February 2020 Volume 2

MATERIALS

Contact Measurement Pictures of a portable profilometer measuring surface roughness (see pictures for reference) are included with this article. A profilometer works by pulling a diamond-tipped stylus across the surface that is being measured, for a specific distance and with a specific contact force. The device plugs the stylus’s displacement data into the appropriate surface roughness formula and displays it in Ra format. Other surface roughness formulas are also available. The main advantages of using a contact profilometer are: • Acceptance – most of the world’s surface finish standards are written for contact profilometers • Surface independence – contacting the surface is often an advantage when dirt, foreign material, surface reflectance, or color can cause inaccurate results from a non-contact profilometer • Resolution –The stylus tip on some profilometers can be as small as 20 nanometers which allows for a resolution that is significantly better than white-light optical profiling. Objectivemeasurement is kingwhen it comes to determining surface roughness, however, perception of manufacturing process results can get in the way of that objective measurement. As you will see in the comparative method photos, there are visual surface roughness comparators used by many skilled personnel in the industry to get a rough estimate of surface roughness without taking a measurement. But what can be taken away from this image? Just because steel is removed using a certain process–be it Blanchard grinding, surface grinding, or even CNC milling–the surface roughness results don’t have to be limited to a predefined range. In fact, it is possible to hold Ra to a range of 8-16 even on a Blanchard grind (see the image of 16BL in the comparative method photos) and especially so on a surface grind (see 16G in the comparative method photos). It is also possible to hit Ra values for milled edges in a range of 16-32, even though those milled edges may look completely different than a surface ground face. The cutting tool marks or grinding stone marks do not necessarily mean anything with respect to surface roughness–it’s more about how we go about the process of grinding or milling.

In conclusion, here are the main points of this article: • There are internal standards for surface roughness on all ground plate. • Ra is by far the most common surface roughness measurement. • Surface roughness is important for ground plate because it sometimes correlates with size/parallelism. • Surface roughness is also important in other situations for wear resistance and avoiding crack formation. • A contact profilometer should be used to measure surface roughness. • Cutting tool and grinding stone patterns do not necessarily indicate good or bad surface roughness. ■

Steve Janiszewski, Forging Products Division Manager, Superior Die Set Corporation

FIA MAGAZINE | FEBRUARY 2020 43