Ra, Rz, Rt Surface Roughness Measuring & Finish

Table of Contents Lapping The Lapping Process Some Figures on Lapping Hand Lapping The Single-Plate Lapping Machine Conditioning Rings and Functional Principle The Working Plate Pressure Plates Workholders Speed of the Working Plates Machine Types Centerless Cylindrical Lapping The Lapping Oil The Lapping Powder Surface Finish Quality Accuracy Aspects of Production Lapping Idle Times The Polishing Table Unit Price Calculation Measuring Laser Flatness Measuring Instrument Checking the Working Plate What Workpiece Types are Lapped? Diamond Lapping

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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, the pressure is usually light at the beginning of the process, increasing as work proceeds, and diminished towards the end. This results in the optimum material removal rate, surface finish, and flatness achieving overall surface finish quality to perfection. The waviness (also known as peaks and valleys) is a calculation of surface irregularities with a spacing greater than the surface roughness. These usually occur due to warping, vibrations, or deflection during the machining process.

Grit size and Surface Finish

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 correlation between the grit size and surface finish goes hand in hand with each other.

To emphasize, the surface roughness is defined by the minute variations in height of the surface of a given material or workpiece. The individual variances of the peaks and valleys average (Ra value), or quantified by the largest difference from peak-to-valley (Rz). Roughness is usually expressed in microns. A surface that exhibits a Ra of 8 consists of the highest peak and valleys that average no more than 8 µm over a given distance. Roughness may be also measured by comparing the surface of the workpiece to a known sample.

The international surface finish standards (DIN , , ISO /1-2. ) 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 are now commercially available (Figure 54).

Flat Surfaces & Readings

Ra is the integer mean of all absolute roughness profile deviations from the centerline within the measurement length. Rz is the absolute peak to valley average of five sequential sampling lengths within the measuring length. Ra compares all dimensions and has no distinguishing value when it comes to separating rejects from suitable cylinders.

Figure 46: Arithmetic mean roughness value Ra

Mean roughness value Ra (DIN ) 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.

Figure 47: Maximum peak to valley height Rt

Maximum peak to valley height Rt (DIN ) is the vertical distance between the highest peak and lowest peak 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.

Figure 48: Mean roughness depth Rz

Mean roughness depth Rz (DIN ) is the average value from the individual roughness depths of five individuals measuring distances in sequence. In other words, the calculation is from five Rt values. The deviation from the mean line, specifically focusing on the highest peak and valley.

Figure 49: Roughness standards table (N Standard), arranged for comparative reading according to A. W. Stahli and DIN /1

Surface roughness designation systems

50: Machine manufacturers&#; comparison tableFigure 51: Surface profile of a turned workpiece of steel. Ra 7.51, Rt 31.1&#; Figure 52: The same surface as in Figure 51 after lapping with Si-C 500 grain,
but the vertical scale increased 10 x. Ra 0.103, Rt 1.09.&#; Figure 53: The same surface lapped with diamond 2-3 my, same magnification as in
Figure 52: Ra 0.009, Rt 0.119
54: Roughness measuring unit with multiple evaluationsFigure 55: Optical polishing machine
FLM 750-P with cooling unit
56: Close-up view of FLM 750-P optical polishing machine

Measuring Equipment

Surface roughness and surface flatness are two quite different concepts and are important to remember. Many of the electronic measuring instruments in use today for determining surface finish quality has 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. To emphasize, it is essential to compare the type of probe (radius), needle pressure, measuring distance, and filtering (cut-off), see DIN Standard .

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 so, bearing ratios are measured at different levels of the surface roughness and specified in %. Visual inspection performing by means of a comparison between a polished surface and an unpolished surface. Seeing the surface texture between the two in the below images.

Figure 57: Shows a matte-lapped aluminum part at a magnification of and with the corresponding measurement diagram.

Figure 58: The same workpiece as in Figure 57, but polished on the polishing table with 4/0 fine paper, material removal approx. 2-5 microns, probe radius 5 microns, measuring pressure 1 mN.

