1. Drills & Drill Press Ops.
MACH 112: Cutting Tools & Measurement

2. After completing this unit, you should be able to…
OBJECTIVES
After completing this unit, you should be able to…
Identify tools for drilling holes
Identify tools for countersinking and counterboring.
Select speeds and feeds for countersinking and counterboring.
Determine required hole sizes for c’bores & c’sinks

3. How a drill cuts

4. Parts of a Twist Drill
Features of a twist drill (Besly Cutting Tools, Inc.).

5. Drill Point Angles
Long point for
soft materials

6. Drill Types Oil-hole Drill Straight Fluted Drill Insert Drill
Spot Drill
Oil-hole Drill
Straight Fluted Drill
Insert Drill
Straight Flute Gun Drill
Spade Drill
Center Drill
High Helix Drill

7. Drill Sizes Numbers Letters Fractions #80 to #1 (0.0135 – 0.228)
A to Z (0.234 – 0.413)
Fractions
1/64 – 3 ½

8. Drilling Hints Examine drill before drilling Set correct RPM’s
Check clearance above and below work piece
Clamp work securely
Always clean drill shank and machine taper before inserting drill
Use shortest drill possible
Always start each hole with a SPOT drill the hole
Pay attention to the chips being produced ie. Shape and color
Never force a drill if more pressure is required

9. Countersinks
A countersink is a tool used to make a conical enlargement of the end of a hole.
The included angles on commonly available countersinks are 60, 82, 90, and 100 degrees.
Most flathead fasteners have an 82-degree head angle
In the aircraft industry, a 100-degree angle is prevalent.
Cutting speed used when countersinking should always be slow enough to avoid chattering.

10. Counterbores
Counterbores are tools designed to enlarge previously drilled holes.
They are guided into the hole by a pilot to ensure the concentricity of the two holes.
They are often used to recess a bolt head below the surface of a workpiece.
The diameter of the counterbore is usually 1/32” larger than the head of the bolt.

11. Counterbores COUNTERSINKING AND COUNTERBORING A two-flute counterbore.
A multiflute counterbore.
Multiflute Counterbore.
Fig. H-101 The counterbore is an enlargement of a hole already drilled.
Pilot MUST have clearance!
A two-flute counterbore.
Two-flute counterbore.

12. Counterbores
When a variety of counterbore & pilot sizes are necessary, a set of interchangeable pilot counterbores is available.
Fig. H-102 Interchangeable pilot counterbore.
Fig. H-103 Pilot for interchangeable pilot counterbore.
Counterbores are made with straight or tapered shanks to be used in drill presses and milling machines.

13. Counterboring Tips COUNTERSINKING AND COUNTERBORING
For most counterboring operations the pilot should have 1/64”of clearance.
If the pilot is too tight in the hole, it may seize & break.
If there is too much clearance, the counterbore will be out of round & have an unsatisfactory surface finish.
The pilot must be lubricated while counterboring.

14. Spotfacing COUNTERSINKING AND COUNTERBORING
Counterbores or spotfacers are often used to provide a flat bearing surface for nut or bolt heads on rough castings or a raised boss.
This operation is called spotfacing.
Fig. H-104 Spotfacing on a raised boss.

15. Cutting Speeds
Speeds are half the speeds used for twist drills of corresponding diameters.
Choice of speeds & feeds is affected by condition of the equipment, power available & material being counterbored.

16. Calculating Sizes

17. Review What three included angles are found on drills?
What drill point angle be used on hard material?
Name four different types of drills
How are English drill sizes identified?
What are the three most common angles found on c’sinks?
How much clearance is normally allowed between a bolt and a c’bored hole?
How much clearance is needed to accommodate the counterbore pilot?
What is spotfacing?

18. QUESTIONS