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Questions and Answers

What does the symbol Ra represent in the context of surface finishes?

Micron values for surface treatment

Total irregularity count

Average roughness in micrometres (correct)

Maximum peak height

Which of the following is true about the RMS method of measuring surface roughness?

It gives a result approximately 11% higher than Ra (correct)

It averages only the negative heights

It disregards negative values completely

It is lower than the Ra method by 11%

What does the capital letter 'N' with associated numbers signify in the old format of surface finish symbols?

Standard manufacturing processes

Micron values for surface finish (correct)

Non-directional surface finishes

Irregularity measurement count

How is the direction of lay indicated in surface finish symbols?

With various symbols like Parallel or Perpendicular Signup and view all the answers

What aspect does the surface finish symbol not account for?

Fishing method Signup and view all the answers

What happens to the Ra value in the new format of surface finish symbols?

Placed in a new symbol position Signup and view all the answers

In conditions where surface finish is specified, what additional information can sometimes accompany the symbol?

Production method or treatment Signup and view all the answers

What determines the size of the 'N' number in the old format of surface finish symbols?

The coarseness of surface finish Signup and view all the answers

What is a consequence of not having a honed finish on specific surfaces of a hydraulic cylinder?

The seals will not function properly. Signup and view all the answers

Which components are particularly sensitive to surface finishes?

Piston rings and sealing surfaces Signup and view all the answers

Why should finer surface finishes be specified sparingly?

They significantly increase production costs. Signup and view all the answers

What does a surface roughness gauge measure?

The average peak-to-valley height of surface irregularities Signup and view all the answers

What material is typically used for the stylus in a surface roughness gauge?

Diamond or sapphire Signup and view all the answers

What is the maximum radius of the stylus used in a surface roughness gauge?

0.013 mm Signup and view all the answers

Which method is NOT mentioned as a way to obtain surface roughness readings?

Peak-to-peak (Rz) method Signup and view all the answers

What is one of the uses of a honing tool?

To hone the inside surfaces of pipes Signup and view all the answers

What is the primary reason for ensuring correct surface finishes on components?

To ensure correct performance and longevity Signup and view all the answers

Which of the following components rely heavily on specific surface finishes?

Piston rings and cylinder liners Signup and view all the answers

What is the effect of not having a honed finish on the inside of a hydraulic cylinder?

Seals would be unable to function properly Signup and view all the answers

Which tool is used to measure surface irregularities?

Surface roughness gauge Signup and view all the answers

What is measured using a tracer head in a surface roughness gauge?

Average peak-to-valley height of irregularities Signup and view all the answers

Which statement is true regarding finer surface finishes on components?

They can increase production costs unnecessarily. Signup and view all the answers

What is the maximum radius of the stylus used in a surface roughness gauge?

0.013 mm Signup and view all the answers

In which units is the reading for surface roughness typically measured?

Microns Signup and view all the answers

Study Notes

Surface Finishes and Their Importance

Surface finish is crucial for the proper function and lifespan of many components.
Components that heavily rely on specific surface finishes include piston rings, cylinder liners, bearings, seals, valves, gears, and plastic injection molds.
A hydraulic cylinder exemplifies the necessity of precise surface finishes:
- A honed finish on the inside of the cylinder and the piston rod is critical for proper seal function.
- Honed pipes are more expensive than standard steel pipes and less readily available.
- Honing is achieved using a tool with honing stones attached to a rotating mandrel.
Finely finished surfaces are not always necessary and should be specified sparingly to avoid unnecessary costs.

Measuring Surface Finish

Microscopic irregularities (e.g., grooves, undulations) are present on surfaces even if they appear smooth to the naked eye.
Surface roughness is measured using a surface roughness gauge or tester.
This device consists of a tracer head, an amplifier, and a decoder.
The tracer head contains a diamond or sapphire stylus with a maximum radius of 0.013 mm.
The stylus moves across the surface, and its movements are translated into electronic fluctuations, providing a reading on the meter in microns.
The average peak-to-valley height of irregularities is measured over a specific sample length.

Surface Finish Measurement Methods

Two methods are used for measuring surface roughness:
- Arithmetic mean deviation (Ra) method
- The root mean square (RMS) method
The Ra method calculates the average deviation from the zero line, which represents the average peak-to-valley height of irregularities.
The RMS method incorporates both positive and negative deviations from the zero line, resulting in a value approximately 11% higher than the Ra method.

Surface Finish Drawing Symbols

Surface finish symbols on detail drawings indicate three possibilities: obligatory, optional, or prohibited finish.
The old format uses capital letter 'N' with numbers 1-12 to represent different micron values.
- Smaller micron values (finer surface finish) correspond to smaller 'N' numbers.
- Both upper and lower 'N' limits can be specified.
The new format uses 'Ra' values in micrometres placed on a new position within the symbol.
- An 'Ra' value can also have upper and lower limits.
Additional information can be added around the symbol, including:
- Production method
- Treatment
- Coating (above the symbol's tail)
- Machining allowance
- Direction of lay
- Sampling length
The symbol should be placed horizontally or vertically (right reading) and never upside down.
Different manufacturing processes and machining methods produce a range of 'N' or 'Ra' values.

Direction of Lay

The direction of surface irregularities, or lay, is indicated using symbols:
- Parallel
- Perpendicular
- Crossed
- Multidirectional
- Circular
- Radial
- Particulate or non-directional