Tapping and thread milling are two ways of making threads in a hole but they’re used in different situations. The main difference comes down to speed versus flexibility.
Tapping
Tapping is the faster option if you need to make lots of threaded holes that are the same size. Roscamat electric tapping machines, like the example seen here, https://www.cotswold-machinery-sales.co.uk/roscamat-tapping-machines/electric-tapping-machines/roscamat-tiger-electric-tapping-machine/ are made for this kind of work and they produce threads quickly even in tougher materials like steel. The downside is that each hole size needs its own tap, which means more tools and more time spent changing them. Also, once a hole is tapped, you can’t really adjust the thread size as it’s fixed. Tapping mainly works for internal threads and is better for through holes, as blind holes can need extra finishing to achieve clean threads at the bottom.
Thread Milling
In thread milling, the tool moves in a circular motion to cut the thread and this means you can adjust the size more easily if you need to. This is useful when you need a precise fit. One tool can be used for different thread sizes, so you don’t need as many tools and it can also create both internal and external threads. The only real drawback is that you need a high-speed machine to do it properly. It’s important to stick to manufacturing standards when using thread machines.
If you want speed and are making lots of the same holes, go with tapping. If you need flexibility and accuracy, thread milling is the better option.