Quality Scan: Improving Surface Measurement Productivity
By Brad Ludwick
Hommel Etamic America
Rochester Hills, MI
Nearly 70% of the investment in form and surface-measurement equipment in 2011 will be used for checking roughness, waviness, flatness, and contour. While not the largest part of total investment in test, measurement, and inspection equipment, this can hold the key to determining the acceptability of precision parts that must function correctly in complex assemblies.
Tolerances have become tighter, and product life cycles are expected to be longer than ever. Time spent selecting the right instruments and determining the proper evaluation procedure required for a robust manufacturing process is therefore worthwhile.
First, determine the parameters to be measured based on product design, as indicated by the part print. The types of parameters to be evaluated play an important role in determining what surface measurement equipment to use.
As budget constraints tighten, cost/benefit analysis of the measurement process is important. Equipment cost and the time necessary to measure a feature must be weighed against the value of the result. That value can be determined by factors such as the effect of surface finish on part function, the ability to make machine adjustments if a bad part is found, and the likelihood of producing a bad part.
Knowing the type of process that produces the surface, whether milling or grinding, and the type of tool used in the process can help determine the type of equipment needed and the evaluations that should be performed. From this information, you can better judge the parameters that should be measured.
Careful evaluation is also needed to select the proper measurement equipment. Are full analytical tools required, or will a simple pass/fail value be sufficient? In many cases, simple roughness-only checks based on part-print tolerances can be done on the shop floor. More complex, full-blown analysis will require a more capable instrument, and often must be carried out in a lab environment.
Lab-oriented equipment requires more training for the operator or technician, who will be asked to provide a detailed evaluation of parts. A machine operator is probably only going to want to know if the results are in tolerance, and will be using a robust instrument that can deliver reliable measuring results. On the shop floor, results will be demanded immediately, because those results determine the capability of a process.
A more costly part with extensive measuring requirements would justify a more capable measuring instrument. It’s possible to under-invest in metrology if the equipment you purchase isn’t consistently capable of doing what must be done to provide the quality and process capability you need to deliver parts that perform as desired.
In automotive engines, for example, the cylinder bore surface is more important than ever due to emissions restrictions. Cylinder bore surface finish is critical in maintaining proper sealing and lubrication to meet today’s emissions standards. In this case, it may pay to investigate more-sophisticated technology that can provide a complete map of the cylinder’s surface at production rates. Users must evaluate life-cycle cost of the proper equipment, and compare that to the value of the information gained.
Additionally, users should consider whether the manufacturing process can be altered or improved to change the measured value. Measurement results not only indicate acceptability of a part, but can point to ways the manufacturing processes can be improved to yield higher-quality parts.
Other considerations include the type of environment in which measurement takes place. Temperature and humidity are not usually major factors in surface finish measurement, but noise and vibration are. Eliminating vibration is an important factor in any surface-finish application.
Part fixturing is also important in determining the type of equipment to use for surface-finish measurements. In some cases, the part can be taken to the instrument, where the part should be held in an appropriate fixture to provide proper orientation and steadiness. In other cases, it’s necessary to take the instrument to the part. The fixture can be built around the measuring instrument, and a robust fixture will provide orientation to the part, as well as protect the measurement unit in shop-floor environments.
It’s important to define measurement equipment requirements based on current part-print tolerances, production rates and processes, and where you will be doing the measuring. Metrology instruments from 10–15 years ago may not be up to today’s demands for speed, reliability, and accuracy. And investing in the right equipment can yield advantages in tuning your manufacturing process and customer satisfaction. ME
This article was first published in the March 2011 edition of Manufacturing Engineering magazine. Click here for PDF.