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Friday, 5 October 2012

Mythbusting Flatness (part 1)

Flatness is possibly one of the more widely misunderstood aspects of precision woodworking, by experienced and novice woodworkers alike. Most woodworking courses tend to start with sharpening and tuning - because you can't work without sharp well tuned tools. I would suggest that a better starting point might be a theoretical understanding of flatness, because with that you can better understand what you are aiming to achieve when sharpening and tuning - and save yourself bags of  time and effort in the process.

Right, goatee beards and berets at the ready - let's bust some myths!

Perfectly flat! - BUSTED!

From an engineering perspective, absolute flatness is a conceptual state that is approached to within a given tolerance. Nothing has ever been made that is absolutely 100% flat, however we can get extremely close to it. Good quality reference surfaces like straightedges and surface plates will have a stated tolerance, the smaller the tolerance the closer the tool is to flat, straight, square etc. The smaller the tolerance gets, the more expensive it becomes to achieve.

It is worth mentioning that the tolerance on a straightedge is one of the factors that needs to be taken into account when expressing the measurement taken against it. Avoid buying cheap tools described as 'precision' but without a stated tolerance, you are genuinely better off with a banana. A straightedge with a 0.003" tolerance might sound impressively straight, but if you go to take a 2 thou measurement against it, you could be out by 75%. Would you buy a tape measure that gave you two feet plus or minus 18 inches?

I have scoured the suppliers for a straightedge that is both reasonably priced and sufficiently accurate and settled on the Bowers Metrology 465 Engineers Straightedge (same company group as Moore & Wright) which gives accuracy of 0.0005" over it's 500mm length and still comes in on the right side of £75 (at the moment). 

Rub it on a flat thing. - PLAUSIBLE! (sometimes)

A common myth is that rubbing something on a big flat abrasive surface will automatically make it flat too. However, when you think about it, every slight variation in pressure as you move the object over the surface will have a cumulative effect. The flat surface prevents you from making it concave, so what this process actually achieves is altering the condition of the surface in the direction of convexity. If it was concave to begin with then you will indeed be making it flatter, if it was flat or even minutely convex then the only possible outcome will be greater convexity. Abrading on a larger abrasive surface is a useful technique, but only when it is understood and used appropriately to reduce concavity or intentionally introduce convexity.

When it comes to making wood flat with a hand plane we use 'stop shavings' or shavings within the length of the workpiece to introduce very slight concavity (equivalent to the depth of cut over the length of the sole). These are followed by 'through shavings' that move us back in the direction of convexity - too many through shavings and you will end up with a convex workpiece. Although the tools are different and we are cutting rather than abrading, the principles are exactly the same. Approaching flatness from known concavity in a controlled manner.

Concavity can be introduced or increased by working the object over an abrasive surface that is smaller than itself. In the case of a chisel this can be achieved by working across the width of the stone, or if lapping a plane by using a piece of abrasive that is shorter than the sole. Once the surface is uniformly slightly concave it can then be worked on a larger abrasive surface to bring it back towards flat.

I've seen the light! - Plausible! (But it ain't a measurement)

Seeing light under a straightedge or square is not a very satisfactory method of inspection because light is subatomically thin. Take a bench plane that is flat to British Standard of 3 thou on centreline for example. An optic fibre core has an external thickness of 1/3 of one thou (that's the thickness of the tube as well as the hole for the light inside it). In other words you could stack eight optic fibres on top of each other and slide the lot through a three thou gap. A feeler gauge on the other hand gives you a definitive yes or no answer about whether the gap is bigger or smaller than the thickness of the feeler. The only thing light can do is tell you whether or not you need to bother getting the feeler gauges out. In other words it's a quick initial check, seeing no light is an automatic pass, but seeing light only identifies a need for proper measurement.

A word about tolerances

Tolerances are usually expressed as 'X thou maximum total deviation on centreline' if you think of this as 'would an X thou feeler guage go under it?' you are using the appropriate numbers. They can also be expressed as +/- Y thou, this gives you the deviation up or down, so to check for it you would need a feeler gauge twice as thick as Y thou. You can also have unilateral tolerances that only allow deviation in one direction, for example =/- Z thou allows for a concavity of Z thou with no tolerance for convexity.

If you have any questions or comments please feel free to post them here and I will do my best to answer them. In part two we will get a little deeper into the specifics of tool tuning and sorting out some of those eBay horrors.


  1. Very Interesting stuff - I'm always worried that my veritas straight edge is out when working with musical instruments - Is there any way of checking if a straight edge is flat without buying another straight edge? Do temperature / atmospheric changes affect the the accuracy of a straight edge to a considerable degree?

  2. Hi Otis, You can check them by doing a flip test. Lay the straightedge on a piece of paper and carefully mark a short line with a marking knife at either end and another one in the middle. Turn the straight edge over (i.e. you are still referencing off the same edge) and repeat the middle mark. This will double any error making it easier to see, if you see two lines - it's not straight. Repeat the exercise a few times to be sure and always have the bevel of the knife flush against the edge so that you get a true reading.

    All materials expand and contract to some degree with changes in temperature, that's just physics. Good straight edges are carefully heat treated to make them as stable and uniform as humanly possible. In other words if the whole thing gets minutely bigger in all dimensions at the same time, its straightness will be unaffected.

  3. hi

    so I think I understand the piece of wood becoming convex if you take too many through shavings with a hand plane. But I was wondering if a power jointer actually makes reasonably flat surfaces (in the direction of plausible) when it's properly set up? because the fact that the outfeed table is at the same height as the cutting edge of the blade, and the infeed table is variable, always seemed logical to me in that it should produce a flat surface. Is that a resaonable expectation?

    regards Johannes

    1. Hi Johannes,

      Yes, a power jointer will make a reasonably flat surface so long as pressure is applied to the workpiece in the right sequence when feeding the timber.

      Power jointers can get you within a mm or so of finished dimension and will leave a slightly scalloped surface due to the rotation of the blades. This takes care of the grunt work and a hand plane can then be used to achieve a smooth surface that is accurate to a tenth of a mm.

    2. actually, for most peoples work a cheapo straight edge will be perfectly fine, apart from the occasional obvious piece of garbage the main point of its cheapness will be less rigour in its inspection, flimsier construction and more obviously a coarser finish to the parts that matter, its thats last point that is most pertinent.

      something that is never raised and as a toolmaker/ prod eng is foremost in my mind is that a lot of the reliability/ tolerance of the thing is imparted by the method of manufacture ie the actual machine tools involved, were they nackered pre WW1 vintage in the back streets of Delhi with a man on pricework or something a bit more capable.... therefore perhaps we could say that for general workshop use the 'B' grade type item is perfectly good from hopefully a reputable source. I suppose with the cheaper stuff its whether you can live with the coarser finish or lack of conformity to a stated British Standard grade, ie 'B', 'A', Inspection etc, I know I couldn't.....

      and of course its what you test against, metrology can get involved and demands rigour, and so we must need something a bit more than our 'gannies fluffy slipper to compare against !!

  4. Thank you Richard,

    Now I understand why you pointed out how immaculate the finish is on the surfaces of the new super squares!

    I guess that given the very fine tolerances that your tools are made to, properly accurate metrology tools are an absolute must in your workshop. For us humble woodworkers a good quality Engineer's B grade would indeed be considered definitive.

    My beef is with those that sell bits of ground flat stock and call them 'precision woodworkers straightedges', not understanding that dimensional consistency and straightness are far from the same thing.

    When combined with the unsuspecting purchaser's misguided faith in their accuracy, they can be even more dangerous than a fluffy slipper!