Exploring Advanced Manufacturing Technologies. Steve Krar
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.
Fig. 2-2-1 Single-point grinding combines two technologies to grind parallel diameters, tapers, contours, threads, etc. (Junker Machinery Inc.)
Many conventional OD grinders use a wheel with a desired geometric shape dressed into the wheel face. Once a dressing or truing unit shapes the wheel, that shape is then transferred to the workpiece by movement of one or both of the machine’s slides. Fig. 2-2-2.
On conventional OD grinders, the wheel/workpiece interface forms a line of contact between the face of the wheel and the work. For example, if two pencils are laid side-by-side, with one representing the workpiece and the second the grinding wheel, contact between them forms a line; a wider wheel contacts more of the workpiece.
SINGLE-POINT GRINDING
Single-point grinding uses a process that imitates single-point turning; a single grinding wheel is used to perform a variety of operations. Profiling, plunging, and thread cutting are accomplished by precise CNC control of the X and Z axes through servomotor and ballscrew actuation, Fig. 2-2-3. That control is the key to single-point grinding because the workpiece shape is ground by the coordinated movement of the machine axes and not by the shape that is dressed into the grinding wheel.
The single-point CNC controlled grinding technology allows an operator to completely finish straight sections, shoulders, contours, tapered contours, slots, etc., on a workpiece in a single setup using single or multiple wheels. Single-point grinding produces high accuracy parts, increases productivity, and reduces grinding costs.
Fig. 2-2-2 On conventional grinders the part profile is dressed into the wheel face and then transferred to the part. (Junker Machinery Inc.)
The Grinding Wheel
Most applications use a cubic boron nitride (CBN) vitrified-bond superabrasive wheel rather than metal-bond wheels that are very time consuming and expensive to dress. Advances in vitrified superabrasive bonds now make them practical for single-point grinding, however these wheels tend to be rather coarse. Advantages include aggressive cutting action and reduced frequency of wheel dressing.
In single-point grinding, a narrow .156 to .236 in. (4 to 6 mm) CBN grinding wheel, dressed flat across its face, is used. When the grinding wheel is swiveled one-half of a degree, the contact area between the wheel and work becomes a single-point. The grinding wheel’s angle of attack presents an edge of the wheel that makes the contact between the wheel and work-piece tangential.
Grinding