Tapping

Let's Talk Taps

    * The influence of hole size and preparation are often overlooked when troubleshooting tapping problems.

    * One of the first steps in troubleshooting a tapping operation should be careful evaluation of the drill size and drill condition.

    * Some of the most common user errors are misconceptions about hole size and its relationship to thread strength.

    * The diameter of the hole to be drilled for the threads in a nut or other inside threaded piece is theoretically the minor diameter of the corresponding screw size.

    * Threaded assemblies fail essentially due to one of two different happenings: 1) Stripping 2) Tensile failure of the bolt across its cross section.

    * If the bolt is stronger than that of the nut, it will fail either by stripping or by tensile failure BEFORE the nut fails.  This is actually a good thing as the bold is almost always much less expensive than the "nut".

The most common misconception about hole size is that the strength of the threaded assembly increases with a smaller tap hole size.  Here are some facts that may surprise you:

   1. Beyond 55% thread height of the internal thread there is virtually no increase in assembly strength.

   2. It is rare to specify a drilled hole that will yield greater than 60-65% internal thread height when out of roundness and eccentricity of the internal thread are taken into account.  In general, yields of 72% are common.  Unless the material strength of the bolt is greater than that of the nut.  For instance an aluminum nut with a hardened steel bolt should have a percent of thread up to 75%.

   3. Even the highest quality of drill will have a tendency to cut oversize.  Published drill recommendations actually take this into account.

Other Tapping Tidbits:

The size of the tap drill for American National threads equals major diameter minus1, divided by the number of threads per inch.

Remember that Forming Taps require larger holes than cut taps.

Use the largest possible hole size when tapping.

Watch for deformation of the hole when preparing (drilling) hardened materials such as steel and stainless steel.

Finally always remember to keep your drills sharp!  Also keep feed rates as high as possible while maintaining acceptable drill life.