.olympusmachining.com", "logo": { "@type": "ImageObject", "url": "https://www.olympusmachining.com/images/olympus-logo.png", "width": 300, "height": 100 }, "image": "https://www.olympusmachining.com/images/olympus-machining-headquarters-hanover-pa.jpg", "description": "Olympus Machining LLC is a U.S.-based precision CNC machining shop in Hanover, Pennsylvania, providing CNC milling, CNC turning, and prototype-to-production machining services for OEMs and manufacturers nationwide.", "telephone": "+1-717-634-5094", "email": "info@olympusmachining.com", "address": { "@type": "PostalAddress", "streetAddress": "639 Frederick St, Suite 1", "addressLocality": "Hanover", "addressRegion": "PA", "postalCode": "17331", "addressCountry": "US" }, "geo": { "@type": "GeoCoordinates", "latitude": 39.8007, "longitude": -76.9836 }, "areaServed": { "@type": "Country", "name": "United States" }, "priceRange": "$$", "sameAs": [ "https://www.google.com/search?sca_esv=2f4ed381b998a4d8&sxsrf=AE3TifODgJgBq4b1RAZEZnaHSJwX6ZOvSg:1767447357236&kgmid=/g/11kr8nzb2c&q=Olympus+Machining+LLC&shndl=30&shem=ptotplc,shrtsdl&source=sh/x/loc/uni/m1/1&kgs=100779a69d016c62&utm_source=ptotplc,shrtsdl,sh/x/loc/uni/m1/1", "https://www.instagram.com/olympusmachining/" ], "knowsAbout": [ "CNC Milling", "CNC Turning", "Precision CNC Machining", "Prototype Machining", "Short-Run Production", "Robotics Machining", "Quality Assurance", "First Article Inspection", "ITAR Compliance" ], "hasOfferCatalog": { "@type": "OfferCatalog", "name": "CNC Machining Services", "itemListElement": [ { "@type": "OfferCatalog", "name": "CNC Milling Services", "url": "https://www.olympusmachining.com/cnc-milling" }, { "@type": "OfferCatalog", "name": "CNC Turning Services", "url": "https://www.olympusmachining.com/cnc-turning" }, { "@type": "OfferCatalog", "name": "Precision CNC Machining", "url": "https://www.olympusmachining.com/precision-cnc-machining" }, { "@type": "OfferCatalog", "name": "Prototype to Production", "url": "https://www.olympusmachining.com/prototype-to-production" }, { "@type": "OfferCatalog", "name": "Robotics & Automation Machining", "url": "https://www.olympusmachining.com/robotics-automation-machining" } ] } }, { "@type": "WebSite", "@id": "https://www.olympusmachining.com/#website", "name": "Olympus Machining", "url": "https://www.olympusmachining.com", "publisher": { "@id": "https://www.olympusmachining.com/#organization" }, "potentialAction": { "@type": "SearchAction", "target": "https://www.olympusmachining.com/?s={search_term_string}", "query-input": "required name=search_term_string" } }, { "@type": "WebPage", "@id": "https://www.olympusmachining.com/resources/cnc-machining-tolerances-gdt-guide#webpage", "name": "CNC Machining Tolerances & GD&T Guide: ISO 2768, Cost, and Design Rules", "description": "Engineering guide to CNC machining tolerances: ISO 2768 m/f/c/v grades, GD&T datums and true position, and how tolerance selection drives machining cost.", "url": "https://www.olympusmachining.com/resources/cnc-machining-tolerances-gdt-guide", "isPartOf": { "@id": "https://www.olympusmachining.com/#website" }, "about": { "@id": "https://www.olympusmachining.com/#organization" }, "inLanguage": "en-US" }, { "@type": "BreadcrumbList", "itemListElement": [ { "@type": "ListItem", "position": 1, "name": "Home", "item": "https://www.olympusmachining.com" }, { "@type": "ListItem", "position": 2, "name": "Resources", "item": "https://www.olympusmachining.com/resources/cnc-machining-tolerances-gdt-guide" }, { "@type": "ListItem", "position": 3, "name": "CNC Tolerances & GD&T Guide", "item": "https://www.olympusmachining.com/resources/cnc-machining-tolerances-gdt-guide" } ] }, { "@type": "FAQPage", "mainEntity": [ { "@type": "Question", "name": "What is the default tolerance for CNC machined parts without a tolerance