Although one probably doesn't think about Gullliver's Travels in relation to metrology, when it comes to measuring the very small—as in the micro- or meso-scale—it is not inappropriate to consider Gulliver among the people of Brobdingnag, where he was very small and what we consider to be somewhat insignificant in size takes on enormous scale. As Patrick Lanthier, F25 product manager, Carl Zeiss IMT Corp. (zeiss.com/imt) notes, for example, "Cleaning the part before measuring is critical to collecting reliable data. For example, cement dust can be anywhere from 3 to 100 microns, while a human hair could be 5 to 200 microns. When you are talking about a machine like the ZEISS F25, whose MPE is 0.25+L/666, that small speck of dust or hair becomes a large source of potential error." (Lanthier cites engineeringtoolbox.com for the source of the object size figures.)
Lanthier has some suggestions that should be taken into account when measuring the very small, whether you're doing it optically or with touch. (The F25 machine utilizes two sensors. One is an optical camera sensor based on an objective lens, the type used in Zeiss microscopy. That sensor is used for 2D measurements. Then there is a tactile, passive measuring scanning sensor that is based on silicon-chip technology with integrated piezo-resistive elements. A camera is focused on the tactile sensor so the operator can see the tactile probe as it moves about the part, providing, for example, the means to visually verify the interaction of the stylus and the workpiece. This tactile sensor is used for 3D measurements.)
Here are his recommendations:
• When it comes to handling, realize that this is going to be difficult. If you can handle the part by hand, then be sure to wear gloves so as not to contaminate the part with any dirt, dust or oil. You may want to use tweezers or vacuum tweezers. In any case, be sure to take into account that due to the tiny size, the parts are likely to be fragile, so don't handle too roughly.
• Keep the work area clean. A disorganized space could make finding a small part exceedingly challenging.
• Have some magnification on hand. A magnifying glass, microscope or loupes are useful.
• Clean the part before measuring. Lanthier suggests, "Using a small ultrasonic table top cleaner, or a simple part bath, may be the answer for most part cleaning applications. As there are a myriad of cleaners to use for different materials, an important step in measuring parts is to match the proper cleaning solution to the part materials."
• Be careful while fixturing the part for measurement. "As in part handling, clamping forces are critical to holding the workpiece. You do not want to induce any stresses into the part you are measuring, while at the same time you want to make sure the workpiece is secure," he says.
• Using a CAD model can facilitate the measurement planning, particularly for small parts. That is, "Planning the navigation for a part too small to see is simplified by visualizing your measurements on the CAD model, before the actual measurements ever begin."