Servo - May 2009

ahead of the tap. Even with this
design, I recommend clearing
the chips out of the hole regularly
for cleaner threads and fewer
breakages.

bottom of a blind hole.

Intermediate or Plug

This variation has only 3 to 5 reduced threads and is the most commonly stocked type. It is a little harder to align in a hole and cuts threads almost to the bottom of blind holes. Don’t worry about alignment problems; we will eliminate them forever in Part 2!

or hardened steel, coated taps are preferred or even mandatory to avoid breakage.

Spiral

The spiral provides good transport of chips back out of blind holes. Low spiral taps work for materials that produce short chips, while high spiral taps are meant for long, stringy chips and deeper holes. I find spiral taps are more fragile and would only recommend them for shallow blind holes.

Bottoming

This last variation has just one or two taper threads and is only used to finish threading down to the bottom of a blind hole or to clean up damaged threads; you would never start tapping a new hole with a bottoming tap! You can avoid buying bottoming taps simply by drilling holes a bit deeper than required and only partly threading them with a plug tap.

Oxide

Oxide is a good and inexpensive general purpose coating if you mainly tap steel parts; it helps to reduce friction and prevent material sticking to the tap. It is not recommended for non-ferrous metals, however, I have never had a problem with oxide coated taps in aluminum.

Thread Rolling

This type of tap does not cut metal but forms it under pressure so no chips are produced, making it ideal for blind holes. Only soft and ductile metals work with roll forming taps and fortunately aluminum works very well. These taps have almost no flutes and are very strong; if handled correctly, they are not likely to break. Lubrication is very important with this type of tap; it is a specialty item and is not worth the expense for most builders.

Nitride

Nitride is another cheap and versatile coating, mainly for softer and abrasive materials such as aluminum, bronze, copper, and fiberglass. It increases surface hardness and extends tap life.

Oxide over Nitride

This is my personal favorite, particularly for titanium. It is a little more expensive than either coating alone, but worth it in harder materials.

Recommendation

I recommend using spiral point taps where possible; they produce a very clean thread and are less likely to break than most other types. If spiral points are not available, use straight taps and save a few cents.

Recommendation

The ‘classic’ method of tapping a hole is to start with a tapered tap, move to an intermediate tap once the threads are well started, then finish with a bottoming tap if you are tapping a blind hole. This method sells many sets of taps but is seldom required; most professionals would start with an intermediate tap and only use a bottoming tap if absolutely required.

Another trick is to grind off the point on an intermediate tap so it will cut closer to the bottom of a blind hole (as shown in Photo 2). I hardly ever buy bottoming taps nowadays.

TiN (Titanium Nitriding)

TiN is a general purpose coating with a gold color. It reduces friction and adds a little hardness. I notice little difference to uncoated taps and the higher cost is hard to justify. I have had really bad results with TiN coating in titanium.

Tap Taper

As well as the design categories above, each type of tap can come in a different end taper (also called the end chamfer).

Coatings

TiCN (Titanium Carbonitriding)

TiCN is an improvement on TiN with lower friction and better wear resistance. It has a dark blue or purple color and is mainly used for machine tapping in very difficult materials. It is the most expensive coating and is seldom required

Full Taper

These taps have 7 to 10 reduced height threads at the end of the tap. The reduced diameter makes a tapered tap very easy to align in a hole and start the thread cutting process. By the same token, they will not cut threads to the

Surface coatings on taps improve performance, reduce wear, and reduce breakage. For hand tapping in easy machining metals, uncoated taps work just fine and the often high cost of coating is hard to justify. In hard to machine materials like titanium