Measurement on the shop floor: Portable CMMs

The last five years have witnessed a revolution in portable measuring devices and the reason is obvious: It's better to measure where you manufacture.

The companies who have embraced these technologies are reaping the cost-saving benefits involved in taking the measuring device to the part.  They don't need a dimensional lab with a measuring machine parked in the centre.  They don't have to take large parts off their machine tools.  They save time and money!

It's also much easier for the occasional user to use a portable measurement device since programming is much easier and training time is half that of a direct computer-controlled measuring machine.

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But, as with anything new, there are upsides and downsides; the upsides are obvious.  The downside is that it's very difficult to choose what product is right for the job, so let's have a look at the different measuring devices while also exploring the unique characteristics of portable  measuring equipment, and find out what's right for you.

Portable arms have become the most popular coordinate measuring machine sold today.  One key benefit, in addition to its portability, is a much lower sticker price than a conventional CMM.

The portable arm, being a manual coordinate measuring machine, is extremely suitable for one-offs or low volume inspection on the shop floor.  It does not, however, lend itself to higher volume of inspection for two fundamental reasons:

1. The portable arm becomes tedious to operate and,
2. Repeatability suffers greatly because the probe is positioned manually; this is especially true on components that deviate radically from specifications.

Manufacturers of arms specify accuracies and repeatability figures that are easy to meet on a ring gauge in a lab, but impossible to duplicate on the shop floor.  If you're looking for an accuracy of 30 µm (.0011 in) or less, then a stationary coordinate measuring machine is your best bet;  anything over that, then the arm is a perfect choice, but only if you are using hard probing.

Should you choose a laser scanner mounted to your arm, the accuracy is then pushed out to around the 70 µm (.0028 in) range.  Having said that, the laser scanner has the ability to gather and inspect data faster than any other measurement system using touch probing.  One additional bonus of portable arms is that they are able to measure components much larger than their measuring volume; this is done by a process called “leapfrogging.”

The laser tracker is the big brother of the portable arm.  This technology has been radically developed and improved over the last three years.  Prices of laser trackers now start in the $60,000 range, making it affordable for the average small shop.  A laser tracker becomes cost-effective for parts starting at 2m³; the larger the part, the more cost-effective the tracker is.  Laser Trackers are commonly found in the aerospace and automotive industry measuring everything from the complete aircraft or vehicle down to small fixtures required for inspection and assembly.

The new generations of laser trackers are highly portable; in fact, the latest models are packed in small suitcases and easily transportable on an aircraft.  These devices are more portable than arms while displaying the same accuracy characteristics as a large coordinate measuring machine.  A Laser Tracker can be purchased with a probing system similar to a coordinate measuring machine which greatly simplifies the usage because no special tools are required and part programs will be interchangeable between your coordinate measuring machine and the Laser Tracker.  Other options on the new generation of trackers include laser scanners and robot calibration heads.  In fact, many of our customers use their trackers to calibrate their large machine tools.

To sum up, if you are a busy shop measuring large components where space is at a premium, then you need to consider the portable measuring machine.  It will save you precious machining time, floor space and money.

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