3D Scanning: Reproducing One-Of-A-Kind Prototypes
A laser scanning system helps this shop capture the free-form surfaces on a hand-sculpted original. The resulting digitized models are the basis for CAM applications such as programming a CNC machining center.
Charlie Sikes and Mike Holdridge are in the business of speed. They specialize in making fast cars even faster. Pioneers in their field, these entreprenuers are applying this need for speed to their business as well. They are using a 3D laser scanner to rapidly reproduce handmade prototypes on a CNC machining center. Laser scanning has become a key part of their workflow, increasing their efficiency and helping them take on more jobs with faster turnaround times. Mold shops, die makers and other manufacturers can use the same techniques when 3D shapes must be replicated quickly and accurately.
The two engineers, both racing veterans, came together in 2007 to create The Race Shop, a high-performance engine machine shop in Seymour, Wisconsin. They build complete engines and high-performance engine components for other shops across the country.
So what makes an engine fast? These experts have a simple answer: Airflow. Gas and air burn inside the cylinders of an engine to provide power. The faster air can be pulled in, the more efficiently the engine will perform.
Based on daily experience, most people perceive air as light and easy to move through. There is even the saying "thin as air." However, at the high speeds and pressures of a performance engine, air behaves like a very different substance. Its characteristics resemble those of a thick, gooey gel. An engine that can pump this thickened air more efficiently will perform better and win more races.
The cylinder head controls the flow of air as it enters the cylinder, mixes with the gas, and leaves the cylinder as exhaust. As a result of design and manufacturing constraints, mass-produced cylinder heads usually have sub-optimal airflow. In a process called "porting," experts such as Mr. Sikes modify the shape of stock cylinder heads to increase airflow and performance. Porting is considered the main factor determining the power output of modern racing engines. In practice, the process calls for as much art as science.
The Race Shop starts with stock castings from suppliers such as Edelbrock, Profiler, RHS and TA Performance. These mass-produced castings act as a "canvas" for Mr. Sikes’ work. He uses a die-grinder with carbide burs to hand-cut the shape of one combustion chamber’s intake and exhaust ports. The cuts are then finished with sanding rolls. After each modification, Sikes measures the airflow with a SuperFlow 600 Flowbench, a machine that pumps air through the cylinder to simulate engine conditions. In a V8 engine, there are two cylinder heads, each featuring four combustion chambers. Rather than devote time hand-porting all four, Mr. Sikes focuses his efforts on manually "sculpting" one combustion chamber and pair of intake and exhaust ports so that it performs as near to perfection as he can achieve.
Once Mr. Sikes achieves his goal for airflow, he has a one-of-a-kind, high-performance combustion chamber and set of ports. He now needs to repeat this design multiple times to complete the modified cylinder head. Also, most customers want enough pairs of cylinder heads to outfit several race cars—often a dozen or more.
Repeating this manual cutting process for each cylinder would be very time consuming. It would also be nearly impossible to precisely replicate the optimal profiles for each cylinder head by hand. Similar challenges face machinists and mold makers who must hand-create a master, then replicate it. They often turn to CNC machining to meet this need. In this case, The Race Shop uses a Centroid A532, a simultaneous, five-axis CNC machining center designed specifically for machining cylinder heads. With this machine, the shop can replicate Mr. Sikes’ hand-cut design at other locations on the modified cylinder head, thus making enough heads to complete the customer’s order on time.
In this description, one key piece is still missing.To reproduce the part using the CNC machine, the shape of the hand-built prototype must first be captured digitally. One commonly used method to digitize 3D shapes involves a touch probe mounted in the spindle of a CNC machine.
When touch probes contact a surface, the slight impact triggers an electronic switch inside the body of the probe that sends a signal to the CNC unit. That signal causes the CNC to halt movement of the machine and record its axis positions at that instant. The numerical values of these axis positions represent the coordinates of a single 3D point.
Because the machine may take some time to stop once the CNC unit has received the probe’s signal, the recorded point may not be sufficiently accurate. This condition is typically resolved by taking another probe hit at each point so that the CNC head moves a smaller distance for the second contact. Continuing with the probing routine, the CNC head will retract to a stand-off distance and then move forward to seek out a new point on the target surface. For complex shapes like a cylinder combustion chamber, conducting these probing routines calls for some judgment and manual intervention from the operator.
When The Race Shop opened in 2007, it used this method to produce digital 3D models, which could be imported into the shop’s Mastercam CNC programming software. Although thoroughness and accuracy were sufficient, time was an issue. According to Mr. Sikes, digitizing a cylinder head with the touch probe typically took 3 to 4 hours. The process occupied the programmer’s time, too, because experienced judgment was required to ensure that enough points were captured for a usable 3D representation of the part. Another problem was that this method tied up the CNC machine, thus reducing its revenue-generating output. Fortunately, the shop’s Centroid machine has specialized features that facilitate digitizing cylinder heads. With a general-purpose CNC machine that lacks these features, touch-probe digitization of cylinder heads can take even longer—as long as 18 hours or more.
In May 2008, Mr. Sikes and his partner started searching for other technologies that could help them digitize their prototypes faster and handle more business. After some research, they began experimenting with 3D laser scanning. Ultimately, they adopted a system from NextEngine (Santa Monica, California).
