My machining career started at our family machine shop. When I began, we had only conventional, or manual, machine tools. Bridgeports from the 1960s, a Monarch 10EE toolroom lathe built in 1946 and a Lodge & Shipley lathe with a placard announcing that it conformed to the specifications of the War Production Board from World War II were some of the machines I used while learning the craft.

I learned how to tap holes by hand with a tap wrench and tap block. Sometimes, I would "power tap" with the Bridgeport or use the tailstock of a lathe. Other times, I employed methods that even the saltiest of machinists might question, like using a ½"-dia. drill motor to make threads in large castings.

Rarely having any trouble, it is fair to say I was adept at tapping holes. However, when our first CNC machine tool arrived in 1995, tapping became the bane of my existence. Broken taps, bad surface finishes, oversized holes and scrapped parts quickly were the norm. It was as if I had forgotten how to tap holes.

Mitsubishi Hitachi Power Systems Americas has the rigid tapping option on its controls but uses tension compression holders like this one to give another level of protection from errors in programming or setup. Image courtesy of C. Tate

After research and consultation with experts, along with practice on the machine, I realized I had to change my approach. Tapping methods used on conventional machines were not going to work on the machining center.

Sensory Focus

When tapping on a conventional machine, a machinist constantly adjusts to sights and sounds, and this ability to closely interact with the tapping operation allows a lot of leeway in tap selection. With conventional machines, it was common for us to apply inexpensive hand taps for everything, including power tapping. Tapping on a machining center prevents a machinist from being actively engaged in the process and removes the ability to use his senses for guidance while the tap is in the hole.

With a machining center, a machinist can't stop in the middle of a tapping operation when something sounds or feels wrong. You know that something is incorrect only after a tap is broken or threads are bad. Having the correct tap geometry is critical when tapping on a machining center. There is no time to react to a poor situation caused by using the wrong tool. I learned fast that taps have features designed for specific applications. Using a tap in an application for which it was not designed yielded lousy results and frustration.

Holding and driving taps on conventional machines was done using collets, drill chucks, tap wrenches (held by hand) and, in the crudest setups, adjustable wrenches.

When I purchased the machining center, I bought the rigid tapping option, which is intended to eliminate the need for special toolholders. So I tried to hold tools on the machining center like I had on conventional machines. I was met with frustrating results. My first attempt used a 3-jaw keyed drill chuck. After failing to thread a hole with a
3⁄8-16 tap on three consecutive tries, I knew that other methods were needed.

Thread milling is the first choice for threadmaking at Mitsubishi Hitachi Power Systems Americas. Making threads that have a large depth-to-diameter ratio can be much easier with a thread mill than with a tap. Image courtesy of C. Tate

The second setup incorporated a standard double-angle collet, which worked much better until the tap started to wear, increasing cutting forces to the point that the tap began to slip in the collet. The result was poor thread quality and a grimace as I wondered how much of the tap was going to exit the hole. I tried a few other methods and finally found relief when my tap salesman suggested I use a tension compression holder.

Tension compression holders are spring-loaded and allow axial movement of the tap, which mitigates undesirable effects associated with tap movement. These holders also are designed specifically for holding taps, so they don't have issues with movement in the collet. Technically, a tension compression holder is not needed for rigid tapping. However, the holder does protect against small inaccuracies in the system. Because the holders are easy to use and one size accepts a variety of tap sizes, I started using them all the time.

Lubricious Threading

Tapping on any machine tool requires some type of lubricant or cutting fluid. On conventional machines, I used various waxes, cutting fluids and oils made especially for threading. I could have used the same methodology for the machining center, but it is not practical to stop the machine midcycle and apply oil to the tap or hole. Also, the petroleum-based products floating around in the coolant caused other issues, and tapping with water-soluble coolants is normal practice anyway.

It did not take long to realize that tapping operations are sensitive to coolants. I ran the machine for several months using a coolant that worked well with general milling and drilling, but when I tried tapping 6-32 threads in aluminum, I experienced sporadic results. Thread quality was poor, and I could not trace the problem to the tap, tapholder or machine. After tapping a few dozen holds with old-fashioned tapping oil, I figured out that the coolant did not provide the necessary lubricity. I ended up changing brands and carefully monitoring the concentration each day, which eliminated the problem. Getting high-quality threads requires not only getting coolant to the hole but having the proper formulation and concentration.

I learned many lessons about tapping, but the most valuable lesson was how to thread mill. Thread milling can be a far better way to thread holes than tapping. I became familiar with thread milling at a trade show. I tried thread milling on my machines, and now I reach for the thread mill before the tap. With thread milling, I do not need to reverse the spindle. There are typically no coolant or toolholder issues. And broken thread mills do not usually stay stuck in holes like broken taps.

Tapping is one of the most difficult machining operations to master. Many variables, such as tap geometries, tap coatings, cutting fluids, cutting speeds and toolholders, play significant roles in success. Working with an applications engineer at a good tap manufacturer is the best way to correctly set all the key variables.