To assess part conformance and process control, manufacturers typically use either in-process gaging or final inspection.
IN-PROCESS GAGING VS. FINAL INSPECTION
In-process gaging is a technique that measures the part while it is still in the process of being produced. The data collected during production is then used to adjust the process or part conformance.
Final inspection is a technique used to measure a finished part after production has been completed. This is typically done in an inspection station or a quality control lab where the part can be measured by multiple pieces of equipment such as calipers and coordinate measuring machines. While the accuracy, reliability and speed of final inspection have improved throughout the years, this techniques still holds a few drawbacks.
PROBLEMS ASSOCIATED WITH FINAL INSPECTION
A major problem associated with final inspection is that the determination of part conformance between the part and the lab is unknown. In some cases, manufacturers are continuing to add cost to a part not knowing if the already performed operations were within the proper specification. Sometimes, machine operators also measure the part within the overall process before adding the additional manufacturing cost.
Furthering compounding the problem is multiple machining operations. For example, if a defect appears within the first operation, problems will be caused further down the line. Adding defected parts to machining costs is considered a waste. The best way to avoid wasting cost is to remove the defected part from the process before any manufacturing costs are invested.
EVOLUTION OF TECHNIQUES
Due to the problems associated with past techniques, gaging, final inspection and process control has been evolving. New techniques are being created to gain benefits and minimize problems that have appeared in the past.
In this guide we will go over the techniques that once were, how they currently are now and where we believe they may be in the future.
LOOKING AT THE PAST
Before repeatable machining methods were introduced, machining repeatability relied solely on the skills of the machinist. Machinists that had more experience, skill and patience were able to relate to the consistency of parts being produced. During this time, machinist would check parts multiple times and follow the process of “cut, measure, adjust and repeat.”
As time went one, repeatable machining methods started to appear such as CNC machine tools. These types of methods made it so that the machinist’s skill was no longer the sole factor of part quality. Repeatable machining methods allowed machinists to set up the machine, run a few parts to make sure everything was within the proper control limit and then release the remainder of the process to an operator. An operator would then load the rough parts and unloaded finished parts.
While these new methods were a step in the right direction, many existing variables, such as casting variation, ambient temperature changes and tool wear, were still a problem. Because of this, operators would still have to check parts within a set interval to assure parts were in conformance. This type of in-process gaging is was led to the adjustments made to ensure the quality of a part today.