callout?", "acceptedAnswer": { "@type": "Answer", "text": "If a drawing does not specify tolerances, most U.S. precision CNC shops default to ±0.005 inch (±0.13 mm) on linear dimensions. Some shops use ISO 2768-m (medium) as the fallback: ±0.1 mm for dimensions up to 6 mm, ±0.2 mm for 6–30 mm, ±0.3 mm for 30–120 mm, and ±0.5 mm for 120–400 mm. Always state the default tolerance block on the drawing to avoid ambiguity." } }, { "@type": "Question", "name": "What is the difference between ISO 2768-m and ISO 2768-f?", "acceptedAnswer": { "@type": "Answer", "text": "ISO 2768-m (medium) is the general-purpose grade for machined parts: ±0.1 mm to ±0.5 mm depending on dimension range. ISO 2768-f (fine) is roughly twice as tight — ±0.05 mm to ±0.2 mm — and requires finish passes, slower feeds, and more inspection time. Fine grade adds 15–40% to machining cost compared with medium. ISO 2768-c (coarse) and ISO 2768-v (very coarse) are rarely used on precision CNC parts." } }, { "@type": "Question", "name": "How does GD&T true position affect CNC programming and inspection?", "acceptedAnswer": { "@type": "Answer", "text": "True position (position tolerance) defines a cylindrical tolerance zone around the theoretically exact location of a hole or feature, referenced to specified datums. In CAM programming, the machinist must maintain the hole center within that zone relative to the datums, not just hold linear ± tolerances on X and Y. On the CMM, true position is calculated from actual measured coordinates versus the datum frame, producing a single composite value rather than separate X and Y deviations. Tighter true-position tolerances (e.g., Ø0.005 inch vs Ø0.020 inch) require more rigid fixturing, slower machining, and 100% CMM inspection." } }, { "@type": "Question", "name": "Why do tight tolerances increase CNC machining cost?", "acceptedAnswer": { "@type": "Answer", "text": "Tight tolerances increase cost in four ways: (1) machining time — finishing passes, slower feeds, and smaller stepovers are required; (2) tooling — sharper, more wear-prone inserts and end mills must be changed more often; (3) fixturing — custom hard fixtures or pallet systems replace general vises to reduce setup variation; and (4) inspection — 100% CMM verification or SPC sampling replaces go/no-go gauges. As a rule of thumb, each halving of the tolerance band roughly doubles the machining and inspection cost on that feature." } }, { "@type": "Question", "name": "What datum strategy works best for CNC machining?", "acceptedAnswer": { "@type": "Answer", "text": "A good datum strategy for CNC uses machined features that are accessible in the first operation as the primary and secondary datums. Common practice: mill a precision reference face and two perpendicular edges in Op-1, then use that face as Datum A (primary, 3-point contact), one edge as Datum B (secondary, 2-point contact), and a second edge or corner as Datum C (tertiary, 1-point contact). This creates a repeatable Cartesian frame for every subsequent operation. Avoid using cast or raw-stock surfaces as datums unless the design intent specifically requires it, because as-cast variation can shift the entire part frame." } } ] } ] }, { "@context": "https://schema.org", "@graph": [ { "@type": "Organization", "@id": "https://www.olympusmachining.com/#organization", "name": "Olympus Machining LLC", "url": "https://www.olympusmachining.com", "logo": "https://storage.googleapis.com/gpt-engineer-file-uploads/SFKHg3gA6NVscESUCStAZeugW5B3/social-images/social-1762640845290-Olympus-Machining-Gear-Logo_Logo-ONLY.png", "description": "Olympus Machining LLC is a precision CNC machining shop located in Hanover, Pennsylvania, providing CNC milling and CNC turning services for OEMs and manufacturers nationwide.", "telephone": "+17176345094", "email": "info@olympusmachining.com", "address": { "@type": "PostalAddress", "streetAddress": "639 