location, form and orientation of planar surfaces. This is the most common application of profile. In this application the profile is applied to a planar surface, and the following conditions apply:
The profile callout is applied to a true profile.
The true profile is related to the datum referenced with basic dimensions.
The tolerance zone is a uniform boundary centered around the true profile.
All elements of the profile must be within the tolerance zone simultaneously.
The tolerance limits the form, location and orientation of the surface.
Tolerance Analysis
Figure 47 Profile Used To Tolerance A Surface Location
2.29.2 Inspecting Profile of A Surface
There are many ways a surface could be inspected. One way is to use a special gage as depicted in the diagram (Refer Fig 48). First, the part is located in the datum reference frame. Then the dial indicator is used to measure the distance between the toleranced surface and the true profile. Depending on the dial indicator reading of the part surface, the part surface will be determined to be in or out of the profile tolerance zone. The number of points to be checked is determined by the inspection plan.
Tolerance Analysis
Figure 48 Inspecting Profile Of A Surface.
2.30 Profile Of A Line Control
Introduction:
The basic concept of profile of a line and surface are same. But differs in the manner that line profile is 2D while surface is 3D. It is typically used as a form control or in conjunction with a profile of a surface control. The symbol for it is shown in Fig 49
Figure 49 The Profile Of A Line Control.
Definition:
The basic concepts of a profile of a surface and a line are same. The basic difference is that the tolerance zone for the profile of a surface is three dimensional while that of a line is two dimensional. A profile of a line control is a geometric tolerance that limits the amount of error for line elements relative to the true profile. The tolerance zone is same as that for surface profile. The tolerance zone is two dimensional ; it is two uniform lines applied at ant cross section of the surface. Profile of a line provides control in one direction only. Therefore, profile of a line is often used as a part of a multiple simple segment control of a surface.
Profile Of A Line And The Coordinate Tolerance Used To Control Form Location
In this example, a profile of a line is used with a coordinate
tolerance. The coordinate tolerance locates the surface, and the profile call out refines the form. The profile of a line control specifies two datum references. Therefore the profile of a line control affects the form and orientation of the line elements. ( Fig 50) the figure shows an example of a profile of a line and a coordinate tolerance used to control the location , orientation and form.
Tolerance Analysis
In the figure the following conditions apply: Profile callout is applied to a true profile The coordinate tolerance locates the surface.
The profile of a line control refines the form and orientation of the line elements in one direction.
Figure 50 Profile Of A Line Used With A Coordinate Tolerance
2.31 Surface Finish
Tolerance specifications are imposed on dimensions to ensure functional and assembly requirements of mating parts. Tolerances determine to a large extent the manufacturing processes required to produce the part. Surface quality is another important factor that affects the performance of mating parts relative to each other as well as choice of manufacturing processes. Tolerances and surface quality are interrelated in the sense that both are direct outcomes of manufacturing processes. A manufacturing process such as lapping and honing that produce small tolerances also produce smooth surfaces. Therefore in specifying tolerances a designer should consider the requirements of surface finish in addition to functional and assembly requirements. For example an interference fit made on a rough surface may have a reduced area which results in subsequent reduction of the interference force between mating parts. Higher surface quality results in higher production costs. Thus designer would normally leave a surface as rough as is feasible.
Surface finish can be evaluated quantitatively by using various measures. The most popular measures are surface roughness and waviness. The measure of the irregularities over a sampling length is defined as surface roughness, whereas the measure of large variations over a wavelength
Tolerance Analysis
There are three methods of calculating the surface roughness R of a surface. Let us define an imaginary mean surface such that the total variations (measured by the sum of the areas between the mean surface and profile of the actual surface) above the mean surface are equal to that beneath it.
The roughness average Ra measures the average of the absolute displacement (variation) relative to the mean surface:
Ra =
∫
L dx y L 0| | 1Where IyI is the absolute value of the roughness function y(x). The roughness average Ra is also known as arithmetic average (AA). It is usually measured using a planimeter.to calculate the area below and above the mean surface.
Ra values are usually expressed in micrometers or micro inches and its value can vary quite considerably without affecting the surface functions.
Another measure of surface roughness is given by the RMS (root mean square) value Rq which is still an averaging method and is given by:
Rq2 =
∫
L dx y L 0 2 1The third method of roughness is given by the maximum peak - to - valley height Rmax. Sometimes Rmax is evaluated at various locations over the length of the surface and an average is calculated.
Y Root Mean Square roughness Rq X
Mean surface Length
Fig 51 Root Mean Square Roughness (Rq)
Roughness Measures
The table below recommends the specifications of surface roughness for functional processors. These values given are only a guide and a designer can make his own selection depending upon the process. However from the point of production economy it is better not to specify values finer than that are really necessary for satisfactory functioning of the process.
Guide to surface finish from various process µ m
ROUGHNESS HEIGHT RATING,