How Flat Does Flat Need To Be?

All of the work I did lapping the soles of my hand planes brought me to the question of “How flat does flat need to be?” and it’s been rattling around in my head ever since.

My ultimate concern was how the flatness of the plane’s working surface might translate to the surface of my woodworking projects, so I decided to err on the side of caution and switch to a “flat” 1/4″ glass lapping plate.  What if I had continued using the granite lapping plate with the .003mm concave gap?  Sure, the sole of the plane would have been curved at the heel and toe.  Would it have worked well enough to create a flat surface on a piece of wood?  Was the curve on the heel and toe actually acceptable and I’m just too obsessed with “flatness” to let it go?

I looked into “flatness” by visiting the National Institute of Standards and Technology website – www.nist.gov .  These are the people responsible for standardizing measurements and measuring methods for the whole country.  These are the people who make sure your gallon of gas is actually a gallon.  Surely these are the people who had the information on what was acceptable for “flatness.”

As it turns out I couldn’t locate an NIST official definition or what an acceptable industry standard of “flatness” might be.  I did come across publication #73-239 “Gage Block Flatness and Parallelism Measurement” by J.S. Beers and C.D. Tucker written in 1973. Their opinion of “flatness” in a gauge block (something that is supposed to be as flat as flat can be) is that “Typically commercially available reference flats, plane to  within 1 or 2 micro-inches over a 2.5 inch diameter are usually adequate.”  When you consider one micro inch is .000001 inches (1 millionth of an inch), that sounds awful flat to me, and if Beers and Tucker are OK with that standard, I think I can be as well.

I also looked at the “flatness” of some straight edges.  For example, the Veritas straight edge I used to measure the concave in my granite slab boasted of being machined to within .0010mm and another from Garrett Wade claimed to be machined to within .0005mm.

Then I thought about my .003mm gap which couldn’t be much more than the .0010mm standard of the straight edge I used.  If that .003mm gap caused me such problems, how could I trust the straight edge?

I took a closer look at the K-D Tools #161 feeler gauge I used to measure the gap in the first place.  I pulled out the .003 gauge and noticed something I didn’t notice when I originally used it.  There was a second number right below .003 and it is .076.  Now when I look at it, it is just as clear as a daylight.  What I thought was .003mm was actually .003″ and the other number is .076mm.  That gap is nowhere near as small as .003mm.  My feeler gauge can’t even measure as small as .003mm because it only goes down to .038mm, still a long way off when you are talking about “flatness.”

Originally I thought this concave gap measured .003mm but it was really .003" or .076mm

No wonder the sole of my plane took on the shape of the granite plate so quickly.  Lesson learned: Make sure you know how to ready your measuring tools.  As I have said all along, I want to share my mistakes as well as my successes.

My original question still stands though. How flat does flat need to be? What is an acceptable amount of “gap” on a surface whether it be a hand tool , machine, or woodworking project?  As far as a hand place goes, it does need to be much less than .076mm.

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