Quality Scan: Great Tools: The Best Defense Against Downtime
By Scott Walker
Mitsui Seiki USA Inc.
Many OEMs in today’s time-sensitive manufacturing environment demand that their top tier suppliers have the equipment producing their parts available 98% of the time. The aerospace industry, for example, has an aggressive production schedule currently and is relying on its supply chain to provide parts as needed for assembly lines—no sooner and absolutely no later—akin to the automotive industry. As such, the machines servicing those customers must be ready and able to produce on demand.
Many high-level shops have schedules for planned downtime to, for instance, change coolant, add oils, perform qualification work, conduct laser positioning, and in the case of the aerospace industry—make sure the asset is operating as designed to FAA requirements. These shops also have the flexibility to move up or push back the maintenance functions to fulfill high-priority orders.
Then there’s the unplanned downtime. It happens, and those are the quality-related events that can test a customer relationship if delivery is blown. How does a manufacturing engineer grapple with these unplanned events? Plan for them. Yes, even though they can’t be put on a spreadsheet, since you don’t know the day or the hour they will happen, you do know they will happen. Generally to make these events tolerable, you need to ensure that you have common spare parts on hand such as proximity switches, solenoids, pumps, covers, and such. Also, key components that could put a production asset out of commission for an extended period of time should be readily available—spindles, ballscrews, and gears. Have people, preferably on site, who know how to switch out these parts and fix the machines. Maintain superb documentation. And, have a highly responsive machine tool supplier with experienced technicians on call.
While all of those aspects are essential, the ultimate insurance to keep unplanned machine tool episodes to a minimum is to have well-made equipment for your critical, expensive components and demanding customers in the first place. Exceptional machine tool quality is the result of using the best construction materials, components, and highly skilled assembly techniques. For example, incorporating a toolchanger that can change 2 million times before it wears out versus 100,000 times. If the requirement is to have availability at a minimum of 98% of the time, machines need to perform at that level for 75,000 hours. That’s what you are buying with high-end equipment. Furthermore, for parts costing $40,000, such as a titanium component for a jet engine versus a $10 bracket, you will need the better built assets to get improved part accuracy.
Manufacturing engineers face many challenges. But one that can be the most troublesome is chasing accuracy. The culprit could be deficiencies within the machine tool’s volumetric accuracy that go undetected until a part or parts are placed in the farthest reaches of the machine’s work envelope. You want to make sure that the machine’s dynamic volumetric capability is 80% tighter than the part tolerance. For instance, if you have to produce a part to 0.002" (0.05 mm), the machine tool must be capable of true positioning to 0.0004" (0.010 mm).
Volumetric accuracy has to be built into a machine before it’s purchased. To reduce pitch, yaw and roll that affect the machine’s volumetric accuracy, the machine has to be hand-scraped along its ways. Also, when heavily massed machine tool components are placed together during assembly—for example, when a four-ton column is placed on the machine’s bed—the column compresses and bends the bed. For the column to track straight, a slight bow is produced in the way system to compensate for this distortion. Then, when the column is added to the machine tool, the column’s weight compresses and allows the axis to run flat or straight. The easiest way that a buyer can make sure that a machine tool’s volumetric accuracy is adequate is to cut sample parts where they are located in the work envelope and to check them to verify that tolerances are met. Only machine tools designed and built with the precision to achieve high-volumetric accuracy can hold tight tolerances regardless of where the cutting tool goes within the envelope.
That’s how you plan for unplanned machine downtime. Consistent, 98% machine tool availability is possible and that’s what is required in the most active industries right now. ME
Editor’s Note: This is the last month in which the Quality Scan column will appear. This column will be replaced by a new “Advanced Manufacturing Now” column. The first installment of the new column appears in this issue on page 12.
This article was first published in the December 2014 edition of Manufacturing Engineering magazine. Click here for PDF.
Published Date : 12/1/2014