Greenfield Tap and Die along with Cutting Tools Chicago aka General Cutting Tools tapping tech tip of tap design and applications. Today, it seems as though there are as many tool designs as there are part materials to be tapped. However, a closer look at the geometry of the various specialty taps developed for these materials reveals a few basic design philosophies:
- The most common and easiest to machine materials are soft, ductile materials that produce long, continuous (stringy) chips. They range from non-ferrous such as aluminum and copper, to mild to medium alloy steels, and also include some free machining stainless steels. These materials are easy to cut, produce minimal heat, and are not very abrasive. Taps for these materials are designed with medium to high hook cutting geometry and minimal or no thread relief. They are made from general purpose high speed steel such as M1 or M7, with an oxide, nitride surface treatment or Titanium Nitride (TiN). Most GP taps or those for stainless, fall into this category.
- The second group of materials includes heat-treated ferrous materials, generally above 275 Bhn. These materials are generally abrasive and do not produce chips easily, and heat generated during cutting becomes a consideration. Taps for these materials are designed with low or negative hook cutting geometry, relief to generate a chip, and require heat-resistant cobalt or tungsten based HSS such as M42, Rex 45, or T15. Thin film coatings TiN, TiCN, TiN+CrC/C and TiAIN are often preferred for lubricity, heat resistance, and abrasion resistance. Heavy duty and HP taps for hard alloys are designed with these features.
- Tough alloys such as nickel and titanium make up the third group. While generally not hard, these materials exhibit a toughness that makes it very difficult to produce a chip. They also have more elastic memory (closing in) which causes friction and heat, are quite abrasive, and easily work harden. The chips produced are generally long. The taps designed for this group are similar to those for hard materials, but the hook and relief are often higher to reduce friction and heat. When tapping materials such as titanium, higher H limits are recommended to overcome shrinkage.
- The last category includes materials that produce very broken chips or powder, such as cast iron or brass. Because these materials are cast, they are abrasive but soft. The taps for this group normally feature a low or neutral hook, and no relief, other than the back taper, is required. General purpose HSS is often sufficient, and either a nitride surface treatment or one of the thin film coatings such as TiN or TiCN helps with abrasion resistance.