2.2
Tube Engraving Setup
In this section, a Swiss-type micro-engraver is developed as a general solution to create special designed patterns on a thin flexible Nitinol tubes [1]. In the literature, it has been shown that patterned Nitinol tubes can provide the required features for designing sen- sorized continumm robots [2, 3] and CTRs with improved stability [4, 5].
There are many challenges in generating precision patterns into the surface of Nitinol tubes including accurate depth of cut, precision profiling and smooth surface finishing. Since the workpiece is superelastic, there should be a special fixation in order to provide con- stant support in all orientations of the workpiece throughout the machining process. In the literature, non-contact machining methods such as laser cutting and Electrical Discharge Machining (wire-EDM) are commonly used to address the problems in machining flexible tubes. To date, only cutting-through patterns or straight slots can be made into the walls of continuum robots, as shown in [3, 4]. For our work, we are interested in machining helical slots along the surface of the CTRs in order to integrate FBG sensors in a helical layout for real-time shape and force sensing. The depth of these slots needs to be precisely controlled to protect the fiber from the forces and pressures of surrounding structures. In recent years, a Swiss-type CNC lathe that combines the advantages of a CNC mill and a lathe became a popular solution to generate helical cuts in a cylinder structure, as shown in Fig. 2.2(a, b). However, in a normal configuration, there is almost no support on the workpiece except for the clutch. As a result, the machining precision decreases as the rigidity of the workpiece drops. For flexible Nitinol tubes used in CTRs, it is almost impossible to engrave patterns with desired accuracy. Building a custom made jig for this type of machine could be an option, but the cost is quite high.
The promising research potentials in patterned CTRs and deficiencies in current machining processes motivated us to build a desktop micro-engraver, specially for creating various patterns on the surface of flexible tubes. As shown in Fig. 2.2(c, d), a support block was designed to provide constant support in all orientations of the workpiece during the ma-
2.2. TUBE ENGRAVING SETUP 34
Figure 2.2: (a) A commercially available Swiss-type CNC lathe, Citizen-Cincom K16 (Marubeni Citizen-Cincom Inc., United States). (b) A view of the machine’s full size. (c, d) The CAD design of our desktop micro-engraver (e) The actual setup of the developed desk- top micro-engraver. The machining process can be monitored using a microscope through the viewing window on the support block.
chining process. The alignment between the end mill and workpiece was maintained by the inner structures of this block, while the workpiece rotates and translates. A new block can be easily custom made for different tube diameters, since it was made out of ABS us- ing a 3D printer. In order to observe the tool-workpiece interface in real-time, a viewing window was designed and a microscope was placed on top of the block. A wide-angle entry from the end mill side was designed for the application of pressurized air. Silicon lubricant was constantly sprayed into the support block to minimize the friction between the block and the Nitinol tube. It also serves as machining coolant to reduce tool wear. To find the required tool offset for machining, a multimeter continuity test was used to probe
2.2. TUBE ENGRAVING SETUP 35 for contact between the end mill and the workpiece.
Since the hardness of Nitinol is quite high (58-64, Rockwell C scale), an AlTiN-Nano coated carbide mill bit (944215-C6, Harvey Tool, United States) was used in this setup. This mill bit features a ball end and three V-shape helical flutes for maximized chip loading. The diameter of this end mill is 0.015 inch and the cutting length is 0.023 inch. It is special designed for micro-milling exotics &aerospace alloys. The formulas for calculating the machining speed and feed rate are:
RPM = (3.82×SFM) / D (2.1) IPM = RPM×IPT×T (2.2) where, RPM denotes revolutions per minute; SFM is surface feet per minute; D is the diameter of the end mill; IPT is chip load per tooth; and T is the number of teeth of the end mill. According to the datasheet of the chosen end mill, and the hardness of the Nitinol alloy, these numbers are: SFM = 40∼75; D = 0.015 inch; IPT = 0.00006; T = 3. As a result, the speed and feed rates for machining Nitinol tubes are: RPM = 10000∼19000; IPM = 0.9144 m/s; and the depth of cut is 0.1524 mm.
To provide the accuracy and repeatability required for patterning CTRs, high-precision lin- ear stages (T-LSR300B, Zaber Technologies, United States), and rotary stages (T-RS60A, Zaber Technologies, United States) were chosen to generate the cutting profiles. The rotary spindle (200 series rotary tool, Dremel, United States) was mounted on one of the linear stages for accurate depth control. The entire system has a portable size (40 x 40 cm), and weighs less than 7 kg. The performance of this Swiss-type micro-engraver for creating helical patterns on Nitinol tubes will be discussed in detail in Chapter 7. Although this technology was designed for engraving helical patterns, it can be generalized for creating much more complex 3D patterns since all the three axes are computerized. In this case, significant upgrades on the user interface are needed, in order to automatically generate