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Introduction to Numerical Control _NC_

VIEWS: 5 PAGES: 19

									Basic Hole Making
Machine Planning for Hole
Operations
®Not all holes are equal!
®Some holes are rough holes
®Some holes require tight tolerances
®Some holes are compound features
®Consider hole attributes when selecting
 hole making operations
Attribute 1: Basic Shape of Hole
®“Simple” thru hole
®“Simple” blind hole
®Sub-features
  ®Threads
  ®Counterbore
  ®Countersink
  ®Spotface
Attribute 2: Size of Hole
®Diameter
  ®“Standard”
  ®“Non-standard”
®Very Large
®Very Small
Attribute 3: Tolerances of Hole
®Dimensional Tolerances
  ®X – Y Location
  ®Diameter
®Geometric Tolerances
  ®Perpendicularity/Angularity
  ®Circularity/Cylindricity
  ®Straightness
  ®Other
®Surface Finish
Hole Making Operations
Center Drilling
®Mark the X – Y location of the desired
 hole
®Prevents twist drill “wander”
®Especially helpful when:
  ®Workpiece is not flat
  ®Holes are not perpendicular to the part
   surface
  ®Close positional tolerances are required
Drilling

®Twist drill is most common drilling tool
®Creates or enlarges non-precision holes
  ®Drilling is a roughing operation
  ®May produce coarse, helical feed marks
  ®Poor dimensional accuracy
®Creates small burrs on entry, coarse
 burrs on exit
Drilling (Cont.)
®Twist drills always follow existing holes
®Twist drills require workpiece material
 on all sides of the hole
®Twist drills only cut on the bottom of the
 tool
®Cutting forces are greatest at “break
 through”
Drilling Tools
®Four “series” of twist drills
     ®Fractional - 1/64” - 4” in 1/64 steps
     ®Letter - A (.234”) to Z (.413”)
     ®Number - #1 (.228”) to #97 (.0059”)
     ®Metric - .5 to 80mm
®Common Twist Drill Wear Patterns:
     ®Burnt corners - spindle RPM too high
     ®Broken edges/cracks - Feed too high
Common Drilling Techniques
(with twist drills)
®Center Drill all holes requiring accurate X – Y
 location
®Peck Drill deep holes (more than 3X drill
 diameter)
®Step Drill (drill pilot hole) large holes
  ®Select pilot hole slightly larger than the drill web
  ®Do not select pilot hole that is excessively large
Boring
®Boring is a single point cutting operation
®Produces a precise internal cylindrical
 surface
®Enlarges and straightens existing holes
®May be used to modify existing hole
 location and orientation
Boring (Cont.)
®Produces close tolerance holes
®Good surface finish is possible
®Good roundness
®Accurate diameters which may have
 non-standard diameters
Reaming
®Uses a multi-fluted cutting tool
 called a Reamer
®Existing hole is enlarged and
 accurately sized
®Requires an appropriate size hole
 in the workpiece prior to reaming
  ®Refer to stock allowance charts
Reaming (Cont.)
®Produces close tolerance holes
  ®Good surface finish
  ®Good roundness
  ®Accurate Diameters
®Does not modify existing hole
 location or orientation
  ®A reamer follows an existing hole
Tapping
®Tap produces uniform, internal helical
 threads.
®Requires an appropriate size hole in the
 workpiece prior to being tapped
  ®Refer to “tap drill” reference charts
®Lubrication is essential during tapping
®Produces small chips
  ®Use pecking motion to break chips
Counter Bore

®A counter bore is a cylindrical
 enlargement of a top of an existing hole
Spot Face

®A spot face is the smoothing and
 squaring of surface around the top of an
 existing hole.
Countersink

®A countersink is a conical enlargement
 of a top of an existing hole.

								
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