Frederick St, Suite 1", "addressLocality": "Hanover", "addressRegion": "PA", "postalCode": "17331", "addressCountry": "US" }, "sameAs": [ "https://www.google.com/maps/place/Olympus+Machining+LLC" ], "hasCredential": [ { "@type": "EducationalOccupationalCredential", "name": "ITAR Registration — DDTC", "credentialCategory": "Government Registration", "recognizedBy": { "@type": "Organization", "name": "U.S. Directorate of Defense Trade Controls (DDTC)" } }, { "@type": "EducationalOccupationalCredential", "name": "CMMC Level 1 Compliance", "credentialCategory": "Compliance Certification" } ] }, { "@type": [ "LocalBusiness", "ManufacturingBusiness" ], "@id": "https://www.olympusmachining.com/#localbusiness", "name": "Olympus Machining LLC", "image": "https://storage.googleapis.com/gpt-engineer-file-uploads/SFKHg3gA6NVscESUCStAZeugW5B3/social-images/social-1762640845290-Olympus-Machining-Gear-Logo_Logo-ONLY.png", "description": "Olympus Machining LLC is a precision CNC machining shop located in Hanover, Pennsylvania. We provide CNC milling, CNC turning, and prototype-to-production machining services for OEMs and manufacturers nationwide.", "address": { "@type": "PostalAddress", "streetAddress": "639 Frederick St, Suite 1", "addressLocality": "Hanover", "addressRegion": "PA", "postalCode": "17331", "addressCountry": "US" }, "telephone": "+17176345094", "email": "info@olympusmachining.com", "url": "https://www.olympusmachining.com", "priceRange": "$$", "areaServed": "United States", "aggregateRating": { "@type": "AggregateRating", "ratingValue": "5.0", "reviewCount": "15", "bestRating": "5", "worstRating": "1" }, "review": [ { "@type": "Review", "author": { "@type": "Person", "name": "Michael R." }, "reviewRating": { "@type": "Rating", "ratingValue": "5", "bestRating": "5" }, "reviewBody": "Olympus Machining delivered precise aluminum components for our robotics end-of-arm tooling on time and at a fair price." }, { "@type": "Review", "author": { "@type": "Person", "name": "Sarah T." }, "reviewRating": { "@type": "Rating", "ratingValue": "5", "bestRating": "5" }, "reviewBody": "We needed ITAR-compliant machining for defense hardware and Olympus Machining handled it professionally." }, { "@type": "Review", "author": { "@type": "Person", "name": "Dave K." }, "reviewRating": { "@type": "Rating", "ratingValue": "5", "bestRating": "5" }, "reviewBody": "Great CNC shop for prototype work. They provided DFM feedback that saved us time and cost." } ], "hasOfferCatalog": { "@type": "OfferCatalog", "name": "Olympus Machining CNC Services", "itemListElement": [ { "@type": "Offer", "itemOffered": { "@type": "Service", "@id": "https://www.olympusmachining.com/cnc-milling#service", "name": "CNC Milling Services", "url": "https://www.olympusmachining.com/cnc-milling", "provider": { "@id": "https://www.olympusmachining.com/#organization" } } }, { "@type": "Offer", "itemOffered": { "@type": "Service", "@id": "https://www.olympusmachining.com/cnc-turning#service", "name": "CNC Turning Services", "url": "https://www.olympusmachining.com/cnc-turning", "provider": { "@id": "https://www.olympusmachining.com/#organization" } } }, { "@type": "Offer", "itemOffered": { "@type": "Service", "@id": "https://www.olympusmachining.com/prototype-to-production#service", "name": "Prototype to Production Machining", "url": "https://www.olympusmachining.com/prototype-to-production", "provider": { 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Tolerances & GD&T Guide: ISO 2768, Cost Impact, and Practical Design Rules A vendor-neutral engineering reference for specifying tolerances and geometric dimensioning on CNC machined parts. Covers ISO 2768 general tolerances, GD&T datums and true position, perpendicularity and concentricity callouts, and how each tolerance band drives machining