Quality Control Automation
Project brief: reduce destructive functional testing and associated scrap rate and costs by implementing a semi-automated, network-connected quality control system for in-line dimensional inspection of injection molded plastic components at a contract manufacturer in China.
Early into my tenure at Boa Technology, I taught myself to use SolidWorks to produce 3D printed part fixtures, and to program the in-house optical CMM. Used together, the fixtures and programs cut part inspection times in half. Soon I was asked to expand this system to collect process capability statistics, and soon after to overhaul the quality control (QC) system at the contract manufacturer in China.
Converting the formerly test-heavy QC system to inspection-based pushed quality checks upstream, offering the following benefits:
- Reduces the need to rely on root cause analysis to retroactively discover part non-conformance.
- Reduces scrap by catching individual part non-conformance before assembly into finished products.
- Reduces production costs and man hours by enabling the reduction of test frequencies, or the elimination of tests entirely.
Design requirements
The design of the inspection fixtures was guided by a few notable constraints:
- Parts must be held securely enough for the CMM to locate key features to define part coordinate systems.
- The parts must sit evenly on the fixture for the CMM to establish datums and/or reference planes.
- Engineering drawings allow some part-to-part variation.
- Fixtures will be used by unskilled workers in a factory environment.
- The CMM must have a clear line of sight to all part features being measured.
Hence, the fixtures must constrain the part without over-constraining it, must account for part-to-part variation stemming from allowable flash and dimensional tolerances, must be intuitive to use and machinable from tough plastics, must be transparent (for backlight operations) and must not contact the part surfaces being measured.
Shown below are examples of the resulting fixtures currently in place at the contract manufacturer.
This fixture measures a part produced on an 8-cavity mold. Two parts per cavity are used, each held in an orientation that provides visibility into a different set of dimensions. The small triangles on the fixture match corresponding triangles on the parts, providing a visual cue for how the parts are to be loaded. The fixture incorporates other features to ensure parts can only be loaded in a single orientation.
Fixture in use
This image shows the above fixture mounted in the CMM. The green tint comes from the backlight that proves indispensable for many inspection programs.
This fixture holds an organically curved part in such a way that the top rim of the part is co-planar to the stage of the CMM. The thru-cuts in the fixture exist so that a ball end mill can machine curvatures into overhanging bosses on the fixture.
The drawing makes use of GD&T callouts for clarity and control of relevant dimensions, ensuring the fixture can be properly duplicated by any machine shop. Click image to enlarge.
This fixture also holds an organically contoured part. The top surface matches the curvature of the part, and requires being machined with a ball end mill. As such, the protruding bosses visible to the left must all be filleted where they meet the top surface. However, these fillets interfere with part geometry, so the pockets adjacent to the bosses were modeled so a square end mill can retroactively remove the fillets.
Internet of Things
Once fixtures and inspection programs had been developed, I worked to network-connect the system so that inspection data would be automatically uploaded, in real-time, to a cloud database accessible by both the contract manufacturer and Boa Technology headquarters. Software packages were used to automatically generate control charts and provide notification if the molding process was drifting out of control.
Control chart for a part produced by a new mold. Operators are required to select from a list of assignable causes when any dimension measures out of tolerance. This provides valuable information for the contract manufacturer.
Results
After implementation, a joint study involving the contract manufacturer determined that the updated QC system cut production costs for a single product suite by roughly $80,000 annually. Boa Technology currently produces over 10 different product suites.
In 2017, I gave a presentation at the WESTEC technical conference in Los Angeles, CA, detailing the design process and the quality and financial outcomes. For more information on this project, please click below to download the presentation slides.
Shortcomings of the CMM
The optical CMM does not have the ability to measure dimensions that reference undercut geometry unless the part is cross-sectioned. The process involved in preparing cross-sections is too labor-intensive and time-consuming to be viable on the production line, however. For these dimensions, I instead designed precision GO/NG gauges that are simply inserted into a part then rotated to check for dimensional conformance.
The central boss in each of these gauges locates the gauge in the center of the part. The overhanging lobes are what gauge the part. The three topmost thin raised features reference the part datum. Each gauge has a pair to complete the GO/NG gauge set.