{"id":2104,"date":"2026-09-13T21:56:10","date_gmt":"2026-09-13T13:56:10","guid":{"rendered":"https:\/\/skypumold.com\/?p=2104"},"modified":"2026-09-14T00:30:21","modified_gmt":"2026-09-13T16:30:21","slug":"cnc-machining-tolerances-guide","status":"publish","type":"post","link":"https:\/\/skypumold.com\/ja\/cnc-machining-tolerances-guide\/","title":{"rendered":"CNC Machining Tolerances: How to Specify Them for an Accurate Quote"},"content":{"rendered":"

CNC machining tolerances can make the difference between a quote that is fast and reliable and one that requires days of clarification. When a drawing says only \u201ctight tolerance required,\u201d a manufacturer must price the uncertainty: extra setups, slower machining, more inspection, and a higher risk of rejected parts.<\/p>\n

This guide explains how to specify tolerances so your CNC machining supplier can understand the function of the part, select a suitable process, and provide a more accurate quotation.<\/p>\n

What Is a CNC Machining Tolerance?<\/h2>\n

A tolerance is the permitted variation from a nominal dimension or geometry. For example, a diameter specified as 20.00 \u00b10.02 mm may measure from 19.98 mm to 20.02 mm and still meet the drawing requirement.<\/p>\n

Tolerances can control more than size. They may also define form, orientation, and location\u2014including flatness, perpendicularity, parallelism, position, runout, and profile. The achievable result depends on the complete manufacturing situation: material, feature geometry, wall thickness, tool access, heat treatment, surface finish, batch size, fixturing, and the inspection method.<\/p>\n

Why Tighter Is Not Always Better<\/h2>\n

It is tempting to apply the smallest possible tolerance to every dimension. In practice, unnecessarily tight tolerances usually increase cost without improving part function. They may require additional operations, special tooling, slower cycle times, temperature-controlled measurement, more detailed inspection, or a higher scrap allowance.<\/p>\n

A better rule is simple: apply tight tolerances only where the assembly, sealing, motion, alignment, appearance, or service life requires them. Use practical general tolerances for non-critical dimensions.<\/p>\n

General Tolerances vs. Critical Tolerances<\/h2>\n

General tolerances<\/strong> apply to dimensions that do not have an individual tolerance next to them. They are normally stated in the drawing title block or in a general note.<\/p>\n

Critical tolerances<\/strong> are assigned directly to features whose variation affects fit or function. Typical examples include bearing seats, sealing diameters, hole patterns, locating faces, precision pockets, datum surfaces, and mating interfaces.<\/p>\n

Separating the two helps the machine shop focus process control and inspection on the features that matter most.<\/p>\n

What to Include in a CNC Machining RFQ<\/h2>\n

1. Units, Drawing Revision, and File Priority<\/h3>\n

State whether the drawing uses millimetres or inches, identify the current revision, and explain which file controls if the 2D drawing and 3D model conflict. The 3D model is useful for geometry, but the 2D drawing should communicate tolerances, datums, surface finish, threads, and inspection notes.<\/p>\n

2. A Clear General Tolerance Note<\/h3>\n

Include a general tolerance rule for unspecified dimensions. If you reference a standard, state the exact standard and edition used by your organization. Do not assume every supplier will interpret a short note such as \u201cISO tolerance\u201d in the same way.<\/p>\n

ASME Y14.5 is widely used for geometric dimensioning and tolerancing (GD&T). ISO 2768-1 covers general tolerances for linear and angular dimensions without individual indications. For general geometrical specifications, note that ISO 2768-2 has been withdrawn and replaced by ISO 22081. Standards evolve, so confirming the edition avoids ambiguity.<\/p>\n

3. Identify Function-Critical Features<\/h3>\n

Mark the dimensions that affect the part\u2019s function. If a bore receives a bearing, a face creates a seal, or a hole pattern locates another component, make that relationship clear. A brief functional note can help the engineering team understand why a tolerance is important and propose a manufacturable solution when necessary.<\/p>\n

4. Define Datums and Use GD&T Where Appropriate<\/h3>\n

Dimensions need a stable reference. A datum system tells the manufacturer how the part is located for machining and how it should be set up for inspection. Without meaningful datums, a tight positional or orientation tolerance may be difficult to manufacture and impossible to verify consistently.<\/p>\n

Use GD&T when it communicates functional requirements more clearly than multiple coordinate dimensions. Avoid adding symbols that do not serve a functional purpose.<\/p>\n

5. Specify Fits, Threads, and Mating Parts<\/h3>\n

For shafts and holes, define the required fit or provide the mating dimension and functional intent. For threads, include the thread system, nominal size, pitch, tolerance class, depth, and whether the requirement applies before or after coating. Also identify inserts, press fits, and sealing features.<\/p>\n

6. State Surface Finish Requirements<\/h3>\n

Surface roughness and dimensional tolerance are related but not interchangeable. A part can be within size tolerance and still have an unsuitable surface. Apply roughness requirements only to the relevant faces and include any direction-of-lay or sealing requirements when needed.<\/p>\n

7. Define Inspection and Reporting Needs<\/h3>\n

Tell the supplier which features require inspection and whether you need a standard inspection report, first article inspection, material certificate, CMM report, or another record. Also identify the agreed measurement condition or method for features that can be interpreted differently.<\/p>\n

Typical Information by Feature<\/h2>\n
\n\n\n\n\n\n\n\n\n\n
Feature<\/th>\nUseful information to specify<\/th>\nWhy it matters<\/th>\n<\/tr>\n<\/thead>\n
Mating bore or shaft<\/td>\nDiameter tolerance or fit class, surface finish, mating part<\/td>\nControls clearance, interference, and wear<\/td>\n<\/tr>\n
Hole pattern<\/td>\nDatum references, true position, hole size and depth<\/td>\nControls assembly alignment<\/td>\n<\/tr>\n
Sealing face<\/td>\nFlatness, finish, sealing area, coating condition<\/td>\nControls leakage and contact<\/td>\n<\/tr>\n
Thin wall<\/td>\nMinimum thickness, critical zones, acceptable distortion<\/td>\nGuides fixturing and machining sequence<\/td>\n<\/tr>\n
Thread<\/td>\nStandard, size, pitch, class, depth, gauge requirement<\/td>\nPrevents assembly and inspection disputes<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/figure>\n

Five Common Tolerance Mistakes<\/h2>\n
    \n
  1. Applying one tight tolerance to the entire drawing.<\/strong> This can make non-critical features unnecessarily expensive.<\/li>\n
  2. Sending files with conflicting revisions.<\/strong> A model and drawing that disagree will delay review and create quality risk.<\/li>\n
  3. Using GD&T without functional datums.<\/strong> The inspection setup may not represent how the part works in the assembly.<\/li>\n
  4. Ignoring the effect of finishing.<\/strong> Anodizing, plating, heat treatment, grinding, and polishing can change dimensions or the condition in which they must be measured.<\/li>\n
  5. Leaving inspection expectations until after production.<\/strong> Reporting and measurement requirements should be included at the quotation stage.<\/li>\n<\/ol>\n

    Pre-RFQ Tolerance Checklist<\/h2>\n