Producing accurate threads in machining and workshop practice is critical both for workpiece quality and for the life of the tap. Choosing the wrong tap or drilling an incorrect hole diameter can lead to tap breakage, loose threads and significant loss of time.
In this article, guided by Temak Teknik, we cover in detail the parameters for correct tap selection, tap types, tap drill size calculation and the steps for hand tapping.
1. Key Parameters to Consider When Selecting a Tap
When selecting a tap, you should first consider your production conditions and the structure of the material to be machined:
- Production Quantity and Method: For small batches, hand taps or simple machine taps can be used. In series production, CNC or automatic tapping machines should be preferred.
- Cutting Direction: Depending on the application requirements, determine whether a right-hand or left-hand thread is to be cut.
- Material Structure: The hardness, toughness and softness of the material, and the type of chip it produces, directly affect tap selection.
- Tolerance Classes: The tolerance class marked on the tap (e.g. 6H, 6g) must match the technical drawing, and the threads produced must always be checked with a thread gauge.
2. Tap Types and Applications
Taps are divided into different categories according to their helix angles and chamfer forms:
- Form A Taps: Have a 6–8 pitch chamfer length and a 5° chamfer angle. Preferred for through holes in short-chipping materials.
- Form B Taps: Among the most common types in the field. Have a 3.5–5.5 pitch chamfer length and an 8° chamfer angle.
- Form C Taps: Have a 2–3 pitch chamfer length and a 15° chamfer angle. Used especially as spiral flute taps for short-chipping materials and in blind holes.
- Form E Taps: Preferred for narrow blind holes where the thread must be cut right to the bottom of the hole (1.5–2 pitch, 23° chamfer angle). Note: The shorter the chamfer, the shorter the tap life will be.
3. Tap Drill Size Calculation and Preparation
There are two vital steps to observe before tapping:
Drill Diameter Calculation
Before tapping, the diameter of the hole to be drilled is calculated with the following practical formula:
Hole Diameter = Nominal Diameter − Pitch
- Example: For a metric 10 (M10) tap, the pitch is 1.5 mm.
Hole Diameter = 10 − 1.5 = 8.5 mm
If an arbitrary drill bit is used instead of 8.5 mm, the threads may not engage or the screw will fit loosely (with play).
Countersinking and Lubrication
- Countersinking: The hole must always be countersunk after drilling. This allows the tap to start easily and prevents material bulging and tap breakage.
- Use of Cutting Oil: Cutting oil must always be used during the operation. Lubrication facilitates chip evacuation, reduces friction and ensures a smooth thread surface.
4. How to Hand Tap, Step by Step
Hand tapping generally uses sets of 3 hand taps:
- No. 1 Tap (One Ring): The taper (starting) tap.
- No. 2 Tap (Two Rings): The intermediate tap.
- No. 3 Tap (No Ring): The bottoming (finishing) tap that completes the thread.
Application Steps
- Starting and Squaring: After countersinking the hole, the No. 1 tap is positioned. To ensure the tap enters the part at exactly 90°, it is checked from two different angles (with a square or by eye) as it is started.
- Lubrication: A few drops of cutting oil are applied to the tap flutes.
- Advancing While Breaking the Chip: The tap wrench is turned evenly. When the tap begins to bind, it is turned back slightly every half/full turn to break the chip.
- Using the Remaining Taps in Sequence: After removing the No. 1 tap, the operation is completed by running the No. 2 and then the No. 3 tap, taking care to follow the same thread path.
Important Tip: Make sure the force you apply to the tap wrench is equal and balanced on both handles. Axial misalignment easily causes small-diameter taps to break.