Thread Making--Screw Taps
Screw Taps are the tools for making medium and small size of threads. They have simple structure, easy to use and can be manually operated. But they also can work in machines. They are used widely for making parts’ threads. So let’s talk more about the knowledge about the screw taps.
What’s screw tapping?
Screw tapping is cutting the inner threads on steel, aluminum, titanium, plastic, etc…material with screw tappers.
（1）Which factors can affect the performance of the screw taps?
Cutting blade material;
Style of Screw tappers;
Cutter Bars of Screw Tappers;
Depth of the Holes
（2）Thread Pitch：The thread pitch is the distance between corresponding points on adjacent threads.
（3）Lead: Lead is a specification for the travel distance of a screw after a complete revolution. In short of “S”
（4）Nominal diameter of the thread：Pipe threads’ nominal diameter is the inside diameter (British Unit). The other kind of threads’ nominal diameter is the threads’ major diameter (Metric Unit).
（5）Threads’ Medium Diameter：The medium diameter is the most important because it controls the fit and strength of all the thread assembly. The medium diameter is on the node, and the width of this position matches the width of the adjacent groove.
Naming of threads
British Threads (BSPT/TPI)：British thread is marked in Inch Unit, which is developed by the United States, the United Kingdom and Canada according to the unified system.
Metric thread: the ISO (International Organization for Standardization) system is the global standard for metric thread.
How to design high performance screw taps?
During thread tapping, we must consider these factors: parts’ design, screw taps’ design&application. The goal is to reduce the cutting pressure while the enhance the cutters’ strength in maximum.
（2）Balance the options: Must balance all aspects of your application
（3）Important points for designing the thread taps
1）For softer sticky materials that form long chips
Simplify the structure of the thread taps;
Expand the front angle and the hook shape angles;
Expand the rear angle and interference;
Collapse blades easily
The screw tappers are fragile
Larger space for cutting chips
2）For Rigid Material
The screw tap has a heavy-load structure;
The front angle and the hook shape angles are small;
Shovel back and rear corner are small;
Cutting pressure is higher;
Design blade to robust to reduce the blade collapse;
The cross section is large;
Limited capacity space
（4）The factors for designing the screw taps：thread taps’ groove shape、cutter material、Surface hardening. These design features must be kept balanced in order to provide proper cutting, chip control, lubrication, and torsional strength.
The shape of the screw taps
（1）Types of screw taps’ cutting surface
Choose the hook screw taps correctly;
②Choose the smaller or negative hook thread tap correctly
（2）Screw Taps Cutting Tapered teeth
More tapered teeth from the screw taps, the longer life it will have during thread making. The testing shows that the cutting tools’ life will double if adding half of cutting teeth thread. Unlike other tools, the screw taps can only change when the number of chip groves and the length of the tapered teeth changes.
（3）Reverse Tapered teeth：like other cutting tools, thread taps also have reverse tapered teeth.
The advantage of the thread clearance：
Speed cutting and less heat accumulation;
Winding on the screw taps, less bonded materials( less sticky);
Higher tapping speed can be used;
Compensation for the plastic deformation of the workpiece
The disadvantage of the thread clearance：
The cutting edge becomes brittle and easy to collapse;
Insufficient rigidity of the spindle and clamp (including the floating handle) can make the thread too poor;
Very small chips during the reversal may be embedded to collapse the cutting blade
（5）The tolerance of the thread taps
Every thread taps have its dedicated pitch diameter;
The thread taps marked with H or D are mainly from USA
H/D tolerance shows the thread dimension of the screw taps.
Currently, screw taps will mark with its thread grade
The Universal HP Screw Taps Series
Infy that the thread tap is the correct size to the component fit grade
Class 3B screw taps are suitable for Class 2B components
The screw taps grade marked with "X" indicates a large tolerance for precision screw taps, electroplating, or heat treatment components, or for materials close to (elastic memory).
Electroplated tapping thread
For the inner threads, you must select the screw taps of the larger tolerance level
A larger medium diameter will make the thread size slightly larger
The large increase after plating will bring the thread size back to the specified value
The cutting process of the screw taps
（1）Hole types and cutting chips treatment
（2）Extrude cutting, spiral discharge screw taps
Best suitable for blind and deep hole processing
Recommended materials for producing adhesive chips
Perfect for intermittent cutting
The cone core of the spiral discharge groove is very thin, and it is the most fragile part of the silk cone design. Therefore, the speed should be 30% ~ 40% lower than the straight discharge chip groove screw tap, so as to avoid breaking.
（3）Pull out cutting
（4）Indirect discharge of dander groove screw taps
– Taper (Form A)“A”
– Plug (Forms B & D)“B/D”
– (Form C) “C”
– (Form E) “E”
（5）Thread punch taps：whatever making the threads in blind hole or through hole, there is no cutting chips during production.