Metric, UNC and UNF threads can all be machined on CNC parts, but they are not interchangeable. For a reliable quote, specify the standard and complete thread callout on the drawing, then identify whether the feature is internal or external, blind or through, and how it will be inspected. The mating component and assembly environment should drive the choice.
Metric vs UNC vs UNF at a glance
| Series | Typical drawing example | Pitch notation | Selection starting point |
|---|---|---|---|
| ISO metric (M) | M6 × 1 – 6H, internal thread | Millimetres between crests | Use when the mating fastener and drawing follow the metric system. |
| UNC | 1/4-20 UNC-2B, internal thread | Threads per inch; coarser Unified series | Use when the mating fastener calls for UNC, often in North American assemblies. |
| UNF | 1/4-28 UNF-2B, internal thread | Threads per inch; finer Unified series | Use when the mating fastener calls for UNF or a finer pitch is functionally required. |
These examples explain notation; they are not a promise that every size, class or material is available without review. ISO metric thread dimensions and tolerances are governed by the applicable ISO 261 et ISO 965 documents. Unified inch thread designations are covered by ASME B1.1. State the required standard edition where compatibility or inspection is critical.
How to choose the thread series
Start with the mating part
Match the screw, nut, fitting or existing assembly first. Metric and Unified inch threads can look similar in a CAD preview but differ in pitch and fit. Do not substitute one because the nominal diameters seem close. If the mating hardware is not fixed yet, tell the supplier the operating loads, available wall thickness, material and assembly method so the choice can be reviewed.
Compare coarse and fine only within the same system
UNC and UNF are Unified inch series. For the same nominal diameter, UNF has more threads per inch than UNC. Fine pitch may help with adjustment or limited engagement length in a suitable material, while a coarser pitch can be more forgiving in some assembly conditions. Neither is automatically stronger or cheaper in every part; check the actual fastener, material, engagement and service conditions.
Review coatings and final fit
Anodizing, plating and other treatments can change thread fit. Note whether a thread must be masked, machined after finishing or checked with a gauge after finishing. If the mating component is supplied separately, state the intended fit and acceptance method on the drawing.
What a complete thread callout includes
- Thread standard or series: for example ISO metric, UNC or UNF.
- Nominal size and pitch: millimetres for metric pitch or threads per inch for Unified threads.
- Internal or external tolerance class if the design requires one.
- Usable full-thread depth or length, and blind versus through-hole.
- Location and datum relationship when thread position matters to the assembly.
- Coating, insert, gauge and inspection requirements where applicable.
For a blind hole, usable thread depth is not the same as total drilled depth. Allow for the drill point and tool runout or tap lead. For an external turned thread, show the start and end location, mating length and any relief or shoulder requirement. An ambiguous callout can change both process planning and price. For other manufacturability checks, review our Guide de conception pour l'usinage CNC.
Threads on CNC turned and milled parts
External or internal threads on rotational parts may suit Tournage CNC. Threaded holes in prismatic parts may be produced with tapping or thread milling during Fraisage CNC. The process depends on diameter, material, blind-hole clearance, location and quantity. A supplier should confirm the method against the actual drawing rather than assume one method for every thread.
RFQ example and inspection question
Instead of writing only “M6 thread,” send a callout such as “M6 × 1 – 6H, internal, through; inspect with an appropriate thread gauge,” and include the mating fastener specification if a special fit is required. For an inch-series example, “1/4-20 UNC-2B, internal, usable thread depth per drawing” makes the pitch and class clearer. These are illustrative callouts; the final tolerance and depth must follow your design requirements.
If you need custom threaded parts, upload a STEP model and dimensioned drawing for a quote. Include material, quantity, thread callouts, finish and any gauge or inspection requirement. Our team can review the manufacturability questions before pricing. For the rest of your file package, use our CNC RFQ file checklist.
