EU
Share:
Table of Content
Table of Content
CNC drilling and CNC boring both make holes. But they do very different jobs. Drilling adds a new hole feature to a part. Boring does not change the hole feature. It only improves the accuracy of a hole that already exists.
This guide covers the key differences, typical tolerances, tooling, and when to use each. If you need to decide which process fits your part, or whether you need both, you will find the answer here.
CNC Drilling vs CNC Boring
Drilling and boring are not interchangeable. They serve different purposes at different stages of the same workflow. The comparison below sets out how they differ across the factors that matter most on the shop floor.
Purpose
CNC Drilling: Drilling creates a new hole from solid material. It establishes the position, diameter, and depth of the feature in a single operation.
CNC Boring: Boring refines a hole that already exists. It adjusts the diameter and improves internal geometry, but it cannot originate the hole itself.
Cutting Tool
CNC Drilling: Drilling relies on a drill bit, of which several types are in common use. Twist drills, spade drills, and core drills each suit specific materials and applications.
CNC Boring: Boring relies on a boring bar fitted with a single cutting insert. Adjustable heads allow one bar to machine a wide range of diameters without a tool change.
Order of Operation
CNC Drilling: Drilling is the first machining step in any hole-making sequence. It may itself be preceded by centre-drilling or spot-drilling to establish accurate location.
CNC Boring: Boring follows drilling, casting, or forging, because it requires a pre-existing hole to work from. It cannot cut directly into solid stock.
Hole Diameter Range
CNC Drilling: Drilling diameter is limited by the size of the bit fitted. Each new diameter requires a separate tool, and very large holes are impractical to produce by drilling alone.
CNC Boring: Boring has no fixed upper limit within the machine’s capacity. A single adjustable boring head can cover a broad range of diameters.
Hole Shape and Geometry
CNC Drilling: Drilling produces straight cylindrical holes. Speciality bits such as spot drills and countersinks add conical features, but the underlying shape remains cylindrical.
CNC Boring: Boring produces cylindrical, tapered, stepped, and profiled bores. The single-point tool traces whichever path the CNC program defines.
Axial Length Control
CNC Drilling: Drilling extends hole depth by advancing the bit into the workpiece. Feed rate and dwell time control the final depth reached.
CNC Boring: Boring does not increase hole length. It acts on diameter and finish only, not on depth.
Material Removal Rate
CNC Drilling: Drilling achieves a high material removal rate. Multiple cutting edges strip material rapidly, which keeps cycle times short.
CNC Boring: Boring has a lower removal rate. A single cutting edge takes small increments per pass in order to preserve precision.
Accuracy and Tolerance
CNC Drilling: Drilling holds a typical tolerance of ±0.05 to ±0.1 mm, which is adequate for most standard clearance holes.
CNC Boring: Boring achieves ±0.01 mm or better, and is required for bearing fits, sealing surfaces, and precision assemblies.
Surface Finish
CNC Drilling: Drilling produces an internal finish of Ra 3.2 to 6.3 μm. This is sufficient for fastener and general clearance holes.
CNC Boring: Boring produces Ra 1.6 to 3.2 μm. This finish suits holes that must seal, rotate under load, or accept a press fit.
Cost per Hole
CNC Drilling: The cost of drilling is low. Bits are inexpensive and cycle times are short.
CNC Boring: Boring costs more per hole. Tooling is more expensive, setup takes longer, and each hole requires additional machining time.
Quick Comparison Table
| Feature | CNC Drilling | CNC Boring |
|---|---|---|
| Purpose | Creates a new hole | Refines an existing hole |
| Cutting tool | Drill bit | Boring bar with insert |
| Cutting edges | Multi-point | Single-point |
| Order of operation | First step | After drilling or casting |
| Hole diameter range | Limited by bit size | Wide range from one head |
| Hole shape | Cylindrical | Cylindrical, tapered, stepped |
| Length control | Extends depth | Does not extend depth |
| Material removal rate | High | Lower |
| Tolerance | ±0.05 to ±0.1 mm | ±0.01 mm or better |
| Surface finish | Ra 3.2 to 6.3 μm | Ra 1.6 to 3.2 μm |
| Cost per hole | Low | Higher |
| Typical application | Fastener and clearance holes | Bearing seats, seals, precision fits |
Important: Drilling sets the position of a hole. Boring only adjusts diameter and depth. If a hole is drilled in the wrong location, boring cannot fix it.
When to Use CNC Drilling vs CNC Boring
Choosing between the two comes down to one question: does the hole already exist?
If the answer is no, you need drilling. If the answer is yes and the hole needs to be more accurate, you need boring.
Use CNC drilling when:
- You need to create a new hole in solid material
- The hole is for a fastener, bolt, or screw
- Position accuracy is the priority
- Speed and cost matter more than surface finish
- The hole will be used as a pilot for boring or reaming
Use CNC boring when:
- The hole needs to meet tight dimensional tolerances
- You need a specific surface finish inside the hole
- The hole is a bearing seat, hydraulic bore, or precision fit
- A cast or forged hole needs to be corrected and refined
- The drilled hole is close but not accurate enough for assembly
Note: Drilling comes first to establish the hole. Boring follows to bring it to final specification. The two processes work together, not against each other.
Custom Quality Parts By Aria
Contact Us: Sales@madearia.com
Written By
Gavin Leo
Custom Quality Parts By Aria
Send your specs. We’ll get back with a quote in 12 hours.