Inside the process: CNC grinding for high-precision HSS taps

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Although solid carbide taps often last longer – especially in hard alloys – high-speed steel (HSS) still accounts for about 97% of cutting tap production. The trade-off: carbide’s superior wear resistance comes with greater brittleness, increasing the risk of breakage under pressure.

For HSS taps, the center holes and drive square are produced before heat treatment. Once hardened, the shank is ground between centers, using the pre-machined centers as datums. The drive square is left in its milled condition (no post grind).

The tap is never clamped by the square – for grinding or for cutting. The square serves only as a drive feature. The tap blank is held between centers for shank and geometry grinding. In actual use, a tapping attachment on a lathe or CNC machining center clamps on the ground shank to guide the tool, while a floating driver engages the square to transmit torque and allow axial float for alignment and clean retraction after threading.

“Holding between centers” is a common method in grinding, where a cylindrical workpiece is supported by two conical points called centers, one in the headstock (the “live” center, which rotates) and one in the tailstock (the “dead” center, which is stationary). This method ensures the workpiece’s features are concentric and allows for accurate grinding with precise alignment. The tailstock typically should have a hydraulic system to apply pressure to ensure the workpiece is firmly and concentrically captured between the two centers. On smaller taps, using minimum pressure or an adjustable spring-loaded system is needed to avoid bowing the shank.

Tap manufacturing typically uses dedicated flute, thread, and chamfer grinders, with thread grinding controlling final accuracy and tool life. On HSS taps, conventional wheels (alumina/ceramic – non-CBN, non-diamond) are generally favored over CBN because they’re less expensive, easier to dress frequently for sharpness and profile control, and better suited for short runs or frequent wheel shape corrections.

Modern CNC thread grinding relies on autonomous rotary diamond roll dressing. By repeatedly refreshing the wheel with a roll with the inverse of the required wheel form, the grinding process preserves the exact flank angles and root/crest radii in a prolonged unattended manufacturing process. Common dressing roll manufacturing includes reverse-plated (or electroplated), sintered, and CVD-sintered designs.

Current CNC tap grinding solutions fall into two categories: integrated thread/chamfer grinders and multi-axis grinders that finish threads, chamfers, and flutes in one clamping. Multi-axis systems enable the (less common) use of superabrasive wheels – CBN for HSS and diamond for carbide – supported by dedicated rotary dressing and truing methods.

There are also purpose-built CNC grinding machines designed to only grind the flutes on cutting taps. These machines offer automated processes for high-volume tap production by focusing on straight and spiral flute grinding operations. Depending on the model, optional stations can be added to perform chamfer or spiral point grinding in the same setup. Rolling taps are typically ground on multi-axis CNC grinders in one setup.

MyGrinding Inc.
https://www.mygrinding.com