I have worked at many spring plants and they all have had a culture and ways of doing things. Trends, managers, owners and factory software may vary, but one rock-solid practice was present at each factory–shop floor specifications.
Many times called a “spec card” or “spec sheet,” this document was the road map for any given product. While the shop routing or shop traveler describes the steps needed to produce the spring, the specifications describe how the finished product is to look and act.
I have had many debates over the years about the need for this document, but as an engineer and former coiler, I can and would testify that the specifications and their role to the quality of any manufactured part are critical. When I was a spring maker, I was handed a package of instructions that asked me to do something, and to make sure the result looked like the customer’s expectations. I never gave a thought to who prepared the paperwork or what it took to make those judgments. As an engineer, I do that for a living and have some firm beliefs in the necessity for this piece of paper.
When a blueprint is received, its purpose is to describe the product required, including dimensions and any load expectations. It also may include most processing needs such as shot peening, grinding, pressing and special coatings or identification. This data can run the gamut from a handwritten sheet with no drawing, to a meticulously drawn instruction complete with quality information and processing specifics. However, some blueprints have been around for decades and may include blatant errors that were, for whatever reason, never corrected.
Also, wire types have changed considerably over the years with improved materials being available. There are also sizes that are not readily available and, depending on the application, may dictate a re-design of the part with current stock sizes to keep the price reasonable.
This means that any errors that require correction must be approached in a way that will deliver the critical needs for part function and, at the same time, be sure the shop floor can produce the part, order after order. One of the functions of a design engineer is to screen the blueprint and describe what the customer expects and what exceptions to the blueprint are allowed. This makes the engineer the needed filter for giving the operator nothing but the pertinent data needed to make the part. This means both removing any redundant information as well as correcting errors that don’t make sense.
For example, a blueprint may call out 44 coils for a given extension spring.
When the engineer calculates the design, the result may show that with all other dimensions and spring rate entered, there must be 41 coils, not 44. The engineer must then make a judgment if the 41 coils will be acceptable to the customer, or if another dimension can be adjusted to make everything fall into place.
The whole purpose of the design calculation is to verify that what shows on the blueprint can, indeed, be made. If anything does not match the blueprint, an exception should be noted to the customer before the order is placed to warn them if a dimension needs to be altered (wire size, wire type, body diameter, free length, etc.) to meet the most critical need for the function of the part. This also includes tolerances, which can sometimes be a highly critical need for part function, especially in “fits in” or “fits over” situations.
After the engineer has received approval for any changes, those changes are then used to create the final document that gives the shop floor the filtered, improved and negotiated dimensions and loads that are standardized and shows any differences from the original blueprint. In most factories, both the blueprint and spec sheet are given to the shop floor operators as their instructions on what to make, and how to make it. These two documents are two halves of a whole — the blueprint which shows what the customer expects, and the specifications generated by engineering in a standardized format that is familiar to the shop floor and has the filtered details.
At this point I must mention that any changes to an engineered product, such as a spring, needs to be approved by the customer. Although customers may not know the science behind the spring, they do understand the science of their product and expect clear and honest communication on any issue that affects the function of that product. Clear communication can only improve the process as it relates to the shop floor and that helps eliminate errors and rejections.
By: Randy DeFord, Engineering Manager Mid-West Spring & Stamping