Thread Milling vs Tapping: When to Use Each

Thread milling vs tapping is a choice most shops make hole by hole, based on material, size and what the part is worth.

When Should a Threaded Hole Be Thread Milled Instead of Tapped?

Tap it by default, because tapping is faster. Thread mill it in hard or tough materials, in large threads, and anywhere a broken tap would scrap an expensive part.

An r/Machinists poster who threads 1018 steel and 932 bronze with plug taps wanted to switch to thread milling and asked how other shops decide. The answers followed a pattern: form taps where allowed, cutting taps on steels, and thread mills for tough materials and holes where a broken tap isn't an option.

Why shops default to tapping

Speed. A tap makes the whole thread in one in-and-out move. Several replies called thread milling slow and said it isn't their choice for production runs.

Form taps push material into shape instead of cutting it, so there are no chips to pack the hole, and one shop runs them on aluminum with cutting taps on steels. For the tradeoffs, see roll taps vs cut taps.

When thread milling wins

• Hard or tough materials. Replies named titanium and stainless, and one shop threading 50 HRC tool steel called tapping "unpredictable and unreliable." See also why a rigid tap strips threads in titanium.
• Expensive parts. One reply said titanium is always thread milled "because no one wants to scrap a 20k blank." Modern Machine Shop adds that thread milling makes short, broken chips that don't bind in the hole the way tap chips can.
• Bottoming threads. A thread mill has a flat bottom, so it can cut full threads closer to the bottom of a blind hole than a tap can.
• Adjustable fit. The thread diameter comes from the toolpath, not the tool. One reply said they can "easily tighten or loosen a thread when needed."
• Large threads. One thread mill covers a range of diameters, and a small machine that can't tap a big hole can often thread mill it (MMS). One reply said their shop thread mills anything over 3/4 in.

Where thread milling struggles

Depth is the main limit. MMS puts it at about 2.5 times the tool diameter, because the cutting force on a thread mill isn't balanced and the tool deflects. Cycle time is the other cost on high-volume parts.

Thread milling vs tapping for designers

You usually don't need to pick. Call out thread size, class and depth, and let the shop choose unless your spec requires a method. For titanium, see how deep you can tap a blind hole in titanium.

At DigiForge

Our CNC service machines metals including Stainless 304, Stainless 316, Alloy Steel 4140 and Titanium Grade 5. If a thread must be milled or formed, put it on the drawing. To know how we'd thread a specific hole, ask us when you quote.