Tighten the Reign on Threading

May 19, 2003
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The fastener industry is a cost-conscious one. Covering all the bases, from having an original threading standard to investing in equipment, can make the difference between go and no go. The trouble with a screw thread is that it's not a simple element. But where does one begin the process to make the best fastener possible?

Hilliard Cox, president of Frank J. Cox Sales Ltd. (Brampton, Ontario, Canada), said it is absolutely critical for the manufacturer to know what thread is being produced. The tolerances on the gaging involved on making a metric thread and exporting it to Europe can be different than the gaging on the same thread being used in the United States. Once it has been determined what thread is being made, the next step is to obtain a copy of the standards, such as those put out by the American National Standards Institute (ANSI), and follow them.

Cox said the difficulty in interpreting specifications, to make the fasteners correctly, is an international problem. "Many books have pages of thread data -- some are good and some are bad. If a manufacturer doesn't have the original document, an ANSI standard for example, they run the risk of having problems pop up along the way, and these problems never show themselves until the machines spit out a couple hundred thousand parts," said Cox.

One way to avoid this aggravation is to buy the specification sheet. Cox has seen companies running a $200,000 job that won't spend $20 to get the standard, and it cost them $50,000 or $100,000 worth of bad parts. Cox said, "Avoid using someone's interpretation and get the document. They're written in plain English and are very easy to understand."

Customer-provided drawings with thread specifications that make no real reference to the standards leave everyone second guessing all the way down the line, including the people supplying the gages and cutting tools for the job, said Cox. "Everyone just assumes it is a standard everyday Class 2 thread. As a result, a lot of problems start at that very point."

With the specification in hand, compare it against the customer's component drawing to verify the customer has interpreted the specification correctly. If there is a discrepancy, consult with the customer. Resolving issues upfront will reduce scrap and rerun time. Even though drawing issues are resolved, don't overlook tool setup. "Verify the machine tool setup because often the threading tool itself is not verified relative to form," said Trent Duncan, president and CEO of Lone Star Gage and Calibration Inc. (Houston).

The lack of understanding of the dimensional metrology behind the screw thread can cause difficulty as well. For example, the most measured element on a screw thread is the pitch diameter. When a company makes bolts they spot check them with a pitch micrometer. "The manufacturer finishes the job and ships it to the customer where it is probably rejected because he's using thread ring gages or a thread comparator. It's the difference in the metrology involved that's causing the problem. The errors can be quite dramatic," said Cox. He suggests that as much as one-third of the product tolerance can be eaten up by differences in the way a product is measured or gaged. If running close to a borderline, that could be the difference between acceptability and scrap.

Use the correct equipment
"It surprises me the number of companies producing fasteners that have no means of determining if the thread forms are correct, because they don't have basic equipment such as an optical comparator. They just assume the tool they are using is correct and will automatically produce the right thread," said Cox.

Joe Greenslade, president of Green-slade and Co. Inc. (Loves Park, IL) agrees with Cox's observation and says owners and managers need to make investments to produce the best product possible. "Because gages don't produce product, there's a big reluctance to invest in modern and accurate gaging," he said. "The gages tend to help refine and reduce variation of process by giving their operators better analytical tools."

Greenslade also stresses that both capital equipment and maintenance for that equipment should be viewed as an investment and not an expenditure. Management cannot demand better output from operators if they're not given the right tools and if those tools have not been adequately maintained. "You can only use so much duct tape and wire to hold the equipment together and still expect good output," said Greenslade. Keeping tools in the correct condition avoids tool wear and then thread-form breakdown.

Both fixed and limit gages have their advantages. "There are two schools of thought," said Rolf Knittel, president of Leitech-US (Madison, WI). "Attribute gages check all elements of the thread. And on certain applications, it is the right thing to do where you have a critical, highly stressed application. However, for the screw that holds the shelf up in the refrigerator and other normal commercial practices, the normal fixed limit gaging is more than adequate."

Duncan said, "Unfortunately with most gaging, it doesn't check all of the thread form, so if there's not some means by which to verify that the tool is still correct, an incomplete thread form could be the result."

When it comes to sufficient inspection, Duncan said many people don't cover all of the bases because by using a plug or ring gage or an element gage of some type, only one element is being checked. "If you only look at what you want to look at, it leaves all of the other characteristics incorrect," said Duncan. "An ISO requirement is to verify design intent. Make sure that whatever the process, a sufficient number of processes are being used to verify what is being produced, from the form to the diameter to the lead. All of these things need to be verified, not 100%, but such that you're in control of your process to verify design intent."

When it comes to deciding what type of gage to use, whether one argues for or against a specific method or a combination of both, gaging keeps the process within specification limits.

SPC and threading
Greenslade said that statistical process control (SPC) has been a big push in the fastener industry for 5 to 10 years. SPC has made a significant contribution toward decreasing the nonconforming materials that have been shipped. "In the past, the fastener industry has used rudimentary measuring devices-calipers, micrometers, fixed limit ring gages-and that was state-of-the-art just a few years ago," he said. "With SPC, you need specific numbers to do an analysis. So, it has pushed the gaging industry in general to understand repeatability and reproducibility and the implications of design decisions on how to improve that performance." He encourages all manufacturers of fasteners to learn about the practical aspects of SPC and not be intimidated by statistics and formulas. "The bottom line is, if you control every input in your process and control the process, your output is better," he said.

Another way to reduce variation is to limit the number of sources used and have assurances from those suppliers on the consistency and quality of the raw materials and tools they sell. Create an inspection process or seek documentation from the supplier. "The manufacturers themselves can have the best equipment and do all of the procedural things that they need to do, but if the tooling and raw materials are not closely controlled, then all of the manufacturer's efforts can be for naught," said Greenslade. He estimates that more than 90% of manufacturers' purchases are made because of mandates from the end customer and not from wanting to do a better job. "It helps many people at the operational level, but the extent to which modern gaging has been used during the past 10 to 15 years is in response to mandates and not self improvement," said Greenslade.

In the end, correct threading processes come down to education, whether it's learning how to use and read a gage, choosing the correct gage for the job or checking jobs thoroughly before completion. Cox said, "You can't expect people to blow the whistle before the job is done if they don't know something is wrong."

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