1. Calculate the CLA/Ra value for the wave form below.
2. What is the difference between surface texture and integrity?
ans. Surface integrity refers to all of the properties of the surface of a material, while surface texture on refers to the geometry of the surface.
3. Describe roughness, waviness and lay.
ans. Roughness is semi or completely random variation in the surface height, these are typically smaller in size. Waviness is a period or larger variation in surface height. This can be caused by warping or buckling, ripples, etc. Lay refers to a direction of a roughness pattern. For example when cutting with a lath the roughness will be different in the axial and radial directions.
4. What methods are used for measuring surface roughness?
ans. Surface roughness is normally measured with an instrument that drags a stylus across the surface (called a profilometer). The movement up and down is measured and used to calculate a roughness value.
ans. Cutoff is the length of the surface that the stylus of the profilometer is allowed to move over.
6. Two different surfaces may have the same roughness value. Why?
ans. A surface roughness value gives an indication of the rms value, but this can come in many forms. A regular looking roughness pattern may have the same roughness value as a shallower wave form with an occasional deep pit.
7. What will be the effect of a difference between the stylus path and the surface roughness?
ans. If the stylus path does not align with the lay of the roughness, then the roughness reading will be lower (or higher) than expected.
8. When is waviness a desirable and undesirable design feature?
ans. Waviness of a surface can be desirable when the surface is to have a rough appearance. If there is a moving mechanical contact between two surfaces waviness can lead to premature wearing of the parts.
9. Given the figure below indicating stylus height values for a surface roughness measurement, find the Ra and Rq value.
10. How are surface roughness and tolerance of the process related?
ans. Surface roughness is a good indication of the ability of a process to control final dimensions. Therefore if the process cannot control the surface roughness, it will be unlikely that the dimensions can also be controlled.
11. How are tolerances related to the size of a feature?
ans. The tolerance/surface roughness graph is based on an important concept in manufacturing. There is a relationship between the scale of a dimension and the scale of a tolerance. In other words, if we make two parts in the same machine, but one is twice the size of the other, then its tolerance must be twice the size. Here we can see the more precise processes are near the bottom with a ratio of tolerance to dimension of 1/10000, the highest is about 1/10. Note: polishing and lapping are used to finish the production of gage blocks.
• Roundness is of particular importance when designing components for fit and function.
• Most of the methods considered so far are suited to measuring with single points, but a round shape is a collection of points, with each point having significant influence if out of tolerance.
• Precise roundness measurement equipment is expensive
• Two fundamental methods for measuring roundness are,
- Intrinsic - uses points on the round surface to measure from
- Extrinsic - uses a separate round surface for a reference (e.g. a precision bearing)
• Three methods for Intrinsic roundness testing are shown below,
• All three of the intrinsic methods are inexpensive
• The Intrinsic methods all have an important limitation. In particular, if the deformation of the round is small, the methods will deal with it reasonably, but if the deformation is large enough to make the shape non-cylindrical, then the results will err significantly.
• When using The Flat Plane, or the Centre to intrinsically measure roundness, the diameters can be directly obtained, but when using the Vee block, some additional calculations are required.
• The vee block method has particular disadvantages,
- a number of angles are required (the standard angle is 90°)
- only suitable for regular odd lobed figures
• The centre support method also has disadvantages,
- The part may be bowed, or warped
- off centre or degraded centre holes will decrease reading quality
• The features of this methods are,
1. the reference datum is not points on the object, but a separate precision bearing
2. The axis of the part being measured is aligned with the machine bearing axis
3. A stylus is moved in to contact the part, and then it moves about in a circular path
4. The deflection of the stylus is amplified onto a polar plot to be used in evaluation of the part
• We can measure the out of roundness value as the minimum distance between two concentric circles that enclose/envelope the trace profile. This distance must obviously be divided by the magnification.
• Only roundness deviations are amplified. This creates distortions in the trace.
• The Talyrond machine also uses a low pass electronic filter to reduce the roughness that is shown on the plot. But this still shows the lobing.
• Eccentricity - the talyrond can also be used to detect concentricity. A simple example is a bearing race shown below.
• An example of the part discussed above, is now shown in a trace from the Talyrond
