I have received a specification of flatness from Grizzly.
Dear Leo,
Thank you for your email dated January 22, 2014.
A table saw table has an acceptable flatness of 0.015” – 0.020” over the entire table surface. A jointer has an acceptable tolerance for the tables of 0.002” per foot of bed length.
If we may be of any further assistance, please do not hesitate to contact us. You are a valued customer, and we look forward to serving your future woodworking and metalworking needs.
Sincerely,
Vince C.
Technical Service
Grizzly Industrial, Inc.
EN #901
ETC#R
So – the flatness spec of .002 per foot for a jointer surface sould be taken as .002 per linear foot of jointer table surface. Mind you – that does not mean a single line – but it does mean the entire surface on a linear foot. If your jointer table it 6” wide – it means the length plus the width to be within .002. The .002 is to be cumulative along the linear surface – 2 feet would be .004 – and 3 feet would be .006 I would apply the same spec to a table saw fence.
The table saw “surface” of .015-.020 is again the entire surface – not – a single line.
When someone tries to use a “straight edge” that is a single line. That is NOT flatness – that is straightness. It is not a trustworty method of straightness either. BEST you can derive from that is a "relative GUESS" about straightness of a single line. You are not even close to flatness there.
Also – Flatness is never measured in reference to anything. It is relative to itself only. It is flatness – no datum. When using a straightedge – (which is probably not straight) – to try to measure flatness – the not so flat straightedge is the Datum – or reference point and even at that can only represent a single straight line.
I have some of my engineering books at home that describe how to measure flatness, but the process is not within the capabilities of a home hobby shop.
What I am saying here, is that it is not possible for the home hobby shop to measure flatness, unless you have a LARGE surface plate, plan to disassemble you machine, have precision jackstands, and precision measuring instruments. The thing being measured for flatness must be mounted upside down on the jackstands in 3 points, and the “underside”, entire surface needs to be scanned for total indicator deviation.
Anything other than this method is not flatness, and anything other than this does not relate to the machine builders specs.
Here is a simple explanation of flatness
http://www.toolingandproduction.com/features/2007_December/1207_flatness_and.aspx
I have been all over the forums and read hoards of discussion on flatness. I can say with a high level of assurance, there is very low percentage of people that understand what flatness is. Therefore – just because you read it on a forum somewhere does not mean it is correct information.
Even in manufacturing machine shops, not many people understand flatness.