type of curve interpolation. To take advantage of the capability, one requirement is a CAM system capable of outputting NURBS tool paths.
DIGITAL DRIVES
Machining centers for HSM generally use digital servo drives to maintain accuracy at high feed rates. The alternative, analog drives, may include lag time on the order of 10 milliseconds. A machine moving at 90 in/min. will move .015 in. in that time. Digital drives execute motion commands within a tighter margin, making it possible to combine high feed rates with high precision.
Fig. 2-1-28 The Look-Ahead feature of high performance controls allows the CNC to read ahead a number of program blocks. (Kelmar Associates)
High Resolution Feedback
Another advantage of improved processing power in the CNC system is the ability to use higher resolution feedback to monitor and control axis positions. This is particularly useful where the goal of HSM is to produce a smooth finish with little need for subsequent polishing.
PROGRAMMING
High-speed machining makes the tool path a more significant factor in the process, Fig. 2-1-29. Taking lighter cuts with a smaller step-over increment is only one consideration. An effective tool path also protects the tool by keeping cutting load steady and keeps a high feed rate by avoiding sharp changes in direction.
Programming a smaller depth of cut for roughing with a faster feed rate using positive rake cutters will assist the machining process. Finishing is recommended using Z-level machining (climb cut, pick over, and conventional cut) to produce better surface finishes.
Decisions made during programming can also affect the quality of the workpiece. If the purpose of HSM is to machine a smooth surface, the tool path may contribute to this goal.
Fig. 2-1-29 The tool paths created for machining a part. (FeatureCAM/Engineering Geometry Systems)
HIGH FEED RATES
A CNC with look-ahead capability will try to protect the tool, work, and machine from the effects of sharp changes in direction at high feed rates by slowing the feed in advance of the turn, Fig. 2-1-30. This slowing down sacrifices efficiency and may visibly affect the surface of the part. To keep the tool path fast and effective, avoid slow-downs by making direction changes more gradual. There are a variety of ways to machine with smoother motion such as rounding corners, smoothing reversals, and machining in circles.
Another approach to keeping the feed rate high does not involve direction changes, but instead changes in the feed rate. Feed rate optimization may allow the program to keep a higher average feed rate where the profile of the cut changes often.
Fig. 2-1-30 In high-speed machining, the feed rate should be kept as fast as possible. (Modern Machine Shop)
SUMMARY
▪HSM uses high spindle speeds, high feed rates, and light depths of cut to increase productivity, reduce lead time, reduce warping, increase part accuracy, and improve surface quality.
▪High-speed machining begins at 12,000 surface feet per minute (sf/min.) and may be as high as 18,000 sf/min. and feed rates of 600 in/min. when machining aluminum.
▪The key factors that affect the efficiency of a HSM system are the machine tool, the controller, spindle, toolholder, cutting tool, and programming.
▪The CNC control, cutting tool, machining center and other components must be designed with the goal of using the higher spindle speed productivity.
▪In a toolholding system consisting of the spindle, toolholder, and cutting tool, the toolholder is the most important link because it has the greatest effect on the overall concentricity and balance.
▪A balanced toolholder is critical for producing high-quality surface finishes, extending spindle life, and reducing or eliminating vibration that can affect the metal-removal process.
For more information on HIGH SPEED MACHINING see the Websites: www.turchan.com www.mmsonline.co
(Steve Krar, Consultant – Kelmar Associates)
Over the years, many developments helped to improve the metal-removal rate and increase the flexibility of conventional OD cylindrical grinding operations. The development of superabrasive wheels greatly increased metal-removal rates, however the parts produced were limited to the shape of the grinding wheel. Therefore these parts had straight or angular forms and it was not possible to produce contour forms without dressing the wheel to the form required.
Single-point grinding is a process that combines two technologies - superabrasive grinding wheels and high-precision servo control - to provide a contour grinding process that resembles a computer numerical control (CNC) outside diameter (OD) turning operation. It allows one machine to perform several operations such as grinding parallel diameters, tapers, contours, and threads without removing the part from the machine. Performing more operations on a part in one setup reduces the amount of workhandling between operations. For many medium OD grinding applications, it is a means of combining several grinding applications and machines into a single step.
THE GRINDING PROCESS
The basic idea for single-point grinding comes from the modern CNC turning center where a single-point cutting tool can be used to perform various operations. For example, one single-point tool can profile, face, plunge, and cut threads.
A single-point OD grinding machine is similar to a turning center since two axes of movement are generally involved in both metal-removal processes. On turning centers a form tool can be used to cut profiles, or a single-point cutter can be programmed to follow a desired profile through the coordinated movements of the X and Z axis, Fig. 2-2-1.
CONVENTIONAL GRINDING
Production OD grinding traditionally is composed of process-specific steps. For complex workpieces in a medium-sized batch, these steps are often sequential. The work moves from one process-specific machine to the next. For example, a plunge or step grinding machine will finish bearing races and shoulders, a form grinding machine will clean up tapers and profiles, a thread grinding machine will cut threads, and so on.
Individually, each process step is performed very quickly. An analysis of the total throughput time, however, reveals that significant savings could be made if work handling between operations could be reduced or eliminated. Additionally, keeping a workpiece on a single machine provides better workpiece accuracy because concentricity (dimensional relationships) between workpiece features can be maintained.
