A beginner's guide to Thread Milling

Thread Milling is cutting a thread with a rotating cutter that interpolates a helical path, rather than with a tap. It is used for threads in hard material, large threads and threads that must be concentric with another feature. This entry explains what it is, what the numbers on a data sheet mean, and what to put on a drawing.

It is written from the point of view of the buyer and the drawing office: the values that are realistic in production, the ones that cost money, and the way the result is verified. Where an industry sets the requirement, aerospace is used as the example: traceability and a first article before anything enters the batch.

In plain terms

Thread Milling is cutting a thread with a rotating cutter that interpolates a helical path, rather than with a tap. If you have not met the term before, the useful way to think about it is this: it is chosen when a part needs threads in hard material, large threads and threads that must be concentric with another feature.

The terms you will hear

• Thread size range – M3 to M100 and above.
• Typical tolerance – 6H with a standard cutter.
• Surface finish – Ra 1.6 µm on the flank.

What it costs and how long it takes

Cycle time, which is longer than tapping but safer, and a realistic cycle is 1 to 4 minutes per thread. Those two numbers, not the headline hourly rate, decide the price of the part.

What to ask a supplier

• What tolerance will you hold in production, not just on the first part?
• What does the dimensional report cover, and which standard is it written to?
• What happens to the fixtures and programmes if we reorder in a year?

Applications

Thread Milling is used for threads in hard material, large threads and threads that must be concentric with another feature, and in practice it appears most often in hardened parts, large diameter threads, threads in blind holes, parts where a broken tap is expensive.

How it fits into a machining route

The route is fixed before the first part is cut, and it is the route rather than the individual setting that holds the result: cycle time, which is longer than tapping but safer. Where a drawing calls for something the route cannot hold economically, that is raised at quotation rather than after the parts are made.

Standards, documentation and what is issued

The documentation requirement is in-house approval with dimensional reports. Material is supplied against a certificate where the application calls for it, inspection results are recorded with the job, and the records are kept so that a repeat order can be compared with the first instead of being measured from scratch.

Thread plug gauge and a go no go check is the verification step. Where a customer format or a standard report is required, it is agreed before the batch starts, because a report written after the event is a report nobody can use.

Frequently asked questions

What tolerance should be specified for Thread Milling?

6H thread tolerance is realistic in production. Tightening it beyond the functional requirement increases cost without improving the part.

Which material is usually used?

17-4PH stainless steel is the usual choice, supplied against in-house approval with dimensional reports.

How is the result verified?

Thread plug gauge and a go no go check, with the results recorded so a repeat order can be compared with the first.

What is the most common mistake?

Cutter deflection on a small diameter, which tapers the thread.

How long does it take and what does it cost?

A realistic cycle is 1 to 4 minutes per thread, and the cost driver is cycle time, which is longer than tapping but safer.

Related capability

If you are specifying Thread Milling on a drawing and want a second opinion on the route, send the model and the quantity. The first response is a manufacturability review with the likely risk points and a quotation, so the decision can be made on facts rather than on a price alone.

Machining, inspection and documentation are handled in one place, which is why the dimensional report and the material certificate arrive with the parts rather than after them.

Related entries

These entries sit next to Thread Milling in the encyclopedia: Deep Hole Drilling, Reaming and Boring, Inconel 718. Reading them together gives a fuller picture of how the process, the material and the measurement affect each other.