One of our most popular case studies compares various manufacturing methods for a missile component that controls flight. Supersonic missiles are highly sensitive to the exact geometry of control surfaces and precision is mission critical. Canards (French for “duck”) are the pivoting fins attached to the side body of missiles ahead of the main wing that provide stability and maneuverability for a projectile. Supersonic missiles must also shift between subsonic and supersonic speeds and canards affect the airflow against the main wing, altering the center of mass, and shifting the aerodynamic center. Thus, any deviation in geometric specifications will greatly affect flight control, causing extra turbulence and unanticipated movement.
Our case study concludes that Liquidmetal injection molding is superior to CNC machining and metal injection molding (MIM) when it comes to minimal part-to-part variation, extremely tight tolerances, and cost and time considerations. Here we seek to quantify the dimensional consistency more precisely. After successfully casting parts via Liquidmetal’s injection molding process, one of our customers performed coordinate-measuring machine(CMM) measurements to evaluate the precision of the parts. A CMM is a device used to measure the physical geometry of parts; it consists of a probe with three-axes of motion that is used to acquire coordinate positions to a precision of 0.00001″ (0.25 microns). Line profiles were acquired, meaning, for each (x,y)-coordinate position, a z-coordinate was recorded. Effectively, this is a measure of the surface “roughness” (in quotes so as not to be confused with surface roughness at a smaller scale, a component of surface metrology and important in studies oftribology); the data is utilized to quantify the deviations from the nominal, or, how much the points measured vary from the intended dimensions at a given location.
Two line profiles were collected for each part, with each profile encircling the part at a different x-location (e.g., at position x = X0, the surface height, z, was recorded for each y-position along the front side of the part and then continued to the back side at the same X0) (see Figure 1). These actual z measurements are then compared with the average of all the samples. These results are shown in Figure 2. For each sample, the difference between its actual z and the average z for all samples is plotted as a function of line profile position (± ΔZ vs Line Profile Position). For the entirety of the line scan, ΔZ is always less than ±10 μm (0.0004”) for all parts, while the standard deviation is on average 5.5 μm (0.0002”). To reiterate, this means that amongst this random sampling of parts, no part differed from the next by more than 20 μm (0.0004”) for any given location on the part. In fact, we are most likely reaching the limits of the CMM equipment at these magnitudes (this has to do with the model set up from the CAD drawing, physical selection of control datum points, and the slight error/uncertainty resultant from the combination of these.)
Figure 1: CMM measurements are taken along a line around the part at a specified x position. Every point measured, thus, has an (x,y,z)-coordinate, where x is constant, y varies as the machine moves around the part, and z corresponds to the height in relation to the origin.
Figure 2: Plots of dimensional consistency in metric (μm) and English (inches) units. Part-to-part variation is measured as ± ΔZ where ΔZ is the difference between the actual Z of the sample and the average Z of all the samples at the given line profile position. For all samples, ΔZ is less than 10 μm (0.0004”) at all locations. The standard deviation of all samples as a function of line profile position is plotted on the right axis and is less than 5.5 μm (0.0002”) on average.
This remarkable dimensional consistency is superior to CNC machining and is achieved in less time and at lower costs.
Tighter tolerances can be achieved through fine grinding and polishing after CNC machining; however, the cost for tightening up tolerances, for example, from ±0.0005” to ±0.0001” may be 3.3 times more.† Even so, Table 1delineates part-to-part variation, which is not necessarily the same as tolerance, per se (think accuracy versus precision). Thus, some key benefits of this quality, especially in applications such as this where it is essential to have part-to-part dimensional consistency (precision) rather than extremely strict dimensional accuracy (while maintaining specified tolerances, of course), are the time and cost savings from not having to check and measure every part. Instead, parts need only be sampled during quality control because of the confidence of minimal part-to-part variation. Based on these reasons and the CMM measurements, Liquidmetal injection molded parts clearly demonstrate practical and economic advantages.