A #10-32 taper tap is required for the technical program. Taps can dull and break if used improperly, have more than one on hand. Everything that will be tapped in the technical program will be through materials. A handle will be very useful, although aluminum can be tapped on an extremely slow setting on a hand drill.
A tap is a threaded tool that has been fluted to form a series of cutting edges. One end of the tap has been squared so it can be turned by a tap wrench. The act of placing threads into a hole is called tapping. There are three different types of taps and they are used in conjunction with each other. A tapered tap, a plug tap and a bottoming tap (figure 2).
If a deep closed hole needs to be tapped, all three types will be used (figure 3). The tapered tap will take the first third of the taps depth to achieve full diameter, gradually cutting threads into the inner wall of the hole. A plug tap is then used to relieve the threads cut by the taper tap and deepen the lower threads. Full depth is achieved using a bottoming tap.
When tapping through a piece of metal a tapered tap or a plug tap are sufficient (figure 4).
Tapping a hole by hand is a relatively easy process. A tap is always used with a tap wrench (figure 5). Cutting oil can be used on the flutes to tap more easily and preserve your tap especially when tapping stainless steel (figure 6).
Taps are labeled by diameter and number of threads per inch.
For example 3/8”’ – 16NC tap (figure 7) is a tap with an outside diameter of 3/8” and there are 16 threads per inch. The NC stands for National Course which have fewer threads per inch, and NF stands for National Fine and have a more threads per inch. The ones that cause a little confusion are the imperial sizes that are smaller since the diameter measurement doesn’t match the number given. For example 10-32NF, the 10 is an old reference number that is not associated with the diameter but the 32 does still indicate 32 threads per inch. Metric taps are labeled as “8mm-1,” 8mm is the outside diameter, 1 is the number of threads per millimeter. Charts are available, recommending drill bit sizes for tap sizes (figure 9).
Note: When tapping stainless steel, it is recommended to drill the hole 1/64” larger than the recommended size. The heat produced when drilling stainless causes the hole to enlarge and will shrink upon cooling.
Industry standard for aluminum, brass and copper is a 75% thread depth which is what is shown on most tap charts, but for iron, steel and stainless steel a 50% thread depth is acceptable. By increasing the diameter 1/64” the thread depth will not affect the strength of the tapped hole.
When starting a tap it must be aligned perpendicular to the hole that is being tapped (figure 9). Some downward force will be needed initially, once the teeth grab however no further downward force is required. If the tap is not perpendicular to the hole there is a risk of it binding and breaking inside the hole. To avoid cut chips from causing binding in the hole, back out the tap occasionally. Never forcefully turn the tap as they are made from hard alloys which allows them to cut efficiently but are extremely brittle as a result.