What exactly is a "rough" surface? 6

[yaston4]

I am trying to find the definition of a "rough" surface in terms of numbers, equations or something that can help me distinguish between a "rough" surface and a "smooth" surface.

You might ask why this is important? There are some "rough" surface mathematical models I have seen from Williams and Kapoor, I can't not seem to find what is classed as a "rough" surface and what is not?

Can anyone provide any input? Thanks.

 

[JordanL]

Surface roughness is usually measured in micrometers or microinches. Wikipedia has a good description of it with a chart. You can find similar descriptions in the machinery's handbook.

Different machining processes can produce varying surface finishes with different roughnesses. There is no point (to my knowledge) that a surface becomes "rough" or a surface becomes "smooth", but a constant gradient of more and less rough/smooth.

Perhaps a better description of what you are trying to achieve could help me clarify more if needed.

 

[Zinov]

Forget Williams and Kapoor....these are academics
Rough or smooth depends on the intended use of the surface.
How to measure roughness : profilometer (contact, contactless),comparison (existing samples)
Profilometer can provides at least 30 parameters to measure the roughness of a surface:
Ra,Rt,Rz,Rv,Ry,Rp,Rq...etc
Selection of the right parameter depends on the intended use.
Most known factor is Ra and Rz.
You have an ANSI standard with complete definitions and equations
Thanks

 

[Tmoose]

Are your interests related to the "shakedown" mentioned here ?

http://www-mech.eng.cam.ac.uk/Tribology/research/10.shtml

 

[drawoh]

yaston4,

Surface finish requirements are absolutely relative to your application. I suggest you order a surface finish comparator. This will connect the numbers in your handbooks to actual surface finishes.

Who are Williams and Kapoor? I can find a fluid mechanics book by Baljeet S Kapoor. My college book, Fluid Mechanics With Engineering Applications, by Daugherty and Franzini, has a table on page[ ]213 showing absolute roughness in feet and millimetres. Maybe you need a better textbook.

--
JHG

 

[macmet]

We recently had to spec a surface roughness. We did a few tests to find a roughness value that worked using some known comparison samples and then used the attached table to find a way to manufacture the part to our spec.

Not sure if that is useful to you, it is not the academic approach.

 

[psv1990]

Roughness is entirely dependant on your application. In one case, pipe roughness is a function of friction or in the case of milk, it is a function of surface adhesion of bacteria.

 

[geesamand]

Roughness, from a process point of view, is only half of the picture. Roughness measurements are certainly a measurement of the difference between peaks and valleys of the surface, but equal roughness surfaces can have very different characteristics. For example, a surface that started a bit rough but has been polished, may have all of the peaks knocked off but there are still valleys that form the roughness of the material. From a contamination point of view, maybe the peaks are harmless and the valleys are what you need to eliminate or at least round them out.

Once you know these things you can determine the material finish processes required and the roughness value to assign to it.

Also keep in mind that electropolishing is a popular process that puts a fairly good luster on the surface. While I believe electropolishing is useful and economical, I've seen situations where a 125Ra surface has been electropolished and while it looks smooth, it's really just a shiny version of the same rough surface. So don't let looks deceive you.

 

[SomptingGuy]

Yep. There sure are some things that you just can't polish.

- Steve

 

[Pud]

some things that you just can't polish

...but you can always roll it in glitter.


www.tynevalleyplastics.co.uk

It's ok to soar like an eagle, but weasels don't get sucked into jet engines.

 

[SomptingGuy]

Sorry to the OP. I took a bit of a left turn in this thread and Harry high-sided it completely. A new repost for me though, and one I will be using in meetings. Very good.

- Steve

 

[mfgenggear]

yaston

surface finish is as explained from the previous post, is determined from the application, function of the part.
eg: seal surface, bearing surface, gear surface.

most drawing's I have seen specify Ra micro finish. which depending on the type of part & size, will determine the method of inspection.
for my application, small fine pitch gears, most surface finish are measured with a profilometer. as noted by others.
test blocks can be used to calibrate & test the profilometer.

I would like to add that micro finish specified can & will dictate the manufacturing method.
if the surface finish specified is an 4 micro or better, the the supplier will have to use the right tools to obtain it.
use tight surface finish only, and only when it is required, or the cost of the parts will increase.
Normally a 63-125 is adequate for most applications. and I would consider a smooth finish.

in my world of machined, case harden gears, & shafts, a 250 micro finish is a rough surface. 125 or 63 micro finish is normally standard.
for my type of work. however the same parts can & do mix in different micro finish call outs.
gear profile can be between 4-32 micro, journals can be 4-32 micro surface finish, non functioning areas can be 32-125 micro.

some surfaces can be 2-4 light bands, for flatness & micro combined.

I have given example of different specified surface finishes.

HTH

Mfgenggear
if it can be built it can be calculated.
if it can be calculated it can be built.

 

[yaston4]

Wow! Thanks everyone, your posts have been fantastic and really help. I will do my best to respond and make some further comments.

JordanL, Zinov, thanks for your explanations and information. Zinov, I am Williams and Kapoor is my best friend... no just kidding

Tmoose, yes part of my interests in this is with regards to shakedown.

SomptingGuy, sorry I didn't understand your post :-(

Everyone else, thanks again!!