MCAD Modeling Methods-3D Input Technology31 Dec, 2005 By: IDSA ,Mike Hudspeth
Reverse-engineering tools save time and money.
My boss asked if we had any 3D models of one of our older products—we needed to use it as the foundation for a new product. The product had been on the market since the early 1980s, well before our company even had CAD software. We were in luck. We found drawings of parts created in CAD by a company we had bought in the late 1980s. The problem was that they were 2D drawings, not 3D models, and they weren't very up-to-date. You know how that happens—changes are made, but no one takes the time to update the drawings.
My boss asked me how long it would take to model the parts. I explained that because of the out-of-date drawings, I'd have to spend a lot of time measuring actual parts. It could take weeks to get to where we felt confident we had halfway accurate models. There was an alternative, he said in a moment of managerial inspiration. He knew of a company that performed 3D scanning. Maybe it could supply us with at least a surface model to go from. We contacted the company, with mental images of Starfleet officers whipping out tricorders and scanning our parts. Sure enough, the company could supply us with fully parametric SolidWorks files, in just more than a week. We were thrilled. Why wouldn't we be? We didn't know anything about 3D scanning. We do now.
Pulling Back the Curtain
There's a saying that any sufficiently advanced technology is all but indistinguishable from magic. Unfortunately, we began our 3D scanning project with somewhat unrealistic expectations. We waited the week and in came our part files. They were fully parametric SolidWorks files, as promised. But as we looked closer, we noted some disturbing differences between the models and the actual parts. Some blends were different. Features were missing. Geometry bore a none-too-faithful resemblance to the actual part. In short, we were less than confident in our newly acquired data. Because the parts that we were building had to mate up with existing production parts, we could little afford to gamble. We knew we couldn't trust our old drawings. The production parts were bound to have warpage and deformations, so they were only somewhat better. To top it off, now our 3D scans were scaring us to no end. We were in quite a pickle.
What is 3D scanning and what is it capable of? First, there are many different ways to get a 3D scan. There are physical probes, laser probes—a whole slew of different kinds of probes. But whatever the underlying technology, they all do essentially the same thing. They record the locations in 3D space of points on the surface of a specific object. Once they collect this data, they can output what is known as a point cloud—a collection of all the points the 3D scanner has identified. When you look at one, you can see the rough outline of the object, but it looks a little funny (figure 1). You can usually bring the point data into modeling software without too much trouble. When we asked for the raw data, it came in as an STL (stereolithography) file.
Figure 1. This is data from a 3D scanner—believe it or not, this can be turned into very nice surface data.
We called the scanning company and told them about our problem. They were understandably distressed. They claimed the problems we were seeing were due to inherent limitations in the process. Features at the bottom of a cavity are very difficult to scan, especially for a laser scanner (figure 2). For this reason they didn't in fact use the laser, but rather a CMM (coordinate measuring machine).
Figure 2. This knockout pin location was missing from the SolidWorks model created from scanned data. The scanning company told us that this kind of feature couldn’t be seen by its equipment.
A CMM captures data via mechanical means. A probe located at the end of an articulated arm or boom (figure 3) sends data to a computer that documents each point as it is sensed. This is as easy as touching the probe to whatever place on the physical model you wish to record. This can be a long and tedious process. Imagine digitizing a work of art, say Michelangelo's David. You might want to reproduce his nose. It's not just a matter of touching the nose and you're done. The more points you record, the more accurate your point cloud will be. Touch, record, touch, record, all the way from the tip to the bridge and down around the nostrils. You may end up with literally thousands of points, just for the nose. Imagine doing the whole sculpture, and you begin to see the problem. You could end up with a very accurate point cloud, but labor cost will be high. That's a good reason to send out the job. Outside contractors have to be competitive with their pricing, and they scan things all day, every day, so they generally know what they're doing.
Figure 3. A coordinate measuring machine (in this case a Wenzel) uses a probe to touch the surface of a physical model and record the position of contact. This is the most accurate type of 3D input device.
Laser scanners were invented to make the 3D scanning process more reliable. They are noncontact so they can scan flexible objects like the human body. Recently I came across a story about the movie Zathura, which was released late last year. In the movie, the girl who plays the big sister is frozen solid. The producers had to create a life-size dummy of her. They performed a traditional cast of her face and head so they could get the fine detail, but the rest of her body was laser scanned. She was scanned in several poses, and the producers chose the one they liked best. This brings up another advantage of laser scanners—they're fast. They can complete a fairly detailed scan in just minutes. Laser scanners range from small hand-held models to large, immobile units. A designated pattern, usually a line or a grid, is drawn in laser light across the surface of a physical model and the reflections recorded. Lasers do the job faster, but are somewhat more limited because they can't reach into areas that a physical probe can access. Also, their resolution isn't as tight as a CMM's.
There are other kinds of 3D scanners. Structured light scanners use a shadow instead of a laser. The object to be scanned is lit by a static light and a shadow passes over it. The shape of the shadow tells the scanner about the shape. It is limited by undercuts in the object that it can't see. I found a Web site that uses this technology for the ultimate in custom dress design, accurate to 1/16th of an inch. Like something out of Buck Rogers, holographic scanners use light from different sources and wavelengths to record 3D data. I once watched a special on the Discovery Channel about the death of Pompeii. Scientists used a kind of radar or sonar to map where bodies had been covered by ash. By sensing the difference in the returning signal, they identified empty cavities in the ground and injected plaster to preserve images of the victims' final resting positions. Sad but fascinating, 3D scanning helped them understand something of what the victims went through. Technically, MRIs and CAT scans are medical applications of 3D scanning technology.
Once you've got your point cloud data, you need software to lay out surfaces based on the points. Most scanners come with their own proprietary software, although many 3D modelers can do the job as well. Alias StudioTools is pretty good for that, as are Unigraphics NX and ICEM Surf. Essentially they all select points and build a surface that is either described or influenced by them. You usually get to decide.
So how did our story end? We bit the bullet and trusted the 3D scanned parametric models because they were the sources of information that most closely matched what we were producing. We mixed in a little of the data from the old drawings, mostly tolerances and critical dimensions, and crossed our fingers. I don't know that 3D scanning gave us instant market share, but it did improve our time to market, and that's what makes or breaks a product introduction.
Mike Hudspeth, IDSA, is an industrial designer, artist and author based in St. Louis, Missouri.