For fastener manufacturers, the secret to good threading is about investing in the right equipment and learning how to use it correctly.

One of the most annoying issues a fastener manufacturer can face is the correct threading of its parts. Put simply, the quality of a part’s threading can mean the difference between a useful product and a piece of trash. The question, then, is how does a fastener company begin to tackle this frustrating, yet essential, aspect of their trade?

To begin with, it’s necessary for the manufacturer to know the exact thread specification required for the part being produced. Tolerances can vary in unexpected ways. For instance, a part with a metric thread made for use in the U.S. is different from a part with a metric thread made for use in Europe. International differences are just one reason a part’s threading can be in error. The manufacturer must follow the standards such as those put forth by the American National Standards Institute (ANSI.) Without a strict set of standards, the quality of the part is jeopardized. As Hilliard Cox, president of Frank J. Cox Sales Ltd., Brampton, Ontario, CA, puts it, “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.”

The easiest way to avoid this dilemma is by purchasing a specifications manual, most of which are written in plain language and are easy to understand. Amazingly, many companies simply follow the customer’s specifications without an additional guide. The problem is that customer-provided drawings oftentimes make no real reference to the existing standards, leaving much of the threading to guesswork. Says Cox, “Everyone just assumes it is a standard everyday Class 2 thread. As a result, a lot of problems start at that very point.”

Once the proper guide has been obtained, comparisons should be made against the specifications provided in the customer’s plans. Any points of difference should then be discussed with the customer before going any further. Taking the time to go over the plans thoroughly to begin with can save a great deal of scrap and rerun time, not to mention aggravation.

Another potential pitfall in the threading process arises from the company’s lack of familiarity with the dimensional metrology. Differences in metrology can cause problems in screw threads. For example, when differing measurement devices give contrary readings on a like part. Cox explains that “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 metrology involved that’s causing the problem.” Reportedly, as much as a third of a product’s tolerances can be devoured by the differences in the way a product is measured. If tolerances are already close, that little bit lost in measurement differences could be what makes the part a candidate for the scrap pile.

This brings us to the next culprit in threading problems, the lack of the right equipment. In order to determine if the thread is correct, and to refine and reduce variations in the product, companies need basic gages such as optical comparators. Still, many firms forego investing in these essentials. Joe Greenslade, president of Green-slade and Co., Inc., Loves Park, IL, says, “Because gages don’t produce product, there’s a big reluctance to invest in modern and accurate gaging.” The key to understanding the importance of both equipment capital and maintenance, Greenslade says, is to view them as investments instead of expenditures. “You can only use so much duct tape and wire to hold the equipment together and still expect good output. Keeping tools in the correct condition avoids tool wear and then threadform breakdown.” Not only is it important to keep the tools in good condition, it’s also important to have the right tools. The fact is that there are a variety of gages and measures, each with their own advantages and disadvantages. The manufacturer must determine which tool – or combination of tools – is needed for the job. 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.

Many within the fastening industry credit Statistical Process Control (SPC) with decreasing the non-conforming materials that are shipped. This is because SPC has forced the hands of fastener companies to invest in better measuring devices. Not necessarily the best gages, but ones that are better than the rudimentary instruments used in the days before SPC. Even gage makers were forced to adapt to steeper standards, as Greenslade explains. “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 all the implications of design decisions on how to improve that performance.” To Greenslade, SPC offers the industry a reason to gain greater control over the gaging process and improve their product. Still, he concedes that more than 9 out of 10 manufacturers’ purchases are made because the end-customer mandated them and not from the manufacturers’ desire to improve their standards.

Ultimately, ensuring good threading comes down to a matter of education. Whether reading a gage correctly, knowing which gage works best for a specific job or taking the time to check job quality, if these practices are not taught and learned, bad threading – and the useless parts it creates – will continue to be a reality for many fastener manufacturers.

Source: Tighten the Reign on Threading
Gillian Babicz
Quality Online, Feb. 2001
http://www.qualitymag.com/articles/2001/feb01/0201f3.asp