time, tooling, fixturing, and inspection cost. Written from the perspective of Olympus Machining, an ITAR-registered precision CNC machine shop in Hanover, Pennsylvania. ## ISO 2768 General Tolerances for CNC ISO 2768 is the international standard for general tolerances on linear and angular dimensions without individual tolerance callouts. It defines four classes — **m (medium)**, **f (fine)**, **c (coarse)**, and **v (very coarse)** — applied by a default block on the drawing title sheet. Most precision CNC shops in the U.S. either follow ISO 2768-m or default to an in-house ±0.005 inch (±0.13 mm) linear tolerance when no standard is specified. Dimension Range (mm) ISO 2768-m (medium) ISO 2768-f (fine) Typical CNC Fit 0.5 – 3 ±0.1 mm ±0.05 mm General bracket, cover plate 3 – 6 ±0.1 mm ±0.05 mm Standard housing feature 6 – 30 ±0.2 mm ±0.1 mm Mating faces, mounting pads 30 – 120 ±0.3 mm ±0.15 mm Base plates, chassis rails 120 – 400 ±0.5 mm ±0.2 mm Large structural frames 400 – 1000 ±0.8 mm ±0.3 mm Weldment-machined bases Values per ISO 2768-1:1989. Always place the callout "ISO 2768-m" or "ISO 2768-f" in the title-block tolerance block. ## GD&T Basics for CNC Machined Parts Geometric Dimensioning and Tolerancing (ASME Y14.5 / ISO 1101) defines the form, orientation, location, and runout of features relative to datums. On a CNC shop floor, GD&T callouts determine how the part is fixtured, which features are probed first, and how the CMM report is structured. ### Datums (A, B, C) Datums are theoretically exact planes, axes, or points that establish a coordinate frame. In CNC programming, the machinist physically creates this frame by probing or indicating the datum features during setup. A well-chosen datum strategy: - Uses features machined in the same setup or an earlier operation to minimize stack-up. - Aligns the primary datum face parallel to the machine table (3-point contact). - Uses two perpendicular edges for the secondary and tertiary datums (2-point and 1-point contact). - Avoids datums on thin, flexible, or non-machined surfaces that vary lot-to-lot. ### True Position (Position Tolerance) True position defines a cylindrical tolerance zone centered on the theoretically exact location of a hole, pin, or feature, relative to the specified datums. The tolerance value is prefixed with the diameter symbol (⌀) and is typically 1.5–3× the linear tolerance on each axis. For example, a hole located at 50 mm from Datum B and 30 mm from Datum C with a true position of ⌀0.15 mm means the actual hole center must fall within a 0.15 mm diameter circle around the theoretical center, not simply within ±0.075 mm on X and Y independently. - • **MMC modifier:** When the tolerance is applied at Maximum Material Condition (Ⓜ), bonus tolerance is available as the feature departs from MMC. This makes inspection more forgiving and can reduce cost. - • **LMC modifier:** Least Material Condition (Ⓛ) is used when minimum wall thickness or minimum clearance is the design driver. - • **Regardless of feature size (RFS):** No bonus tolerance; the stated zone applies at all sizes. Most expensive to machine and inspect. ### Perpendicularity, Parallelism, and Angularity These orientation controls define how closely a surface, axis, or center plane aligns with a datum reference. Control What It Constrains Typical CNC Impact Common Value Perpendicularity Surface or axis at 90° to datum Requires face-milling in same setup; avoids re-chucking error 0.05–0.1 mm Parallelism Surface or axis parallel to datum May require finish pass on both faces in one clamping 0.05–0.1 mm Angularity Surface or axis at specified angle Requires tilted fixture or 4th/5th axis; adds setup time 0.05–0.1 mm Concentricity Median points of feature align with datum axis Turned parts: verify on lathe with indicator; costly on milled bores 0.02–0.05 