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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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