Surface Improvement Technology
Surface improvement technology, as it is applied to metal removal, has grown substantially in the past few years. Tighter tolerances, new workpiece materials, and new and improved abrasives are the main reasons for this growth. This type of surface finishing (metal removal) has several names describing very different processes. However, they all have one common function; all are low-velocity abrading processes using lower cutting speeds (compared to grinding) and lower pressure to minimize heat, resulting in excellent surface, size, and geometry control.
- Polishin is the use of abrasives to create a smoother surface with little, if any, geometry improvements (seal areas, low loading-bearing areas).
- Lapping is the process of folding the workpiece material. It incorporates a loose abrasive for very fine surface finish and small amounts of stock removal. (Generally used on flat applications.)
- Honing is generally associated with inside diameter surface finishing. It uses abrasives to improve surface finish and geometry characteristics such as roundness, taper, and sizing. (High wear areas and applications needing close tolerances.)
- Superfinishing generally incorporates vitrified abrasive products such as diamond and CBN. It applies a light pressure with a high reciprocating action to achieve improved surface finish and geometry characteristics (outside diameter applications with high load areas and tight tolerances).
- Micro-finishing has grown significantly in the past few years. Generally associated with abrasive tape as the media, it has replaced most polishing applications where high loads and tight tolerances are required.
Other surface finishing methods such as brushing and buffing improve the workpiece appearance and sometimes are incorporated with other processes to enhance results. These processes have deviations within their category which allow varying process results.
CHOOSING THE PROPER PROCESS
In most cases, the process can be selected by an experienced person. However, it is becoming more common in areas of reliability, efficiency, and performance that a certain amount of development is incorporated into the discussion. To properly define process and machine type, first establish the workpiece condition coming to the process and the desired process results.
In an example, a decision had to be made regarding the use of superfishing or micro-finishing processes. The workpiece was designed with a load-bearing outside diameter, close surface finish requirements, and additional geometry requirements such as roundness and flatness. The incoming workpiece's outside diameter (O.D.) varied in surface finish and geometry due to improperly maintained grinders.
Reviewing the factors affecting the selection, superfinishing was eliminated due to the incoming part condition (an oil hole or groove which could damage stone tooling and cause irregularities in the surface finish), unmanned machine, and surface finish requirements. Abrasive tape microfinishing, using a hard back-up shoe, was chosen.
The incoming workpiece condition varied in surface roughness. Abrasive stone superfinishing requires a rough incoming surface to dress the stone; without this condition the stone loads up and becomes dull. Abrasive tape, on the other hand, indexes new abrasive each cycle which allows a more forgiving incoming surface requirement. If damaged by oil holes or other part characteristics, the abrasive again is indexed and new abrasive is in place for the next incoming part. Workpiece geometry such as concave or convex surfaces can wear a stone's contact surface improperly; abrasive tape, with a hard backup and new abrasive media indexed in place each cycle, eliminates this problem.
This machine will be placed in an unmanned transfer line. Abrasive tape can be monitored for breakage and non-indexing, while stone cannot. The superfinishing stone process could have been chosen in different circumstances: manned machine, simple design, high production rate, lower tool cost, and less tool wear.
CHOOSING THE PROPER METHOD
The following outline will serve as a checklist for various aspects of the micro-finishing /superfinishing selection process.
Incoming workpiece condition
Outgoing workpiece conditions
Production rate per hour
Estimated lifetime production
Surface finish process
Workpiece handling method
Workpiece orientation during finish operation
Gauging and measurement
Chose proper surface finish measurements that best apply to this application (RMS, RA, RZ, etc.).
Various abrasive materials