One of the single biggest cost drivers for machined parts is the length of time it takes to machine it. The rigidity and strength of the actual cutting tools often determines how much time it takes. Very simply, the shorter a tool is, the faster it can feed, and the less the part will cost to make. The selection of these cutting tools is determined by the design of the part and a few simple rules can really help reduce machining time.
The larger a tool that can be used in that corner, the faster it can feed through the material. As the length of that corner increases, the length of the tool must increase as well and that tool must be fed much more slowly to avoid deflection and breakage. The relationship is worse than linear. For every doubling in length, the feedrate is more than cut in half. When figuring costs, assume that a double of the ratio equates to a double of the cost of that feature. A good ratio is less than 3:1. Once you get up to 4, 5, or 6 to one, the feedrates are much slower.
Sometimes you just need to have a long small radius because of assembly issues. There are still options to reduce the cost of features like this. Figure 3 shows how you can make a virtually square corner with very little intrusion into the surrounding walls. This is a great technique if for weight or assembly reasons you can't tolerate a larger radius.
The key to this feature is to not put the center of the radius on the intersection of the inside edges. Put the center point inboard and then you can adjust it to fit your application. Use the biggest radius that fits the application as well.
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