“Machining,” according to a dictionary, refers to the various processes that change a piece of raw material into a desired final shape. That explanation makes it seem simple enough, but it is not so when it comes to machining titanium. Titanium can be machined in many different ways…there’s turning, boring, milling, drilling, grinding, sawing, water jet, chemical milling, tapping, electric discharge machining, and that is just to name a few.
Because of the increasing demand for titanium in recent years, it has become necessary for machine shops to adapt to it. Titanium is significantly harder than other common metals and the low thermal conductivity makes it difficult to machine. In addition, it requires the proper application of flood coolant to dissipate any heat.
There are several guidelines that should be followed when it comes to machining titanium. It is important to maintain sharp tools, which should be carbide tipped. The use of generous flood coolants is necessary to maximize heat removal and not compromise the titanium. Machine speeds should be decreased and feeds increased when machining titanium. Interruptions in the feed of titanium should be avoided along with removing turnings frequently. This will help avoid backup that could damage both the machine and the titanium.
Let’s go back to the different ways to machine titanium. Commercially pure titanium and several of the titanium alloys can be turned and bored easily. The term “turning” refers to the machining process where a cutting tool moves linearly while the titanium work piece rotates. This process cuts the external surface of the titanium. “Boring” refers to a similar process but it results in cutting the internal surfaces. Carbide tipped tools, which are extremely hard and therefore stay sharper longer, are used whenever possible in turning and boring. They offer high production rates and longer tool life than non-carbide tipped tools.
Drilling of titanium can be done with the proper set-up. Using sharp drills and maintaining maximum drilling force is the key to success. It is important to set-up the drill with the bit being no longer than necessary. This ensures maximum drilling force while still allowing the titanium chips to exit through the flutes of the drill. High speed steel drills can work for titanium alloys, but carbide tipped drills still provide the best results when drilling commercially pure titanium and deep hole drilling.
The machining process called “milling” is challenging with titanium than some of the other machining processes discussed. Milling refers to the use of rotary cutters to remove material from a piece of titanium by feeding it at an angle to the tool. Milling is the most popular machining process in most machine shops because of its ability to create parts of precise sizes and shapes. There are two different types of milling machines and they are classified by orientation: vertical and horizontal.
The picture above shows titanium connecting rods being made for the TiChero. They are being made on a Mori Seiki MH-50 Horizontal Mill using Iscar cutting tools. It is called a horizontal mill because the cutting tools are on a horizontal axis. It is common for teeth to fail and chip in the milling process. Carbide tipped tools are usually the tool of choice when machining titanium; but when used in the milling process and compared to high speed steel tools, the performance increase does not always outweigh the price difference. Depending on the milling job, high speed steel tools may be more cost effective than carbide tipped.
The newest application used in machining titanium is water jet cutting. This tool has the capability to cut a variety of materials, including titanium, by using a high-pressure jet of water. Not to be confused with a pressure washer, a water jet mixes water with an abrasive to cut the material. Because water jet cutting does not create the same heat as traditional cutting, it allows for higher cutting speeds and produces smoother edges. Unfortunately, water jet cutting can only be used on thinner pieces of titanium, up to a maximum of three inches thick.
These are just a few of the processes used to machine titanium. As you can see, it is no simple matter. It is a complicated process requiring precise machining tools and techniques.