Learn how to specify CNC machining tolerances, surface roughness, and finishing requirements for better CNC parts in this practical guide for engineers, product developers, and procurement teams.<\/p>\n
Introduction<\/h2>\n
CNC machining can deliver highly accurate, functional, and production-ready parts \u2014 but the final result depends heavily on how tolerances and surface finishes are specified.<\/p>\n
This guide helps engineering and procurement teams understand:<\/p>\n
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How CNC tolerances affect cost, lead time, and inspection<\/p>\n<\/li>\n
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When to use standard tolerances vs. tight tolerances<\/p>\n<\/li>\n
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How surface roughness impacts part performance<\/p>\n<\/li>\n
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How finishing processes may affect dimensions<\/p>\n<\/li>\n
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What to prepare before submitting CNC projects for quotation<\/p>\n<\/li>\n<\/ul>\n
Designed for: Engineers, product developers, purchasing teams, hardware startups, OEMs, and industrial manufacturers.<\/p>\n
<\/figcaption><\/figure>\n<\/p>\n
Why Tolerance and Surface Finish Matter<\/h2>\n
In CNC machining, tolerance and surface finish are not just technical details. They directly affect whether a part fits, functions, assembles, and performs reliably in its intended application.<\/p>\n
A part can be beautifully machined but still fail if a critical hole is out of tolerance. Likewise, a dimensionally accurate part may still underperform if the surface is too rough, too smooth, or incompatible with its final coating.<\/p>\n
For engineers and procurement teams, the key question is not simply:<\/p>\n
\u201cHow tight can this part be machined?\u201d<\/strong><\/p>\n
The better question is:<\/p>\n
\u201cWhich features truly require tight control, and which can follow standard machining tolerances?\u201d<\/strong><\/p>\n
Over-specifying tolerances can increase machining time, inspection complexity, scrap risk, and cost. Under-specifying tolerances may create assembly problems, rework, or performance failures.<\/p>\n
This guide provides a practical framework for choosing tolerances and surface finishes that are precise enough for function \u2014 without adding unnecessary cost or complexity.<\/p>\n
Key Terms Engineers Should Know<\/h2>\n
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<\/colgroup>\n
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Term<\/p>\n<\/th>\n
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Definition<\/p>\n<\/th>\n
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Importance<\/p>\n<\/th>\n<\/tr>\n
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Tolerance<\/p>\n<\/td>\n
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Allowable dimensional variation<\/p>\n<\/td>\n
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Controls fit, function, assembly, and inspection<\/p>\n<\/td>\n<\/tr>\n
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Standard Tolerance<\/p>\n<\/td>\n
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Default tolerance without special requirements<\/p>\n<\/td>\n
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Reduces drawing complexity and cost<\/p>\n<\/td>\n<\/tr>\n
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Tight Tolerance<\/p>\n<\/td>\n
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Stricter control for critical features<\/p>\n<\/td>\n
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Improves precision but may increase cost and lead time<\/p>\n<\/td>\n<\/tr>\n
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Surface Roughness<\/p>\n<\/td>\n
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Surface texture, often measured as Ra<\/p>\n<\/td>\n
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Affects friction, sealing, appearance, fatigue, and coating<\/p>\n<\/td>\n<\/tr>\n
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As-Machined Finish<\/p>\n<\/td>\n
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Surface left after cutting<\/p>\n<\/td>\n
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Common for prototypes and internal features<\/p>\n<\/td>\n<\/tr>\n
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Post-Processing<\/p>\n<\/td>\n
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Secondary finishing operations<\/p>\n<\/td>\n
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Improves appearance, corrosion resistance, or performance<\/p>\n<\/td>\n<\/tr>\n
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GD&T<\/p>\n<\/td>\n
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Geometric Dimensioning and Tolerancing<\/p>\n<\/td>\n
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Defines form, orientation, location, and runout<\/p>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
CNC Machining Tolerances<\/h2>\n
Understanding CNC machining tolerances helps engineers specify the right level of precision for part function, fit, manufacturability, and cost.<\/p>\n
Standard vs. Tight CNC Tolerances<\/h3>\n
Most CNC parts do not need every dimension to be tightly controlled. In many cases, standard machining tolerances are sufficient for non-critical features such as external profiles, clearance surfaces, covers, brackets, and visual housings.<\/p>\n
Tight tolerances should be reserved for features that directly affect assembly, motion, sealing, alignment, or product performance.<\/p>\n
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<\/colgroup>\n
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\n
