Exploring Advanced Manufacturing Technologies. Steve Krar
motor transmits force through a magnetic field instead of mechanical linkage.
▪Requires no contact between moving parts, except in the way system. Reduced contact between moving surfaces translates to reduced wear and reduced vibration.
Fig. 2-1-3B Linear guides are found on newer machining centers. (Fadal Machining Centers)
Fig. 2-1-4 A linear motor is a rotary motor unrolled flat. (Ingersoll Milling Machine Co.)
▪High accuracy at high feed rates lets the machine position, interpolate, and contour more accurately.
▪Faster machining for large workpieces since the linear motor is unaffected by its length of travel.
HIGHER CUTTING SPEED
On many machining centers, the metal-removal process is the key to HSM. Only higher speeds and corresponding feed rates during the actual machining process can produce improved time reductions, Fig. 2-1-5. Functions such as rapid traverse and tool change time are fast enough and therefore they do not have a great effect on the metal-removal process.
SPINDLES
A high-speed spindle is the most important part of the high-speed machining process, Fig. 2-1-6. In order for high-speed spindles to operate effectively, they require higher performance slide/servo requirements of the machine tool. The CNC control, cutting tool, machining center, and other components must be designed with the goal of using the higher spindle speed productivity. A conventional machining center can be retrofitted with a faster spindle, better cutting tools, and programming changes to produce some of the benefits of HSM.
Fig. 2-1-5 Higher speed and feed rates can reduce cycle times. (The Association for Manufacturing Technology)
Fig. 2-1-6 The high-speed spindle is the main component of a high-speed system. (Modern Machine Shop)
Spindle Compromise
A high-speed spindle presents a tradeoff between cutting force and cutting speed.
▪High-speed spindles generally have direct-drive motors, meaning the motor must fit inside the spindle housing and therefore their size is limited.
▪A limiting factor in high-speed spindles is the bearing that trades stiffness for speed. This is why most high-speed machining generally use light depths of cut.
Types of Spindle Bearings
▪Hybrid Ball Bearings - Composite ceramic bearings take the place of all-steel ball bearings in most high-speed spindles, Fig. 2-1-7.
•In a hybrid ball bearing, the race is still steel but the balls are long-life ceramic balls that deliver more stability at high speeds.
•The balls are lighter and stiffer, so they deflect less from centrifugal force, improving efficiency and reducing vibrations and stresses.
Fig. 2-1-7 Ceramic bearings are in most high-speed bearings. (Fadal Machining Centers)
Fig. 2-1-8 In hydrostatic bearings there is no contact between moving parts. (Fisher Bearing)
▪Hydrostatic Bearings - Higher-power high speed spindles, and spindles with high DN number, are equipped with hydrostatic bearings where there is no contact between moving parts, Fig. 2-1-8. Instead of steel or ceramic balls, a fluid, most likely water, supports the spinning shaft. Their advantages are stiffness, low runout because the fluid pressure tends to hold the shaft on centerline, and low maintenance.
OTHER NON-CONTACT BEARINGS
Air Bearings
An air bearing spindle’s superior runout characteristics that combines low runout with high speeds make it possible to machine effectively with delicate tools. An air bearing is a low stiffness bearing best suited for the lightest cuts. Where high-speed drilling generally implies light cuts taken quickly, effective milling with an air bearing is limited to very light cuts.
Magnetic Bearings
The spindle shaft is supported by a dynamic magnetic field and its stiffness can be digitally controlled to offer stiffness comparable to that of a ball bearing, Fig. 2-1-9. Magnetic bearings do not require a separate system to deliver air or hydraulic fluid.
Fig. 2-1-9 The spindle shaft of a dynamic magnetic field that serves as a bearing. (IBAG)
RETROFIT SPINDLE
Gain some of the advantages of high-speed machining by retrofitting a high-speed spindle to an existing machining center, Fig. 2-1-10. There are various types of retrofit spindles available such as replacement, fixed centerline, and secondary. Each spindle has options that are unique to them.
Fig. 2-1-10 Retrofit high-speed spindle options. (Modern Machine Shop)
Replacement Spindle - The machine’s existing spindle is replaced with a high-speed spindle. This option gives the retrofit spindle access to the machine’s entire work zone. However the machining center may lose the capability it once had to perform slow, deep cutting.
Fixed Centerline Spindle - An independently powered spindle that is mounted in the original spindle. The high-speed spindle can be removed to let the original spindle take slower, heavier cuts.
Fixed Centerline Options
Air powered spindle - If HSM is limited to very light milling and drilling with small tools, particularly in softer materials, then a spindle powered by air may be sufficient.
Spindle speed increaser - This device can increase spindle speed at the cost of some lost Z-axis travel. The increaser, shown in Fig. 2-1-11, uses planetary roller bearings in place of gears. An 8X version of this increaser delivers speeds up to 50,000 r/min.
Secondary Spindle - An independent high-speed spindle that is attached alongside the main spindle, Fig. 2-1-12. This leaves the original spindle available for slower, heavier cuts.
▪The secondary spindle is not centered along the X axis. On a typical vertical machining center, this causes the secondary spindle to lose access of 30-40% of the work zone in the X direction.
Fig. 2-1-11 A spindle speed increaser can deliver speeds up to 50,000 r/min. (Koyo Machinery)
TOOLHOLDERS
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