mm Runout (Total) Surface variation when rotated about datum axis Turned parts only; checked on V-block or spindle runout gage 0.02–0.05 mm ## Standard Tolerance Bands in CNC Practice The table below maps common tolerance callouts to what a precision CNC shop actually holds on a 3-axis or multi-axis mill, and on a CNC lathe with live tooling. Tolerance Band Milling (3-axis) Multi-axis / 5-axis CNC Turning Relative Cost ±0.005″ (±0.13 mm) Routine, single setup Routine Routine Baseline (1.0×) ±0.002″ (±0.05 mm) Finish pass, good fixturing Routine Routine +15–30% ±0.001″ (±0.025 mm) Finish pass, careful chip control Routine with probing Routine with sharp insert +40–80% ±0.0005″ (±0.013 mm) Controlled environment, rigid fixture Achievable Achievable on ground ways +100–200% ±0.0002″ (±0.005 mm) Difficult; temperature control required Difficult Grinding or honing often needed +300–500% Relative cost assumes the feature is critical and must be verified. Non-critical features at tight tolerances may be quoted at medium-grade price if the shop is confident in process capability. ## How Tolerance Choices Impact Machining Cost Tolerance selection is the single largest cost driver a design engineer controls. The relationship is nonlinear: halving the tolerance roughly doubles the cost on that feature. The four cost mechanisms are machining time, tooling wear, fixturing complexity, and inspection rigor. ### 1\. Machining Time Tighter tolerances require lower feeds, smaller radial stepovers, and more finishing passes. A pocket milled at 0.250″ depth of cut and 40 ipm feed might slow to 0.050″ depth and 15 ipm when ±0.001″ wall position is required. On complex 3D surfaces, toolpath density increases and spindle time can triple. ### 2\. Tooling and Tool Life At tight tolerances, tool wear becomes a significant error source. A 0.375″ carbide end mill with 0.002″ flank wear may still hold ±0.005″ but will fail ±0.001″. Shops compensate by reducing tool life limits, using premium coated cutters (AlTiN, ZrN), or switching to solid-carbide micro-grain end mills — all of which raise tooling cost per part. ### 3\. Fixturing General vises and toe clamps hold ±0.005″ adequately. For ±0.001″ and tighter, shops invest in custom hard fixtures with precision locating pins, vacuum chucks for thin parts, or pallet systems that maintain sub-0.0005″ repeatability across batches. Custom fixture amortization is built into the per-part quote. ### 4\. Inspection At ±0.005″, a caliper or micrometer check is sufficient. At ±0.001″, features are typically CMM-verified on every setup and sampled per lot. At ±0.0005″, 100% CMM inspection is common, and each part may spend as much time in the inspection room as on the machine. Tolerance Machining Time Tooling Cost Fixture Inspection Total Cost Multiplier ±0.005″ Baseline Baseline Vise / standard Spot-check 1.0× ±0.002″ +20% +10% Vise + locating CMM sample 1.3–1.5× ±0.001″ +60% +30% Custom fixture CMM per lot 1.8–2.2× ±0.0005″ +120% +80% Precision hard fixture 100% CMM 3.0–4.0× ## DFM Rules for Tolerancing CNC Parts - **Tolerance only what matters.** Calling out ±0.001″ on a non-critical mounting ear forces the shop to slow down everywhere. Use title-block defaults for general features and local callouts only for mating surfaces, seal lands, and bearing bores. - **Use GD&T position with MMC where possible.** A ⌀0.010″ true position at MMC allows bonus tolerance as the hole is drilled larger, making the part easier to machine and inspect without loosening the assembly fit. - **Avoid concentricity when runout or position will suffice.** Concentricity inspects median points and is difficult and slow on a CMM. Total runout or position tolerance is usually functionally equivalent and much faster to verify. - **Design for one or two setups.