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In Rhino 4.0 Essential Training, author Dave Schultze shows how the 3D NURBS-based modeling tools in Rhino 4.0 are used to engineer products from toy robots to full-sized aircraft. This course concentrates on using Rhino 4.0 for industrial design and rapid prototyping, with a review of common 3D terminology using specific examples. Along with a comprehensive exploration of the Rhino interface, the course includes an introduction to building 3D objects with Rhino's three primary entities: the curve, the surface, and the solid. Exercise files are included with the course.
In this video, we'll define the two terms, Units and Tolerance, and explain their relationship. Unfortunately, some people start modeling and forget to check these two critical settings for a new project, or when using someone else's file. You can then end up with serious accuracy problems later. Sometimes, these errors can be so intrinsic that the project really needs to be remodeled with more appropriate settings. Yup! Sorry to tell you, but sometimes you do need to start over. Let's start off by examining Units first. We access those from the Tools options.
It's actually an icon that looks like a yellow gear. We can click that as well. I'm going to select, under Document Properties, Units, and right now we are in Millimeters. So, that's actually a pretty handy unit to use and is valuable for like product design whereas feet or meters would work better for large-scale projects. Let's take a look at some of these units we can set. So, here is all the familiar ones, Millimeters, the Kilometers, all the way down to Lightyears and Parsecs.
I haven't done a project that big yet. The good news about this is you can set the Units upfront and then actually change them anytime, multiple times during the course of a project, although you probably wouldn't want to, but Rhino will react by asking you if you'd like to scale the file. That can take a lot of problems away right off the bat, at least for changing Units. Let's now talk about the tolerance. This can be a little difficult to understand, but probably the best way to look at it is the accuracy of the file.
So, in this case, we've got three decimal places, and in general, a smaller number with more decimal places means more accuracy. A larger setting means lower accuracy. So, I've got a demonstration prepared; hopefully, it will make this even more clear. So, this file was generated with a high tolerance of three decimal places, and I decided to go ahead and put some thickness on this sail. You're seeing a radius of 0.4 units around.
Let's come up to the corner. This is where you usually have problems, but this is nice and clean. So, the 0.4 radius that ran around the entire perimeter, works fine with a tolerance that was far more accurate than it needed to be. Let's take a look at some low tolerance issues and how to identify them. In this case, I lowered the tolerance to 1, pretty high, and then ran a Fillet of 0.4, actually the same size. So, see, we have a tolerance level of a differential of about 1,000.
Let's see what happens. So, here is the result. We cannot generate geometry accurately or reliably. That's smaller than the tolerance. So, that's probably the biggest reason to have the tolerance as high as you possibly would need, although we have to be careful. You don't want to have the tolerance too high. It can add so much additional calculations to the software that it can slow down, especially with larger files. That's kind of the relationship between Units and Tolerance, and I'm going to tell you now the best way to start a brand-new file, if you have this option.
I'm going to go to File > New. When we get to this Template File interface, Rhino actually is organizing the units and tolerances together, in pairs. For example, we have Small Objects - Millimeters, which is what I've done in this file. This note will tell you exactly where it's best suited. For example, this says a small object millimeter is best for using objects smaller than a truck and for objects which must be built to manufacturing tolerances.
So, these are pairing up the units and tolerances into a logical connection. This is probably the best way to start a brand-new file. However, if you get a file from someone else, you definitely would want to check what units you're working in and the tolerance thereof before you do too much work. I'll go ahead and cancel out of this. Units and tolerances often get confused or ignored, and they're really not that hard to understand. I suggest use the Templates via the File > New command and just go with Rhino defaults.
However, if the project is expected to have intricate details, then by all means, increase that tolerance by a factor of 10 or so. One final tip: even though you can change the tolerance at any time in Rhino, that's not a good solution and can easily make problems much, much worse. A changed tolerance only affects objects created from that point forward, so it does not fix older objects. Then you actually have the far worse problem of different pieces of geometry with different tolerances, definitely a situation to avoid.
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