Methods of Treatment of Various Threads: 5 Ways Compared

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  5 methods of treatment of various threads at a glance
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  How single-point turning and thread milling work
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  Tapping and thread rolling: when each one wins
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  Thread grinding and what to do after the thread is cut
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  How to choose between the methods of treatment of various threads

Single-point turning cuts the thread with one insert that follows the helix of the part. The machine synchronizes spindle rotation and Z travel, so the lead stays constant over the whole length. Cutting speed, depth of cut and the number of passes set the result. Most shops run 6 to 10 passes in steel and 3 to 5 in aluminum.

This is the go-to method for external threads on shafts, fittings and turned parts. A 4,000 mm maximum processing size on our lathes means long lead screws and tie rods stay on one setup, which keeps the pitch from wandering between ops. Threads come out at class 2A or 6H cut without a second operation.

Thread milling uses a rotating cutter that travels around the bore in a helical path. The tool is smaller than the hole, so one cutter can produce any diameter above its minimum. Chip breaking is easier than tapping because the tool never packs the flutes. Blind holes with a shallow relief benefit most.

The trade-off is time. A thread mill makes several revolutions per hole, while a tap makes one pass. On M6 and larger, though, the mill wins on tool life and on the ability to fix a thread by re-running the same path.

Once the path is proven, we keep it in the program and re-run it on later batches.

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  External, long partsSingle-point turning holds lead over long lengths.
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  Large or odd diametersOne thread mill covers a wide size range.
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  Blind holesMilling clears chips better than tapping.

Tapping is the fastest way to put a thread in a hole up to about M20. A tap cuts or forms the thread in one pass, and the cycle time is a few seconds. For soft metals and short holes, nothing beats it on cost per hole. The catch is chip evacuation in blind holes and the risk of tap breakage in hard material.

Form tapping displaces material instead of cutting it. There is no chip, the grain flows with the thread, and the thread is stronger in fatigue. It needs a slightly larger pilot hole and works best in aluminum, brass and low-carbon steel. In 316 stainless or titanium, the torque climbs fast and the tap can snap.

Thread rolling does the same thing on external threads, but with two or three rolls pressing the blank. The blank diameter sits between the minor and major diameter, and the material flows up into the crest. No material is removed, so the thread is stronger than a cut one.

Rolled threads suit high-volume fasteners and any part that sees cyclic load. Setup cost is higher because you need the right blank size and roll set, so it pays off on runs of hundreds, not ones and twos.

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  Cut tappingCheapest per hole in soft metal, M1 to M20.
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  Form tappingNo chips, stronger thread, needs a bigger pilot hole.
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  Thread rollingBest fatigue life on external threads at volume.

Thread grinding comes after heat treatment. Once a part is hardened to 50 HRC or above, a cutter will not touch it, so the thread is ground with a profiled wheel. The wheel is dressed to the thread form and the part indexes one lead at a time. It is the slowest method and the most accurate.

Use it for tooling, gages, lead screws and any thread that must hold a grinding class such as 4H. Grinding also removes the decarburized skin left by hardening, which is where fatigue cracks often start. That is a real gain on aerospace and medical parts, not just a cosmetic one.

Threads are not the last op on most parts. Deburring, anodizing, plating and laser marking all follow. Plating adds thickness, and a class 2B thread can go tight after zinc or nickel. We mask threads or cut them with allowance when the finish is thick.

Laser marking needs a minimum character height of 1.5 mm to stay legible after anodizing. If the thread callout and the mark sit on the same face, plan the order so the mark is not cut through by a later deburr pass.

A quick note on inspection: we check threads with go/no-go gages and record the result.

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  Hardened partsGrinding is the only option above about 50 HRC.
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  Thick platingCut with allowance or mask the thread.
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  Marking1.5 mm minimum character height.

Start with the material. Aluminum, brass and mild steel tap and roll cleanly. Stainless 316, 17-4PH and titanium fight the tap, so milling or single-point turning is safer. Hardened tool steel leaves grinding as the only path. The material usually decides the method before the drawing does.

Then look at the hole. A blind hole with a shallow relief favors thread milling because chips clear. A through hole in soft metal favors tapping. A tapped hole smaller than M2 needs a floating holder and a peck cycle, and even then the tool life is short.

Volume sets the rest. One-off prototypes go to turning or milling because there is no tooling to buy. Runs in the hundreds or thousands justify rolling or a dedicated tap. Our shop runs from one prototype to 10,000+ part runs with no minimum order quantity, so the same part can switch methods as the quantity grows.

Finally, check the class of fit. A loose class 2B thread is fine for a bolt, but a positioning thread on a gage needs a tighter class and probably grinding. Tell us the fit class and the function, not just the size. That one detail changes the process plan.

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  Soft metal, through holeTap it.
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  Hard material or blind holeMill or single-point it.
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  High volumeRoll it, after the tooling is paid off.