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RussZHC
07-07-2010, 09:42 PM
...thumb nail actually.

I read reference a few times to the idea of dragging a thumb/finger nail across a surface (or two mating surfaces etc.) and my question would be if anyone has ever bothered to assign a rough mathematical value to this reference point?

For me right now it maybe more important to have a practical idea...so for example if dragging the nail across a "scratch" on an otherwise "smooth" surface, just causes the nail to catch...can it say be polished out? Does it require fine cloth/paper (say 400 grit or finer)? Does it require machining?

doctor demo
07-07-2010, 09:48 PM
It would depend on what ''the surface'' is made of.

Steve

Deja Vu
07-07-2010, 09:56 PM
It depends mainly on the condition of your nail. Is it cracked? Has it been kept clean? Was it touched up with a file lately? Roughly handled tools are always less reliable for accurate measurements.

So, it depends.:D

Black_Moons
07-07-2010, 10:20 PM
Catch/nocatch is very vauge, Id have to say, If it does not catch, its small enough it could be polished out.. with a reasonable amount of effort

if it does catch, its clearly worse then that, but how much worse? Who knows. If you must, take a sample peice of material thats the same, and gouge it and see how long it takes to polish out :P

Liger Zero
07-07-2010, 10:35 PM
When was your thumb last calibrated? And by who?

boslab
07-08-2010, 02:10 AM
Roughness comparison blocks are for just that, rubbing your thumb accross!, obveously with a surface texture measuring device then really accurate CLA measurements can be made but its quite acceptable to compare in the shop or on site with your nail.
The measurement of surface roughness is not as straightforward as most other engineering measurements, such as length or angle or hardness; and in order to get good accuracy, expensive instruments are needed. These are often unsuitable for workshop use, where roughness of machined surfaces can usually be assessed conveniently and with sufficient reliability using visual and tactile (fingernail) comparison with a standard set of surfaces machined in the same way.

Such sets of surfaces are called roughness comparison specimens. They are simple to use, inexpensive, and hence widely used in many different applications. See here

http://www.rubert.co.uk/AboutUs.htm

davidwdyer
07-08-2010, 06:28 AM
A little off subject here but.....

I understand the original "rule of thumb" was one of the first "womens liberation" laws passed in England.

It stated that one could no longer beat his wife with anything bigger around than his thumb.

Was this a good law or not? :)

RobbieKnobbie
07-08-2010, 06:44 AM
When was your thumb last calibrated? And by who?

I send my nails out every six months to get calibrated by NIST tracable standards.

Last time they came back they were all chewed up.

Dave Converse
07-08-2010, 06:50 AM
Fun answers for sure. But I did read some time ago that the smallest scratch we could detect with a human fingernail would be .002". Therefore, nothing felt when raking a fingernail across a scratch would indicate that the scratch is less than .002 deep. Not very precise, but you asked.................

RussZHC
07-08-2010, 07:19 AM
I know this is all sort of vague as is further information below but the reference has come up before and I think it has value.

"Steel", not hardened (old Sheldon lathe spindle)

The scratch is, IMO, about what a "scratch all" would do in marking most of the steel I use right now (scrap so that too is wide open in terms of content, mfg process, quality) but not as deep as the mark a heavy duty automatic center punch would leave on the same material.

jugs
07-08-2010, 08:44 AM
A little off subject here but.....

I understand the original "rule of thumb" was one of the first "womens liberation" laws passed in England.

It stated that one could no longer beat his wife with anything bigger around than his thumb.

Was this a good law or not? :)

That 'Law' in England is a myth http://womenshistory.about.com/od/mythsofwomenshistory/a/rule_of_thumb.htm (http://womenshistory.about.com/od/mythsofwomenshistory/a/rule_of_thumb.htm)

but
NOTHING that promotes or allows violence to another can be good.


john
:)

AD5MB
07-08-2010, 09:34 AM
It stated that one could no longer beat his wife with anything bigger around than his thumb.

Was this a good law or not?

before the invention of rebar...

oldtiffie
07-09-2010, 07:56 AM
First, the OP:

...thumb nail actually.

I read reference a few times to the idea of dragging a thumb/finger nail across a surface (or two mating surfaces etc.) and my question would be if anyone has ever bothered to assign a rough mathematical value to this reference point?

For me right now it maybe more important to have a practical idea...so for example if dragging the nail across a "scratch" on an otherwise "smooth" surface, just causes the nail to catch...can it say be polished out? Does it require fine cloth/paper (say 400 grit or finer)? Does it require machining?

Next, the best response so far:


Roughness comparison blocks are for just that, rubbing your thumb accross!, obveously with a surface texture measuring device then really accurate CLA measurements can be made but its quite acceptable to compare in the shop or on site with your nail.
The measurement of surface roughness is not as straightforward as most other engineering measurements, such as length or angle or hardness; and in order to get good accuracy, expensive instruments are needed. These are often unsuitable for workshop use, where roughness of machined surfaces can usually be assessed conveniently and with sufficient reliability using visual and tactile (fingernail) comparison with a standard set of surfaces machined in the same way.

Such sets of surfaces are called roughness comparison specimens. They are simple to use, inexpensive, and hence widely used in many different applications. See here

http://www.rubert.co.uk/AboutUs.htm

Here are my "scratch blocks" - which are surprisingly accurate. I used to use these may years ago and I needed another set - so I bought it. I use the nail on my middle finger - it is very sensitive - as it is on most people. The "scratch blocks" are for "comparison" purposes to see which "scratch block" the finish on the job feels most like.

http://i200.photobucket.com/albums/aa294/oldtiffie/Surface_finish/Surface_roughness_card1.jpg

I suggest that reading these links will clear matters up quite nicely as it is subject that seems to be little understood by many who should know it:

http://i200.photobucket.com/albums/aa294/oldtiffie/Surface_finish/Machining_finishes1.jpg

http://i200.photobucket.com/albums/aa294/oldtiffie/Surface_finish/Machining_finishes2.jpg

http://i200.photobucket.com/albums/aa294/oldtiffie/Surface_finish/Machining_finishes3.jpg