There’s no special reason for a rail gauge of 1425 mm, it could easily be a round figure like 1400, 1500 etc. How the standard sizes are arrived then? Well, there’s a standard way of doing that too. All numbers are created equal, and are an ultimately arbitrary assignment of scale; once that scale is assigned, there are preferred numbers within that scale. How this works is interesting, and provides insight into the nature of standards.
Charles Renard designed a standard for use with decimal scales; a way of subdividing them by 5, 10 or more such that the ratios remain close to constant. Therefore, following Renard, we’re going to sell widgets delivering the following levels of awesome: 1 Gr, 1.6 Gr, 2.5 Gr, 4 Gr, 6.3 Gr and 10 Gr.
If you are designing something, from an experiment to a widget to a presentation, choose preferred numbers. Like magic, your data will have a meaningful spread, your parts will be more likely to be interchangeable, and your ratios will have a mysterious, aesthetically pleasing quality: the Golden Ratio is 0.618, and is the basis of the series of preferred numbers. The Reynard numbers are a compromise between the aesthetic and mathematical ideal of harmonic proportion, and the realities of actually measuring and building stuff.
Basically, if you work with decimal scales, burn these numbers into your heart: 1.6, 2.5, 4, 6.3.
Nesting the parts while machining using CAM
While working on a recent CAM project using CAD model, we found few interesting techniques. In the first job, we requested the operator to use a 1/4″ Ball nose cutter of 1″ long. We were expecting there could be some tool flexing because for the first cavity profile cut. But he used almost 1 1/2″ long cutter. Finally, once the machining is done there is a clear cut draft left on the part which is almost 3 degrees. Also, there was not enough overpass of ball nose at the bottom of the part leaving a radius there. It was not too bad other than doing some manual cleanup of the part after machining.
In our second batch of CAM, we nested four parts as shown on image from two separate CAD models. So, we have to make a single pre-machining block to make two separate parts of quantity two each. Also, we tried to machine only up to 4 mm around the part using 1/4″ cutter. This saved us almost 50% of machining time as we eliminated machining unneeded stock. Also, we ensured the cutter overhang is only 22mm just more than nose overpass. This didn’t cause any tool flex and squeak.
Nesting the Parts example
Another interesting thing is after doing the rough cut with 1/4″ cutter, we lost the datums where we set original zero. So, for finish cut we have not changed X and Y coordinates. But, asked the operator to reset Z with respect part top surface which was intentionally left unmachined before. This helped us in quickly running the finish cut using 1/8″ cutter without much time on reference point set.
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