CNC Machining Tolerances
How We Review Drawings Before Quoting
Tight tolerances are expensive when they are applied to the wrong features. Loose tolerances cause assembly problems when they are applied to the wrong features. The fastest way to avoid both is a tolerance review before the job is quoted and machined. We review every drawing we receive, and we ask for the information listed below so the quote reflects what your part actually needs.
What a Tolerance Callout Tells the Machinist
A dimension on a drawing is a range, not a target. A callout such as Ø12.000 +0.010/−0.000 means the finished diameter must fall between 12.000 mm and 12.010 mm. The machinist must decide how to hold that band on the lathe, which cutting sequence to use, and how to check it before the part leaves the cell.
Bilateral tolerances
Example: 25.00 ± 0.02 mm. The allowed variation is split above and below nominal. These callouts suit features that must sit centered in a mating part.
Unilateral or limit dimensions
Example: 25.00 +0.02/−0 mm. Variation is allowed in one direction only. These are common for bores and shafts that form press or sliding fits, where the fit depends on which side of nominal the size falls.
Geometric tolerances (GD&T)
GD&T controls form and relationship, not only size. Position, concentricity, runout, perpendicularity and flatness each affect how a part assembles and how it is inspected. A position tolerance tied to datums A and B, for example, requires a fixture and a measurement strategy that a simple size check does not provide.
When a drawing specifies concentricity or runout between two turned diameters, we machine both features in the same setup wherever possible. Each additional setup adds a source of error. A shaft with a 0.01 mm runout requirement can drift if it is flipped between operations.
Equipment Tolerance Ratings and What They Mean for Your Part
The values below are machine-level ratings from our equipment inventory, including ±0.01 mm, ±0.02 mm and ±0.05 mm classes. They describe what a machine type can achieve under suitable conditions. They are not a guarantee that every finished part will hold those values.
| Process | Equipment Type | Qty | Listed Precision | Typical Use |
|---|---|---|---|---|
| CNC Milling | 3-Axis CNC Machining Centers | 25 | ±0.01 mm | Brackets, housings, mounting plates |
| CNC Milling | 4-Axis CNC Machining Center | 1 | ±0.01 mm | Multi-sided components |
| CNC Milling | 5-Axis CNC Machines | 8 | ±0.01 mm | Complex profiles, multi-face parts |
| CNC Turning | CNC Lathes | 43 | ±0.01 mm | Shafts, sleeves, bushings |
| CNC Turning | Gang-Type CNC Lathes | 7 | ±0.01 mm | Small shafts, precision connectors |
| Automatic Turning | Automatic Lathes | 30 | ±0.02 mm | Pins, small turned parts |
| Turn-Mill Machining | CNC Turn-Mill Machines | 15 | ±0.01 mm | Parts combining turned and milled features |
| Swiss-Type Machining | 5-Axis / 6-Axis / Other Swiss-Type Lathes | 10 | ±0.01 mm | Small, slender, or complex small-diameter parts |
| Screw Machining | Screw Machines | 20 | ±0.05 mm | Screws and threaded fasteners |
Machine ratings set the starting point for a feasibility review. The tolerance we can commit to on your part depends on the material, feature size and depth, number of setups, thermal behavior during cutting, and the inspection method available for that feature. A ±0.01 mm diameter on a 6 mm stainless shaft in a single setup is a different job from the same callout on a 150 mm aluminum housing with three datum faces.
Our Tolerance Review Process
Before we quote, we check the drawing against five criteria. Select each item to see what we review and why it matters.
If a callout is not achievable as drawn, we say so in writing and propose alternatives, such as a change in datum scheme, a different process, a relaxed non-critical dimension, or a tighter inspection plan on the features that matter.
Matching Inspection Methods to Tolerance Bands
Inspection capability is part of tolerance capability. A ±0.02 mm diameter can often be checked on a calibrated micrometer during the run and confirmed on the CMM for the first article. A position tolerance of 0.02 mm to a datum scheme requires CMM measurement with a documented datum setup. We agree on the inspection method during the quote stage, so both sides know which features are measured, how, and what report is provided.
| Inspection Equipment | Qty | Suitable For |
|---|---|---|
| Coordinate Measuring Machine (CMM) | 1 | 3D dimensional and geometric checks, position, profile, perpendicularity |
| Optical Measuring Systems (2D Vision) | 5 | Hole position, profile, small-feature dimensions |
| Concentricity Measuring Instrument | 1 | Concentricity between turned diameters |
| Height Measuring Instruments | 2 | Step heights, shoulder positions |
| Surface Roughness Tester | 1 | Ra verification on machined surfaces |
| Micrometers | 10 | Precision outside diameters |
| Digital and Dial Calipers | 30 | General dimensional checks during production |
| Pin Gauges | 1.0–16.0 mm sets | Hole diameter and fit checks |
| Thread Ring and Plug Gauges | M1.0–M16 | External and internal thread verification |
Which Process Fits Which Tolerance Requirement
Choosing the process is often the most important tolerance decision. Where a feature requires tolerances tighter than the process comfortably holds, we recommend a secondary operation such as finish grinding or a reduced-stock finish pass. We do not promise a tolerance we cannot repeat across the run.
| Requirement | Recommended Approach | Reason |
|---|---|---|
| Coaxial turned diameters with concentricity control | CNC turning, same setup where possible | Fewer re-chucking errors |
| Small slender shafts with tight diameters | Swiss-type machining | Guide bushing supports the cut close to the tool |
| Prismatic parts with multi-face position tolerances | 3-axis or 4-axis milling, fixture-based | Repeatable datum location across faces |
| Turned parts with cross-holes or flats | Turn-mill machining | Milling and turning in one setup reduces stack-up |
| Threaded fasteners in high volume | Screw machining, ±0.05 mm class | Cost-efficient for the tolerance class |
Why Unnecessary Tight Tolerances Add Cost
A tolerance tighter than the function requires increases cycle time, adds inspection steps, raises scrap risk, and can force a slower process. A ±0.005 mm callout applied to every dimension on a bracket turns a standard milling job into a longer, more expensive job without improving how the bracket works.
A simple review test
Identify which features touch another part, carry a load, seal a fluid, or set an alignment. Those features usually deserve the tightest callout. Cosmetic faces, clearance bores and non-mating edges usually do not. When a tolerance appears to exceed the functional need, we ask whether it was intentional before we price it.
Information We Need for an Engineering Review
Send the following with your RFQ. With this information we return a feasibility note that identifies achievable tolerances, flags risk features, recommends a process and inspection method, and provides a quote that matches the requirement.
- Native CAD file (STEP or Parasolid preferred) and a 2D drawing with GD&T
- Critical dimensions and features clearly marked, with their function noted
- Datum scheme and intended fixturing, if known
- Material specification and heat treatment, if any
- Required surface finish and any coating requirement
- Acceptance criteria, including sampling plan, first-article requirement and inspection report format
- Expected annual quantity, and whether the order is prototype, repeat or production
Request an Engineering Tolerance Review
If your drawing contains tight dimensions, multi-datum GD&T, or an assembly that depends on a specific fit, send it to us before the RFQ is finalized. Mark the critical features, list the acceptance criteria, and tell us how the part will be used. We will review the tolerances, confirm what we can hold, and quote based on that review.
Please include your part number, quantity, material and the critical dimensions you need held. YJCNC will reply with a feasibility assessment and a quotation.
- Email: info@yjcncmachining.com
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- Address: Yabian Community, Shajing Street, Baoan District, Shenzhen, China
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