Measurement tools for Additive Manufacturing: Pros and Cons

Giles Gaskell is Hexagon Manufacturing’s Commercial Business Manager and has been delivering the Introduction to Basic Measurement and Metrology for Additive Manufacturing session at AMUG for several years now. As part of that session Giles uses his vast experience to give a rundown of the pros and cons of the metrology machinery on the market currently.

The session began with a brief history of measurement including the old definition of yards, which used to be measured by a rope from the nose to the tips of the fingers of a particular construction site’s foreman, until King Edgar the Peaceful came along in the 900s and created the first standard for measurement that was the yardstick. The yardstick was taken from Edgar’s measurements and implemented on construction throughout England.

Moving through the centuries, standards were often created for manufacturing during warfare, geometric dimensioning and tolerancing for example is thought to originated with Stanley Parker’s methodology during the manufacture of planes in WWII.

During the session, Giles ran several tests to demonstrate how inaccurate human measurement systems can be. He asked the 50 or so attendees to tie two knots in a lanyard that was, at their best guess, 12 inches apart. After collecting and a quick sight analysis he picked out the longest and shortest, the range was 18 inches to 8 inches.At the same time, he’d asked three volunteers to use callipers to run some measurements, and although much closer than the guesswork, you can see in the image below, the range is still significant.

A mathematician would say 2+2 equals 4, a statistician would say between 2 and 5, but a mechanical engineer would say 12 to be on the safe side.

It’s for this reason that we now have a range of measurement technologies and being an expert in those, Giles ran through the pros and cons of those for measuring parts that are additively manufactured.

Coordinate Measuring Machines (CMM) were introduced in the late 1960s for jet engines. They are the most accurate machines for measuring holes, but they have limitations in measuring complex surfaces, for that you require optical scanners.

Optical scanners, such as laser and structured light scanners, are increasingly being used for data collection in manufacturing. Laser scanners often used by hand gather enormous amounts of data very quickly, while structured light scanners give higher quality data because they are repeatable. However, optical scanners have limitations in measuring high-precision machining, holes and narrow slots, and hidden features (often common in additively manufactured parts).

For hidden features you’re going to need CT Scanning, which can do volume data analysis, porosity detection and analysis extremely accurately. However, CT is often costly and cannot measure inside dense materials (see below). GE, who are at the forefront of CT Scanning are still doing destructive testing on aircraft blades because they are almost impossible to scan.

Ultimately Giles suggest for a large majority of cases that a combination of the tools above will prove sufficient and even using callipers alongside a surface plate can vastly improve your measurement of parts. But one thing is for sure, you cannot stick your head in the sand when it comes to measuring parts.

“If you can’t measure your parts, you can’t sell them. Or, at least, you can’t sell them again. The worst thing is if your customer has better measurements tools than you do.”