Exploring Advanced Manufacturing Technologies. Steve Krar
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For industry to operate effectively, the material that produces the final product must be machined and formed quickly and accurately. The key factors that affect the efficiency of a metal-removal process are the machine tool, the controller, spindle, toolholder, cutting tool, and CNC programming. High Speed Machining (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.
In virtually all metal-removal operations, manufacturers are trying to reduce the amount of time a part is moved from machine to machine and perform more operations in a single workpiece setup. This has led to the development of new machine tools such as the turning center with live tooling and special workholding fixtures where both turning and milling operations can be performed in one part setup.
(Steve Krar, Consultant—Kelmar Associates)
High-speed machining (HSM), in order to be most effective, must involve the correct selection of machine tools and controls, cutting tools, and programming. 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. This requires a machine that can produce a spindle speed of 8,000 revolutions per minute (r/min) or higher.
The speed in High Speed Machining (HSM) is the speed at which CNC machining can replace the operations of polishing, assembly, unused shop capacity, and other manufacturing delays. Run fast enough, and machining centers become an economical alternative to more dedicated systems for a variety of production parts. If after careful evaluation, the cycle time of each operation can be reduced even by a small amount, it could produce big savings in production time and cost.
The goal of High-Speed Machining should not only focus on the speed of machining but also the flexibility it provides. Batch jobs can be run with little advance notice, streamlining inventories, Fig. 2-1-1. The speed can let CNC machining centers compete effectively for parts that would once have required a more dedicated manufacturing process. The key factors that affect the efficiency of a HSM system are the machine tool, the controller, spindle, toolholder, cutting tool, and programming.
Fig. 2-1-1 High-speed machining focuses on speed and flexibility. (Cincinnati Machine, a UNOVA Co.)
MACHINING CENTERS
HSM allows CNC machining centers to compete with a dedicated manufacturing system such as a transfer line. The machining centers can deliver these benefits:
▪The reduction or elimination of non-cutting time by minimizing tool-change time
▪Produces more parts than machining with a slower spindle and deeper cuts
▪Better surface finish that can eliminate operations of grinding and hand finishing
▪Minimal warping of monolithic (large) parts such as those common in the aerospace industry
▪The production of single complex parts that replaced sections formerly made up of a number of parts
▪Freedom to change part number – To set up a dedicated system because of a design change might require months of downtime while machining centers can be updated in a matter of days.
▪Fast response to engineering changes - Any machining center can be equipped to run multiple part numbers, giving the manufacturer flexibility to respond to customer needs.
Fig. 2-1-2 High-feed rate contouring requires high responsive ways and drive motors. (Modern Machine Shop)
Complex, high-feed-rate contouring requires high response from the ways and drive motors of the machine, Fig. 2-1-2. HSM may also affect the choice of machine hardware. How well the machine manages heat may be a factor. And the freedom to take lighter cuts might permit a different method of contouring.
MACHINE TOOL WAY SYSTEMS
The way system is the part of the machine tool that holds linear motion on track in each axis. There are two basic types:
1.Box or hardened ground ways usually found on conventional machines consist of a box-shaped stationary way that mates with a slide. A thin film of oil is pumped between them to keep the slide moving, Fig. 2-1-3A.
2.Linear guides found on newer machining centers have a linear bearing system that rolls along a guide way. This guide way is usually shaped in a way that helps the bearing grip it, Fig. 2-1-3B. Most machining centers designed for HSM use linear guides.
Fig. 2-1-3A Conventional machine tools are usually equipped with box ways. (Fadal Machining Centers)
Linear Guide Advantages and Disadvantages
Linear guides are a low-friction way system that reduces axis reversal error. Compared to box way systems, linear guide systems have a shorter life since linear bearings have moving parts that wear out. Linear guides provide less damping than box ways. Additional damping in the machine structure can make up for this. The linear bearing design can also influence vibration.
▪Virtual Ways
The advanced state of control technology allows some machining centers to use interpolation only - no way-system hardware, to achieve linear motion in X, Y and Z axes.
▪Thermal Stability
Single-setup machining and dry machining can both lead to large variations in machine temperature. Machine features for minimizing thermal distortion may become important. Some machining centers circulate coolant through the spindle and/or ball screws.
LINEAR MOTORS
A linear motor, an alternative to the rotary motor, can be thought of as a rotary motor unrolled flat, Fig. 2-1-4. It does not require ball screws to move machine slides. The lack of a ball screw makes the linear motor stiffer. On conventional machines, the ball screw and drive train are sources of backlash.
The linear motor has been applied to some CNC machine tools and offers high feed and high acceleration rates.
Advantages
▪Less vibration,