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Design Recommendations
The good practices in
designing for the milling process
should be followed.
If a machined hole is
necessary, we list the following design recommendations according
to Bralla in his book Design for Manufacturability Handbook.
- Through holes are
preferable to blind holes because of easier clearance for tools
and chips, especially when secondary operations such as reaming,
tapping, or honing are required.
- When blind holes are
specified, they should not have flat bottoms. The preferred drill
bit generates a pointed hole, and if other bottom shapes are specified,
secondary operations are required.
- Avoid deep holes because
of chip-clearance problems and the possibility of deviations from
straightness.
- Avoid designing parts
with very small holes if the small size is not truly necessary.
Small drills are more susceptible to breakage.
- If the part requires
several drilled holes, dimension them from the same surface to
simplify fixturing.
- Design parts so
that all holes can be drilled from one side or from the fewest
number of sides.
- Use the same size
of holes, fasteners, and other screw threads as much as possible
so that the number of drill spindles and drill changes can
be minimized.
The following recommendations
are accounted for in the WebCAD software as rigid rules that can't
be broken:
- The drilled entry
surface should be perpendicular to the drill bit to avoid starting
problems and to help ensure that the hole is in the proper location.
- The exit surface
of the drill also should be perpendicular to the axis of the
drill to avoid breakage problems as the drill leaves the work.
- Standard drill
sizes are used to avoid the added cost of special drill
grinding.
- There are
limitations as to how closely two simultaneously drilled
holes can be spaced.
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