In the world of digital manufacturing, where machines are capable of making parts to micron accuracies, being able to measure precisely and reliably is essential to qualifying every part that we make. Let’s take a closer look at one of our important measurement tools, the CMM, and how it works for you.
What is a Coordinate Measuring Machine?
At its most basic a coordinate is a point, a fixed singular location in three-dimensional space. A series of points can be used to define the parameters of a complex shape. Therefore a coordinate measuring machine (CMM) is any device that is able to collect this set of points for a given object and to do so with an acceptable degree of accuracy and repeatability.
What are the components of a CMM?
The foundation of the system is just that: a heavy base plate or table which serves as the foundation for an object placed on it to be measured. This is often a massive slab of granite or some other dense material that is stable, rigid, immune to fluctuations caused by the environment, and ground with a very flat top face.
To this table is mounted a moveable bridge or gantry. Vertical posts support a horizontal beam, and on this beam will be suspended another vertical column that holds the measuring probe. The bridge or gantry is able to move along the X-axis. The vertical spindle can move along the bridge thus defining the Y axis. And the probe on the vertical column can move up and down which defines the Z-axis over the table.
At the end of the spindle is the probe. There are different technologies available that can be used as a probe, partly depending on the objects to be measured and the degree of accuracy required. In our case, a precise sphere of ruby is mounted on the tip of the stylus.
Finally, the tip of the probe communicates its information to a computer which interprets the data with specialized software to create a 3D map of the part in question from the cumulative set of points.
How does the probe work?
The exact size and position of the tip must be precisely known in advance to establish the “zero” point for all subsequent measurements. That is one of the reasons for using a ruby sphere, since it is a hard substance that does not fluctuate in size due to temperature or humidity. The tip is mounted on a spring-loaded stylus. That stylus in turn has an angular rotation of 105° and a circular rotation of 360°. As a result, the entire machine is considered a 5-axis CMM.
The stylus is connected to exquisitely sensitive electronics that detect even the slightest deviation in electrical resistance coming from the probe. Each time the spherical tip contacts a solid object and is forced to deflect, that generates an electrical pulse which is sent to the computer which maps out a point on our imaginary X-Y-Z space. Hundreds or thousands of such points are collected, depending on the geometry and size of the part.
Are there drawbacks to the CMM?
All touch probes must make physical contact with an object to take a reading. This might not be desirable for extremely delicate surfaces which might potentially be damaged, although this concern is virtually non-existent for machined metals and plastics such as those Star works with.
The size of the probe tip must be suitable for the features it’s intended to measure. A tip that is too large will be unable to resolve fine features smaller than itself. Also, the tip must be very clean, at the microscopic level, to insure that no foreign contaminants get between the ruby tip and the work piece.
Very soft and pliable rubbers or elastomers might be so yielding under the touch of the probe that the surface deflects slightly, causing an errant reading. Other techniques involving lasers or visible light would then be employed for a more accurate measurement.
The reliability of the CMM is dependent upon a very rigid physical frame that does not distort over time or due to environmental conditions. Our Nikon Altera ceramic bridge CMM, using a Renishaw PH10M Plus probe, is guaranteed to maintain its accuracy for 10 years without requiring adjustment or calibration – a testament to the robustness and reliability of its design.
What does this mean for you when you’re considering making a product?
Ultimately, reliable measurements of complex shapes and surfaces may rely upon a combination of tools and techniques, each one of which is optimized for a specific application. We use a variety of advanced tools to insure that every part we manufacture meets and exceeds your exacting requirements. If in doubt about how we will measure to your specifications, contact us and talk to one of our quality control experts.