Almost every machined part has a threaded hole somewhere, and there are two fundamentally different ways to cut it. Tapping is fast and cheap. Thread milling is flexible and forgiving. Knowing which one your shop will reach for — and why — tells you a lot about cost, risk, and whether that part is going to come back with a snapped tool buried in it.
Two Ways to Make a Thread
Tapping drives a tap — a tool shaped like the finished thread — straight down into a pre-drilled hole. The tap’s geometry is the thread; one plunge cuts (or forms) the full profile in a single pass. It’s the threading method most people picture.
Thread milling uses a smaller rotating cutter that spirals around the inside of the hole, generating the thread through controlled CNC motion (helical interpolation). The tool is smaller than the hole, and the toolpath creates the thread, not the tool’s full shape.
Where Tapping Wins
- Speed. For small, standard threads in high volume, nothing beats a tap. One quick plunge per hole.
- Cost per tool. Taps are inexpensive and universally stocked.
- Small diameters. Below about #6 / M3.5, thread mills get fragile. Tapping is often the only practical route for tiny threads.
- Simple, friendly materials. Aluminum and free-machining steels tap beautifully.
The catch: a tap is matched to one specific thread size and pitch, it cuts the hole bottom-up with limited control, and — the big one — when a tap breaks, it breaks inside the hole. Hardened tap steel is miserable to remove and frequently scraps the part. In a deep hole in expensive titanium, a snapped tap can vaporize hours of prior work.
Where Thread Milling Wins
- Hard and gummy materials. Titanium, Inconel, hardened steel, stainless — exactly the materials that snap taps. The lighter cutting forces of a thread mill are far safer.
- Big threads. A 2-inch tap is a huge, expensive, torque-hungry tool. A single thread mill can cut it — and many other sizes — comfortably.
- One tool, many sizes. A single-pitch thread mill cuts any diameter of that pitch. Fewer tools in the carousel, less inventory.
- Near-bottom threads in blind holes. A thread mill can cut threads much closer to the bottom of a blind hole than a tap, which needs clearance for its lead-in.
- Recoverable failure. If a thread mill breaks, the tool is smaller than the hole — you can usually back it out and save the part. That single fact justifies thread milling on high-value work.
- Adjustable fit. Because the toolpath sets the diameter, the machinist can tweak the cut to dial in thread fit class without changing tools.
The cost: thread milling is slower per hole and the tooling and programming are more sophisticated. For a thousand M3 holes in an aluminum bracket, insisting on thread milling just burns money.
Side by Side
| Tapping | Thread Milling | |
|---|---|---|
| Speed per hole | Fast | Slower |
| Best for | Small, high-volume, soft materials | Large, hard materials, high-value parts |
| Tool flexibility | One size per tap | Many diameters per cutter |
| If the tool breaks | Usually scraps the part | Often recoverable |
| Programming | Simple | Helical interpolation (more complex) |
| Fit adjustment | Fixed by tap | Adjustable in CNC |
Form Tapping: The Third Option
Worth a mention: form (roll) tapping doesn’t cut at all — it cold-forms the thread by displacing material, producing no chips and a stronger thread through work-hardening. It’s excellent in ductile materials like aluminum and mild steel, and the no-chip aspect is a real advantage in blind holes where chip evacuation is a headache. It won’t work in brittle materials, and it needs a slightly larger pre-drill than cut tapping.
What This Means for Your Part Design
You usually don’t specify the threading method — the shop chooses it. But your design drives the choice, and a few decisions make threads cheaper and more reliable:
- Don’t over-spec thread depth. Threads engaged beyond ~1.5× diameter add almost no strength and a lot of tapping risk. Call out only what you need.
- Allow clearance at the bottom of blind holes where you can — it widens the shop’s options and reduces broken-tap risk.
- Use standard sizes unless function demands otherwise. Standard means a tap is on the shelf.
- Flag hard materials and high part value. If the blank is costly, the shop will lean toward thread milling for safety — and you’ll want them to.
These trade-offs sit right alongside the rest of machinable design — see our notes on designing CNC-friendly features and how threading callouts ripple into your quote.
Got parts with critical or tricky threads?
DFW Machine matches the threading method to your material, volume, and part value — tapping where it’s smart, thread milling where it’s safer. Send the print and we’ll tell you which way we’d cut it and why. Request a quote →
The Bottom Line
Tapping is the right call for small standard threads in friendly materials at volume — fast and cheap. Thread milling earns its slower cycle on hard materials, large diameters, and parts too valuable to gamble on a tap that might break off inside them. A good shop owns both and picks deliberately. If your quote comes back higher than expected on a threaded titanium part, this is often why — and it’s the cheap insurance that keeps your part out of the scrap bin.