*2.1. Technical Approach*

A collection of machine tools available at OMAX, CBA, and other facilities were selected for the project. One of the early tasks was to define a reference part that was suitable to be machined and/or fabricated with all the selected machine tools. A decision was made to use one the flexures investigated at NASA/JPL as a key component of prototype microsplines for asteroid grippers developed under the Asteroid Redirect Mission [5,6]. As shown in Figure 1, the flexure consists of 11 full-length and 2 half-length spring-like elements. The length and width of the flexure elements were 36.3 mm and 0.5 mm, respectively. The separation between each element was 0.76 mm. The aspect ratio (length/width) was therefore 72.6. The widths of the element and gap between them are 0.51 mm and 0.76 mm, respectively. The aspect ratios of the length-to-width and length-to-thickness were 71.5 and 47.7. The above are the dimensions of the full-scale flexure. Small-scale flexures with 0.5, 0.4, and 0.33 were also machined using several tools. The flexure is extremely delicate and flexible. It took only very weak side force exerted onto the flexure element during machining to deflect them permanently. During the course of machining, strengthening tabs were used to support the delicate elements (Figure 1a). Figure 1b shows the tool path with several color coded curves: green—traverse without cutting, magenta—lead in and out, and blue—cut at quality five level. The tool path shown in Figure 1b included two steps: (1) machine the flexure with the tabs in place and (2) remove the tabs. Figure 1c shows the final part with the tabs removed.

(**c**) Finish Flexure

**Figure 1.** DXF of flexure selected as the reference part for the MicroCutting Project.

The flexure, machined or fabricated from materials that were most suitable for the individual tools, were then inspected qualitatively and quantitatively to compare the performances of these machine tools.

#### *2.2. Micro Abrasive Waterjet Technology*

The AWJ is amenable to micromachining as the diameter of the AWJ can potentially reduce to micro scales [7]. The μAWJ technology was developed and commercialized under the support of an SBIR Phase II/IIB grant. Several novel processes were developed and incorporated into the MicroMAX for meso-micro machining of 2D and 3D parts. The main focus was to downsize the AWJ nozzle capable of meso-micro machining. As such, several challenges were present as the three-phase, supersonic slurry flow inside the nozzle transitions from a gravity-dominated flow to a microfluidic flow. Novel processes and apparatus were developed to enable μAWJ technology for industrial applications [4].

At present, the 7/15 nozzle with an orifice and a mixing tube ID of 0.007" (0.18 mm) and 0.015" (0.38 mm) is the smallest production nozzle whereas the 5/10 nozzle is currently used for special applications. Garnet with 240 mesh can be readily used with these nozzles. For garnets finer than 240 mesh, a proprietary process was developed to enhance their flowability. Success in developing the AWJ technology led to the culmination in the release of the MicroMAX as a new product debuted in 2013. It has a position accuracy and repeatability of 15 μm and 5 μm, respectively. A MicroMAX

version II with the incorporation of a Rotary Axis for machining axisymmetric features was released for production in 2016.

## *2.3. Machine Tools and Participants*

The participants in the Microcutting Project included MIT/CBA (www.cba.mit.edu), OMAX (www.omax.com), Formlabs (www.formlabs.com), Datron (www.datron.com/), (Moog Inc. (http: //www.moog.com/), and BMF Precision Technology Co, Ltd. (http://bmftec.com/). Machine tools available at the facilities of the participants and used in the project included CBA Digital Fabrication Facility (http://cba.mit.edu/tools/index.html) Beam Dynamics Model LMC10000 CO2 laser system— 1.2 m × 1.2 m cutting area, 30.5 m vertical travel, 500 w (1550 W peak), 25 μm overall accuracy, 51 m/min max cutting speed (91 m/min traverse speed) Sodick SL400G Wire EDM—X, Y, Z Axis travel, 400 × 300 × 250 mm; wire diameter range: 0.051 to 0.30 mm.

Zund G-3 L-2500—Repeatability ± 0.03 mm, position accuracy ± 0.1 mm/m, working area 1800 mm × 2500 mm, high speed router 3.6 kW 50,000 rpm.

FabLight 3000 Fiber Laser—3 kW laser, working area of 6.35 m × 1.27 m and tubes of diameter 12.7 mm to 5.1 mm. Capable of cutting steel, stainless steel, spring steel, aluminum, copper, brass, titanium. Repeatability of 15 μm, accuracy of ± 20 μm/m.

