Drilling, Design Rules
engineering fundamentals Drilling, Design Rules
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Drilled Part Design
The following are guidelines for drilled part design.
  1. Advantages of drilled holes include accuracy and sharpness of edges. Since machining is expensive compared to other manufacturing processes, drilling to create a hole should be justified by looking at alternatives. Before adding drilled holes to a design, ask yourself whether the hole is needed and/or whether it can be cast, molded, or pierced with sufficient accuracy instead of drilled.
     
  2. Specify standard drill bit sizes. Unusual hole sizes bring up the cost of manufacturing through purchasing and inventory costs.
     
  3. Through holes are preferred over blind holes. This has to do with the fact that a blind hole does not provide as much leeway for chip exit and cooling. Operations such as reaming and threading after drilling are more easily conducted on a through hole.
     
  4. Do not specify flat-bottomed holes. Twist drills create cone-bottomed holes and flat-bottom holes cause problems with reaming, etc.
     
  5. If possible, do not specify holes that are smaller than one-eighth inches in diameter. Drills for smaller holes tend to break and for convenient mass production, are not recommended.
     
  6. For large holes, try to cast in a preliminary hole that must only be bored out to specification. This saves material, transportation cost, and drilling cost.
     
  7. When dimensioning holes, it is better to use rectangular rather than angular (or polar) coordinates. Angular coordinates will require the machinist to set up a dividing head or to re-dimension the part, both of which take time.
     
  8. Minimize the number of drilled hole sizes so that tool changes are minimized.
     
  9. Minimize the number of directions on the part that holes must be drilled from.
     
  10. The entrance and exit surfaces of a drilled hole should be perpendicular to the hole axis. The reasons for this are as follows:
     
    1. Upon entrance of the drill, the drill tip will wander if the surface that the tip contacts is not perpendicar to the drill axis.
    2. Exit burrs will be uneven around the circumference of the exit hole. This can make burr removal difficult.

     
    Bad and good examples of entrance and exit lands are shown in the figure below.

  11. Intersections of drilled holes with other cavities should be avoided if at all possible. If interesection with a cavity is unavoidable, the drill axis should at least be outside of the cavity, as shown below.


  12. On drawings, multiple holes in a flat surface should be located from the same horizontal and vertical datums.
     
  13. If there are protrusions surrounding a drilled hole, it may be difficult to bring the drill press head close to the entrance surface, resulting in a drill bit that is prone to wandering, chatter, and other instabilities. This problem can be solved by providing a fixture with a drill bushing close to the drill bit. However, part design must allow for this fixture, as shown below.


  14. Drilled Hole Depth:
     
    Deep, narrow holes with length to diameter ratios of larger than three should be avoided. Deeper holes are possible but the drill will tend to wander and possibly break. One way to avoid a deep, narrow hole is to use a stepped entrance. Blind holes should be drilled to a depth 25% deeper than the actual hole in order to provide space for chips.
     
  15. For bored hole part design, see design of parts for boring.

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