Hole Machining Operations Operations which may be completed on drilling and boring machines. Hole Machining Operations

Tolerances of Hole Dimensional Tolerances Geometric Tolerances X – Y Location Diameter Depth Geometric Tolerances Perpendicularity/Angularity Circularity (or oval) Straightness Surface Finish Rough with twist drill Smooth with reamer

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 Flutes Shank Tip Angle 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 Hole Machining Operations

Drilling (Cont.) Twist drills always follow existing holes

Sizes of Twist Drills 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 Below: all-in-one step drill

Drills specially for wood Spur Bits (fig. 20-9 text) Cuts clean, large holes in wood. Forstner Bits (fig. 20-10 text) Cuts flat bottomed hole (good counter boring tool for wood projects) Hole saw (fig. 20-11 text) Combination drill and saw: drill in center aligns saw blade-type cutter.

Multi-Spindle Drilling

Tapping Tap produces uniform, internal helical threads. Requires an appropriate size tap drill hole in the workpiece prior to being tapped Refer to “tap drill” reference charts

Tapping: example What size tap drill is needed for 10-24 screw? 10-24 screw has nominal diameter = .190 inches For coarse (UNC) threads need No. 25 drill. No. 25 drill – look up twist drill size chart: No. 25 drill diameter = 0.1495 inches (to tap a .190 diam. screw need a .149 hole)

Reaming Uses a multi-fluted cutting tool called a Reamer (ref. Figure 20-14 text) Existing hole is enlarged and accurately sized

Reaming (Cont.) Produces close tolerance holes Good surface finish Good roundness Accurate Diameters Applications: sliding fits

Hole entry treatments: Counterbore & Spot Face Counterbore creates recess for screw head. Spot face creates a smooth and square surface around the top of an existing hole.

Hole entry treatments: Countersink A countersink is a conical enlargement of a top of an existing hole. Required for a flat head screw.