Surface finish (Evaluation Length)

[Kostas M]

Hello Everyone,

I need your help regarding the parameters that I need to choose for measuring the surface finish on an inside diameter.

The length that the finish needs to be maintained is 12mm and the requirement is Rz5. According to the spec for Rz5 the cut-off length has to be 0.8mm.,
My question is the evaluation length. How many cut-off lengths do I have to use? If the length that the finish needs to measured is 9.6 mm and the cut-off length 0.8, do I have to use 12 cut-off length? (9.6/0.8=12)
I have found a lot of tables that specify for 0.8 cut-off length, the evaluation length have to be 4mm which means 5 cut-off lengths.

Thank for your assistance in advance
Kostas

 

[Jen Kirley]

Hello Everyone,

I need your help regarding the parameters that I need to choose for measuring the surface finish on an inside diameter.

The length that the finish needs to be maintained is 12mm and the requirement is Rz5. According to the spec for Rz5 the cut-off length has to be 0.8mm.,
My question is the evaluation length. How many cut-off lengths do I have to use? If the length that the finish needs to measured is 9.6 mm and the cut-off length 0.8, do I have to use 12 cut-off length? (9.6/0.8=12)
I have found a lot of tables that specify for 0.8 cut-off length, the evaluation length have to be 4mm which means 5 cut-off lengths.

Thank for your assistance in advance
Kostas

Hello Kostas, I am sorry for the delay in this response.

I don't want to over-complicate the method for inspection when the requirement seems simple: the surface will have a finish of x.

Finish is usually measured with a profilometer, so I welcome any and all Covers to add their expertise because I am not aware of cutoff unless it is concerng an area to which the finish requirement does not apply. A profilometer will send a stylus along a length determined necessary for that instrument to calculate roughness detected over that distance. It is my understanding that any and all surface identified by specification as requiring this R value would be able to fall into this kind of measurement.

Do I understand your question correctly?

 

[Miner]

Kostas,

Let's look at the reason for the various lengths:

• Cutoff length - this is used to separate roughness (Rz) from waviness. Too large a cutoff, and you start measuring waviness instead of roughness.
• Evaluation length - this is set up as multiples of the cutoff length to allow averaging of multiple roughness measurements to minimize measurement variation. Five is a common default number used for averaging, but is not a definitive requirement. As Jen said, you do need to control Rz on the entire surface, so there is no reason you cannot specify the evaluation length to be 12x the cutoff length. The point is that this now becomes a business decision on your part to minimize risk. You are in control of this.

 

[Kostas M]

Hello Mike and Jen,

Thank you very much for your answers.

I did my research and found the ISO 4288 spec for Surface texture: Profile method -Terms, definitions and surface texture
parameters.
Since the Rz is the average of the peak to the valley differences of the cut off lengths, if you use more cut-off lengths and there is just a area with bad finish on the surface the reading will be lower. If you measure the finish only on the rough area using less cut-off lengths, the reading will be higher.
According to the table of the spec, depends on the call-out of the drawing, you set the parameters on the profilometer. The table shows that there is a limit. For example if the specification is Rz 5, you need to use cut-off length 0.8mm X 5 (Total 5 cut off lengths) in order for the evaluation length to be 4mm.

Thanks
Kostas