Positional Stages and Tooling

A schematic view of the machining setup is shown in Figure 1. The block to be machined is held in a metal plate that is attached to linear, compact, precision XYZ positioners [7]. The positioning stages employ precision ground leadscrews which have 5 $\mu$m of accuracy over the full 2 inches of travel. The resolution of movement of the stages is 0.1 $\mu$m, while the grating period and readout of the linear optical encoders sets the effective resolution to 1 $\mu$m. The drive systems for these positioners employ DC servo motors with rotary encoders for closed loop positioning feedback. The stages are made out of special-alloy aluminum tooling plate for good stiffness and long-term stability.

 

Figure 1: Schematic view of the micro NC machine.

Motion control of the positioning stages is accomplished with the PC-bus based Unidex 500 series controller [7]. The U500 interface card in the PC is connected to the DR500 amplifier/drive chassis. The amplifier and control box has cabling to control the XYZ drives. A digitizing joystick is also part of the system. The joystick allows the user to control the stages for coarse adjustments, tool registration and workpiece inspection. During actual machining, the joystick is disabled. The Windows based toolkit software has the option of loading user-supplied NC code and the option of single-stepping or free-running modes of operation.

Depending on the actual features to be cut, the removal of metal is accomplished with three different machining strategies and five different tools: milling with two endmills, broaching with an endmill insert, and scraping with two saws. Figure 2 shows a photograph showing the top view of the five tool holders.

 

Figure 2: Top view showing arrangement of tools.

The endmills are held in high speed, precision air bearing spindles that run at 70,000 rpm [8]. Air bearing spindles have low vibration and exhibit smaller temperature rise compared to electric motor driven spindles. For a given tool and workpiece material, there is an optimal cutting speed (CS) of machining, which is available from a machinery handbook [9]. For a given CS, measured in feet per minute (fpm), the spindle speed required, N, is given by $N = 3.82{CS\over D}$ rpm, where D is the diameter of the cutting tool in inches. For small diameter endmills, required spindle speeds can be quite high. If higher spindle speeds are not available, the feed rate needs to be proportionately slower, resulting in increased time for machining. For high speed steel (HSS) endmills on brass, nominal CS is 100 fpm [9]. The required spindle speed for a 5 mil endmill is 76400 rpm. With regards to precision machining, the high spindle rpm that is a necessity for fabrication with small tools has some ancillary benefits as well: (1) High spindle speeds also has the effect of reducing the chip size by requiring a smaller feed rate per tooth, f_T (since $f_T \propto {1\over N}$). Smaller chip loads result in improved surface finish. Good surface finishes are crucial for cutting down on waveguide losses. (2) High spindle rpm can reduce or eliminate secondary operations (such as deburring, or using finish passes) by improving surface finish of the workpiece in the primary machining operations. (3) The reduced cutting forces due to high spindle rpm gives better control while machining thin walls or brittle materials. (4) Reduced cutting forces reduces the heat transfer into the workpiece. A greater percentage of the heat generated by cutting is carried away with the chip in high speed machining. Reduced workpiece distortion leads to more accurate parts. (5) The high level of precision in the motion of the positioning stages is achieved in the absence of significant loads on the stages. High spindle speeds with small radial feeds are helpful in reducing the load, and hence in maintaining accuracy of the finished work. (5) High spindle speed is less likely to excite vibrations in the workpiece.

Dry, purified house air is used to power the turbines of the air bearing spindles to a constant speed of 70,000 rpm. The house air which usually has a pressure range between 85 to 105 psi is sent to a refrigerated air dryer [10]. Coalescing compressed air filters [11] are used at the inlet and outlet of the dryer. After passing through additional filters to remove any remaining particulates, the air enters a regulator which sets the output pressure to 80 PSI. A pressure switch/alarm detects any drop in pressure from the house air supply. Air for the spindle bearings is sent through a check valve. A backup nitrogen tank container is used to provide air for the spindle bearings should the house air fail during machining operations. The nitrogen tank can provide 25 minutes of bearing air in the absence of house air, so that spindles can be spun down safely. The spindle turbines are run directly from the regulated house air supply. As shown in Figures 1 and 2, two spindles are available for machining. The house air pressure is adequate for powering both spindles at the same time. However, the usual machining operation proceeds by using one of the spindles at one time.

The smallest endmill available commercially is of the order of 4 mils in diameter. Another problem machining with endmills is that they are not effective in cutting pockets where the depth is large compared to the diameter of the tool used. The typical cutting length of commercial endmills is only about 2 diameters (lengths of upto 3 diameters are sometimes available on special order). Thus endmills cannot be used for small and/or deep waveguide sections. For features smaller than 5 mils, thin saws are used in the micro NC machine. Small amounts of material are removed in repeated passes by ``scraping'' (broaching) type of operations where the workpiece is moved against a single saw tooth of a stationary blade. By exposing more of the tooth from the clamp, deeper cuts can be achieved. For example, a 5 mil saw was used to cut a waveguide section 18 mils deep. The saws, however are only capable of cutting straight sections of waveguide. Two saws of thickness 5 mils and 2 mils are used in the machine. The two saw blades are oriented so that their cutting motion is $180^{\circ}$ from each other, that is, one saw cuts in the positive Y direction, while the other cuts in the negative Y direction.

Another tool that is used in the micro NC machine is a special order carbide milling insert with a sharp $90^\circ$ corner [12]. This tool is used to cut diagonal feedhorns [13]. The insert was clamped in a stationary mount, and the horns cut by moving the workpiece in linear ramping motions only in the X-Z plane.

During actual fabrication, there are no tool changes involved. The workpiece is carried by the precision positioning stages to each tool. The relative coordinates of the tool positions to the workpiece are initially carefully measured by using target marks made at the same location by all tools on a corner of the workpiece. An electrical edge-finder is also used to determine tool offsets from the edges of the workpiece. In conventional milling machines many tool changes are required to fabricate parts, which can result in considerable errors in the fabricated parts.

For the sake of clarity, two other components that make up the precision machine system are not shown in Figure 1: a lubrication arrangement and a microscope viewing station. Lubrication for the machining operation is provided using a biostable, water soluble machine oil [14] that is delivered onto the work using a fixed clamp. The lubricant carries out the chips to a collecting pan and is recycled for use within a closed cycled system consisting of a gear pump, a water aspirator, a filter and a reservoir. Antifoaming additives are added to the lubricant mix to prevent foaming of the water soluble oil mixture. A specialized adjustable microscope mount allows the user to watch the actual machining operation with appropriate magnification. The microscope is mounted parallel to the Y-axis to allow for continuous monitoring of the machining operation.