Oxford Solid State Micromachining Laser—532 nm diode-pumped solid-state laser, 150 mm X-Y travel, 50 mm Z travel, 1 micron resolution. It has spot size of approximately 20 microns. The laser outputs approximately 6W of power at 10 kHz and can quickly cut through materials typically up to 0.5 mm thick. It's used for fine cutting, ablation, engraving, and marking

OMAX 5555 JetMachining Center at CBA—X-Y Travel 1.4 × 1.4 m, Tilt-A-Jet, MAXJET 5i Nozzle (10/21), 7/15 Mini MAXJET 5 Nozzle, Precision Optical Locator, pneumatic drill OMAX Corporation MicroMAX—A MicroMAX equipped with a Tilt-A-Jet (TAJ), a Rotary Axis (RA), and a Precision Optical Locator (POL) is available at the OMAX facility (https://www.omax.com/omax-waterjet/micromax). For meso-micro machining, the 7/15 and 5/10 nozzles have been used routinely for cutting demo parts and conducting in-house R&D. The nozzles were driven by an EnduroMAX 40 hp crankshaft pump (Model 4060V) with pressures up to 410 MPa. For these small nozzles, an excess flow control valve was installed to drain a part of the water through the high-pressure pump. At pressures below about 70 MPa, the Bernoulli vacuum was too weak to entrain all the abrasive into the mixing chamber. Vacuum Assist accessory was used to remove the excessive abrasive to mitigate clogging of the nozzle.

AWJ machining was controlled by an IntelliMAX Software Suite featuring an extensive tool set to streamline production. The Suite is based on a precision cutting model in which each engineering material is assigned with a machineability index according to its properties derived from the results of extensive cutting tests. The intuitive Suite that is easy to use consists of a PC-based CAD-LAYOUT and CAM-MAKE. A set of steel slats spaced 25 mm apart is installed inside each JetMachining Center to support the workpiece. A 10 cm thick polyurethane honeycomb placed on top of the slats is often used to provide a firm support to workpieces made from thin materials. Since the AWJ exerts very low force onto the workpiece, it can be secured by relatively simple fixtures such as carpenter clamps. A number of options is available to secure the workpiece depending on the setup. For small parts, thin tabs are incorporated into the tool path to prevent losing it into the tank below. Machining is carried out by setting a standoff distance of 0.76 mm between the tip of the nozzle to the top surface of the workpiece. The tool offset is set to on half of the exit diameter of the AWJ. Depending on the thickness of the workpiece and the required edge quality of cut from Q1 for raw cut to Q5 for precision cut, the cutting speed is set intelligently by MAKE according to the machineability index. The cutting speed also varies according to the shape and curvature of the tool path. For example, the traverse of the nozzle speeds up in straight segments and slows down during corner passing to maintain the same kerf width.

Formlabs-Form 2 Printer—Build Volume: 145 ×145 ×175 mm, Layer Thickness: 25, 50, 100 microns, Laser Spot Size: 140 μm, Laser Power: 250 mW, Wavelength: 405 nm (violet), automated resin system, self-heating resin tank, auto-generated supports. Machine size: 350 mm (L) Ã—330 mm (W) Ã—520 mm (H). *JMMP* **2020**, *4*, 19

Moog, Inc. —These parts were made from metal powders, an aluminum alloy and stainless steel, using laser powder bed fusion. The equipment included:


They were then trimmed to correct thickness with a φ0.25-mm wire EDM (Mistsubishi MV2400S) for 30 min approximately.

Boston Micro Fabrication (BMF) Material Technology Inc.—Digital Lighting Processing (DLP), similar to Stereolithography Appearance (SLA), was used to fabricate the flexures (https://web.archive. org/web/20140221025534/, https://thre3d.com/how-it-works/light-photopolymerization/digital-lightprocessing-dlp). It is a 3D printing process working with photopolymers. In DLP, a 3D model is constructed and 'sliced' through software. Once the sliced images are received by the printer, curable liquid, e.g. monomer or pre-cured resin, is exposed to a pattern of UV-light, in order to selectively solidify a cross-section of the designed parts. The cured cross-section is then lowered below the surface level of the liquid, allowing the liquid to backfill for curing and bonding of subsequent cross-sections. The process is repeated until all the slices of the 3D model and hence the printing parts are completed. The liquid is then drained from the vat, followed by demolding and post-curing of the parts. The nanoArch Micro Scale 3D Printing System InP140 (https://bmftec.com/) nanoArch®is the first commercialized high resolution, multimaterial 3D micro-fabrication equipment based on PμLSE (Projection Micro Litho Stereo Exposure) technology, which is designed for scientific R&D of functional composite materials.

Datron-Neo Milling Machine (https://www.datron-neo.com/us/datron-neo-simple-milling/ overview/). Masking tapes and super glue were used to secure the workpiece onto the substrate.