** Every additional setup introduces stack-up error. If perpendicularity of 0.05 mm is required between a face and a bore, machine both in the same clamping or on a tombstone with rigid pallet referencing. - **Match tolerances to the process.** A 3-axis mill with a 0.250″ end mill cannot hold ±0.0005″ on a deep, thin wall — that is grinding territory. Talk to your machinist about process capability before freezing the drawing. - **Specify surface finish with tolerance.** A 32 µin Ra finish on a bore that must also be ±0.0005″ on diameter will likely require a separate honing or lapping operation. Call out the finish only where it is functionally required. ## Tolerance Stack-Up in Assemblies When multiple machined parts mate, individual tolerances add statistically or worst-case. A common mistake is specifying ±0.001″ on every feature in an assembly of ten parts, producing an impossible overall fit. - • **Worst-case stack-up:** Add all linear tolerances algebraically. If Part A is ±0.002″ and Part B is ±0.002″, the gap can vary by ±0.004″. - • **RSS (root-sum-square):** For independent normal distributions, total variation ≈ √(t₁² + t₂² + … + tₙ²). Two ±0.002″ features stack to ±0.0028″ RSS — more realistic for production. - • **GD&T bonus tolerance:** MMC modifiers effectively loosen the tolerance zone when the feature is not at its maximum material limit, improving assembly fit without loosening the drawing. For critical assemblies, perform a tolerance analysis in CAD (e.g., CETOL, TolAnalyst, or manual spreadsheet) before releasing the drawing. Share the analysis with the CNC shop so they understand which features drive the assembly and which are cosmetic. ## Inspection Methods by Tolerance Band Tolerance Typical Inspection Tool GD&T Verification Report Type ±0.005″ Caliper, micrometer, height gauge Go/no-go pin, surface plate indicator Certificate of Conformance ±0.002″ Micrometer, bore gauge, test indicator Indicator on surface plate, optical comparator Dimensional report, CMM sample ±0.001″ CMM, bore mic, air gauge CMM with datum alignment, true position report CMM report with GD&T callouts ±0.0005″ CMM, air/electronic gauge, roundness checker CMM with scanning probe, form analysis Full AS9102-style FAIR ## Related Resources - • [CNC Machining Surface Finish Guide](/resources/cnc-machining-surface-finish-guide) — how Ra values and post-processes interact with tolerance bands. - • [How to Reduce CNC Machining Costs](/resources/reducing-cnc-machining-costs) — tolerance selection as the primary DFM cost lever. - • [Precision CNC Machining](/precision-cnc-machining) — our shop capability overview and typical tolerance bands. - • [Quality Assurance & Inspection](/quality-assurance) — CMM capabilities and first article inspection process. ## Need Tolerance Guidance for Your Next Part? Send your drawing to Olympus Machining and our CAM engineers will review tolerance feasibility, suggest GD&T callouts that reduce cost, and flag features that may require grinding or lapping instead of machining. [Request a DFM Review](mailto:info@olympusmachining.com) Last reviewed: May 12, 2026 [CNC Milling](/cnc-milling) [CNC Turning](/cnc-turning) [Quality Assurance](/quality-assurance) [Prototype to Production](/prototype-to-production) [Industries Served](/industries-served) [Materials Machined](/materials-machined) [Robotics & Automation Machining](/robotics-automation-machining) [Precision CNC Machining](/precision-cnc-machining) [ITAR CNC Machining for Mid-Atlantic Aerospace OEMs](/itar-cnc-machining-mid-atlantic-aerospace) [![Olympus Machining LLC logo — CNC machining shop Hanover, PA](/assets/olympus-logo-DbWcTX06.webp)](/) Olympus Machining LLC is a precision CNC machining shop in Hanover, Pennsylvania, providing CNC milling and turning services for OEMs and manufacturers nationwide. 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