This document summarizes key concepts related to surface quality and accuracy in machining. It defines different aspects of surface texture including roughness, waviness, lay, and flaws. It describes common methods for measuring and characterizing surface roughness numerically and visually. Surface roughness values like arithmetic average, root mean square, and maximum peak-to-valley roughness are defined. The document also discusses how surface finish affects functional properties such as wear resistance, fatigue strength, and corrosion resistance. Finally, it introduces process capability and how the CpK value is used to determine if a process can reliably meet specified tolerances.

1. Chapter IV:
Surface Quality and Accuracy
គុណភាព​និង សុក្រិតភាព ផ្ទៃទលិតទលមេកានិច
Prepared by Sry Vannei.

2. 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

3. 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

8. 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.

9. 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)

10. 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.

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

17. Statistical Process Control
When 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±0.003
Has 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:

18. 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 parts
S = standard deviation of
measurements from a
sampling of n parts

19. When CpK ≥ 1, then one can conclude that at least 99.73 percent of
the parts produced will fall within the range specified by the LSL
and USL. In plain English, this means that the process is centered
sufficiently close to the target dimension value and that the spread
of measurements is smaller than the tolerance range for that feature.
If CpK < 1, then one can conclude that fewer than 99.73 percent
of the parts produced will meet the design specifications. In
this case, the manufacturing engineer can consider alternative
processes, or he or she can work to improve the existing process in
order to get the defects to an acceptable rate.