The Technique of Lapping
Surface Finish Quality
With a given grit size and fluid viscosity, varying the lapping pressure produces a higher or lower material removal rate, a thicker or thinner film, and a rougher or finer surface finish. In practice, therefore, pressure is usually kept light at the beginning of the process, increased as work proceeds, and diminished towards the end. This results in the optimum material removal rate, surface finish and flatness.
As an example, on a steel part hardened to 60 HRc, lapped using silicon carbide 500 grit, a pressure of 250 g/cm squared will produce a surface finish of about Ra= 0.2 my (N4) or Rz 0.6-0.8, whereas by reducing the pressure to 50 g/cm squared a surface finish of about Ra= 0.05 my (N2) or Rz 0.2-0.3 can be obtained.
The international surface finish standards (DIN 4762, 4768, ISO 4287/1-2. 4288) are applied in an analogous manner, in that the surface finish quality is specified in Ra values or in the even more accurate Rt values. In practice, the more realistic value Rz is also specified, which is determined by averaging 5 separately measured Rt values. Suitable measuring equipment for acquiring these values is now commercially available (Figure 54).
The mean roughness value Ra (DIN 4768) is the arithmetic mean from all values of the roughness profile R within the measuring distance lm. It therefore specifies the average deviation of this surface profile from the mean line.
The maximum peak-to-valley height Rt (DIN 4748) is the vertical distance between the highest and lowest points of the roughness profile R within the overall measuring distance lm. In other words, this is the height difference between the highest mountain and lowest valley within the measured range.
The mean roughness depth Rz (DIN 4768) is the average value from the individual roughness depths of five individual measuring distances le in sequence. In other words, it is calculated from five Rt values.
Surface roughness designation systems
Many of the electronic measuring instruments in use today for determining
surface finish quality are equipped with microprocessor control systems and printers (Figure 54). However, the true value of the results obtained is open to dispute, as most are only approximate, and vary according to the device concerned. It is essential to compare the type of probe (radius), needle pressure, measuring distance and filtering (cut-off), see DIN Standard 4768. It is also very important to consider the material of the workpiece, its microstructure, hardness and type of machining, as well as the direction of the measuring distance with respect to the machining traces. Even when applied with a pressure of only 1 mN, a diamond probe with a radius of 5 microns will compress the surface of a non-ferrous part to about 50% of the roughness depth.
The porosity of the microstructure must be taken into account in the case of
oxide ceramic and sintered metals. Frequently, the bearing ratio is measured at different levels of the surface roughness and specified in %. Visual inspection is performed by means of a comparison between a polished surface and unpolished surface.
Figure 57: Shows a matte lapped aluminum part at magnification of 1600 and with the corresponding measurement diagram.
Figure 58: Same workpiece as in Figure 57, but polished on polishing table
with 4/0 fine paper, material removal approx. 2-5 microns, probe radius 5 microns, measuring pressure 1 mN.