Application of Threads Striker: Thread Milling in CNC Work

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  Key takeaways
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  What the application of threads striker means on a CNC machine
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  Why the application of threads striker spread through hard-alloy work
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  Which parts suit the application of threads striker, and which do not
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  How we run the application of threads striker at GreatLight
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  Material behavior in the application of threads striker
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  Thread milling against tapping: when each one fits

The application of threads striker describes a machine that forms threads by milling rather than by a tap. The tool is a single-point or multi-tooth cutter that moves along a helical path. The control links the X, Y and Z axes with a circular interpolation command and a Z feed, so the cutter climbs around the bore as it advances one pitch per revolution.

That path is the whole idea. A tap cuts with every tooth at once and must reverse out of the hole. A thread mill cuts one small arc at a time, so the load on the spindle stays low. The machine never has to stop and reverse at the bottom of a blind hole, which is where taps usually fail.

The striker part of the name comes from the older shop term for a tool that strikes or forms a thread in a bore. On a modern vertical or horizontal machining center, the same job is done by a rotating cutter on a helical path. The outcome is the same thread form, produced with less torque and better chip control.

Thread mills cover a range of diameters with one body. A mill sized for M6 can cut M6, M8 and M10 threads at the same pitch family by changing the helical radius. That single fact changes how a shop plans tooling for a mixed batch of parts.

The method started in European and North American shops that cut stainless, titanium and nickel alloys. Taps in those materials break, seize and chip. A broken tap in a finished aerospace or medical part can scrap the whole workpiece, and removal is slow and risky.

Carbide tooling changed the economics. A solid carbide thread mill holds its edge at cutting speeds that would destroy a high-speed steel tap. The tool also tolerates the interrupted cut of a helical path, which is gentler than the full engagement of a tap.

CAM software removed the last barrier. Tool libraries now store thread mill geometry, and the helical path is generated from a thread callout. A programmer picks the thread size, the class and the depth, and the software writes the G02 or G03 spiral with the correct lead.

The result is a process that is predictable. Cutting forces are low and steady, so a small machine can cut a thread in a hard alloy that would stall a tap. Surface finish on the flank is usually better than a tapped thread because the cutter shears the material rather than pushing it.

Thread milling fits large or awkward parts first. If a part is too heavy to rotate, or the thread sits on a face that is hard to reach, a helical path on a 3-axis or 5-axis machine is easier than turning the part for a tap. The same applies to threads that sit close to a wall or a shoulder.

It also fits threads in thin-wall bosses. A tap pushes outward and can distort a thin wall. A thread mill cuts with low radial force, so the wall holds its shape. Threads in castings with interrupted bores behave better as well, because the cutter does not grab at a void.

It does not fit every job. A shallow through hole in mild steel with a common pitch is faster and cheaper to tap. Deep small threads, below about M3, are also a poor fit, because the thread mill shank becomes too slender to survive the cut.

High-volume parts with one thread size are another weak case. A tap cycle is seconds shorter per hole, and across 10,000 parts that time adds up. Thread milling earns its place when the material is hard, the part is expensive, or the thread size changes often.

We run thread milling on our 16 simultaneous 5-axis machining centers and on the 16 mill-turn centers, where a thread often follows a turned diameter. For simple plates and brackets, the 27 three-axis machines handle milled threads without a rotary table.

A typical setup for a stainless part uses a cutting speed around 80 to 120 m/min for carbide, a feed per tooth of 0.03 to 0.06 mm, and a radial step-over of 0.2 to 0.4 mm. We climb mill and keep the helical lead matched to the pitch so the flank does not rub.

Thread depth is checked with go and no-go gauges and, on critical parts, with a thread micrometer. We hold ±0.005 mm on the bore before threading, because the pitch diameter of a milled thread follows the pre-drill size closely.

Our maximum processing size is 4,000 mm, so a long frame with threads at both ends can be milled in one setup instead of being moved between machines. That reduces the chance of a re-clamp error shifting a thread axis.

Aluminium is forgiving. Grades such as 6061 and 7075 cut fast with a thread mill, and the main risk is a burr at the entry. A chamfer of about 0.5 mm at the hole mouth removes it before the thread is cut.

Stainless grades 304, 316 and 17-4PH work-harden if the tool rubs. Keep the feed per tooth up and the radial engagement light, and the cutter stays in clean shear. A dwell at the bottom of the helix is a common mistake that hardens the root.

Titanium TC4 and Inconel are where thread milling pays for itself. Taps in these alloys fail by chipping or by galling onto the flanks. A thread mill with a wear-resistant coating and a rigid holder cuts the same thread with far less risk.

Plastics and copper alloys are usually tapped, but a thread mill helps on PEEK and on beryllium copper where a tap can tear the flank. In those cases we slow the helix slightly and use a sharper edge geometry.