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Tolerance Type<\/p>\n<\/th>\n
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Best Used For<\/p>\n<\/th>\n
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Typical Impact<\/p>\n<\/th>\n<\/tr>\n
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Standard Tolerance<\/p>\n<\/td>\n
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General dimensions, non-critical features, prototypes<\/p>\n<\/td>\n
Higher cost, longer lead time, more inspection<\/p>\n<\/td>\n<\/tr>\n
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\n
Ultra-Tight Tolerance<\/p>\n<\/td>\n
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High-precision optical, aerospace, medical, or metrology-related parts<\/p>\n<\/td>\n
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Requires engineering review and advanced quality control<\/p>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
Practical recommendation:<\/strong> Only apply tight tolerances to features that truly need them. Keep the rest of the part under standard tolerance whenever possible.<\/p>\n
Common CNC Tolerance Reference<\/h3>\n
The following table provides a practical reference for CNC machining tolerance planning. Actual achievable tolerance depends on material, part geometry, feature size, machine setup, tool access, and inspection method.<\/p>\n
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<\/colgroup>\n
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Level<\/p>\n<\/th>\n
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Range<\/p>\n<\/th>\n
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Best For<\/p>\n<\/th>\n
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Note<\/p>\n<\/th>\n<\/tr>\n
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General<\/p>\n<\/td>\n
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\u00b10.10\u20130.20 mm<\/p>\n<\/td>\n
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Non-critical parts<\/p>\n<\/td>\n
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Fast and cost-effective<\/p>\n<\/td>\n<\/tr>\n
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Precision<\/p>\n<\/td>\n
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\u00b10.05\u20130.10 mm<\/p>\n<\/td>\n
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Functional prototypes<\/p>\n<\/td>\n
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Common for mechanical parts<\/p>\n<\/td>\n<\/tr>\n
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Tight<\/p>\n<\/td>\n
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\u00b10.025\u20130.05 mm<\/p>\n<\/td>\n
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Holes, mating surfaces<\/p>\n<\/td>\n
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Needs review and inspection<\/p>\n<\/td>\n<\/tr>\n
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\n
Very Tight<\/p>\n<\/td>\n
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< \u00b10.025 mm<\/p>\n<\/td>\n
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Precision assemblies<\/p>\n<\/td>\n
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Confirm case by case<\/p>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
Note: <\/strong>Tight tolerance is not a universal requirement. It should be applied selectively based on the function of each feature.<\/p>\n
<\/figcaption><\/figure>\n<\/p>\n
What Drives CNC Tolerance Capability?<\/h3>\n
CNC tolerance is influenced by more than machine accuracy. The full manufacturing process must be considered.<\/p>\n
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<\/colgroup>\n
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Factor<\/p>\n<\/th>\n
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How It Affects Tolerance<\/p>\n<\/th>\n<\/tr>\n
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Material Type<\/p>\n<\/td>\n
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Aluminum is generally easier to machine tightly than some plastics or difficult alloys<\/p>\n<\/td>\n<\/tr>\n
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Part Size<\/p>\n<\/td>\n
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Larger parts may be harder to hold to very tight tolerances across long distances<\/p>\n<\/td>\n<\/tr>\n
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Wall Thickness<\/p>\n<\/td>\n
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Thin walls can deflect during machining<\/p>\n<\/td>\n<\/tr>\n
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Feature Depth<\/p>\n<\/td>\n
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Deep pockets and long holes may increase tool deflection<\/p>\n<\/td>\n<\/tr>\n
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Tool Access<\/p>\n<\/td>\n
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Limited access may require longer tools or special setups<\/p>\n<\/td>\n<\/tr>\n
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Setup Count<\/p>\n<\/td>\n
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Multiple setups can introduce alignment variation<\/p>\n<\/td>\n<\/tr>\n
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Heat and Stress<\/p>\n<\/td>\n
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Machining can release internal material stress<\/p>\n<\/td>\n<\/tr>\n
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Inspection Method<\/p>\n<\/td>\n
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Tight tolerances require appropriate measurement tools and procedures<\/p>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
Engineering insight:<\/strong> If a part has thin walls, deep cavities, long slender features, or multiple critical datums, tolerance review should happen before production begins.<\/p>\n
