From: Dave Baker
Newsgroups: rec.crafts.metalworking
Subject: Re: Surface finish on turned parts???
Date: 11 May 1999 21:01:13 GMT

>From: Jeff O'Malley <jomalley@pdq.net>
>
>	I have been asked a question about surface finish that I don't have a
>clue about.  The print calls for a 4 surface finish.  I think this
>refers to a 4 micro inch finish.  Am I right in this?? or is there
>another numbering system for surface finish that I need to know about?
>Somewhere I have seen the term rms applied to surface finish whazzit
>mean??
>	Finally, how does one go about producing this 4 finish on turned
>parts?  Abrasive cloth? tumbling in a media?
>	This one has me buffaloed.  Help!!
>			Jeff O'Malley
>	P.S.	Where would I look in Machinerys Handbook to find out what the
>different surface finishes are and how they are achieved??


Surface finishes are achieved by turning to a very small amount over finished
size - say top drawing limit and then applying liberal quantities of "Turner's
Friend". This wonderful product can be purchased in sheet or roll and in
different grit sizes. By applying finer grades of Turner's Friend any desired
surface finish can be achieved with the minimum of fuss. Simply go down to your
local engineer's supply store and ask them for "Turner's Friend" by name.

Seeing as most engineering specs these days are drawn up on computers by spotty
oiks fresh out of college who have no real idea how the world in general works
nevermind the bits they are designing, your target surface finish of 4
whatevers may conveniently be described as "shiny". No-one receiving the parts
will have the equipment to measure the actual surface finish and even if they
do you simply say that on your surface finish tester it gave the desired
result. This way everyone stays happy at minimal expense and the bits will work
if a half arsed decent experienced engineer checks them and decides they are
ok.

Chatting to a colleague in the trade recently he told me the story about a
Touring Car race team who drew up a design for a fabricated aluminium
suspension strut. The spotty young cretin who drew it called for the position
of the mounting holes to be to a tolerance of 0.001 mm !!! On a welded
fabricated part !!

Needless to say they asked many engineering companies to quote on this part
only to get the reply "try someone else mate - it's beyond our capabilities".

My colleague said no problem - he could do the job. He welded up the struts and
drilled the holes to within half a mm or so - about the best he reckoned any
reasonable man could do. Given that any tolerance you specify on a car
suspension part is going to disappear under all the other tolerances on spring
rate and length etc he felt it was a damn good job. No-one ever got back to him
to say they weren't acceptable. It's doubtful whether with all the angles
involved anyone was even able to measure and check them.

I think we call it "pragmatism".


Dave Baker at Puma Race Engines (London - England)  - specialist flow
development and engine work. Web page at
http://hometown.aol.com/pumaracing/myhomepage/index.html