Although laser scanning is not a new technology, recent developments have made it an effective tool for many machine shops. One of these developments is the use of multiple laser stripes to cross-validate scan data, which enables users to create acceptable scanned data more readily. (See the sidebar below) This ability makes 3D scanning a practical technique for machinists working under tight timeframes. Other developments in optics and laser technology have reduced the cost of these systems while improving the quality of scanned data.
Laser scanning allows The Race Shop to capture a dense sampling of 3D points more quickly than prior touch-probe techniques. This capability is especially valuable when scanning the combustion chamber, an "organically" shaped area that previously took the greatest portion of capture time.
However, touch probes still have a place at this shop. In the case of a cylinder head, the stylus of the touch probe is small enough to fit inside the intake and exhaust ports and provide measurements of the interior surface. Although
this procedure takes more time than laser scanning methods, it is the only practical way to capture such hard-to-reach surfaces.
To capture other surfaces, Mr. Sikes places the cylinder head on the automated turntable and clicks a button to start the scan. The scanner takes about 2 minutes to automatically sweep an array of laser stripes across the target. When the laser operation is finished and the scan is completed, a 3D model appears on Mr. Sikes’ computer screen.
The system includes ScanStudio HD, a software application that helps the user visualize the results of a scanning session and assembles multiple scans into 3D models. Once a scan is complete, Mr. Sikes rotates and inspects the 3D model. To keep the data file manageable, he uses the software’s trimming tools to delete the non-critical area surrounding the combustion chamber.
Creating Machinable Surfaces
Laser scanners can capture millions of points. To display the model on screen or prepare toolpaths for machining, these points must be connected together to form a surface. There are two distinct ways to represent the surface of a 3D model. One is to create a mesh of points. The other is to generate mathematically defined curves that fit the points. These curves are called NURBS (non-uniform rational B-splines).
A mesh surface connects the scanned points together in a cohesive shape using tiny polygons. Each vertex represents a discrete point from the original set of points captured by scanning. Meshes are commonly used in graphics and finite element analysis. Meshed surfaces are gaining acceptance because more and more CAM programs can use this mesh to create toolpath files for CNC machining. For use in CAM software, mesh surfaces are usually imported as STL files.
In contrast, surfaces generated with NURBS do not contain points from the original scan. These surfaces consist of large, curved patches that approximate a surface within a desired tolerance. Using surfaces defined by NURBS is an excellent way to represent free-flowing organic shapes. NURBS files are typically smaller than those in other formats.
The modeling software used by The Race Shop has the ability to automatically produce NURBS-based surfaces from scanned data. Conversion from mesh to NURBS is an automated process. The result can be saved as an IGES or STEP file. Most CAD and CAM software packages accept files in these formats.
Another option is to convert the scan into a parametric solid. Instead of representing the model by its surface, a parametric solid represents the model as one constructed from a set of basic shapes, dimensions and logical operations that describe the part. This type of representation allows the model to be easily modified in CAD software such as SolidWorks. This is one reason designers often choose parametric solids for their applications. Parametric solids are considered ideal for parts with well-defined features. Software from the laser scanner manufacturer is available to help users convert a 3D scan into a solid CAD model. Unlike conversions of data into mesh and NURBS surfaces, converting scan data into a parametric solid is not completely automated. However, it produces a CAD model that is highly accurate to the original physical sample.
Although a CAD model is ideal for some applications, The Race Shop finds mesh and NURBS surfaces more suitable. According to Mr. Sikes, the free-flowing surfaces of his handmade combustion chambers are difficult to represent with the basic geometric shapes used in CAD modeling. More important, mesh and NURBS surfacing techniques allow the scan data to be delivered to the shop’s CAM system in just a few minutes.
The current generation of CAM software enables the creation of tool paths from mesh and NURBS surfaces, as well as from parametric solids. When using a high-resolution mesh created from closely spaced points, the resulting toolpath produces workpieces that are very similar to those produced with toolpaths derived from NURBS or a solid model. However, machining from a high-resolution mesh surface adds to cycle time because the machine must move to a large number of discrete point coordinates. Historically, laser scan data has produced large STL files, but this is changing. Scanning software can vary the size of a mesh in flat and slowly changing areas to fit a user-specified tolerance. This automatic process reduces STL file size without diminishing quality and helps reduce machining time.
The Race Shop uses mesh surfaces to cut its cylinder heads. Mr. Sikes saves these mesh surfaces in the STL format and imports them into Mastercam. He copies the combustion chamber multiple times to fill in all four slots on the cylinder head, and then generates roughing and finishing toolpaths that are downloaded to the machining center. "Previously, we used the probe on the CNC machine to get the chamber data. It was a slow and tedious process. Now we can scan, trim, polish and make a surface in less than an hour," Mr. Sikes reports.
The result is a net savings of 2 to 3 hours per job. Moreover, the CNC machine spends less time being used as a digitizing device. In the first four months using a hybrid of laser scanning and touch probing, The Race Shop has completed eight different custom cylinder designs for customers.
"We’ve cut surfaces from the laser scans; they are accurate to the original and produce the engine performance we’re looking for," Mr. Sikes says.
His company is using this same technique to design and manufacture a custom engine for its shop car, a 1962 Chevy Impala with a twin turbo-charged big block Chevy engine. At the moment, it’s capable of speeds in excess of 200 mph on the drag strip. By re-tuning the cylinder heads on this engine, the shop expects to achieve even faster performance.
About the author: Peter DeLaurentis is an application engineer and product designer at NextEngine, Inc. (Santa Monica, California).
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