Guides / Technical

Thread Whirling vs Single-Point Threading on Medical Screws

October 25, 2026

Bone screws are mostly thread by area, and how the thread is cut decides cycle time, surface finish and how many parts survive the lot. Two methods dominate cut threads on slender medical screws: single-point threading and thread whirling. This guide explains how each works, where each wins and what to ask a shop before it quotes.

How single-point threading works

A single-point tool is a small insert with the shape of the thread. The lathe spindle turns the part, the tool moves along the axis at a feed equal to the thread pitch, and the tool cuts one helical groove. The tool takes several passes, each a little deeper, until the thread reaches full depth.

The method needs nothing beyond a normal lathe and a threading tool. That makes it flexible: it can cut almost any thread form, any pitch and any length. The price is time, and on a slender part the cutting force pushes the screw sideways with each pass.

How thread whirling works

Thread whirling puts a ring of carbide inserts around the bar. The ring rotates at high speed, and the part turns slowly. The inserts, set off-centre and tilted to the helix angle, cut the thread in one pass or very few. On a Swiss-type machine the bar passes through the ring, so the thread is cut close to the guide bushing, where the part is best supported.

The method needs a whirling unit, a tool ring matched to the thread form and a machine with the spindle and axis control to run it. Setup takes longer than a plain threading cycle, and the ring is specific to a pitch and form. Once running, it cuts a coarse thread in a fraction of the time.

Where each method wins

Factor	Single-point	Whirling
Passes per thread, typical	Many, often 6 to 12	One to two
Slender, long threaded section	Part flexes with each pass	Cut close to support
Coarse pitch, bone-screw style	Slow	Strong fit
Fine pitch, short thread	Fine	Little advantage
Special form or odd pitch	Flexible	Needs a matching ring
Low volume	Cheaper	Setup rarely pays back
Tooling needed	Standard inserts	Whirling unit and ring

These are typical comparisons, not promises. A shop that runs a screw family weekly can make either method work.

Surface finish and thread quality

Whirled threads tend to show an even, repeatable surface because the insert engages the part gently at high surface speed, and each insert takes a small chip. Single-point threads depend more on the tool edge, the feed per pass and how well the shop controls chatter. On titanium, built-up edge and galling can leave a torn flank if speeds and coolant are wrong. Both methods leave burrs at the thread start, the thread runout and any cross hole, which a separate step has to remove. See our guide on deburring medical parts.

Thread form for bone screws is governed by standards such as ISO 5835 for hex-drive metal bone screws and by ASTM F543 for the screws and their test methods. The standards set the geometry and what a test must show, not the process that cuts it. If a shop switches from one process to the other and the geometry still meets the standard, the part is within the standard but not necessarily within your validated process.

Material matters more than method

Titanium alloys such as Ti-6Al-4V ELI cut with a long, stringy chip and low thermal conductivity. Heat stays in the cutting edge, so tool life falls sharply with speed. Stainless steel such as 316L work-hardens and tends to smear. Cobalt chrome wears inserts quickly. A whirling ring spreads the work across several inserts, which can help with tool life on the harder alloys, but the right insert grade and coolant delivery decide the result. Our titanium, 316L and PEEK guide covers the bar stock side.

What to ask a shop

1. Which process do you plan for this thread, and why? A good answer refers to length, pitch and volume.
2. Is the whirling unit installed on the machine that will run my part? Some shops outsource whirling to another site, which changes the supply chain.
3. How will you inspect the thread? Go and no-go gauges, an optical comparator or a thread measuring system, with the plan for each lot.
4. Where do burrs form and how are they removed? The answer should name the step, the method and the acceptance check.
5. What happens if you change process later? The answer should be a written notification before the change, not after.

For how the thread sits next to cannulation and drive geometry in the price, read what a bone screw quote hangs on.

Where thread rolling fits

A third method sometimes enters the conversation. Thread rolling forms the thread by pressing hardened dies into the bar and displacing material, so no chip is made. It is fast, it leaves a smooth flank and it works well on fine threads in ductile material. It struggles on high-strength alloys, on sharp coarse forms with deep roots and on parts where the displaced material has nowhere to go. Many bone screw designs are cut for those reasons, and some designs use rolling. If your drawing or your regulatory file names one process, the shop has to follow it. If it names none, ask the shop what it plans and why, then confirm that the plan matches what was used on any test specimens in your file.

Failure modes worth knowing

Four problems account for most rejected screw threads on implant alloys.

• Torn or smeared flanks. Titanium and stainless steel stick to the tool when speed, feed or coolant is wrong. The flank looks rough under magnification and the screw may bind in its test block.
• Burrs at the start and runout. A burr that detaches in the body is a clear risk. Both methods leave them.
• Chatter marks on slender parts. Visible as a regular pattern on the thread flank, and often a sign of a loose guide bushing or a worn tool.
• Pitch drift over a long thread. A thread cut with a worn tool or in a warm machine can finish at a slightly different pitch than it began. A gauge that checks only a short section will miss it.

None of these depends on the method alone. They depend on how well the shop controls tool wear, coolant and inspection, which is why the plan for each lot matters as much as the process name on the router.

Inspection of cut threads

A screw thread has several characteristics that all matter: major diameter, pitch diameter, pitch, flank angle and root radius. Inspection plans often combine a functional gauge with a periodic optical check of the profile. Gauges answer the question of fit. They do not show a torn flank or a small root radius that raises stress. A profile projector or optical comparator at regular intervals shows both. For critical screws, ask whether the plan includes a section or a profile check on first articles.

If your drawing calls out a thread class, check that the shop reads it the same way you do. Thread classes and tolerance classes differ between metric, unified and the special forms used on bone screws.

Making the call

Choose single-point when the thread is short, fine, unusual or low in volume, or when the shop has no whirling unit. Choose whirling when the screw is long and slender, the thread is coarse and the volume repays the setup. If you are not sure, ask two shops for both methods and compare cycle time, setup and inspection.

We match buyers with machining suppliers and the buyer pays us nothing. We do not machine parts and we do not qualify a supplier for you, because that stays in your own quality system. To send a screw drawing, request a quote and we reply within 24 business hours.

Sources

• ISO 5835, implants for surgery, metal bone screws with hexagonal drive connection
• ASTM F136, wrought Ti-6Al-4V ELI alloy for surgical implant applications

Standards and regulations change. Check the current text at the source before you rely on it in a submission.

Frequently asked questions

Keep reading

• Wire EDM for Surgical Instrument Jaws and Cutting Features
• Swiss Turning for Medical Device Components: Where It Fits and Where It Does Not