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Types of Holes in CNC Machining: A Design Guide for Engineers and Buyers
A working reference to the main hole types used in CNC machined parts, how they differ, and how to pick the right one for cost, function, and tolerance.
Not every hole on a drawing does the same job. Some pass a bolt through, some locate a dowel pin, some seat a bearing, and some exist only to let a screw head sit flush. Picking the wrong hole type — or leaving the callout vague — is one of the quiet reasons CNC quotes come back higher than expected, or parts fail inspection. This guide breaks down the main types of holes used in CNC machining, when to use each one, and how they affect tolerance, tooling, and cost.
Why Hole Type Selection Matters in CNC Machining
Every hole type calls for a different tool path, a different cutter, and a different tolerance band. A through hole is one of the cheapest features a shop can cut; a reamed hole with a tight positional tolerance can multiply cycle time on that same feature. Understanding hole types before you finalize a drawing keeps quotes predictable and avoids redesign after first article inspection.
How Hole Geometry Affects Cost and Tolerance
Depth-to-diameter ratio, wall thickness around the hole, and whether the hole is open on both ends all change what a CNC shop can realistically hold. A shallow through hole in aluminum is fast and forgiving; a deep blind hole in titanium needs peck drilling, slower feeds, and tighter process control — all of which show up in the quote.
Common Types of Holes in CNC Machined Parts
Through Holes
A through hole passes completely through the part wall, exiting the opposite face. It is the simplest, fastest, and cheapest hole type to machine because chips clear freely and there is no depth control beyond wall thickness. Through holes are used for fasteners, dowel pins, wiring, and fluid passages.
Blind Holes
A blind hole stops inside the material and does not exit the opposite side. It is used where a through hole would compromise strength, sealing, or appearance — for example, a threaded mounting point on a sealed enclosure. Blind holes need extra drilled depth beyond the functional depth and require chip-clearing strategies like peck cycles. For threaded blind holes specifically, see our threaded holes guidelines.
Counterbore Holes
A counterbore is a stepped hole: a larger-diameter, flat-bottomed recess machined at the entrance of a smaller hole, sized to seat a socket head cap screw below the part surface. Counterbore holes are common in assemblies where a flush or recessed fastener head is required for clearance or appearance.
Countersink Holes
A countersink is a conical recess, typically 82° or 90°, cut at a hole’s entrance so a flat-head screw sits flush with the surface. Countersinks are common on panels, covers, and sheet-thin parts where a protruding screw head would interfere with mating hardware.
Tapered Holes
A tapered hole changes diameter gradually along its depth rather than staying constant. Tapered holes are used for self-locking tool shanks, valve seats, and press-fit pins that need a controlled interference fit along the hole’s length.
Reamed Holes
Reaming is a finishing pass that enlarges a pre-drilled hole to a tighter diameter tolerance and smoother surface finish than drilling alone can achieve. Reamed holes are specified where a dowel pin or bushing needs a precise slip or press fit.
Spotface Holes
A spotface is a shallow, flat-bottomed recess machined just deep enough to create a clean, flat bearing surface for a washer or fastener head on a rough or curved casting surface.
Cross Holes and Interrupted Holes
A cross hole intersects another hole or an internal cavity partway through, interrupting continuous tool contact. These require extra care in feed rate and tool selection, since the cutter engages and disengages material inconsistently as it crosses the intersection.
Blind Holes vs Through Holes: Key Differences
| Factor | Through Hole | Blind Hole |
|---|---|---|
| Chip evacuation | Easy — chips exit the far side | Harder — requires peck drilling |
| Depth control | Not critical | Critical, drawn dimension required |
| Typical cost | Lowest | Higher, more cycle time |
| Common use | Fasteners, dowels, fluid passage | Sealed enclosures, threaded mounts |
| Tap type (if threaded) | Taper or plug tap | Bottoming tap |
Choosing the Right Hole Type for Your Application
| Hole Type | Typical Application | Achievable Tolerance |
|---|---|---|
| Through hole | Fasteners, wiring, fluid lines | ±0.05 mm (standard) |
| Blind hole | Threaded mounts, sealed housings | ±0.05–0.1 mm depth |
| Counterbore | Flush socket head screws | ±0.05 mm depth |
| Countersink | Flush flat-head screws, panels | ±0.1 mm angle/depth |
| Reamed hole | Dowel pins, bushings, precision fits | ±0.01–0.02 mm |
| Tapered hole | Tool shanks, valve seats, press pins | Per taper standard (e.g. Morse taper) |
Practical Manufacturing Examples
Aluminum enclosure
Through holes for the four corner fasteners, one blind threaded hole for a ground stud, and a counterbore on the lid for a flush socket screw.
Stainless steel bracket
Reamed dowel holes for precise fixture alignment, paired with standard through holes for the mounting bolts. See our stainless steel machining guide for tolerance notes specific to this material.
Sheet-thin cover plate
Countersunk holes around the perimeter so flat-head screws sit flush with the outer surface, avoiding interference with the mating housing.
Common Mistakes When Designing Holes for CNC Machining
- Calling out a blind hole without specifying functional depth, forcing the shop to guess drill depth
- Using a reamed tolerance on a hole that only needs a standard through-hole fit — this adds unnecessary cost
- Placing cross holes too close to a part’s outer wall, risking breakout
- Mixing countersink angles (82° vs 90°) without noting the fastener standard on the drawing
- Ignoring minimum wall thickness around a hole, which can cause deflection or breakthrough during machining
Checking hole types, depths, and tolerances against your part’s overall GD&T and tolerance requirements before quoting is the fastest way to avoid rework — a step outlined in the ASME Y14.5 standard for dimensioning and tolerancing referenced by most CNC shops, including ASME’s Y14 series.
Table of Contents
Frequently Asked Questions About Hole Types in CNC Machining
What is the difference between a counterbore and a countersink?
Which hole type is cheapest to machine?
When should I specify a reamed hole instead of a drilled hole?
Can a blind hole be threaded like a through hole?
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Not Sure Which Hole Type Fits Your Part?
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