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Meeting the challenge of machining titanium

Posted on 06 Jun 2024. Edited: John Hunter. Read 658 times.
Meeting the challenge of machining titaniumMachining titanium is fundamentally different from machining cast iron or steel. For the best results, tool technology and the overall process must be designed in a holistic way which is where the expertise of the technical consultants at Mapal can identify the optimal combination of precision and cost-effectiveness.

The material properties of titanium are valued in aerospace applications, the automotive industry and also the medical sector, despite the fact that the material is notoriously difficult to machine due to its extremely low thermal conductivity. By way of comparison, with steel machining, 10% of the temperature remains in the workpiece, 15% causes stress on the cutting tool and by far the largest proportion, while 75% of the heat is transferred into the chips and removed with them.

In the case of titanium machining, the chips only absorb 25% of the heat with 60% going into the tool and causing a high thermal load on the cutting edge or the cutting material, which ultimately results in considerably shorter tool lives. In this way, the cutting material costs become the focus of attention.

Wear and tear

If the cutting speed is too low, this can lead to the material sticking and if the cutting speed is too high, the risk of abrasion and tribochemical wear increase sharply and the cutting material is burnt. One way to ascertain the condition of the cutting edge is to look at the width of the wear mark — in a stationary range, it grows slowly and continuously. If this range is exceeded and the machining enters the transient range, a rapid and incalculable failure of the tool cutting edge occurs. This happens when the selected cutting speed or feed is too high. There is a difference of up to 100% in tool life between the stationary and the transient range.

It is very important for Mapal’s area field service to help run in processes, to check the width of the wear mark and to show the customer when the end of the stationary range has been reached, for reliable and optimum machine running times. As a general rule, Mapal recommends replacing the tool when wear is approximately 0.2mm. A carbide milling cutter can still be reground then, but not at higher levels of wear.

MAPAL has incorporated process knowledge of titanium machining into the development of its latest tool technologies. The focus is on wear and tear criteria and their influence even beyond the most suitable cutting material. To ensure optimum heat resistance, Mapal uses innovative cutting materials, for example selected carbide grades and matching coatings that produce as little friction as possible. The micro and macro geometry with extremely positive tool geometries, polished rake faces and measures for efficient cooling pave the way for cost-efficient machining. However, careful balancing of the cutting data is essential for the cost of titanium machining.

When machining steel and cast iron, higher cutting speeds often mean higher productivity and lower overall costs, which are made up of machine costs and cutting material costs — ultimately, the machine costs get lower the quicker and more efficiently the machine operates. Although the cutting material costs increase in this case, an optimum in terms of overall costs is nevertheless achieved at a relatively high cutting speed.

Machining titanium

Conversely, when it comes to titanium, higher cutting speeds are not practical. Tobias Gräupel, technical expert (indexable tools) at Mapal, proves this with an economic efficiency calculation that optimises the cutting data of titanium machining from a cost point of view. A milling operation with a NeoMill-Titan-2-Corner with four cutting edges that machines TiAl6V4 with a cutting depth of 4mm and a cutting width of 24mm is considered. An examination of different combinations of feed rates and cutting speeds leads to a clear recommendation in terms of the machining values for titanium machining.

By way of comparison, when machining a steel part, doubling the cutting speed from 200m/min to 400m/min resulted in an overall cost saving of 24% per part. When machining a titanium workpiece, increasing the cutting speed from 32m/min to 50m/min results in a cost increase of 259%.
Mr Gräupel added: “An increase in the cutting speed is clearly reflected in the overall costs. They skyrocket”. The optimum cost is achieved with a machining operation with high feed rates and low cutting speeds. If, on the other hand, the cutting speeds were maximised in addition to the high feed rates, the costs would be more than four-times as high. These economic efficiency calculations are vital especially for larger lot sizes.”

Furthermore, when producing large quantities, the overall costs play a major role in making investment decisions. Inefficient cutting parameters cause the total costs in titanium to shoot up — only the combination of innovative tool technology and optimum process design leads to the best solution. The technical consultants at Mapal are always on hand to help production managers achieve optimum results regardless of machining and production batch sizes.