BEST PRACTICES IN FIVE-AXIS MACHINING FOR THE JOB SHOP

At a recent WESTEC show in Los Angeles, we heard one world leader in aerospace tell us, “In five years or less, our company will do nothing but design and assembly.” As the business model of many major aerospace, automotive and other prime OEMs has evolved, this is becoming a reality in many market segments. What some shops will see as a nightmare of capital equipment purchasing and leveraging their grandchildren to the bank, others will see as this generation’s opportunity for American ingenuity to once again rise up and win the day.

You’ve seen many opportunities in five-axis machining for your shop — free-form surfaces, advanced mold-making, impeller/blisk production, structural parts for aerospace, medical implants and more jobs have long been options for the job shop. So, you decide to take the plunge and get into this world. The resources are many to help you make that transition. Let’s look at some of the challenges and best practices in five-axis CNC machining.

Tool orientation is perhaps the biggest challenge faced by the five-axis programmer and machinist. Today’s CNC has a system frame function that easily orients the tool to any new coordinate system. This means the tool can be retracted out of an unknown tool orientation via handwheel and it simplifies the manual movements in an inclined plane of operation. Accounting for the tool tip in a change of orientation is a complementary challenge, but the CNC has a built-in five-axis coordinate transformation function that means the tool tip location tracks the part location during the programmed motion of any rotary axis. This keeps the tool tip in precise and constant perpendicularity or parallel to the surface being machined.

Hand-in-hand with that challenge is the part surface programming and, here again, the CNC offers totally independent machine tool kinematics in all axes of motion, as well as the ability to integrate those motions with the machine’s physical position coordinates. NC programs are thereby made independent from the machine’s kinematics. Should the need arise for manual programming while machining, the CNC uses a “rotate tool center point” function to provide full five-axis motion in jog mode. In this way, the tool tip follows the part, providing for much easier set-up and recall.

Programming with standard surface vectors in enabled by the CNC orientation change within a plane of operation, which provides greater circular interpolation for a simple function that controls the orientation change. This avoids any violation of straight side wall contours while flank milling. The programming of intermediate set points also becomes obsolete, which further contributes to significant improvements in contour surface quality. Maintaining the correct angle on all sides and corners is further enabled by the CNC, so easy programming of square pockets is done by simply defining the start and end vector.

To avoid contour violation while flank milling, the CNC utilizes a function that provides precise machining on sloping, flat walls in a single program block. Accounting for 3D cutter compensation is done by using the tool length and radius compensation in a five-axis program, wherein the combined 3D face and flank milling even considers the bottom contour geometry while machining. On a curve, CNC dual spline interpolation programs a second contact point on the tool to produce higher accuracy and speed as linear approximation errors disappear. The old point-to-point process is simply ancient history now.

Today’s CNC can recognize an axis limit and prevent over-travel conditions by automatically switching to an alternative axis position. When dealing with an orientation change within a pole cone, the CNC uses singular tool orientation to automatically switch to linear interpolation of rotary axes. This avoids excessive C-axis movements in the vicinity of the pole cone and provides continuous tool path velocity even in the area of the pole cone. Using a spine compressor to provide smoothing during five-axis machining means non-continuous orientation changes at block boundaries are smoothed and that faster movement along the tool path plus improved surface finish are enabled.

Finally, high-speed cutting parameter coordination is executed by a CNC cycle for easy set-up and user-friendly activation of advanced motion control features. Excessive programming time is eliminated, because the adaptation of the CNC set-up is done according to the particular machining technique being employed.

Still think five-axis machining isn’t in your future? With today’s CNC onboard your five-axis machine, you might want to think again. Any questions

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Randy Pearson is the sales support manager for U.S. dealers and OEMs of Siemens Industry, Inc., Drives Technologies, Motion Control — Machine Tool Business, 390 Kent Avenue, Elk Grove Village, IL 60007, www.siemenscnc.com. A veteran of the machine tool industry, his interest is the training aspect on CNC machine tools, which he conducts through seminars and classes at votech schools and shops, and at Siemens training facilities. For questions or comments on this column, contact Randy at 847-640-1595 or [email protected].