Advances in CNC Technology
Aerospace shops need to know what’s changed—and how it can benefit them.
By Rick Schultz
Aero Structures Program Manager
Fanuc FA America
Today, you’ll struggle to find someone using a rotary phone, a phone with a cord, or a dedicated car phone. Reason being is that technology in all industries is rapidly advancing. As consumers, we are very quick to adopt new technology and buy the latest trendy gadget. When an innovative cell phone or tablet computer is introduced, people will ignore budget constraints and wait in lines overnight to buy one.
Just as in all other industries, CNC technology has come a long way in the last 20 years. However, adoption of this new CNC technology has been very slow, especially compared to consumer industries. There are many valid reasons to remain very conservative with CNC technology since the resulting parts manufactured by CNC machines are often critical components in tightly controlled industries—like aerospace. Most people are more comfortable knowing that the manufacturing of a particular part that passed extensive testing is not allowed to change without going through significant retesting when they are flying at 30,000 feet.
Due to the slow adoption of new CNC technology and limited qualified manufacturing resources, even new parts are typically programmed and manufactured using traditional legacy techniques. However, there are many advanced features that are simple to implement with minimal investment. The hard part as a programmer or manufacturing engineer is finding the time to investigate and be comfortable with a new paradigm. It’s possible to bridge that resource gap by using the CNC vendor’s expertise as a resource to help find ways to easily implement these technologies into your manufacturing.
If these modern advanced features are used, programs will never have to be “reposted” since the CNC will drive tool-tip motion that matches the surface defined in the CAD system instead of the poor approximation used in the legacy processes typically used today. Cycle times will be reduced significantly and part accuracy will increase. The difficulty of cross-training operators and programmers for different machines will be reduced and operational flexibility will be increased. The changes required to use these features are relatively simple and have benefits for every aspect of the manufacturing process.
Looking at the Whole Process
To update your current programming and machining processes and reap the benefits of the technology advancements of the last few decades, it’s important to view the process as a whole instead of focusing on a specific aspect of manufacturing the part. For aerospace parts, the process typically requires four or five-axis contouring and the geometry is modeled using a CAD system. Below is the definition of the four common steps for generating a part using the traditional CAD-CAM-post-CNC processes:
- Accurately model the part in a CAD system.
- From the accurate CAD model, a CAM system generates a map of the part as an approximated series of points that are within a defined tolerance around the CAD model surfaces.
- With traditional and commonly used programming, a complex post-processor takes the CAM-generated series of points and orientations and translates them into a specific machine setup calculating the physical axis positions so the planned tool tip ends up at the CAM-generated points. Common features and processes used include “inverse time—G93” and “pivot point programming.” In this process, any changes in the machine, tool, or fixturing typically require the program be “reposted” with the new information.
- In traditional processes, the CNC simply connects the points calculated by the CAM and post-processor.
For a common five-axis gantry style machine, when the CNC connects the dots the tool tip will actually scallop into the material between every programmed point due to “pivot-point programming.” The common traditional solution is to redo the CAM-Post processes at a tighter tolerance resulting in significantly more points and lengthier programs. The closer the points are together, the less the scalloping effect is noticeable in the part. If the servos are tuned too aggressively, the machine will run “rough,” due to the scalloping motion and the machine tool builder or control manufacturer will typically de-tune the servo system to make it less responsive (and therefore less accurate).
The Modern Process
Modern controls have the ability to bypass this madness and effectively translate the CAM-generated series of points back into the original smooth surface defined in the CAD system without a major change in programming. The majority of the changes required to take advantage of the power currently available are actually simplifying the systems between the CAD system and the CNC. In simplifying these steps to take advantage of the modern advanced CNC features, significant costs are reduced, the time from model to manufacture is reduced, and manufacturing flexibility is increased. All are good things.
Below is a description of the modern process utilizing the advanced CNC features:
- Accurately model the part in a CAD system.
- From the accurate CAD model, a CAM system creates a map of the part as an approximated series of points and orientations that are within a defined tolerance around the CAD model surfaces.
- The modern post-processor takes CAM-generated series of points and reformats them into G-code. The result is exactly the part definition from the CAD model and CAM approximation with feature activation prior to the series of points. Since the data output by the post-processor is part specific and not machine specific, there is no need to repost if anything changes at the machine. This also makes the post-processor generic so the same post can be used for multiple parts and multiple machines.
- The CNC features Tool Center Point control (TCP) and Tool Posture Control (TPC) calculate how to position the physical machine axes to achieve the CAM-generated point and orientation.
These features eliminate the scalloping effect both at the tool tip and at the tool side. The CNC also has features like Fanuc’s Nano Smoothing or the new High Speed Smooth TCP with Fairing to look ahead in the part program and generate a spline curve from the programmed series of points and command the motors to follow the calculated spline curve. The spline curve is effectively reverse translating the series of points generated by the CAM approximation back into the original CAD surface. The result is a significantly smoother, more accurate tool tip path. A racing analogy can be used to describe this process. Instead of a driver going around a race course only using straight lines with end points where the driver is allowed to turn the steering wheel, allow the driver to determine the optimal way to go through the course at the highest speed possible without ending up in the grass.
Using the processes and features described above, part programs are definitions of the part rather than machine-specific motion and commands. The information in the part program is the tool location and orientation relative to the material surface required to produce the part. The CNC takes care of the math to translate that information into specific machine motion to produce the part regardless of tooling, fixture location and orientation, and machine kinematics.
There are other concepts that different organizations have developed to attempt to improve the errors and difficulties caused by the traditional programming process. Some systems drastically change the programming language well known by operators and programmers throughout the industry. Some systems add additional processors between the CAM and CNC to manipulate feed rates or even change the point locations defined in the programs. These systems require significant investment in training, support, and product costs on top of the current investment for the end user.
Rather than reinvent the wheel or try to add an additional system to make up for the problems and lack of flexibility caused by traditional CAD-CAM-post-CNC processes, utilize the advanced features available in the control already driving your machine to drastically reduce costs, improve part accuracy, reduce cycle times, and increase manufacturing flexibility. ✈
This article was first published in the 2012 edition of the Aerospace and Defense Manufacturing Yearbook.