Can a Multi Start Thread Make an 8 mm Plastic Screw Affordable?

A customer planned to order five plastic assemblies, each with an 8 mm-diameter external thread running for 51 mm. The original design called for medical-grade PPSU, one thread start, and 5 mm of travel per revolution. For this multi starts thread project, the challenge was not simply cutting an M8-sized feature. The combination of a large lead, a long, slender plastic section, custom tooling, and a five-piece quantity made the original design expensive to prototype.

Our first manufacturing plan used a Swiss-type lathe with a thread-whirling attachment. The custom whirling tool was quoted at about $500, and machines setup added roughly $200 before material, machining, and inspection.

After a design-for-manufacturing review, the customer reduced the required lead from 5 mm to 2.5 mm. We changed the geometry to an 8 mm nominal diameter, 1.25 mm pitch, and two starts. A matching two-start tap and die were available, so the project no longer needed a custom whirling tool. One polycarbonate (PC) assembly prototype, including tool preparation, cost about $170.

How a multi starts thread increases travel per revolution

Lead, pitch, and the number of thread starts are connected by one simple formula:

Lead = pitch × number of starts

A single-start thread has one continuous helix, so its lead equals its pitch. A two-start thread has two intertwined helices with starting points 180 degrees apart. With a 1.25 mm pitch, two starts produce 2.5 mm of axial travel per revolution.

This arrangement allowed us to increase travel without using a 5 mm pitch. The smaller pitch also left more usable core material than a conventional 60-degree profile scaled to a 5 mm pitch would have provided. The Harvey Performance multi-start thread guide provides a useful visual explanation of pitch, lead, and start count.

A four-start design with the same 1.25 mm pitch would theoretically retain the original 5 mm lead:

1.25 mm × 4 starts = 5 mm lead

That option would still require matching four-start internal and external tooling, accurate indexing of all four starts, and assembly testing. Since the customer could accept 2.5 mm of travel per turn, the two-start design was the simpler and less expensive route.

Why standard M8 tooling could not make the original thread

Standard coarse M8 threads use a 1.25 mm pitch. A conventional M8 × 1.25 thread has one start and therefore advances only 1.25 mm per revolution.

The original part had the same 8 mm nominal diameter but required a single start and a 5 mm lead. Its pitch was therefore also 5 mm. It was not a standard M8 thread, and a regular M8 tap, die, or threading insert could not produce it.

Changing only the flank shape to a familiar 60-degree metric profile would not solve the problem. The 5 mm lead and 5 mm pitch would remain. On an 8 mm-diameter part, that geometry would either remove too much core material or require a shallower custom profile—and either route would need purpose-made tooling.

A large lead also creates a steeper lead angle. The geometry was manufacturable, but it changed the relationship between input torque and axial force and made alignment, friction, clearance, and bending more important. On a 51 mm-long plastic screw, even modest deflection could cause the mating parts to bind.

Why the 51 mm PPSU section was difficult to turn

Medical-grade PPSU is valued for its heat resistance, chemical resistance, dimensional stability, and resistance to repeated steam sterilization. It is still far less rigid than metal, and the proposed geometry left a long, slender section under continuous cutting load.

Single-point thread turning normally builds the thread through multiple passes. As the tool travels along a 51 mm threaded section, radial cutting force can bend the plastic core away from the tool. Deflection, vibration, taper, and thread-form error become harder to control near the middle of the span.

Our engineers suggested reducing the threaded length to about 25 mm to make conventional turning more stable. The customer needed the full 51 mm for the application, so shortening the part was not an option. The machining risk came from the geometry and unsupported length, not from PPSU alone.

Why we first chose Swiss turning and thread whirling

A Swiss-type lathe supports the bar close to the cutting zone with a guide bushing, reducing the effective unsupported length. In thread whirling, several inserts mounted inside a rotating ring cut the thread while the ring is set to the required helix angle.

This process is well suited to long, slender parts, deep profiles, and special threads. Sandvik Coromant lists medical screws and other microcomponents among the typical applications for thread whirling.

