Chapter IV:

Surface Quality and Accuracy

គុណភាពនិងសុក្រ�ិតភាពផ្ទៃ���លិត�លមេ�កានិច

Prepared by Sry Vannei.

Machined Surface Texture:Designer must select a functional surface condition that will suit the operational constraints for either a ‘rough’ , or ‘smooth’ workpiece surface. How smooth is smooth?

2

Surface Texture:SURFACE TEXTURE: Surface texture is concerned with the geometric irregularities of the surface of a solid material, which is defined in terms of surface roughness, waviness, lay, and flaws.1. Surface roughness consists of the fine irregularities of the surface

texture, including feed marks generated by the machining process.2. Waviness consists of the more widely spaced components of

surface texture that may occur due to the machine or part deflection, vibration, or chatter.

3. Lay is the direction of the predominant surface pattern.4. Flaws are surface interruptions such as cracks, scratches, and

ridges.

Stylus contact type instruments are widely used to provide numerical values of surface roughness in terms of the arithmetic average (Ra) or centerline average (CLA), the root mean square (Rq), and the maximum peak-to-valley roughness (Rmax). Other methods of surface characterization include microphotography and scanning electron microscopy. 3

The arithmetic average or CLA is determined as follows:

The root mean square roughness is calculated as follows:

The maximum peak-to-valley roughness (Rt or Rmax) is the distance between two lines parallel to the mean line that contacts the extreme upper and lower points on the profile within the roughness sampling length.

Commonly used surface roughness symbols. (a)Average roughness Ra,(b)(b) root mean square

roughness (Rq), (c) (c) maximum peak-to-valley

roughness height (Rt or Rmax)

SURFACE QUALITY AND FUNCTIONAL PROPERTIES:

The quality of surface finish affects the functional properties of the machined parts as follows:

1. Wear resistance: Larger macro irregularities result in nonuniform wear of different sections of the surface where the projected areas of the surface are worn first.

2. Fatigue strength: Metal fatigue takes place in the areas of the deepest scratches and undercuts caused by the machining operation.

3. Corrosion resistance: The resistance of the machined surface to the corrosive action of liquid, gas, water, and acid depends on the machined surface finish.

4. Strength of interference: The strength of an interference fit between two mating parts depends on the height of micro irregularities left after the machining process.

Anticipated process ‘roughness’ and their respective grades. [Source: ISO 1302, 2001]

Statistical Process ControlWhen a process planner selects machines to perform a given operation on a part, he or she must know whether or not the machine is capable of satisfying the tolerances specified for that part. The process capability study is used to determine whether or not this is the case. For a given feature, a target dimension is specified along with upper and lower tolerance values. For instance, the specification:

2.5±0.003Has a target value of 2.500 in, an upper specification limit (USL) of 2.503 in, and a lower specification limit (LSL) of 2.497 in.CpK is one measure of process capability that provides an indication of both accuracy and precision:

where is usually estimated by S:

Where USL = upper limit on the tolerance LSL = lower limit on the tolerance = process mean, or average value of a set of measurements

= = standard deviation of entire population of

partsS = standard deviation of

measurements from a sampling of n parts

When CpK ≥ 1, then one can conclude that at least 99.73 percent ofthe parts produced will fall within the range specified by the LSLand USL. In plain English, this means that the process is centeredsufficiently close to the target dimension value and that the spreadof measurements is smaller than the tolerance range for that feature.If CpK < 1, then one can conclude that fewer than 99.73 percentof the parts produced will meet the design specifications. Inthis case, the manufacturing engineer can consider alternativeprocesses, or he or she can work to improve the existing process inorder to get the defects to an acceptable rate.