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Tips & Tricks - Design for machined parts
Designing cost-effective machined parts has some distinct differences compared to designing
cast or injection-moulded parts. One particular area is in specifying radii. Consider the part shown in Figure 1, which is modelled in a standard
3D CAD package using the same radius for both the walls and floor.
This is easily modelled using a single CAD feature, but causes problems for the machinist which aren't obvious at first glance.
The wall will be machined with a ball end mill (a cutter with full nose radius), and the floor will be cut with a flat end mill,
which will leave material leftover in the corner area (Figure 2).
This can be avoided by specifying a smaller floor radius (Figure 3). This allows the machinist to use a flat end mill with a radius to
machine the floor area in one operation. In general, the smaller the floor radius can be the better, with a 0 radius being the easiest
of all. This is of course limited by considering stress concentrations and the loading of the part.
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| Figure 1: Equal floor and wall radii |
Figure 2: Material cannot easily be removed |
Figure 3: A smaller floor radius allows faster machining |
| (images sourced from Pro CNC, see the full article
here) |
Note that this is opposite to the rules of designing cast and plastic parts, which need larger floor radii at webs etc. to
ensure complete filling of the mould and streamlined mould flow paths. Plastics in particular are notch sensitive, which can
result in crack initiation at sharp corners.
In terms of the part cost, the equal corner radii will easily cost 10x more than the
unequal corner radii, which is a good incentive when trying to reduce costs and improve manufacturing efficiency of machined parts. |
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