A boring head mounted in a milling machine spindle is the most practical and accurate method of boring holes to precise diameters in a machine shop environment. Unlike a lathe boring bar, where the workpiece rotates, a milling machine boring head rotates in the spindle while the workpiece remains stationary on the machine table. The single-point cutting tool is offset from the spindle centerline by an adjustable amount, and as the spindle rotates, the tool sweeps out a circular path whose diameter equals twice the offset distance. Adjusting the offset by 0.0005 inch changes the bore diameter by 0.001 inch, giving precise, repeatable control over finished bore size.
Buyohlic boring heads are designed for use on Bridgeport-style manual milling machines, knee mills, and CNC machining centers. They accept standard boring tool cartridges and provide fine micrometer adjustment for setting the bore diameter before cutting. Available in R8 shank configuration for Bridgeport-style mills, Morse taper for machines with MT spindles, and BT/CAT configurations for CNC machining centers.
Boring Head Types and Adjustment Range
Standard boring heads with a 2- to 3-inch diameter body typically cover a boring range from 0.5 inch to 4 inches in diameter, depending on the bar length and insert offset used. The micrometer adjustment ring on the head body moves the tool cartridge in precise increments, typically 0.0005 inch per graduation. Reading the graduation correctly and translating it to bore diameter change (the change in bore diameter equals twice the change in tool offset) is the fundamental operating skill for boring head use.
Micro-boring heads, also called fine boring tools, have an integrated single-point tool with an extremely precise micrometer adjustment. They are used for bore tolerances tighter than H7, where the bore diameter must be held within 0.0005 inch of the nominal size. Fine boring tools are the standard for bearing housing bores, hydraulic cylinder bores, and any hole requiring a close sliding or interference fit.
Boring Head Setup for Accurate Bore Sizing
- Pre-drill or rough mill the hole to within 0.020 to 0.030 inch of the final bore diameter before using the boring head
- Set the boring head offset for a light finishing pass (0.005 to 0.010 inch per side) rather than trying to bore to final size in one pass from rough
- Measure the bore diameter after each pass using a telescoping gauge and outside micrometer, or a bore gauge calibrated to the nominal size
- Make the final adjustment and confirm the bore size before removing the workpiece from the machine table, as re-indicating the bore after removal adds setup time
For boring bars for lathe operations, see our boring bars collection. For boring tools more generally, see boring tools. For milling machine toolholding, see our chucks for milling and turning.
Frequently Asked Questions - Boring Heads
The shank configuration depends on your milling machine spindle. Bridgeport and Bridgeport-style knee mills use an R8 taper spindle, so you need an R8 boring head shank. CNC machining centers use BT30, BT40, CAT40, or CAT50 taper tool holders. Machines with Morse taper spindles use MT3 or MT4 boring head shanks. Check your machine's spindle specification before ordering. The boring head body and adjustment mechanism are the same regardless of shank style.
Bore the hole close to final size, then measure the current diameter with a telescoping gauge and micrometer. Calculate the remaining stock to remove on each side (half the total diameter error). Adjust the boring head offset by that amount using the micrometer adjustment ring. One graduation on most boring heads equals 0.0005 inch of tool movement, which produces 0.001 inch change in bore diameter. Take a test cut and measure before committing to the final dimension.
A standard 2-inch body boring head with a 3-inch boring bar can reach bore diameters from approximately 0.5 inch to 3 inches. Larger boring heads with longer bars can reach 4 to 6 inches or more. The maximum diameter is limited by the boring bar length and the machine table clearance below the spindle nose. For very large diameter bores, a dedicated horizontal boring mill is more practical than a vertical milling machine with an extended boring head setup.
No. Boring heads generate significant radial cutting forces that drill press spindles and quill bearings are not designed to withstand. Using a boring head on a drill press results in poor accuracy, excessive vibration, and risks permanent damage to the drill press spindle. Boring heads must be used on milling machines or dedicated boring mills with spindles designed for radial load.
A fine boring operation with a sharp carbide insert, correct cutting speed, light finishing depth of cut, and appropriate feed rate achieves surface finishes of Ra 0.8 to 1.6 micrometers (32 to 63 microinch), which is adequate for most bearing and bushing fits. For smoother finishes, the bored surface is subsequently honed or reamed to Ra 0.4 micrometers (16 microinch) or better.
