Boring is the machining operation used to enlarge and true up an existing hole. Unlike drilling, which creates a hole from solid material, boring works on a hole that already exists, refining its diameter, improving its roundness, straightening its axis, and bringing it to a precise finished dimension that a drill cannot achieve. Boring is the operation you use when a drilled hole is not accurate enough, when the hole needs to be to an exact size not available in standard drill sizes, or when the existing hole needs its axis corrected relative to other part features.
Boring tools at Buyohlic cover the range from single-point boring bars for lathe operations to adjustable boring heads for milling machines, fine boring tools for precision diameter work, and carbide insert boring tools for production rates that HSS cannot match. Whether you are boring a bearing housing to a precise fit, enlarging a shaft bore in a gear, or refining a deep through-hole to exact alignment, the right boring tool makes the operation clean, accurate, and repeatable.
Boring vs Drilling: Understanding When to Use Each
Drilling produces holes from solid material quickly and economically. Standard twist drills are the fastest way to create a through-hole or blind hole from solid stock. However, drills have limitations: they drift slightly from the intended axis, they produce a hole slightly oversize relative to the nominal drill diameter, and they cannot achieve the tight diameter tolerances required for bearing and bushing fits. When a drilled hole is not accurate enough, boring finishes the job.
Boring corrects all three of the drill's limitations. A boring bar with a single-point tool tip cuts the bore to a precise diameter by adjusting the tool radius, straightens the bore axis by cutting concentrically with the machine spindle, and achieves surface finish and dimensional tolerances that drills cannot reach. For bearing bores, bushing fits, and any hole requiring H7 or tighter tolerance, boring is the correct finishing operation after rough drilling. For a deeper understanding of the difference between boring and drilling, see our boring vs drilling guide.
Types of Boring Tools
- Lathe boring bars: Cantilever bars mounted in the tool post, used for internal diameter turning in the lathe. The boring bar projects into the bore, and the single-point insert cuts the inside surface as the workpiece rotates.
- Milling machine boring heads: Adjustable heads that mount in the mill spindle, with an offset tool bit that sweeps the bore as the spindle rotates. The diameter is set by adjusting the tool bit offset from the spindle centerline.
- Fine boring tools: High-precision boring tools with micrometer-adjustable tool bits, used when bore diameter must be held to 0.001 inch or better
- Indexable boring bars: Bars accepting standard carbide insert cartridges for production boring at high speeds and feeds
For boring heads for milling operations, see our boring heads and accessories collection. For boring bars specifically, see our boring bars collection. For related lathe tooling, see lathe tools and accessories.
Frequently Asked Questions - Boring Tools
A boring tool is used to enlarge, true up, and bring to precise dimensions an existing hole in a metal workpiece. Boring corrects the axis alignment, roundness, and diameter of a drilled or rough-machined hole to achieve tight tolerances for bearing fits, bushing fits, and precision locating features. Boring is used whenever a drilled hole is not accurate enough for the intended assembly fit.
Drilling creates a new hole from solid material using a rotating multi-flute tool. Boring enlarges and refines an existing hole using a single-point cutting tool. Drilling is faster but less accurate. Boring is slower but achieves diameter tolerances, roundness, and axis alignment that drills cannot match. In precision work, drilling is used to rough the hole and boring is used to finish it to final dimensions and tolerance.
The boring bar diameter must be small enough to fit inside the hole being bored while still providing adequate rigidity. A general rule is that the boring bar diameter should be no less than 75 percent of the bore diameter. For deep holes, the bar length-to-diameter ratio should be kept below 4:1 to minimize deflection and chatter. Use the largest diameter bar that fits the bore to maximize rigidity.
Boring chatter is caused by vibration in the boring bar due to its cantilever nature. The primary causes are excessive bar overhang (length-to-diameter ratio above 4:1), insufficient clamping rigidity in the tool holder, spindle speed at a resonant frequency, or excessive depth of cut for the bar stiffness. Reduce overhang, increase clamping rigidity, change spindle speed by 20 percent in either direction, and reduce depth of cut to resolve chatter in boring operations.
Standard boring bars and adjustable boring heads require a milling machine or boring mill for use, because the radial cutting force in boring requires the spindle to resist lateral loads that a drill press spindle and quill are not designed to handle. Using a boring head on a drill press causes inaccurate results and risks damaging the drill press quill bearings. Boring should be performed on a milling machine or dedicated boring machine.
