Combined (Cascading) Mistake

In most cases a model maker does not have to be deadly precise. A tolerance for single part or element might vary from 0.5 mm (0.020″) to 1mm (0.040″). A tolerance may be even bigger for larger models and components.  Considering that a minimal reasonable thickness of an element and a wall limit usually should not be less than 0.5 mm (at least model maker has to try to keep it that way), you don’t have to worry very much about a single mistake. Let’s emphasize the word “single” in this last phrase, because there is a treacherous phenomena known as a “combined” mistake, which must not be underestimated nor overlooked.

A combined mistake is a summary of series of single mistakes in self-repeating elements lined up to create a whole part or element.  A single mistake alone might be sometimes microscopical, which often makes it very difficult to compensate for. That single mistake, however, multiplied by number of self-repeating elements is resulting in a combined mistake in overall dimension, which might be quite significant and compromise the whole part, element or an entire model. A combined mistake is like a snow ball – the greater the number of self repeating elements, the more significant a combined mistake will be.

A classic example of an element with a high risk of making a combined mistake is stairs. Stairs is a model maker’s nightmare. Making stairs requires the most precision in preparing a material for single stairs. Often a required precision is beyond the limits of possible if you have to calculate for hundreds of a millimeter. Say you have a stairs containing single stairs of 1.1 mm. You are making it from material that is 1mm thick and you did not compensate for 0.1 mm. After 100 stairs a combined mistake will be 0.1 x 100 = 10 mm!!! (0.4″). Real trouble, isn’t it? There is no way such stair strip can be installed, even if tilted – the gap will be too great.

It is important to understand a danger of a combined mistake, to stay ahead of it and to compensate as it builds up. First you have to do whatever possible to adjust a thickness of material, so it will be as precise as possible. Assembling stairs, measure thickness of the built portion, say, every ten steps. A larger portion of stairs will reveal the value of the initial combined mistake before it will become anymore significant. Compensate for that value by adding a thin piece of material, maximally close to the value of the initial combined mistake and keep assembling. After another ten steps check the overall thickness again, and not only of the next portion, but of the whole twenty steps portion. It will reveal a combined mistake of the next ten step portion and possible remaining combined mistake of the previews one. Remember that each portion might contain its own single mistake that also can summarize and give you another combined mistake at the end. Therefore, you have to check regularly the whole portion that is built so far and to compensate. This way you will reverse the effect of combined mistake’s build up. Series of small, almost invisible compensations along the way will have the same “snow ball effect”, only working to your advantage.

Another classic example of the operation with high risk of combined mistake is hand scoring materials in order for them to depict siding or masonry pattern. I saw many times over how inexperienced model makers moving a ruler with each row, ending a scoring higher or lower than the edge of the wall.  Or even worse – when separately scored wall’s portions are put together, scoring lines do not match. There are also many other operations that anticipate linear or circular division with high risk of combined mistake.

It is important to remember that such division should not ascend from single row to whole. The overall dimension of the part or element has to be marked first and than to be divided into required number of rows using “method of two” or “method of three”.

Unless it is absolutely crucial part of the design, a number of rows (siding strips or brick layers) is not quite important. If it will work for your advantage, it is a good idea to adjust a number of rows so the width of a single row will be rounded up. By sacrificing one or two rows you will earn more definitive value of a single one, reducing the very possibility of a combined mistake.

Besides the two examples that I described, there are numerous similar parts and elements where combined mistake might build up during fabrication or assembly. Evenly distributed windows, doors, openings, road and parking lot markings, corrugated surfaces, grids of any types and many more. Any situation where self repeating elements are lining up should keep a model maker alerted for combined mistake.

A use of modern model making technology, 3D modeling and rapid prototyping plays a great role in reducing the danger of  a combined mistake. While developing a model digitally, a model maker able to input an exact value of the layer, row or material thickness up to thousands of a unit and have the part produced by rapid prototyping machine exactly as ordered. Modern techniques take off the model maker’s shoulders a great deal of hassle related to combined mistake as well as other troubles. However, as long as model’s assembly and fabrication of selected components stays manual, a danger of having a combined mistake is real. Model makers have to be aware of this phenomena,  stay alert, focused  and to know how to predict, prevent a combined mistake and how to compensate for it.