There are many different types of taps, Steven Grantham comments on his blog that there are 38 pages of them in his catalogue. Here’s a few reasons why.
There are many different types of thread, metric fine, metric coarse, UNF (Unified Fine Thread), BA, Whitworth to name just a few. Each of these ranges has a predifined set of sizes and pitches. You need to ensure you have the right type, size and pitch.
Manual vs Spiral Flute
Manual taps have straight flutes and hence need to be progressively worked in to remove the swarf e.g. 1/4 forward, 3/4 back, 1 turn forward, 3/4 back etc. These are designed to be used by hand, however this does not stop you using a lathe or drill press to keep them straight, just don’t turn on the power. The spiral taps have spiral flutes and can work their own swarf out like a drill but are more expensive. To use tapping under power you will need a device that can either stop when it gets to the bottom of the hole or reverse it’s way back out again. These are known as “Auto-reverse tapping heads”.
Taper, second and finishing
Each tap comes in a set of three to be used in sequence for the best results. This is particularly important in harder materials.
Like a finishing tap but for blind holes. A plug tap has a flat end.
The choice of material affects several properties, how hard the tap is, how brittle, how slippy (i.e. how easily it gets rid of the swarf) and price.
The cheapest are carbon steel, these are hard but brittle
The next are HSS, these are slightly less hard that carbon steel but also less brittle. These are ideal for beginners.
Cobalt and carbide are harder still and hence are good for working with harder materials such as stainless steel or cast iron.
TiN (Titanium Nitride) coatings will make your taps stay sharp for longer and hence are good if you have a lot of tapping to be done.
Curious Inventor points out that there different methods of making taps which affects their accuracy and price. Most taps are cut, finding ground ones might require a specialist supplier.
You also need to ensure that you have a hole of the correct size, there are many charts that have the details of what hole size to drill for what thread. Getting the right starting hole size and using all three taps in turn will give you the best results and minimise breakage.
Here’s a metric tapping clearance drill size chart from Engineers edge.