Editor's CommentaryAuxilery spindles continue to become smaller yet more reliable with higher performance.
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As an example, when milling a modified Torx 20 female drive socket in a 316 SST bone screw, a conventional milling process would require 3.3 minutes, running a four-flute, 0.032-inch diameter end mill at 3,000 rpm. When using the high-speed spindle capable of a proper surface speed of 300 sfm, it is possible to run the end mill at 36,800 rpm and reduce the cutting time to 45 seconds. In addition to the tremendous time savings, the part can be completed almost burr-free. This is not possible when cutting at conventional milling speeds.
Installation is simple, as most milling and drilling spindle models can fit into existing 20-mm or 25-mm tool blocks. This compact length allows spindle use in tight tooling situations. If the machine tool mounting bracket allows, these spindles can also be mounted on angles.
One of the main reasons that CNC Swiss machine tools are among the hottest technologies in the precision machined parts industry is the versatility they bring to part production. Much of this versatility is represented by the machines’ ability to perform multiple operations including turning, drilling and milling.
Specialty machining shops and traditional turning shops are looking at Swiss in large part because of its ability to process workpieces complete in a single handling. Moreover, with automatic workhandling such as CNC-controlled bar feeder units, these machines lend themselves to untended or lightly tended operations.
The turning capabilities of Swiss machines are well known. The single-axis plunge of the cutting tool, close coupled to the guide bushing results in high-accuracy turning on long, slender parts. Increasingly, production requirements on Swiss machines incorporate what were once secondary operations, such as drilling and detailed milling.
Most driven tools on CNC lathes are limited to about 5,000 rpm. For larger-diameter tools, this speed limit is not a problem. However, much of the work on Swiss machines is in small diameters. For small-diameter drills and mills, high rotational speed is critical to maximize feed rates and, in combination, provide productive cutting speeds. High-speed spindles can often reduce drilling cycle times (usually the slowest operation) by a factor of five.
Swiss manufacturer of precision high-speed spindles, Ibag, has introduced micro-precision high-speed milling and drilling spindles specifically for the Swiss machine (automatic CNC lathe) market. The new products include a 25-mm diameter 60,000-rpm model (80,000-rpm optional), as well as a 22-mm-diameter 60,000-rpm model.
By using small-precision milling spindles as live tooling on the Swiss machine, it is possible to perform delicate milling operations on the original setup. The high speed, power and rigidity of the spindle allow small end mills to be used at high speeds in stainless steel, brass and other alloys. The use of high-speed cutting techniques (shallow depth of cut and faster feeds) also minimizes burrs.
These spindles are dynamically balanced to ensure that the spindle runs smoothly and quietly with little to no vibration at all speeds. Excessive vibration not only reduces bearing life, it also shortens tool life and can even cause small tool breakage.
The spindles are compact and powerful, using DC brushless motor technology. The DC brushless technology allows the use of a small-sized motor that has higher power and runs cooler than standard AC motor technology. High power and high torque are required when milling tough materials like stainless steel, brass and other alloys.
For thermal stability, the spindles are air-cooled and require no external cooling by water or fans. The bearings used (both front and rear) are sealed hybrid-ceramic (steel races and ceramic balls), with permanent grease-packed lubrication. The spindle design includes an integral positive over-pressure air-seal. The air seal system injects a small amount of compressed air through the stainless steel spindle body, and ensures that dirt, coolant and other contamination does not enter the spindle housing. This protects the spindle bearings, extends spindle life and increases uptime, which is critical for high-production applications.
The maximum allowable taper runout is 2 microns. Higher runout values, even in the micron range, can adversely affect the cutting quality and accuracy of micro-tooling. It is impossible to drill with a tool diameter of 0.020 inch when the runout of the tool may be in the 0.001-inch range. The tool will walk, wear excessively, and in many cases, immediately break when it touches the workpiece. Tool retention on these Ibag spindles uses a precision ER-8 collet system, with a maximum tool shank diameter of 3/16 inch (5 mm).
The spindle can be integrated with the CNC to control operation (on/off and speed), and to monitor any faults by the CNC. As the spindle speed is programmable from the CNC, it is a simple task to change over to a new part by re-programming the required speed for each spindle cutting tool. The CNC interface can use spare M codes to activate the spindle as needed.
The use of micro high-speed spindles, on Swiss-style turning equipment, will provide a means to optimize operations that are traditionally the most challenging, and consume the most cycle time on a small part. With the development of new machines with even smaller footprints, smaller spindles (20 mm and 16 mm) may play an even greater role in making the best efficiency and through-put possible.
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