Correct Length to Measure Surface Roughness

Dear Sirs,
this is my first time in this forum, and this is my first question.
Let' start. Our plant produce some kind of components for engine chains, one of these components are pins. After two stage of the production process, we check the surface roughness, the first after grinding process and the second after finishing process (tipically superfinish). We have in our STD the reference value for Ra - Rz - Rt, the cutoff and the filter to be use, but is not reported the sampling lenght.
We have pins with several lenght starting from 8mm up to 28mm; the question is: can we use the same sampling lenght for all the pins or is better to use a particular sampling lenght in functions of the lenght?
Thanx in advance.
Dario

Now this is a good question! I am not aware of any standards that specify the length over which to check roughness on a pin. The part print would be your best starting point. Usually part prints specify roughness over the full length or just over certain area(s) of the part. If the print does not specify a certain length then you should check the length of the whole pin. After all, you are sampling only a portion of the area anyway. A circle has an infinite number of points therefore a cylinder has an infinite number of lengthwise lines that you can check. Assuming that you use centerless grinding and superfinishing, any significant surface variations will show radially, so it makes sense to check roughness lengthwise.

For those who never saw a device that measures surface roughness, it is a probe that moved linearly on a surface and records the 'bumps' on the surfce. The bumps are recorded based on the size of the probe's tip (the smaller the better) and the cutoff and filter are used to set limits for the probe movement and readings frequency (if I remember right). Ra is the average of the readings taken, Rz is the average of the 5 Max values and Rt is the differrence between the Max & Min readings. Like with any other variable inspection, the more data points you have the more accurate and reliable is your result. In the case of a pin, the probe moves lengthwise on the pin's diameter. Usually the probe is designed to accomodate for slight 'off-center' conditions that might occur in such situations.

I hope that I am accurate enough in my statements and opinion.

DrM2u said:

............... I am not aware of any standards that specify the length over which to check roughness on a pin ...........................

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I thought so but it seems there is. See: Roughness

The length of the path that it traces is called the measurement length. The wavelength of the lowest frequency filter that will be used to analyze the data is usually defined as the sampling length. Most standards recommend that the measurement length should be at least seven times longer than the sampling length, and according to the Nyquist–Shannon sampling theorem it should be at least ten times longer than the wavelength[citation needed] of interesting features. The assessment length or evaluation length is the length of data that will be used for analysis. Commonly one sampling length is discarded from each end of the measurement length.

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harry said:

I thought so but it seems there is. See: Roughness

Click to expand...

Thanks for the quote, it helps. So, the referenced standards seem to be ISO 25178 and MIL-STD-10A, in case any of them apply to the current OP.

Thanks all,
so I've a starting point.

Regards
Dario