This is a post on extremely accurate measuring.
95% of measurements will never needs this sort of detail. It is here though if anyone needs to construct an extremely accurate 3D measurement of a model for things like major hull rebuilds and similar.
Measuring is a vital part of scratch-building and there's some very important rules that apply in measuring anything but especially when it's for accuracy-critical aspects like finding dimensions for 3D design or scratch-build design. As no one wants to read a lecture I'll break this into two parts and write it in point form,
1 Tools
It is impossible to do any measuring without precise, accurate measurement tools and their associate equipment.
- Steel Rule.
This should be graduated in two parts, firstly in fine graduations and then coarse graduations. For metric rules this would be single millimetres and then double millimetres.
- Vernier Gauges.
These are critical and more important than a steel rule. A cheap, accurate Vernier can be picked up for a few bucks. The gauge should have a digital readout. It should never be used for anything other than fine measurement such as scribing along an edge. Although cheap they break amazingly easily.
- Clamps
It is impossible to measure anything that gallops across the bench. You should preferable have a small table vice as well as a flat vice. These vices should have rounded, soft material in them that will not deform the object you measure. Never use a vice as an anvil even if it advertises that it can be used that way. Never over-tighten a vice.
2 Concepts
There are some simple concepts behind accurate measurement that stop the accumulation of measuring error. I won't try and reduce the whole year-long measuring error syllabus in one post, I'll just put the again in point form to clarify things you probably know.
- Datum.
A 'datum' is a group of reference points; data. These are known points.
The reason this is important is that you always measure from a datum point. Always. Measuring from successive points along a model will lead to 'cumulative error' where you cut some piece and even though you were very careful it's too short, too long or something similar. The unavoidable error in each measure has accumulated to give you a poor result.
A datum is not a single data point. In 3D applications it is usually a set of points on a plane. An example of this would be on a vehicle hull where the upper and lower hull join. After checking that it is a flat and not a curved plane we then carefully measure a set of points on it, usually the corners if any. Now we can measure from these known points elsewhere confident our proven base is sound.
You may end up with more than one set of datum in that you'll expand it to get to hard-to-measure-to areas, I'll touch on that in a minute.
- Triangles.
The world is made up of triangles, I'm not kidding. You should always measure to the point being measured to from two datum points. This makes it extraordinarily easy to fix in place with compasses or circles in CAD. To achieve this it is generally best to try and choose your datum points that are as close as possible to forming an equilateral triangle with your measuring point.
Measuring from only two points fixes that new point in only one plane, if you think on it you can get 'swing' to the sides of the line drawn between your two points. You now measure from a third point and this fixes your measured point in space. Once again it's best if your datum points are as close as possible to an equilateral triangle. If you imagine the datum as the 'ground' you have now formed the edges of a three-sided pyramid, locking the measured point.
- Booking & Diagrams
It is essential that you develop a sketch of your measurements and write or 'book' your measurements on this. It doesn't have to be a work of art but it has to be both large enough to fit all the measurements and clear enough for you to look over later and know what it is you're looking at. Because of this you may need multiple views of what you're booking.
It's a surveyor's maxim that 90% of mistakes are Booking Error. Your instruments are accurate, you are sadly not :) If you are tired, your hands are shaking, you are distracted or so on you cannot make accurate measurements. When you write take a measurement it should be repeated, then written down, then checked against the original measurement. I can't stress how important it is to double check everything. A rule is that when you are unsure if you have done something correctly you do it in reverse and check the result against the first measurement. If the answers vary you scrap both measurements again and start afresh.
- Extrapolation vs Interpolation, the perils of angular error.
'Angular error propagates to the value of its extension' means that if you are making a line and your angle is off that error is doubled for every time you double the length of this new line. To avoid this nightmare and unpredictable result you never take two points and extend the line, instead you only ever draw between two points. This is pretty much an iron law.
- Extending the Datum.
Obviously your initial datum is not going to be enough, you might need a point of reference that is floating in mid air. In this case simply glue on a bit of stock material that cannot move out to that point, mark a point on it and measure off the datum as before using three points to fix it and one point to check it. Four measurements in all. This may seem absurd but having a solid datum is immensely helpful when you find an error because you only have to check back along the measurements you made until you hit your known datum.
- Marking Measurement Points.
All this is in vain of you can't be sure of where you marked to. My usual method is to take an awl and put a little divot in the model, making a note to putty it up later. As I generally am measuring off areas I have cut I'm going to be puttying that area anyway.
- Measuring useful points.
Obviously a model is only visible where it can be seen. This means measurement points should be on the places where the planes of the finished model meet. These corners in three planes should be on the outside of the model, never on the inside surface. If you do have to do interior and exterior measurements never have them on the same sketch.
Now, many places you will want to measure to will be floating in the air. Once again a sturdy little form is your best friend. Glue a bit of stock with a glue that is medium-strong but not permanent and which will allow you to remove the form later.
What Glues To Use On What Surface
