What is Ra and Rz in Surface Roughness
Ra and Rz in Surface Roughness
Ra and Rz are roughness parameter. These roughness parameters indicates roughness profile on a surface.
Ra stand for arithmetical mean roughness value and Rz stand for Mean roughness depth.
What is Roughness?
In Surface Texture, it is amount of deviation in direction of normal vector of real surface from its ideal form.
Larger the deviation, more is surface rough, less is deviation mean surface is smooth. Over the rough surface coefficient of friction will be high and over smooth surface coefficient of friction will be less. This is also fact, the surface which have irregularities get rust easily. Maintaining good surface finish over surface it also costly.
Surface Texture (also known as surface finish) is nature of surface defined by three characteristics of Lay, surface roughness and waviness.
What is Lay?
“Lay” is the term used to indicate the direction of the dominant or pattern of texture on the surface.
Example-Parallel, Perpendicular, radial and circular, etc.
All lay in surface finish:-
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| Lay Types in Surface Finish |
Surface Roughness
It is amount of deviation in direction of normal vector of real surface from its ideal form. It is represented by (R), or by symbol in some technical documents.
Some Roughness parameters ass per ISO 4287:-
- Ra – Arithmetic average roughness value
- Rz – Mean roughness depth
- Rz1max– Maximum roughness depth
- Rt – Total height of the roughness profile
- lr– Sampling Length
- In– Total Length of Surface
- Rz1, Rz2,…Rz5- Roughness at 5 sampling length.
- Rz1max – Maximum roughness depth
- Rz – Mean roughness depth: mean value of the five Rzi values from the five sampling
- lengths lr within the evaluation length (ln).
- Rt – Total height of the roughness profile
(Sum from the height Zp of the highest profile peak and the depth Zv of the lowest profile valley within the evaluation length ln).
Amplitude parameters: 1. Peak, 2. Valley
- Maximum profile peak height:- (Pp; Rp; Wp)- It is largest profile peak height Zp/Rp within sampling length.
- Maximum Profile Valley Depth:- (Pv, Rv, Wv)- Largest profile valley depth Zv/Rv within sampling length.
Ra- Arithmetic average roughness value: arithmetic mean of the sums of all profile values.
Rt- Total height of the roughness profile: Sum from the height (Zp) of the highest profile peak and the depth (Zv) of the lowest profile valley within the evaluation length.
Rt= Zp+Zv
Rz- Mean roughness depth: mean value of the five Rzi values from the five sampling lengths lri within the evaluation length.
Rz=Rz1+Rz2+Rz3+Rz4+Rz5/5.
Interpretation of surface texture parameters:-
Ra12.5 Max mean:-
Ra= Profile (Roughness)
12.5= Evaluation Length (ln)
Max= Interpretation of spec limit
As per ISO 4288 [GPS-Surface texture-profile method, terms, definition & method of measurement].
Evaluation length= 5 x Sampling Length.
So accordingly we do setup of roughness measurement. While setup we have to select -Maximum probe tip radius (Rtip). Tip radius depend on sampling length. There is standard table for probe tib radius selection for roughness measurement. Table is as per ISO 4288.
Rz and Rz1max have different meaning here, Rz stand for mean roughness depth, while Rz1mx stand for maximum roughness depth.
Waviness:-
The waviness profile (W-profile) is the profile resulting from electronic low-pass filtering of the primary profile with the cut-off wavelength λ c followed by high-pass filtering with the cut off wavelength λ f . The parameters are designated W and evaluated over the evaluation length In, consisting of several sampling lengths lw. The sampling length Iw corresponds to the cut-off wavelength λ f of the high-pass filter. However, this quantity is not standardized and must be stipulated on the drawing. It should lie between five and ten times λ f.
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| Waviness In surface Textures |
Indication of Surface texture in Drawing
Indication of surface texture in technical product documents is given as per ISO 1302.
Basic Symbol for Roughness:-
Expandable Graphical Symbol:-
- Removal material required
- Removal material not permitted
Complete Graphical Symbol:-
In complete graphical symbol of roughness, only symbol is extended in shape. Below is photo of following:-
1. APA-Any manufacturing process permitted
2. MRR- Material shall be removed
3. NMR- Material shall be removed
Representation of of surface texture parameters in text and in drawing is given as below:-
In text:- MRR Ramax 0.7, Rz1max 3.3
On drawing:-
Rzmax 0.7– maximum mean roughness depth is 0.7 mm (upper Limit).
Rz 0.7– mean roughness depth is 0.7 mm.
Method of Evaluation of roughness measurement
16% rule
All roughness parameters without the “max” suffix represent the mean value measured within the five sampling lengths:
16% of the measured values may exceed the limit.
Step-by-step method:
1. If the first measured value is below 70% of the limit, this is considered to comply.
2. Failing this, take two additional measurements at other points on the surface; if all three measured values are below the limit, this is considered to comply.
3. Failing this, take nine additional measurements at other points on the surface; if a total of not more than two measured values are greater than the limit, this is considered to comply.
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