How to Qualify Accuracy
Todd Grimm posted on March 20, 2012 |
Want to really know the accuracy of a 3D printed part? Don't CMM it; 3D scan it and let a color map ...

3D scanning color mapFor two decades, a pet peeve of mine  has been manufacturers’ claims about 3D printer accuracy. Specs are tossed around with asterisks noting that they are size and geometry dependent. Printer resolution is often passed off as end-product accuracy. Nothing is mentioned about accuracy and material dependence.

But this isn’t a rant about claims; it’s a quickie guide for your own accuracy assessments. It's a way for you to discover the truth, and it’s one that comes from my two passions, 3D printing and 3D scanning.

Long ago, I discovered that CMMs (coordinate measurement machines) aren’t the best tools for 3D-printed part inspection. Like so many tools and processes created for traditional manufacturing methods, it can work, but it isn’t ideal for 3D printing.

The problem is that CMM data assumes that nothing changes between the inspection points. Sure, three points on a machined, flat surface can give you an accurate assessment. But on a 3D-printed part, there is a strong likelihood that the surface varies between those three point. How about three points for a horizontal bore? For a reamed hole, that works. For a 3D printed hole, no way. The simple matter of layering creating flat tops and flat bottoms negates three-point inspections.

I first realized this issue when I adopted 3D scanning in a 2006 benchmark. I had the parts scanned, so I had hundreds of thousands of point with which to work. But I tried to make the reporting match that of a CMM. When I went into the data to do a simple wall-to-wall measurement, I found that small changes in point location would give me very different results. Moving a measurement target 0.010 inch could yield a 0.001- to 0.002-inch difference in accuracy.

After that experience, I now recommend 3D scanning a 3D printed part. To see and report the accuracy, use the nifty color mapping tools. The result is a vivid, 3D display that shows you what’s in tolerance (green), what’s high (red) and what’s low (purple). With the color map, you have full coverage that will show you every defect and every in-tolerance feature.

Don’t worry about this method not being recognized by the quality department. They are moving that way, too. In the world of GD&T (geometric dimensioning and tolerancing), they call this a profile tolerance. Reviewing color maps to a profile tolerance gives the quality department about 90 percent of all the data they need.

The bonus is that it is fast to set up, fast to scan and fast to report. There is no programming needed for setup — just point the scanner at the part and go. Scanning can be completed in a few minutes. For reporting, just import the STL file (or CAD-generated model), identify an acceptable profile tolerance and let the software compare the model to the scan data.

Better results and faster inspection —how can you beat that?

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