Where Tight Tolerances Matter Most<\/h3>\n
Not every surface requires the same level of control. The best CNC drawings identify critical features clearly and avoid applying tight tolerances everywhere.<\/p>\n
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<\/colgroup>\n
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Feature Type<\/p>\n<\/th>\n
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Recommended Control Level<\/p>\n<\/th>\n
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Why It Matters<\/p>\n<\/th>\n<\/tr>\n
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Bearing Seats<\/p>\n<\/td>\n
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Tight<\/p>\n<\/td>\n
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Controls fit, rotation, and service life<\/p>\n<\/td>\n<\/tr>\n
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Dowel Pin Holes<\/p>\n<\/td>\n
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Tight<\/p>\n<\/td>\n
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Controls alignment and repeatability<\/p>\n<\/td>\n<\/tr>\n
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Threaded Holes<\/p>\n<\/td>\n
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Moderate to tight<\/p>\n<\/td>\n
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Affects fastening strength and assembly<\/p>\n<\/td>\n<\/tr>\n
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Mating Surfaces<\/p>\n<\/td>\n
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Moderate to tight<\/p>\n<\/td>\n
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Affects fit and sealing<\/p>\n<\/td>\n<\/tr>\n
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Cosmetic Outer Surfaces<\/p>\n<\/td>\n
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Standard to moderate<\/p>\n<\/td>\n
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Usually more affected by finish than dimension<\/p>\n<\/td>\n<\/tr>\n
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Clearance Holes<\/p>\n<\/td>\n
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Standard<\/p>\n<\/td>\n
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Usually allow assembly flexibility<\/p>\n<\/td>\n<\/tr>\n
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Internal Pockets<\/p>\n<\/td>\n
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Standard to moderate<\/p>\n<\/td>\n
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Depends on part function<\/p>\n<\/td>\n<\/tr>\n
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Thin Walls<\/p>\n<\/td>\n
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Engineering review needed<\/p>\n<\/td>\n
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Risk of deflection and distortion<\/p>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
Best practice: <\/strong>Mark critical-to-function dimensions clearly on the drawing. Avoid vague notes such as \u201call dimensions must be high precision.\u201d<\/p>\n
CNC Machining Surface Finish<\/h2>\n
CNC machining surface finish affects part appearance, functional performance, and the suitability of later finishing or coating processes.<\/p>\n
Understanding Surface Finish<\/h3>\n
Surface finish describes the texture and quality of a machined surface. In CNC machining, surface finish affects not only appearance but also mechanical performance.<\/p>\n
Common surface finish considerations include:<\/p>\n
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Friction and wear<\/p>\n<\/li>\n
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Sealing performance<\/p>\n<\/li>\n
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Paint or coating adhesion<\/p>\n<\/li>\n
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Corrosion resistance<\/p>\n<\/li>\n
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Fatigue resistance<\/p>\n<\/li>\n
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Sliding contact<\/p>\n<\/li>\n
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Visual appearance<\/p>\n<\/li>\n
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Cleanability<\/p>\n<\/li>\n<\/ul>\n
Surface roughness is often measured as Ra, which represents the average roughness of a surface. A lower Ra value generally means a smoother surface.<\/p>\n
However, smoother is not always better. Some applications require a certain texture for coating adhesion, lubrication retention, or non-slip performance.<\/p>\n
Surface Roughness Reference Table<\/h3>\n
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<\/colgroup>\n
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Ra Value<\/p>\n<\/th>\n
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Surface<\/p>\n<\/th>\n
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Best For<\/p>\n<\/th>\n
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Note<\/p>\n<\/th>\n<\/tr>\n
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Ra 6.3 \u03bcm<\/p>\n<\/td>\n
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Rough<\/p>\n<\/td>\n
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Internal, non-critical surfaces<\/p>\n<\/td>\n
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Low cost, fast machining<\/p>\n<\/td>\n<\/tr>\n
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Ra 3.2 \u03bcm<\/p>\n<\/td>\n
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Standard<\/p>\n<\/td>\n
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General CNC parts<\/p>\n<\/td>\n
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Common as-machined finish<\/p>\n<\/td>\n<\/tr>\n
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Ra 1.6 \u03bcm<\/p>\n<\/td>\n
