Unique challenges to measuring microparts

There are several challenges that can arise when you start the process of measuring micro and meso scale parts. Some important factors to consider before you begin the actual part measuring are: part handling, cleaning, and fixturing. Using a coordinate measuring machine allows you the capability of measuring extremely tiny parts completely accurately by using touch (tactile) and optical techniques. Also, carefully following certain steps before you begin the measuring process will help ensure you receive the best possible results upon completion.

Part handling 

Unless you have inadvertently dropped a micro or meso part on the carpet, you may not truly appreciate the extremely small scale and accuracy needed to correctly measure them. To use a common analogy; dropping one of these parts could be like “looking for a needle in a haystack“ since the size of these parts range from uncomfortably small to nearly impossible to physically handle with your own hand.

It is also important to note that because you are working with extremely small parts, having a clean work area is very important. For those familiar with lean manufacturing, a thorough method for organizing a workplace, especially a shared workplace like a shop floor or an office space, is crucial. You should also be prepared to use gloves to keep dirt, dust, and oil from contaminating the part.

Having specific tools on hand to accurately view parts is essential. Some of the most common tools used are a magnifying glass, a microscope, and perhaps even loupes. When you are picking up and moving parts, you may want to use tweezers or vacuum tweezers. One of the most important things to remember when handling parts is how fragile they may be. A force that someone might consider normal could very well be damaging.

Part Cleaning 

Cleaning the part before measuring is critical to collecting reliable data. For example, cement dust can be anywhere from 3-100 microns, while a human hair could be 5-200 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 [source: www.engineeringtoolbox.com].

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 material(s).

Part Fixturing 

Holding a part is integral to getting good results (i.e., repeatability and accuracy) while measuring with a CMM. Typically, you should use a fixture when working with a part. There may be times when you can use the same fixture in manufacturing and in measurement. When making a part, for example, removing part material with a machine tool and measuring the newly created feature on a CMM could be done with the same fixture. Since the introduction of the ZEISS F25, additional fixtures have been added to address some of these needs.

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.

Measuring 

After you have received the part into your work area, cleaned and fixture it appropriately, you will want to begin measuring. One thing that simplifies the measurement of small parts is the use of a CAD model to facilitate measurement planning. Using a CAD-based metrology software, such as ZEISS CALYPSO®, allows you to focus on measuring the part and not editing code or worrying about programming languages. For example, if you have a pre-existing CAD model, you can load it into the software and “extract” features from the CAD model as you plan your measurement. It’s as easy as pointing and clicking to define the necessary evaluations and have a complete measurement plan. Additionally, 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.

On the ZEISS F25, there are two sensors; an optical camera sensor based on an objective lens used in ZEISS microscopy for 2D measurements, and a tactile, passive measuring scanning sensor, based on a silicon-chip technology with integrated Piezo-resistive elements, for 3D measurements. This combination of probing systems gives the operator a great deal of flexibility when determining how to measure the part. Additionally, there is a camera focused on the tactile sensor which allows the operator to see the tactile probe as it moves about the part. This makes it possible in many cases to visually verify the interaction of the stylus and the workpiece.

In summary, when measuring micro parts remember:
Part Handling – The parts that you can measure are very small
Part Cleaning – Dirt, dust and other foreign materials are the enemy of accuracy
Part Fixturing – Find the right balance of security and accessibility