Rough Milling

RAF MILDENHALL, England — A pocket is cut into a block of metal with a horizontal milling machine Nov. 3, 2010. Pockets are cut into blocks of metal so that certain aircraft parts can be fabricated from them. (U.S. Air Force photo/Senior Airman Ethan Morgan)

New strategies for high-speed roughing have been introduced over the past few years, shortening machining times. The metal removal rate is improved by using the full flute length as the cutting surface. Deeper cuts are definitely more efficient at roughing, more so if the feed rate can be increased. And in addition, many of these strategies lengthen cutter life as they are not overloaded by the abrupt changes in direction and cutting motion of conventional techniques.

The problem with conventional methods of milling deep pockets is that optimum conditions only occur when cutting a straight line. Since corners increase the engagement angle of the tool, a lower feed rate must be set. If this lower rate is used throughout the tool path, machining time is increased. But if the feed rate is varied, the tool and machine wears out more quickly.

These new methods often mill in loops instead of in straight lines. Using these “rolling circles” tool paths of constant radius will keep the tool’s engagement angle constant. This, in turn, will preserve the optimum cutting conditions through the entire path. With more consistent feed rate and volume removal, cutting time is shortened and the machine’s life is lengthened.

To carry out optimized roughing, the tool ought to travel with tiny stepovers, high feed rates, and no sharp turns. Some tool path strategies that can help accomplish that includes:

Smart Machining

“Smart machining” is a way to make an optimized tool path by analyzing the layers to be removed by the step-by-step descent of the tool at the Z axis. After milling away each layer, the remaining material forms a terraced slope. The term “rest machining” is used to describe the function to calculate these uncut areas of material and produce tool paths to “clean them up.”

Smart machining includes also helical ramping, a function used for pocket machining. When the tool arrives at the closed area, its path is tailored to the entry angle and geometry, making the cut shorter and safer by detecting and eliminating time-wasting air-cuts.

Knowledge Of Stock Remaining

Many advanced CAD/CAM systems used the “re-roughing” approach. Here is how it works: Start with a large diameter endmill to remove material. Next, use a smaller diameter endmill to reach areas closed to the larger tool. Repeat this process until the part is near-net shape and ready for finish machining. In order to machine a part with cutting tools of different sizes, it is important that the knowledge of stock remaining is built into CAD/CAM system.

Some methods to remove this uncut material include pencil and rest milling. Pencil milling cleans up corners and concave fillets to leave uniform stock at surface intersections. Rest milling is similar to pencil milling except that it is used everywhere, not just corners. Without pencil or rest milling, the finishing tool would have to remove a much larger volume of material at the corners of enclosed areas.

Side Steps

To reduce cutting force and avoid excessive deflection of the cutting tool, roughing cuts are very shallow and side-step distances between adjacent tool paths small. However, when machining with a parallel or zig-zag toolpath, sharp stepover moves are required at the end of every pass. By using simple “looping” toolpaths instead of these sharp turns, feed rates can be increased.

Toolpaths comparision1


(a) Original toolpath with line transition,
(b) & (c) Optimized toolpath with biarc transitions.

Cornering

Machining into corners requires careful considerations of cutter engagement and the appropriate feed rate. The sharp motion of the tool path ought to be smoothed out. If this sharp motions are allowed to remain, the look-ahead function of the software ought to be able to reduce feed rate upon reaching these corners. If the tool motions are fluid during corner machining, more consistent high feed rates can be achieved.

Toolpath applications2

Using the different types of biarc transitions.

NURBS-Based Tool Paths

When machining complex contoured forms, such as dies or molds, NURBS interpolation may let you machine at a higher average feed rate without compromising accuracy. But make sure that the CAM systems does not generate the NURBS spline by using chords for approximation.

Illustrations of conventional linear and NURBS paths3

A conventional linear path (left) and a NURBS path (right) in a sculptured surface machining

Trochoidal Machining

Trochoidal machining is an efficient way to cut a pocket. An end mill with close pitch is is used in a circular movement. These together with a small radial depth of cut enable higher feeds and faster speeds.

Trochoidal milling

Plunge Roughing

In plunge milling, the tool penetrates the material like a drill in the vertical direction, then rises and plunges again about half a diameter away. Since the specialized plunge mill cuts with the end of the tool instead of its periphery, cutting forces are transmitted directly into the spindle. With less side loads and vibration, this method can be used for cutting difficult materials like titanium, or where machine power or torque is limited.


This image shows the material removal in plunge milling.

The Future

In high speed machining, material is removed with a shallow depth and a narrow width of cut. And smaller stepovers will give a smoother surface because scallop height between adjacent passes is reduced, which in turn reduces the amount of polishing needed. And because these methods uses very small tools, high-speed CNC machines can accomplish fine details that used to be done by wire erosion. This saves substantial time and money.

High speed machining lets you do more in less time. But your CAD/CAM system must be able to automate much of the programming required. The goal is having a CAM system that is able to recognize features, and then machine the workpiece automatically.

The next generation of CAD/CAM systems will combine feature recognition with knowledge-based strategies for complete automation. Experienced toolmakers can immediately use a newly purchased system to make things according to their shop practices.


Footnotes

  1. Zhao, Z., Wang, C., Zhou, H., & Qin, Z. (2007). Pocketing toolpath optimization for sharp corners. Journal of Materials Processing Technology, 192-193, 175-180. doi:10.1016/j.jmatprotec.2007.04.096
  2. Ibid.
  3. Chen, Z. C., & Khan, M. A. (2013). A new approach to generating arc length parameterized NURBS tool paths for efficient three-axis machining of smooth, accurate sculptured surfaces. The International Journal of Advanced Manufacturing Technology, 70(5-8), 1355-1368. doi:10.1007/s00170-013-5411-1

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