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Fine<\/p>\n<\/td>\n
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Contact surfaces<\/p>\n<\/td>\n
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Better performance<\/p>\n<\/td>\n<\/tr>\n
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Ra 0.8 \u03bcm<\/p>\n<\/td>\n
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Smooth<\/p>\n<\/td>\n
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Sliding or sealing areas<\/p>\n<\/td>\n
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May need finishing<\/p>\n<\/td>\n<\/tr>\n
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Ra 0.4 \u03bcm or below<\/p>\n<\/td>\n
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Very smooth<\/p>\n<\/td>\n
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Precision applications<\/p>\n<\/td>\n
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Needs process review<\/p>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
Practical recommendation:<\/strong> Use Ra 3.2 \u03bcm or Ra 1.6 \u03bcm for many functional CNC parts unless the application requires a smoother or rougher surface.<\/p>\n
A common mistake is treating machining and finishing as separate decisions. In reality, surface finishing can affect final dimensions, especially for tight-tolerance parts.<\/p>\n
For example, anodizing, plating, powder coating, and polishing can all change surface thickness or edge condition. If a hole, slot, or mating surface requires tight control after finishing, that requirement must be clearly stated before machining begins.<\/p>\n
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<\/colgroup>\n
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Finishing Process<\/p>\n<\/th>\n
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Possible Dimensional Impact<\/p>\n<\/th>\n
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Engineering Note<\/p>\n<\/th>\n<\/tr>\n
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Anodizing<\/p>\n<\/td>\n
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Adds oxide layer to aluminum surfaces<\/p>\n<\/td>\n
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Critical holes may need masking or post-machining<\/p>\n<\/td>\n<\/tr>\n
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Hard Anodizing<\/p>\n<\/td>\n
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Adds thicker, harder layer<\/p>\n<\/td>\n
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Important for wear surfaces but must be planned<\/p>\n<\/td>\n<\/tr>\n
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Plating<\/p>\n<\/td>\n
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Adds coating thickness<\/p>\n<\/td>\n
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Final dimensions should account for plating buildup<\/p>\n<\/td>\n<\/tr>\n
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Bead Blasting<\/p>\n<\/td>\n
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Slightly changes surface texture<\/p>\n<\/td>\n
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Usually minor, but edges may soften<\/p>\n<\/td>\n<\/tr>\n
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Polishing<\/p>\n<\/td>\n
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Removes material<\/p>\n<\/td>\n
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Can affect flatness, sharp edges, and small features<\/p>\n<\/td>\n<\/tr>\n
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Powder Coating<\/p>\n<\/td>\n
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Adds relatively thick coating<\/p>\n<\/td>\n
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Not suitable for very tight mating features without masking<\/p>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
Best practice: <\/strong>Specify whether dimensions apply before finishing or after finishing.<\/p>\n
Tolerance + Finish Decision Matrix<\/h2>\n
Use this matrix during design review or supplier communication.<\/p>\n
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<\/colgroup>\n
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Application Requirement<\/p>\n<\/th>\n
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Suggested Tolerance Approach<\/p>\n<\/th>\n
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Suggested Finish Approach<\/p>\n<\/th>\n<\/tr>\n
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Early prototype for fit check<\/p>\n<\/td>\n
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Standard tolerance<\/p>\n<\/td>\n
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As-machined<\/p>\n<\/td>\n<\/tr>\n
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Visual prototype for customer review<\/p>\n<\/td>\n
Moderate tolerance on critical features<\/p>\n<\/td>\n
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As-machined or Ra 1.6\u20133.2 \u03bcm<\/p>\n<\/td>\n<\/tr>\n
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Sliding or rotating component<\/p>\n<\/td>\n
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Tight tolerance on contact features<\/p>\n<\/td>\n
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Smooth finish, possible polishing<\/p>\n<\/td>\n<\/tr>\n
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Sealing surface<\/p>\n<\/td>\n
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Tight flatness and surface control<\/p>\n<\/td>\n
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Ra requirement should be specified<\/p>\n<\/td>\n<\/tr>\n
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Outdoor aluminum component<\/p>\n<\/td>\n
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Standard to moderate<\/p>\n<\/td>\n
