1.2.1 Observation Methods

 

• Human perception is highly relative. In other words, without something to compare to, you will not be certain about what you are feeling.

 

• To give the human tester a reference for what they are touching, commercial sets of standards are available.

 

• Comparison should be made against matched identical processes.

 

• One method of note is the finger nail assessment of roughness and touch method used for draw dies in the auto industry.

 

 

1.2.2 Stylus Equipment

 

• One example of this is the Brown & Sharpe Surfcom unit.

 

• Basically this technique uses a stylus that tracks small changes in surface height, and a skid that follows large changes in surface height. The use of the two together reduces the effects of non-flat surfaces on the surface roughness measurement. The relative motion between the skid and the stylus is measured with a magnetic circuit and induction coils.

 

 

• The actual apparatus uses the apparatus hooked to other instrumentation. The induction coils drive amplifiers, and other signal conditioning hardware. The then amplified signal is used to drive a recorder that shows stylus position, and a digital readout that displays the CLA/Ra value.

 

• The paper chart that is recorded is magnified in height by 100000:1, and in length by 82:1 to make the scale suitable to the human eye.

 

• The datum that the stylus position should be compared to can be one of three,

- Skid - can be used for regular frequency roughness

- Shoe - can be used for irregular frequency roughness

- Independent - can use an optical flat

 

 

 

 

 

• Where the scan is stopped might influence the Ra value. This is especially true if the surface texture varies within a very small section of the surface. For example,

 

 

 

 

• In both cases 2 and 3 above, Ra would be higher over the longer sample (l2) than over the shorter sample (l1).

 

 

• The bearing surface that the skid/shoe runs on might also have an effected on the measurement.

 

 

 

1.2.3 Specifications on Drawings

 

• The following specification symbol can be used on drawings to specify surface textures desired on a completed part,

 

Waviness height - the distance from a peak to a valley

Waviness width - the distance between peaks or valleys

Roughness width cutoff - a value greater than the maximum roughness width that is the largest separation of surface irregularities included in the measurements. Typical values are (0.003”, 0.010”, 0.030”, 0.100”, 0.300”)

Lay - the direction the roughness pattern should follow

 

• The example below shows an upper limit of 40 micro in. roughness

 

 

 

 

• The symbol below can specify how the roughness is to lay,

 

 

• Standards CLA/Ra values used on drawings are: 1, 2, 4, 8, 16, 32, 63, 125, 250, 500 and 1000 μin.

 

• Stylus travel is perpendicular to the lay specified.

 

• These symbols can be related to the newer GD&T symbols