DRAWING NOTES / CNC MACHINING INSIGHTS
Threaded features are easy to machine when the drawing states exactly what the mating part needs. A clear thread callout prevents assumptions about standard, pitch, class, usable depth and whether the feature is internal or external.
Start with the thread standard and designation
Place the complete designation next to each threaded feature in the drawing view where it is visible. For inch-series threads, identify the nominal diameter, threads per inch and class. For metric threads, identify the nominal diameter, pitch and tolerance class when it matters to the assembly. Avoid a note that only says “tap here” or “threaded hole.”
For example, the drawing should make it clear whether the requirement is an internal metric thread, an external Unified thread, or another defined standard. The applicable standard matters because it establishes the geometry used by the mating component. The ASME B1.1 reference is a useful starting point for Unified inch thread terminology.
Include the information a machinist needs
| Drawing item | Why it matters |
|---|---|
| Thread designation | Defines the series, nominal size and pitch or threads per inch. |
| Internal or external feature | Distinguishes a tapped hole from a threaded shaft or boss. |
| Thread class or tolerance | Communicates fit requirements when the default is not sufficient. |
| Thread depth or engagement | Separates usable thread length from total drilled depth. |
| Blind or through condition | Changes the tool approach and bottom-of-hole considerations. |
| Surface treatment note | Lets the team consider coating build-up or masking before processing. |
Blind holes need a usable-depth note
For a blind threaded hole, a tap cannot normally form full threads all the way to the drill-point bottom. State the minimum full-thread depth required for engagement and, separately if needed, the maximum total hole depth. This gives the machining team a usable requirement instead of asking them to infer it from a section view.
If a screw must bottom out, seal, or sit at a controlled depth, explain that function in a note. Functional intent helps identify whether a thread relief, counterbore, spotface or different fastener length is appropriate.
Plan threads together with finishing
Anodizing and other finishes can affect assembly fit, especially on fine or closely controlled threads. Put the finishing requirement on the drawing or RFQ and identify whether threads need protection, masking, or finishing before the final thread operation. Read our guide to anodizing and heat treatment for CNC parts when choosing a post-machining process.
Use a thread schedule for multi-feature parts
When a part has many threaded locations, a small schedule is clearer than repeating long notes around every view. Number the locations, then list thread callout, quantity, depth, and any special condition. This is especially useful for housings, brackets and assemblies with mixed metric and inch fasteners.
Inspection should follow the functional requirement
If a thread is critical to the assembly, identify the inspection expectation early. It may be a go/no-go gauge, a mating component check, or a documented measurement requirement. State what must be verified rather than assuming every feature needs the same inspection method. For general measurement traceability context, NIST maintains public calibration resources.
A short review before release saves time
Confirm that the drawing uses a single unit system, that every threaded feature has a complete callout, and that finishing notes do not conflict with fit. For a drawing-defined international order, ExactMach coordinates CNC milling, CNC turning, prototypes and custom parts around Shenzhen and Dongguan, with anodizing and heat-treatment support where required.