Deep, small diameter holes are difficult to machine. Smaller diameter drills tend to wander and are prone to break, therefore, they are not recommended for convenient mass production. Chip removal becomes difficult while drilling deep holes.


     

Drills should enter and exit surfaces that are perpendicular to the centerline of the hole. When the drill tip contacts the surface, the tip will wander if the surface is not perpendicular to the drill axis. Exit burrs will be uneven around the circumference of the exit hole. This can make burr removal difficult.
  
      
 

Blind holes should not have a flat bottom. Flat bottomed holes cause problems with subsequent operations (for example: reaming). A standard twist drill creates a hole with a conical bottom. 
  
 

Holes should not intersect a cavity. If an intersection is unavoidable, at a minimum, the centerline of the hole should be outside the cavity. During machining, the drill follows the path of least resistance when it intersects a cavity. There is a good chance that the drill will wander when it re-enters the material.
           
 

When a hole intersects with the side of a feature, at least 75% of area of the hole should be within the material. When the hole is being drilled, there is a good chance the drill will wander if a large portion of the hole is outside the material. The problem becomes more severe when the axis of the hole is on or near the edge of the material.
     
 

When a hole intersects with the side of a feature, at least 75% of area of the hole should be within the material. When the hole is being drilled, there is a good chance the drill will wander if a large portion of the hole is outside the material. The problem becomes more severe when the axis of the hole is on or near the edge of the material.
     
 

Try to avoid pockets and slots that are narrow and deep. Longer tools are more prone to breakage and chip removal becomes difficult, especially when the pockets and slots are blind.
                                   
 

Design milling areas which do not require longer end mills for machining. Longer end mills are prone to breakage and chatter and require longer machining times.
               
 

Edges on the tops of pockets, bosses, and slots should be chamfered and not filleted. For outside corners, chamfers are preferable over fillets. An outside fillet requires a special cutter and a precise setup, both of which are expensive. 

 

Try to avoid features (or faces) too close to each other such that the gap between them is too narrow to allow the milling cutter to pass through. If narrow regions are unavoidable, then they should not be too deep. The size of the milling cutter is constrained by the smallest distance between the faces of the feature. Long, small diameter cutters are prone to breakage and chatter. Larger diameter, shorter cutters are generally preferred.
            
 

Uniform draft features can be more efficiently machined by applying faster milling cycles. Typically, non-uniform draft features require longer machining times. In many instances, a small design element might change the feature from uniform draft to non-uniform draft or vice versa.
 

Milled pockets and bosses should not have a chamfer between the side walls and the base of the feature. Instead, use fillets matching the end radius of a standard ball nose cutter, if required.
 

Sharp inside corners cannot be produced by milling and require more expensive machining methods like EDM. When designing a corner, the edge along the cutter length should match the radius of a standard tool. If a sharp corner is required for mating clearance, then drilling a separate relief hole as shown below may serve the purpose.
   
 

Features should be accessible to the cutting tool in the preferred machining orientation. 

Blind bored holes should be defined with tool relief at the end of the hole.
 
 

Blind axial keyways should have a radius at the end to suit the cutting tool.
  
 

Wherever possible, turned parts should be designed so that a tail stock is not required. This is done by designing the part to be stubby rather than long with a high aspect ratio.
 

The minimum radius on internal corners of a turned part determines the cutting inserts that can be used. It is always recommended to use cutting inserts with larger radii. If possible, inside corner radii should be left to the discretion of the manufacturer.
          
 

Avoid sharp right angles along the OD. The profile must match the taper of the groove tool or be inclined to the turn axis such as to allow use of standard inserts.
 

The width of axial slots and keyways on turned parts should be symmetrical about the turn axis.