Quality Scan: Using CAM to Find Your Five-Axis “Sweet Spot”
By Karlo Apro
Product Deployment Manager
CNC Software Inc.
Author of Secrets of 5-Axis Machining
available from Industrial Press Inc.
In the manufacturing business we pride ourselves on accuracy and precision. These words are mantras of our industry. Yet nothing that moves is ever 100% accurate and that fact leads us to the great irony of five-axis machining. While five-axis systems, if used appropriately, can dramatically improve accuracy and productivity, the additional axes also increase the potential for the stacking of error. Nothing we make is ever perfect. That is why we have tolerances in our designs.
The same applies to our CNC machines. There must be little gaps between components in order for them to move in a linear or rotary fashion. The more components in a machine’s assembly, the more these gaps compound into less and less precise movement. That is the price we pay for five-axis machining’s many advantages. Simultaneous five-axis motion allows you to reach areas that would be either impossible or very inefficient to reach with traditional methods. The drawback is that when you move the machine in the continuous five-axis mode the rotary brakes must be off. The machine is running in the “loose” mode. Today’s five-axis machines use sophisticated technology to keep this motion as accurate as possible; but then again, nothing is ever perfect.
So how can you possibly make a difference in five-axis machine accuracy with CAD/CAM? In the three-axis world, when a programmer is given a new project the first thing he is likely to ask himself is “how will I hold this part?” In the five-axis world a better first question is “where on my machine is the ‘sweet spot’ for this particular part and the toolpaths required to make it?” The sweet spot is a location near the center of the machining envelope where the shortest possible tool can be employed to reach all of the areas that require machining with axis movements that are as close as possible to the center of their range of travel. You accomplish this by creating the toolpaths and then simulating them at various locations until you arrive at the “happy place” where tool movements are tightly choreographed (minimal air cutting), well-supported and all potential interferences have been detected and avoided. Once you are satisfied with the toolpaths, that’s the time to design the fixture.
Of course there are many other CAM programming secrets that can be used to insure more accurate five-axis machining. Here is a handful:
- Clean up surfaces on messy CAD files that result when part geometry is imported via multiple translators.
- Obtain better surface finish by using toolpath optimization algorithms that add and subtract data points to smooth out information presented to the machine’s controller.
- Use trimming toolpaths that oscillate the tool to minimize tool notching and associated inaccuracy.
- Contract the services of an experienced full five-axis programming consultant to write your initial programs and then have him stay around for awhile to get your best programmer(s) up to speed in this new dimension.
- Cut your five-axis teeth on 3+2 machining.
Most newcomers to five-axis machining imagine parts like blisks and rotors when they think of five axis. The reality is that most of five-axis machining is indexing work. Indexing five axis, also called 3+2 machining, is a common sense approach to advanced machining by keeping it simple. Hold the part in one fixture, lock the rotary axes, and attack the part from multiple angles using three-axis linear motion. This type of machining is far more efficient than just three axis because it doesn’t require multiple operations and fixturing. Proficient three-axis CAM programmers can easily master the 3+2 manufacturing process, which is akin to three-axis machining on steroids.
Here are the most important points to remember when it comes to optimizing five-axis machining quality: minimize and control all motion; consider every element of the cutting process; keep the tool as short as possible; design fixtures that allow minimum distance between the workpiece and the machine’s rotary center point; eliminate air cutting, and avoid collisions at any cost.
Also, don’t hesitate to ask for help from your CAM reseller or a contract programmer if you find yourself drifting out over your head. This will get you to your five-axis happy place sooner than later. ME
This article was first published in the March 2014 edition of Manufacturing Engineering magazine. Click here for PDF.
Published Date : 3/1/2014