Surface Finish - Conversion factor to transform Rz Jis in Rz

[Spacewalker]

Hi all,
A fast question.
I know that there are many conversion formula to trasform Ra in others parameter, but I've heard that It's possible to convert a surface roughness value given in Rz Jis into an Rz value. It is true?
There are a rules that can explain this possible correlation?
Thanks for your help.

Dario

 

[Hodgepodge]

Hi all,
A fast question.
I know that there are many conversion formula to trasform Ra in others parameter, but I've heard that It's possible to convert a surface roughness value given in Rz Jis into an Rz value. It is true?
There are a rules that can explain this possible correlation?
Thanks for your help.

Dario

There is no direct conversion. The two different measurements factor in the peaks and valleys of the surface differently. Check out this article in Modern Machine Shop for more information. The article offers a conversion ratio, but it is questionable at best.

 

[Spacewalker]

Thank You Hodgepodge, probably what I've heard It's only speculation ... or an extreme attempt to obtain a value to meet the drawing specification. But anyway, do you think there is no possibility to extrapolate a correlation between these two factors ?

 

[Jim Wynne]

There is no direct conversion. The two different measurements factor in the peaks and valleys of the surface differently. Check out this article in Modern Machine Shop for more information. The article offers a conversion ratio, but it is questionable at best.

The last paragraph of the linked article is applicable to any sort of measurement, and is good advice all by itself:

Communication at the outset of the project can avoid most surprises. The approximate, and sometimes questionable, comparisons can be avoided by developing an understanding of exactly what a parameter on a print means, and how the various parties involved in the production plan to check surface texture. The best way for those involved in the production to be in agreement on the parameters for measurement is to have capable evaluation equipment in both the manufacturer's and customer's facility, making the same check using the same method. If the manufacturer or the customer uses conversion ratios, then both parties should be aware of the use of the ratio and be comfortable with the ramifications.

 

[Hodgepodge]

Thank You Hodgepodge, probably what I've heard It's only speculation ... or an extreme attempt to obtain a value to meet the drawing specification. But anyway, do you think there is no possibility to extrapolate a correlation between these two factors ?

The above mentioned article does offer a ratio that could be used. I would suggest you don't try to skirt the issue. It is best to use the right tool for the job. I would buy a profilometer that measures in Ra & Rz. A multi-tasker is the way to go.

Good luck.

 

[Spacewalker]

The problem are not the parameters we can measure with the profilometer (we have a new hi-level profilometer bought 2 month ago) but understand if It's possible reach an agreement on the parameter to put in the drawing, based on the readings collected from ours Japan colleagues and compared with what we have seen from our side on the same pieces.
At the end, It's always a communication problem ... I will discuss this issue with our Engineering Dept....
Thank you all for support.

 

[bobdoering]

... based on the readings collected from ours Japan colleagues and compared with what we have seen from our side on the same pieces.

Make sure the surface finish is properly called out - Roughness, cutoff and sampling length. Otherwise you are very likely to get different data than anybody else performing the test.