Wednesday, July 22, 2020

CN Woodchip Car - Start From Scratch

I've often wrote about the inability of SketchUp to manage sub-millimetre dimensions. The software is now showing its age, having not been designed to take into account 3D printing back in the early 2000s.

The thing is that as you add parts to your model and merge together shape, small discrepancies creeps here and there. At first, it is unnoticeable. If you enter 0.500mm then you really get 0.500mm, but after a few merges of various components, that becomes 0.498 and some weird angles start to appear. What used to be 90 degrees, will misalign itself on other component. It has been noted by users on SketchUp forums the software have a hard time dealing with very large and very small dimensions.

I've redrawn the model 3 times in the last few days, often from scratch, but always ended up with the same discrepancies. I've worked with CAD softwares for decades now and I now I'm a precision-oriented kind of maniac in this regard. I can't stand odd numbers when dealing with plans and I know, after controlling several parameters, I wasn't the main source of discrepancies.

Most people would think it isn't relevant because most of these errors are several orders of magnitude smaller than what a 3D printer resolution could manage. That's true. However, it becomes a real problem when you try to merge elements together. SketchUp works with faces instead of volumes, so if "parallel" faces aren't really parallel, you can no longer deal with coplanar geometry. In my case, it made it almost impossible to merge together the underframe and walls. This is a big problem because drawing the model using independent components then merging them together later as you go along allows you to manage parts easier instead of fighting with a single part.

Main shell - Almost completed

My first reaction was simply to model the prototype by making it 100 time bigger. That way, I wouldn't have to deal with sub-millimetre dimensions. yet the same issues. Thus, I tried this morning to draw the model in meters. Each millimetre became a meter. In fact, 1.5mm is now 150m. By setting the drawing template to meter eliminates SketchUp inadequacy to address small dimensions. While the idea to enlarge the model was a good one, it only worked when I switched to the metric template (of feet if you draw in imperial units). Thus, the problem isn't about absolute numbers, but related to unit. If you draw 15000mm, you'll get discrepancies. But if you draw 15m, you get absolutely none. Lesson learned! Hope it will help peoples out there using SketchUp! And yes, I know several new intuitive 3D modelling softwares are out there. I'm just trying to make the best of what I have.

Main shell showing the floor depression hiding the metal weight.

That said, redoing the model several times was a good way to improve the design and I'm glad I did. I've reach the point I can redraw almost from memory. Many things have been simplified and I found a way to hide the weight in a way that will look nice when the car is empty while not scrapping all the underframe details. As much as I can, I feel this car must be as straightforward to assemble with being easy to superdetail for modellers wishing it. Maybe I'm wrong to try catering to two different crowds, but this is something worth trying to achieve.

In the next blog about this car, I'll explain how the model will be assembled using - I hope - cool graphics to demonstrate the design.


  1. They must store those dimensions as floating point numbers, and not in a large enough format to avoid rounding errors. That's too bad :(

    1. Probably. It wasn't its original purpose. However, since I switched to metrics, I've got absolutely no issue, which is good to know.