For laser line scanners such as those used on portable articulating CMM arms, it is important to understand and hone your skills at maintaining the correct standoff distance from the workpiece. They all have a zone between their maximum and minimum standoff within which to scan good data. Similarly, these scanners have a maximum angle from the surface of the workpiece in order to get good data. In fact, while close to 90 degree angle is optimum, sometimes a near-exact 90 degrees is too perpendicular to get good data, so slightly off 90 degrees is best. Also, surface conditions and lighting can affect the scan. Shiny surfaces can be particularly challenging, and black surfaces can also present a problem. As scanner technology improves, these factors have become less of a problem. It is common to spray some type of matte finish coating on some parts to make them more acceptable for scanning.
Also, the speed at which you can scan varies from scanner to scanner. Find out what is best for your scanner. Some can scan better but more accurately at lower speeds, and some can be switched between high-speed and high-accuracy modes.
Camera type scanners such as structured light or photogrammetry have their own best practices and operating concerns. For the typical placement of photo-reflective targets it is important to familiarize yourself with the strategies and limits for placement, as well as any issues involving whether adhesive backed stickers are suitable for the surface of the workpiece.
Once you begin scanning, it helps to scan in one direction and minimize overlap as much as possible while ensuring that you get good data over the entire part. As you scan, you look for voids and work to improve your scanning speed, standoff, and angle to minimize voids on your first pass. Multiple passes and overlaps are common, but you may see better results if you can get coverage in one pass with small overlaps.
Dealing with blind spots hidden from the scanner
Voids will occur in deep features such as holes, slots, troughs, etc. For laser line scanners used on CMM arms, it helps to understand how the scanner works. A laser is emitted from one side of the scanner at an angle to the receiving camera on the other side of the scanner. The angle makes a “V” that can experience being cut off inside of holes or other features. This can be overcome to some degree by rotating the scanner 90 degrees and attempting to scan from there, and holes often require rotating the scanner 360 degrees around an angled approach to the hole to get data inside the hole. Of course, switching to the arm’s contact probe may work better to obtain data within the hole. However, one must understand that probed data needs to be probe-compensated where the scanned data does not. The technician must keep the data separated and/or make sure probe compensation is applied in feature-fitting or other means as they go. Learn more in our comprehensive Performance Racing Engine Head Porting & Reverse Engineering video series.
Sketching or Meshing? How about both?
If the parts being RE’d comprise mostly geometric features like flats, holes, slots, or turned lathe shapes that are cylinders, cones, or of constant radii, then the sketching-extruding approach may be the best choice of techniques. If the shape has more free-flowing, organic contours, like airfoils or car body panels, meshing and mesh modeling will probably work best. Recognizing these concepts, it’s likely that most parts, especially when employing dense scanned data, will require a hybrid approach that combines mesh tools and CAD sketching, surfacing, extruding, etc.