DRAWING DETAILS / CNC MACHINING INSIGHTS
A larger circular feature around a hole can mean very different things. It may clear a socket-head fastener, seat a flat-head screw, or create a flat contact surface. Naming the feature and defining the hardware relationship makes a CNC drawing easier to quote and review.
Do not treat these hole features as interchangeable
| Feature | Function | Drawing information |
|---|---|---|
| Counterbore | A cylindrical recess, often for a fastener head. | Main hole size, recess diameter, recess depth and side of the part. |
| Countersink | A conical opening, commonly for a flat-head fastener. | Main hole size, major diameter, included angle and flushness intent. |
| Spotface | A localized flat seating surface. | Diameter, floor condition and the contact function. |
The name is helpful, but it is not the entire requirement. Dimension the geometry that must be machined and inspected.
Start with the mating hardware
When a hole is driven by a fastener, identify the relevant hardware or mating component. This gives the review a functional reference for head clearance and seating. If the head must be flush or recessed, state that. If it only needs clearance, state that instead.
A note such as “for screw” does not establish the head form, seating position, or whether the main hole is through, blind or threaded. Threaded features need their own specification; see our guide to specifying threads for CNC parts.
Define a complete callout
For a counterbore, show the main-hole size plus counterbore diameter and depth. For a countersink, show the main-hole size, major diameter and included angle. For a spotface, show the diameter and floor condition that matters to assembly. A section view helps where several features stack along the same axis.
Separate clearance from precise location
A fastener hole often provides clamp load, not precise location. If it also locates a mating part, identify that function and show the mating relationship. This separates an ordinary clearance hole from a feature whose position affects alignment.
Only assign tight requirements where the function needs them. Our CNC tolerance guide explains why functional dimensions support a clearer inspection plan.
Questions that prevent revisions
- Which side receives the recess or countersink?
- What hardware drives the feature?
- Must the head be flush, recessed or simply clear the surface?
- Is the main hole through, blind or threaded?
- Does the hole locate another part or only provide clearance?
Link inspection to function
If recess depth, countersink profile or spotface seating affects assembly, identify that expectation before production release. See our CNC inspection requirements guide for the information that makes a critical feature reviewable.
Make the hole feature unambiguous
ExactMach coordinates drawing-defined CNC milling, CNC turning, prototypes and custom parts around Shenzhen and Dongguan for international orders. Clear counterbore, countersink and spotface notes give the machining review a shared understanding before production is released.