INCREASING METAL REMOVAL AND TOOL LIFE WHEN MACHINING TITANIUM

Changes in demand require adaptations. As material technologies improve, so must machine, tooling and coolant technologies. Titanium is yet another example of an exemplary material that requires a new machining approach for manufacturers to be profitable.

Discovered in the 18^th century (twice) and then extracted in its pure form in the mid-20^th century, only recently has titanium become a key ingredient in aircraft because of both its characteristics and the economy. Titanium alloys used in aerospace, most commonly Ti-6Al-4V, bring to the table almost equivalent strength to steel, but at 45 percent of the weight. The lighter weight equates to fuel saving for commercial aircraft, an ever increasingly important factor. Moreover, compared to aluminum, titanium is 60 percent more dense and twice as strong, making it an ideal solution for performance applications.

HISTORICAL APPROACHES
While titanium boasts many advantages, material cost and machinability hold many manufacturers back from using this metal. Titanium is a heat-resistant material and, as previously mentioned, is about the same strength as some steels. Because of these attributes, titanium presents similar challenges to that of hardened metals. For a given cutter, the amount of work required to move or shear the metal is relatively high.

During any machining operation, this “work” or power is converted to motion of the metal chip being removed. However, a side effect of this mechanical separation is heat, and adding to this heat is the heat of friction from the cut chip riding up the cutting tool’s rake face. In many materials, the chip of metal absorbs most of this heat. This is not so with titanium, causing the tool tip and surrounding environment to absorb the heat generated (see Figure 1).

The result is a situation with high cutting forces and high heat, neither of which is friendly to tool life. The high forces tend to wear tools, and the heat can become so great that tungsten in the WC (tungsten-carbide) tooling will soften. Therefore, removing a lot of material economically (with high metal-removal rates) becomes cost prohibitive.

Here is the picture: The material is hard to machine and creates heat. The hardness begins to wear the tool while the heat tends to soften it, expediting the wear. In most cases, the cutting speed and therefore rpm are slowed down to compensate.

Now imagine a slow-running spindle, turning a multi-flute tool against a hard-to-cut material. The result is a very large force at a very low frequency. Let’s call it hammering. Under some cutting conditions, each cutting edge could be subjected to forces as high as 2,000 lb at a low frequency of three to five times per second.

Typical in many shops today are low rpm vertical multi-spindle machine tools with low pressure and low-volume coolant whittling away in a pile of titanium chips. This method leads to poor cooling, poor chip dispersion and, consequently, the recutting of chips, low metal removal rates and relatively dedicated operations.

A NEW APPROACH
Typical heavy hogging machines use a large rigid structure, as do newer purpose-built machine tools such as T4 and T2 5-axis horizontal machining centers that use a very rigid machine structure with new technology to enhance the basic construction. T-Series machine tools weigh in at 229,000 lb for the T4 and 132,000 lb for the T2 of Meehanite cast iron. They both use box ways rather than linear guide ways for a more solid connection from column or table to the bed. These ways are over 12 inches wide. The columns are 40 inches wide.

Add to this huge structure the ability to “sense” vibration and change the machine structure. Now an already rigidly built machine can adjust for heavy vibration by stiffening up the slide ways with more friction to reduce the allowed movement by the sensed vibration. This Active Damping System enhances the machines’ capability to make heavy cuts. For example, in Ti-6AL-4V, full slots over 3 in wide and 2 in deep can generate removal rates of up to 25 cu ipm and heavy profiling up to 30 cu ipm.

More recently, the high-torque A/C axis spindle on the T4 has proven that it can take almost half of that metal removal rate in full 5-axis motion. At 14.2 cu ipm, this high metal-removal rate can greatly reduce the amount of time spent semi-finishing or finishing. In addition to the high rigidity of the machine bed, the A/C spindle does not have to be locked in each position. The drives have high torque to hold position while performing 5-axis contouring at relatively high removal rates.

Both of these horizontal machining centers use 1,000 psi through-spindle coolant with 53 gpm flow rate. The pressure helps ensure the force and velocity to remove chips, and the flow rate helps ensure sufficient volume to reduce the heat generated. It has been determined that cutting heat is generated not only by the shearing force of the material at the tool edge but also along the rake face due to friction. Therefore, cooling capability and lubricity are both important factors in reducing heat for extended tool life. However, if the coolant isn’t delivered to the cutting zone – the tool-chip interface – then the coolant won’t be effective.

The latest research that combines this new coolant technology with the T-Series platform has resulted in average tool-life improvements of four times the baseline and as much as nine times the baseline.

Similar to many other industries, changes in demand require adaptations. As material technologies improve, so must machine, tooling and coolant technologies. Titanium is yet another example of an exemplary material that requires a new machining approach for producers to be profitable.