Metric measurements/plans

Discussion in 'Boatbuilding' started by cac, Oct 29, 2006.

  1. cac
    Joined: Oct 2006
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    cac Junior Member

    I've been building an Origami Dingy (http://www.craftacraft.com/blog/2)... nothing earth shattering, but I've been enjoying the little I've gotten done.

    The plans are produced in England and come with detailed metric measurements (and I do mean detailed :). That's great, but has led me to a question...

    Is it "normal" for metric measurements to be taken to the millimeter? I've hit a section with a lot of measurements to 33mm or 78mm or whatever... I would have assumed the nearest centimeter or at least 1/2 centimeter (75mm, 35mm) would be "close enough" for nearly everything. In English measurements we normally would only take things to maybe 1/8" unless special circumstances.

    Anyway, not complaining, just wondering if its "normal" or just a quirk of these plans.
     
  2. raider
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    raider boat-a-holic

    I have been using Metric dimensioning schemes for the last 20 years in the construction equipment and automive design world.

    Normally, we design everything to the nearest half millimeter with a typical tolerance of +/- 0.8 mm for normal parts cut on a laser. The metric dimensioning scheme does have advantages over the English scheme, when it comes to steel and fabrication. But, it becomes a nightmare when it is used in traditional woodworking.

    To answer your question directly, rounding to the nearest half cm would cause dimensional stack-ups and could throw everything off with the tolerance run-out. The metric to English conversion is 25.4 mm to 1 inch (IAW ASTM E380). 1/8" is 3.125 mm and 3/16" is 4.76 mm..... so if you are comfortable with rounding everything off to the nearest 3/16" with a tolerance of a 1/16", then you try iy - but be warned that the run-out could lead to mis-fits. The +/- 0.8 mm tolerance mentioned earlier is the conversion of +/- 1/32" - which has been referred to as the "Human Error Tolerance".

    For me, I can easily lay out dimensions with a Metric ruled tape with far more accuarcy than an English ruled tape. The math is a whole lot easier too when you need to divide sections or calculate areas.

    All that said....... when I leave work, the Metric system stays there. With all of the woodworking and outside design that I do, everything is in Inch units..... go figure.

    PDV
     
  3. cac
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    cac Junior Member

    Thanks for the info... I'm using a metric tape and just using the dimensions given... that's not a (real) problem... I just have to think about it more :)

    I was just wondering more about the "normalcy" of designing something with (to me) odd measurements. I mean if I could reasonably make the measurement 75mm instead of 73mm or 90mm instead of 87mm, then it just seems like it would be easier. I realize that sometimes you need that accuracy, but in cutting a "whatever" I find it hard to believe that 3mm would change things that much on an 8' boat (for a mixture of units) :)

    Anyway, I've been enjoying it, and the design so far is WELL spec'ed out.
     
  4. Raggi_Thor
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    Location: Trondheim, NORWAY

    Raggi_Thor Nav.arch/Designer/Builder

    We use different accuracy for different types of drawings.
    In house construction, 10mm is a normal round off.
    +/- 15mm is often accepted on site tolerances.

    In boat building 1mm is normally OK.
     
  5. jeastman
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    jeastman Junior Member

    Hey

    I've been designing / building ships for over 10 years now in Canada and yes, mm is the normal unit for metric drawings. :)
     
  6. marshmat
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    marshmat Senior Member

    Dimensioning tolerances vary depending on what you're building. A few μm is typical for building a jet engine. 1 mm would be typical of boat plans- yes, the difference between 73 and 75 mm is small, but errors add up and designers are careful to specify tolerances such that the total error, in the end, is still reasonably small. They also take into account the inevitable sloppiness of hand lofting and cutting; by following the given dimensions as precisely as you can, you reduce the effects of things like saw blade width, not-quite-right lofted lines, etc.
    Millimetres are the standard units for the vast majority of technical drawings; when you get into buildings and site plans, then you start to work in metres. I find it increasingly rare to find inch-dimensioned drawings originating outside the United States, currently the only country in the world that does not officially endorse the SI system.
     
  7. jeastman
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    jeastman Junior Member


    I agree. Metric units are the norm now in nearly all countries, even in some shipyards in the US, metric is being used. But whatever, it's only a unit of measurement in the end. Here in Canada, it's nearly all metric but I also use imperial sometimes for US customers.
     
  8. Tim B
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    Tim B Senior Member

    I don't quite see the problem. If the designer has specified 78mm you build it to 78mm. If it's 3 inches, you build it to 3 inches.

    Tim B.
     
  9. cac
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    cac Junior Member

    Not a problem

    I just found it odd... I (me, personally) would tend to just round to the nearest 1/2 centimeter (5 mm) unless there was a strong reason not to.

    I guess either the designer had a strong reason, or just doesn't feel like rounding. Not a problem to build it or whatever, it just seemed "odd" to me.

    If nothing else, I like rounding off :)
     
  10. marshmat
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    marshmat Senior Member

    Being of an engineering/science background I have a hard time seeing how 75 mm is considered 'nicer' than 73 mm.... if your tolerance is +-5mm, then 73+-5 and 75+-5 are still different measurements. I frequently get measurements like 12.257 (+0.013, -0.018) where errors are different in each direction... you just learn to deal with it, if you want to be accurate. Or you round off and get out the glass and filler.
     
  11. cac
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    cac Junior Member

    It's mostly nicer in that my metric tape has larger lines at the 5's :)

    Eyesite counts :)

    I guess I'm just too much of a wood butcher. If its fine cabinetry, I guess very small measurements matter. Machine tools, sure, I understand accuracy there. A handhold cutout on a 8 foot dingy??

    Anyhow, I'm not trying to knock things... I just found it odd and wondered if this was a "fluke" of this design (the Origamy Dingy), or if designers who use metric normally do that. English designs I run into usually round things to a 1/8" or 1/4", which is a good bit larger (easier to see and cut to, for me) than 1 mm.

    I appreciate everybody chiming in on the topic!
     
  12. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    If the measurement is (for exampel) 753mm and you want to round that to 750, this 3mm error (1/8 inch) may very well make the shape of the hull 25mm (one inch) wrong. See picture.
     

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  13. Poida
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    Poida Senior Member

    Maybe hard to explain myself here, but in some cases accuracy is required in others it's not, you decide but the stacking of measurements as described is the problem. It wasn't explained what this is, so I will.

    If your first measurement is 753mm and you mark it off at 750mm your first measurement is 3mm short. May not be a problem. Your next measurement is the same so you mark off another 750mm. Now this measurement is 6mm too short.

    By the time you get to the end of your project you could be 100mm or more out, we now have a very significant error.

    When measuring, and I'm in engineering, even though I would take every measurement to .5mm, I always take each measurement from the root position eg.

    If the first measurement is 753mm I will measure it 753mm. If the second measurement is 753mm from there I add the two together and measure from the start 1.506mm. This prevents compounding errors regardless of how small.

    However as has been suggested if the measurement is 75mm why not measure 75mm it takes the same amount of time and your not likely to end up with one side of your boat 100mm shorter than the other.

    Also check measurements diagonally to obtain squareness.
     

  14. yokebutt
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    yokebutt Boatbuilder

    Unless you have a really crude metric ruler, I just don't see the problem.
     
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