The process was technically suitable, but its fixed cost did not fit a five-piece prototype order. The original profile required a custom whirling tool quoted at about $500, plus approximately $200 for machine setup.

*Material, machining time, inspection, and finishing are not included.

Swiss turning becomes more economical when those fixed costs are spread across a stable production run. For one or five prototypes, the same engineering, tooling, setup, and first-piece work still has to be completed.

A similar cost pattern appears in our custom thread machining case study for a medical bone screw, where a special thread profile also required dedicated whirling tooling.

The two-start tap and die changed the prototype economics

For this multi starts thread prototype, the approved geometry became:

• 8 mm nominal diameter;
• 1.25 mm pitch;
• two start;
• 2.5 mm lead.

This was not an ordinary M8 × 1.25 thread. A standard single-start M8 tap and die would still create only 1.25 mm of travel per turn. The key was finding a matched two-start tap and die that could produce both mating parts.

We machined the remaining part features first, then cut the internal and external threads with the matched tools. Guides and support kept the tools aligned with the 51 mm axis. Torque, chip removal, lubrication, and straightness still required control, but this method avoided repeated single-point turning passes along the slender plastic section.

Because the tools could be reused, the first prototype carried most of the tooling and preparation expense. Additional prototypes would have a lower unit cost as long as the design remained unchanged.

Why the first assembly prototype used PC instead of PPSU

Medical-grade PPSU stock was a significant material expense. Making all five sets before checking the new lead, fit, and motion would have exposed the customer to unnecessary scrap if the design needed another revision.

We recommended one PC prototype first. It was used to verify:

• 2.5 mm of axial movement per revolution;
• smooth engagement between the internal and external threads;
• free movement along the full 51 mm length;
• bending or twisting of the threaded shaft;
• final axial position after assembly; and
• whether the two entry points affected installation orientation.

The first PC prototype, including the matched tooling and preparation, cost about $170. Once the assembly behavior was confirmed, the customer could decide whether to order the remaining parts in the specified medical-grade PPSU.

PC was used only for geometric and functional evaluation. It did not validate the heat, chemical, sterilization, or material-compliance requirements of the final product. Those properties must be verified in the production material. For reference, Ensinger’s medical-grade PPSU data highlights dimensional stability and resistance to common sterilization methods, particularly hot steam.

A two-start drawing must show both pitch and lead

Calling out only “M8” or “2.5 mm lead” does not fully define this thread. The drawing should specify:

• nominal diameter;
• pitch;
• lead;
• number of starts;
• thread form and flank angle;
• right-hand or left-hand direction;
• effective thread length;
• major, pitch, and minor diameter requirements;
• fit and clearance;
• angular location of the thread starts, when functionally important;
• mating internal or external thread; and
• material and operating environment.

For this design, the essential geometry was 8 mm nominal diameter, 1.25 mm pitch, 2.5 mm lead, two starts, right-hand. Writing only “M8 × 1.25” could be mistaken for the common single-start thread and result in the wrong part.

Start position also matters when the assembled part must stop at a specific angle. A two-start thread offers two entry points 180 degrees apart. Diameter and lead measurements alone cannot confirm the final installed orientation.

Prototype inspection can include a fit test with the mating component and a check of axial travel over one full revolution. Production inspection should also cover straightness, major and minor diameters, pitch diameter, lead accuracy, and full-length engagement as required by the drawing.

The practical result

This multi start thread case was not about an impossible part. The original single-start, 5 mm-lead design could be manufactured, but it combined a special profile, a 51 mm slender plastic section, custom tooling, and a very small order. That combination placed too much cost and risk into the first five parts.

Reducing the lead to 2.5 mm allowed us to use a 1.25 mm-pitch, two-start design and an available matched tap and die. Producing one PC assembly prototype first brought the initial validation cost to about $170 and gave the customer a practical way to test the motion before committing to medical-grade PPSU.

The value of a multi-start design is straightforward: pitch, lead, and start count can be balanced to achieve faster axial travel without forcing an oversized pitch onto a small-diameter screw. For low-volume prototypes, checking available tooling and validating one assembly before releasing the full order can save far more than changing suppliers after the drawing is complete.