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Anodizing or powder coating<\/p>\n<\/td>\n<\/tr>\n
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Wear-resistant component<\/p>\n<\/td>\n
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Feature-specific tolerance<\/p>\n<\/td>\n
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Hard anodizing, plating, or material upgrade<\/p>\n<\/td>\n<\/tr>\n
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Precision assembly component<\/p>\n<\/td>\n
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Tight feature-level tolerance<\/p>\n<\/td>\n
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Finish must be reviewed with tolerance stack-up<\/p>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
Drawing Checklist Before Requesting a CNC Quote<\/h2>\n
Before sending files for CNC machining, prepare the following information to reduce back-and-forth communication and improve quotation accuracy.<\/p>\n
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<\/colgroup>\n
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Item<\/p>\n<\/th>\n
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Why It Matters<\/p>\n<\/th>\n<\/tr>\n
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3D CAD File<\/p>\n<\/td>\n
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Provides complete geometry for machining review<\/p>\n<\/td>\n<\/tr>\n
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2D Drawing<\/p>\n<\/td>\n
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Defines tolerances, threads, surface finish, and inspection requirements<\/p>\n<\/td>\n<\/tr>\n
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Material Specification<\/p>\n<\/td>\n
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Affects machinability, strength, cost, and lead time<\/p>\n<\/td>\n<\/tr>\n
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Quantity<\/p>\n<\/td>\n
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Impacts production strategy and pricing<\/p>\n<\/td>\n<\/tr>\n
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Critical Dimensions<\/p>\n<\/td>\n
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Helps supplier focus inspection on important features<\/p>\n<\/td>\n<\/tr>\n
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Surface Finish Requirement<\/p>\n<\/td>\n
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Determines machining strategy and post-processing<\/p>\n<\/td>\n<\/tr>\n
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Thread Specifications<\/p>\n<\/td>\n
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Prevents assembly issues<\/p>\n<\/td>\n<\/tr>\n
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Finishing Requirements<\/p>\n<\/td>\n
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Needed for anodizing, plating, polishing, painting, etc.<\/p>\n<\/td>\n<\/tr>\n
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Inspection Requirements<\/p>\n<\/td>\n
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Defines what must be measured and documented<\/p>\n<\/td>\n<\/tr>\n
1. Applying tight tolerance to every dimension<\/h3>\n
This increases cost and inspection time without improving the part if the dimensions are not function-critical.<\/p>\n
2. Missing 2D drawings for precision parts<\/h3>\n
A 3D model alone usually does not communicate tolerance, surface finish, thread, and inspection requirements.<\/p>\n
3. Ignoring finish thickness<\/h3>\n
Coatings and surface treatments can affect final dimensions, especially for holes, slots, and mating features.<\/p>\n
4. Specifying roughness without function<\/h3>\n
A very smooth Ra value may increase cost. Use it only when required for sealing, sliding, wear, or appearance.<\/p>\n
5. Using unrealistic tolerances for difficult geometries<\/h3>\n
Thin walls, deep pockets, long features, and difficult-to-machine materials may require design review.<\/p>\n
6. Not identifying critical features<\/h3>\n
A supplier cannot know which dimensions are most important unless they are clearly specified.<\/p>\n
How Unionfab Supports CNC Projects<\/h2>\n
Unionfab helps engineering teams and procurement teams manufacture CNC machined parts for prototyping, functional testing, and low-volume production.<\/p>\n
Our CNC machining support includes:<\/p>\n
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CNC milling and turning<\/p>\n<\/li>\n
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Multi-material machining support<\/p>\n<\/li>\n
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Aluminum, stainless steel, titanium, brass, copper, and engineering plastics<\/p>\n<\/li>\n
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Functional prototypes and low-volume production<\/p>\n<\/li>\n
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Surface finishing options including anodizing, polishing, bead blasting, plating, and painting<\/p>\n<\/li>\n
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Engineering review for manufacturability<\/p>\n<\/li>\n
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Tolerance and surface finish discussion for critical parts<\/p>\n<\/li>\n
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Quality inspection support based on project requirements<\/p>\n<\/li>\n<\/ul>\n
Whether you are developing a prototype, validating a mechanical assembly, or sourcing low-volume CNC parts, Unionfab can help you move from design to production with greater confidence.<\/p>\n
Need CNC Parts with the Right Tolerance and Finish?<\/strong><\/p>\n
Upload your CAD files and let our engineering team review your CNC machining requirements.<\/p>\n
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