EAA Sport Aviation

Thread Cutting
With Taps and Dies

BY JOE NORRIS

THREADED FASTENERS ARE COMMON to just about every project.

Occasionally, you will need to create threads in or on a part. You’ll do this with a tap or die.

Taps are used to cut threads inside a hole, while dies are used to cut threads on the outside of a rod or shaft. First, select the proper tap or die size. Each tap or die will cut only one size thread, so you will need different taps and dies for different size fasteners. Thread size is called out by the diameter of the fastener and the number of threads per inch. There are many standards for thread size, but most aviation fasteners are threaded with National Fine (NF) threads. You will occasionally run into a fastener that is threaded with National Course (NC) threads.

Thread size is marked on the tap or die. For example, you’ll see “1/4 x 28 NF” stamped on the tap or die for a 1/4-inch diameter fastener with 28 threads per inch, which is National Fine. Small fasteners are called out by wire size diameter rather than fractional inches, so you might see “ 10-32” on a tap or die. This is a No. 10 fastener with 32 threads per inch. This is a NF thread; 24 threads per inch would be NC for a No. 10 fastener, but usually these small fasteners don’t include the NF or NC in the designation. (A table of specifications for NF and NC fasteners is posted at www. SportAviation.org.)

How do we cut threads with taps or dies? When tapping a hole, first determine what size fastener will be used. Once you know the diameter and thread pitch of the fastener (e.g., 1/4 x 28 NF), select the proper tap. Now comes the most critical choice you’ll make— selecting the proper drill size for the initial hole you will tap. Most tap and die sets come with a chart, or visit www.SportAviation.org for a link to a chart to find the proper drill size. This selection is important; if you use too small a drill, you’re likely to break your tap off in the hole. If you choose too large of a drill, you will not have sufficient thread depth, resulting in a weak connection in your assembly.

One you have selected your tap and drill and drilled the hole,
you’re ready to cut your threads. Place your tap in an appropriate

ILLUSTRATIONS BY JOHN A. JOHNSON

tap handle, then select the correct lubricant for the material you are tapping. For steel, use standard thread-cutting oil. For aluminum, regular kerosene (or jet fuel) will work. Your best results will be achieved with lubricant, but if you don’t have the proper lubricant, it’s better to cut your threads dry than use an incorrect lubricant.

Start your threads carefully, making sure to keep the tap aligned with the hole. Once the thread cutting starts, only turn the tap about half a turn at a time, then reverse a little to clear the chips out of the way. If the tap becomes extremely tight as you start to cut threads, or if it starts to squeak, you may have drilled too small a hole. Proceed carefully to avoid breaking the tap off in the hole.

The tap is chamfered on the end to allow easier starting of the threads. If you are tapping a blind hole, the tap will bottom out and you won’t have threads all the way to the bottom of the hole. Drill your hole about one drill bit diameter deeper than the length of the stud or fastener so that your threads are deep enough. If you can’t drill the hole deeper than your fastener length, follow your normal tap with a bottoming tap. A bottoming tap does not have a chamfer on the end, so it will cut threads to the bottom of the hole after you’ve made the initial threads with a regular tap.

Using a die is a similar process, except now you’re threading the outside diameter of a rod or shaft. Select the die based on the outside diameter of the work and the thread pitch needed. The die will be marked similarly to the tap. The die will also be chamfered on one side to make an easier start, so make sure you put the die in the handle in the correct orientation. To facilitate easy starting, slightly chamfer the end of the rod. Just like when using the tap, make sure your die is square to your work, use the proper lubricant, and reverse your tap every half a turn or so to clear the chips.