Quality Scan: Inspect Your Parts Before You Make Them
By Doug Nemeth
Sales Manager North America
CNC Software Inc.
Internet forums, blogs, and message boards are full of high-minded theoretical discussions about how misleading it can be to measure parts on the same equipment that they were made on. It’s the metrology equivalent of letting the fox guard the hen house. If your machine is out of calibration or has some sort of offset flaw, then the resulting deviations will project themselves into your part, or at least the parts that were cut outside of your machine’s sweet spot, and you may not catch the problem before your customer does.
Practically speaking, however, if your machine has been meticulously calibrated, then doing in-process checks as you manufacture complex geometries, particularly on multi-axis equipment, is an idea that could pay for itself the first time this system flags a problem. If your machine has probes on board, then all you need to do to reap the benefits of in-process gaging is to allocate someone’s time to learning how to use it, which may be less than you might think. To make this even easier, some CAM software is now integrating probing routines into the programming environment.
However, before you start using CAM to initiate the inspection of parts while you make them, you might want to consider something else even further up the line—inspecting your parts before you make them. Many manufacturers are routinely taking models of parts in their CAM system after CNC programming and inspecting them. This goes beyond simulating tool motion and detecting interferences by backplotting or verifying material removal using a color-coded solid model simulation. Nearly all CNC programmers who have these capabilities use them routinely as a check on their own work. What I am talking about is actually inspecting the virtual part in CAM as if it were the finished part itself.
Here are a few of many possible examples:
Visually Inspecting Surfaces—A manufacturer of precision injection molds uses CAM simulation to pre-inspect the surface finish on the EDM electrodes that will be used to cut critical components. He says it’s like taking something the size of a pea and blowing it up to the size of a basketball. With such high magnifications, most toolpath-induced surface blemishes become obvious. This approach has allowed this tool designer to consistently produce high surface finishes important to its medical device manufacturing customer base.
Minimizing Burrs—Burrs are a major concern for a manufacturer of motorized telescope mounts because burrs can affect product performance as well as jeopardize the users’ confidence in the product. Manual deburring is labor intensive and expensive. This manufacturer chamfers the edges on all of its part features to eradicate burrs. When the CNC program is finished, the programmer rapidly flies over and zooms in on all the edges, inspecting them to insure that chamfers are an appropriate size and depth to eliminate nearly all of potential burrs.
He also wants to know if chamfers go in close enough to nearby walls and other features to shave off burrs that might be missed because CAM program defaults don’t allow the tool to run in too closely. When this situation is detected, the CAM software’s “Extend Toolpath” feature allows the programmer to manually extend the toolpath to get in closer without allowing the tool to nick the adjacent feature.
Direct Comparison with the CAD Model—Why wait for a part to arrive at the QC Lab before comparing its data to the CAD model? Some CAD systems allow for the generation of an STL file that can be automatically compared to the CAD model to flag problems too minute to be clearly visible in the CAM software’s verification simulation routine. A job shop we know is using this approach to eliminate the need to measure non-critical features in the QC lab so that parts can go into production sooner.
The point of all this is that significant quality interventions can and should be made during the creation of CAM programs. CAM system users must work with their design and manufacturing counterparts to identify quality issues that can be efficiently monitored at the CAM programming stage.
This is not to suggest that final inspections can be eliminated. Rather that the sooner quality issues are detected and fixed, the less they will cost. So, why not inspect your parts before you make them? ME
This article was first published in the February 2013 edition of Manufacturing Engineering magazine. Click here for PDF.