My first manufacturing job was back in 1973. It was at a dark and dingy shop that produced fittings on screw machines and turret lathes out of different materials, such as brass and copper, but I couldn’t even tell you what industry the parts were for.
The tolerances on most dimensions were about ±0.015 ". The parts were checked with GO/NO-GO gages for the threads and calipers for the other dimensions. The parts were only spot-checked during production and then shipped.
Several years later, I worked as an inspector for another company. A huge stepping stone at that facility was its use of NC mills and lathes. There, I had access to outside and inside micrometers, profilometers to check surface finishes, pi-tapes for checking diameters, an optical comparator and various other inspection equipment. The shop routinely achieved tolerances of ±0.005 " with an occasional ±0.002 ". Previously, that shop only had a final inspection program, but I implemented in-process inspections, which saved untold machining hours and significantly reduced scrap.
Eight years later, I was an applications engineer for a builder of multiple-axis turning centers that could machine to tenths and incorporated in-process inspection capabilities using touch probes. The primary customers were automotive and marine engine builders. At that time, we could also offer customers automated offline probe systems that checked dimensions and automatically updated cutter compensation on the turning center for whatever tool was wearing. There was even a broken tool detector.
Fast forward a few years and I’m employed at a job shop that purchased a large coordinate measuring machine to verify dies being machined. We thought the CMM would help improve part quality and in a way it did. But what it really showed was how close the parts were to being out of tolerance. Not just new parts, but previously made parts we thought were good. Don’t get me wrong, the parts weren’t bad—they just weren’t as good as we thought. Having the CMM forced us to make better parts.
Now I work for a shop that produces aircraft engine and power generation components. We have several large CMMs, a couple of portable CMMs and a variety of other inspection devices to support our quest for quality. Many of the machines have built-in touch probes to verify part quality. Not only do we have in-process and final inspection personnel, but also quality engineers who evaluate and document data for customers. We have to provide 100 percent inspection for many part dimensions, and some parts are so particular they require a cleanliness test.
As my experience shows, improvements in inspection equipment and techniques have evolved dramatically over the years.
However, we still have the human factor to contend with. Despite our best efforts, human error can still creep into any process. With fully automated machines, which are great for production runs, you can virtually eliminate bad parts. Once a machine is programmed and tooled correctly, it can run unattended and repeatedly make good parts.
When I was an inspector, there was a machinist, as good as he was, who occasionally made bad parts. He always had a good part on his bench ready for inspection, but when he made bad parts, he would bury them in the pile of completed parts, hoping to get them past the inspector. Fortunately, I caught them most of the time.
I don’t know if it’s a result of an individual’s poor work ethic, lack of training or what, but until that problem can be corrected, quality will never be a given. It will always be something to strive for. CTE
Related Glossary Terms
Cone-shaped pins that support a workpiece by one or two ends during machining. The centers fit into holes drilled in the workpiece ends. Centers that turn with the workpiece are called “live” centers; those that do not are called “dead” centers.
- cutter compensation
Feature that allows the operator to compensate for tool diameter, length, deflection and radius during a programmed machining cycle.
- in-process gaging ( in-process inspection)
in-process gaging ( in-process inspection)
Quality-control approach that monitors work in progress, rather than inspecting parts after the run has been completed. May be done manually on a spot-check basis but often involves automatic sensors that provide 100 percent inspection.
- numerical control ( NC)
numerical control ( NC)
Any controlled equipment that allows an operator to program its movement by entering a series of coded numbers and symbols. See CNC, computer numerical control; DNC, direct numerical control.
Minimum and maximum amount a workpiece dimension is allowed to vary from a set standard and still be acceptable.
Workpiece is held in a chuck, mounted on a face plate or secured between centers and rotated while a cutting tool, normally a single-point tool, is fed into it along its periphery or across its end or face. Takes the form of straight turning (cutting along the periphery of the workpiece); taper turning (creating a taper); step turning (turning different-size diameters on the same work); chamfering (beveling an edge or shoulder); facing (cutting on an end); turning threads (usually external but can be internal); roughing (high-volume metal removal); and finishing (final light cuts). Performed on lathes, turning centers, chucking machines, automatic screw machines and similar machines.