1. MACH 112: Cutting Tools & Measurement
TAPPING PROCEDURES
MACH 112: Cutting Tools & Measurement

2. OBJECTIVES tOBJECTIVES:
Select the correct tap drill for a specific percentage of thread.
Select the correct cutting fluid for tapping.
Tap holes by hand or with a drill press.
Identify and correct common tapping problems.
Compiled by Ronald C. Schwartz

3. Tap Use TAPPING PROCEDURES
Taps are used to cut internal threads in holes.
tapping can be performed by hand or with a machine.
A tap wrench or a T-handle tap wrench provides driving torque while hand tapping.
Fig. B-134 Tap wrench.
Fig. B-135 T-handle tap wrench.
Compiled by Ronald C. Schwartz

4. Tap Use TAPPING PROCEDURES
Fig. B-136a Hand tapper (Courtesy of Enco).
To obtain a greater accuracy in hand tapping, a hand tapper is used.
Compiled by Ronald C. Schwartz

5. Tap Use TAPPING PROCEDURES
Fig. B-137 Drill press tapping attachment.
Drill presses can be used for tapping with a tapping attachment.
Compiled by Ronald C. Schwartz

6. Thread Percentage and Hole Strength
TAPPING PROCEDURES
Thread Percentage and Hole Strength
Thread strength depends on the work-piece material, percentage of full thread used, and the length of the thread.
Percentage of thread produced is dependent on the diameter of the drilled hole.
Tap drill charts give drill sizes to produce 75% thread.
Common practice is to have a bolt engage a tapped hole by 1 to 1-1/2 times its diameter.
Threaded assemblies are usually designed so the bolt breaks before the threaded hole strips.
Compiled by Ronald C. Schwartz

7. Drilling the Right Hole Size
TAPPING PROCEDURES
Drilling the Right Hole Size
If a thread depth other than 75% is wanted, use the following formula to determine the proper hole size:
Compiled by Ronald C. Schwartz

8. HOW TO DO IT… 3 Hand Tapping
Countersink the hole entrance to a diameter slightly larger than the major diameter of the threads (Fig. B-139).
This allows the tap to be started more easily & protects the start of the threads from damage.
Mount the work-piece in a bench vise so the hole is in a vertical position.
Tighten the tap in the tap wrench.
Fig. B-139 Preparing the workpiece.
Compiled by Ronald C. Schwartz

9. HOW TO DO IT… 4 Hand Tapping
Place the tap in the hole in a vertical position.
Starting the tap.
Start by turning 2 or 3 clockwise turns for a right-hand thread, keeping steady downward pressure on the tap.
Tapping a thread by hand.
When the tap is started, it may be turned as shown.
Compiled by Ronald C. Schwartz

10. HOW TO DO IT… 5 Hand Tapping
After the tap is started for several turns, remove the tap wrench without disturbing the tap.
Checking the tap for squareness.
Place the blade of a square against the solid shank of the tap to check for squareness.
Check two positions 90 degrees apart.
If the tap is not square with the work, it will ruin the thread & possibly break in the hole if you continue tapping.
Back the tap out of the hole &restart.
Compiled by Ronald C. Schwartz

11. HOW TO DO IT… 6 Hand Tapping
Use the correct cutting oil on the tap when cutting threads.
Turn the tap clockwise one-quarter to one-half turn, then turn back three-quarters of a turn to break the chip.
Do this with a steady motion to avoid breaking the tap.
When tapping a blind hole, use the taps in the order starting, plug, and then bottoming.
Remove the chips from the hole before using the bottoming tap.
Be careful not to hit the bottom of the hole with the tap.
Compiled by Ronald C. Schwartz

12. HOW TO DO IT… 7 Hand Tapping
A 60-degree point center chucked in a drill press to align a tap squarely with the previously drilled hole.
Fig. B-143 Using the drill press as a tapping fixture.
Only very slight follow-up pressure should be applied to the tap. Too much downward pressure will cut a loose, oversize thread.
Compiled by Ronald C. Schwartz

13. Common Tapping problems & Possible Solutions
HOW TO DO IT… 8
Common Tapping problems & Possible Solutions
See the entire table on page 75 of your textbook.
Compiled by Ronald C. Schwartz

14. Review Name two types of tap wrenches
Calculate the following tap drill size:
60% thread for a ½-13 UNC
5/8-11 UNC
Why must a hole be countersunk before tapping?
Why must the tap rotation be reversed while tapping?
What precaution(s) must be taken when tapping blind holes?
How can you insure that a tapped hole is square when tapping in a machine?
Compiled by Ronald C. Schwartz