13&14) S Text. Equipment

Question 1

Compare large and small surface texture analysis equipment:

Answer 1

Portable instruments:
- Used by machinists
- Analogue and digital
- Diamond stylus to track the surface
- Robust

Large instruments:
- Much more expensive
- Higher accuracy
- Not portable

Question 2

What are the basic components of a surface texture machine?

Answer 2

The Stylus (mechanical filter) tracks surface. A skid is used as a measurement datum.

Question 3

What are the major issues of importance with the stylus?

Answer 3

Tip dimension and shape

Contact force - dampening the vibrations of running over surface

Dampening of the stylus(dynamics)

Question 4

Why is the Stylus shape important?

Answer 4

It acts as a Mechanical filter (different shape gives different results). E.g 4 sided pyramid

Determines accuracy (sinking into valleys in material or being too large to detect.)

If the sylus is round in shape, it can ‘round off’ the profile of the surface.

Question 5

What is a re entrant feature?

Answer 5

overhang/pocket that the stylus won’t detect

Question 6

What is the main issue with Stylus dynamics?

Answer 6

Trackability is main issue (Keeping the stylus in contact with the surface.)

Question 7

How is stylus deflection managed?

Answer 7

Dampened with spring, stylus deflection can be modelled fairly inaccurately using a cantilever beam model.

Question 8

What are considerations when picking the spring rate to dampen the sylus?

Answer 8

Static 2 body interaction (finding the right force to keep stylus in contact with surface, but not scratch the surface. This force changes with the material being studied. Also, yield stress different on surface compared to the bulk of body.)

Question 9

What are 3 surface texture gauges?

Answer 9

Variable inductance - Older model

Phase grating interferometric gauge. - common new model

Laser Interferometric gauge - less common new model.

Question 10

What are the 2 methods of referencing measurement when analysing surface texture?

Answer 10

Skid Based reference: A skid travels across the same surface as stylus, skid much larger than frequency of peaks so provides good approximation of straight line over surface.

Independent Datum: Independent datum not connected to the stylus arm or surface.

Question 11

What are problems with skid based reference?

Answer 11

When the frequency of surface imperfections is similar to size of the skid the skid will replicate the surface, therefore not providing an unbiased reference. To fix - make skid larger.

Phase lag - imperfections being experienced at different times by the skid and stylus.

Question 12

What is filtering?

Answer 12

Filtering is the separation of roughness and waviness data.

Question 13

How is filtering carried out?

Answer 13

A cut off will be applied to the data. This cut off will be related to the wavelength and amplitude.

The amplitude and wavelength dictate if the imperfection is due to waviness or roughness.

Question 14

What are the 3 types of cut off? How would they be selected?

Answer 14

Selected based on Roughness and/or Waviness Frequencies / Wavelengths Required.

Primary cut off - Lambda c

Lambda S - short wavelength filter

Lambda F - profile filter cut off

Question 15

A filter of lambda C of 0.8mm has been applied, what size will be let in?

Answer 15

0.7 mm allowed, 0.9 mm not allowed.

Question 16

What are the 2 types of profile?

Answer 16

Periodic - regular waves

Non - periodic - random waves

Question 17

How would you chose a cut off for a periodic profile?

Answer 17

1. Estimate graphically (measure) the parameter Rsm of the surface profile.
2. Determine the recommended lc using Table
3. If necessary measure Rsm using the lc determined according to step 2.
4. If the Rsm value from step 3 relates, according to Table Q3, to a smaller or greater lc value than in step 2, use the smaller or greater lc value.

5.Obtain a representative measurement of the desired parameter(s) using the lc value (sampling length) estimated in the preceding steps.

Question 18

How would you chose a cut off for a non periodic profile?

Answer 18

1. Take an unfiltered profile
2. Progressively/iteratively apply a roughness filter starting with the longest lc
3. Calculate the relevant surface roughness parameter
4. Repeat with smaller values of lc and finish with the shortest lc
5. There will come a point where waviness is minimised leaving roughness
6. If lc is too small then the roughness component becomes supressed