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

2. 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

3. 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

4. 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

5. Drilling (Cont.)
Twist drills always follow existing holes

6. 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

7. 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

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

9. Multi-Spindle Drilling

10. 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

11. 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 = inches
(to tap a .190 diam. screw need a .149 hole)

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

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

14. 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.

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