Related

An Introduction to surface roughness measurement

The mathematical definition of Surface Roughness parameters is very strictly defined by international standards. However, getting the &#;correct&#; result depends critically on the selection of a number of parameters that can be set by the metrologist.

Definitions

In any discussion of this type, we need to start with a few definitions. The important ones here are:

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Additional reading:
Power Analyzers and Power Meters

• Roughness: A quantitative measure of the process marks produced by the creation of the surface and other factors such as the structure of the material. Not to be confused with Waviness or Form, which have longer wavelengths.

• Filter: A process to exclude wavelengths above or below a particular frequency. The measurement system is a mechanical filter. Software can perform mathematical filtering.
• Cut-off: The wavelength at which a filter becomes effective. For surface parameters we normally analyse wavelengths between an upper and lower cut-off: these are referred to as Ls or λs (shortest) and Lc or λc (longest). &#;Cut off&#; is also used synonymously with Sample Length as the sample length is always set to Lc. Bandwidth is the ratio of Ls to Lc.

So, where&#;s the problem?

fig1: profile showing a roughness wavelength of 0.25mm with an Ra of about 20μm

fig1: profile showing a roughness wavelength of 0.25mm with an Ra of about 20μm

Look at figure 1. This hypothetical profile shows a roughness wavelength of 0.25mm with an Ra of about 20μm. If it were analysed with a cutoff (Lc) of 0.08mm the calculated Ra would be virtually zero. At Lc =0.25mm Ra would be about 10μm and at Lc = 0.8mm or 2.5mm you would get the &#;correct&#; result of Ra=20μm. If Lc were set to 8mm or more, the calculated Ra would increase, as the large waviness would then be included as well.

Basics

What are the key concepts in filtering a surface roughness profile? The most important concept is to know what you are dealing with &#; this will enable you to choose the appropriate stylus tip and filters. In most circumstances a single measurement will be made on the surface in order to assess the texture.

This measurement must be representative of the surface and appropriate to the purpose of the measurement (e.g. measured normal to the lay of the surface, or in the indicated direction).

cut-off-chart

cut-off-chart

(Recommended cut-offs for different surface finishes)

Selection of Lc/ λc

The choice of λc filter will depend on the process being assessed. There are a number of guidelines available, for example ISO - (see table below). However the choice of cut-off should be made after consideration of the spacing of the profile features (peaks and valleys) caused by the machining process, denoted RSm. As a rule of thumb λc should be set to about five times this spacing.

Selection of Ls/λs/Bandwidth

ISO introduced the concept of &#;bandwidth&#; in the late &#;s. Under this regime the shorter wavelengths used in surface roughness analysis are constrained by a short wave filter (know as the λs filter &#; see ISO :).

The bandwidth is then limited in a controlled way that relates directly to surface features, rather than being limited by the bandwidth of the measuring system. The ISO recommendation is to use a bandwidth of 300:1 wherever possible.

The smallest λs available will be dependent on the data spacing available in the raw profile: for an instrument with 0.25um data spacing, the smallest λs that can be represented is 1.25um. The λs filter should be chosen so that it does not attenuate wavelengths that are likely to be introduced by the machining process. The recommended combinations of λc, λs and stylus tip radius are shown in ISO :.

Filter selection

In general the Gaussian filter is preferred as it is phase correct and has a fast roll-off without &#;ripple&#;. Taylor Hobson Ultra software also offers 2CR and 2CR-PC (phase corrected) filtering. These should only be used when you need backward compatibility with results from older instruments.

The effect of choice of filter is more subtle, causing changes of &#;only&#; a few percent in roughness parameters. However, these can be significant when establishing consistency across sites or between instruments.C

Contact us for a copy of 'A guide to Surface Texture Parameters' booklet of further technical assistance 

Reproduced with kind permission of: Taylor Hobson Ltd

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