HAND TAPS AND WRENCHES
Use of hand taps
Hand taps are used for internal threading of components. Features (Fig 1)
They are made from high carbon steel or high speed steel, hardened and ground.
Threads are cut on the surface, and are accurately finished.
To form the cutting edges, the flutes are cut across the thread. For holding and turning the taps while cutting threads, the ends of the shanks are squared.
The ends of the taps are chamfered (taper lead) for assisting, aligning and starting of the thread.
The size of the taps and the type of the thread are usually marked on the shank
In certain cases, the pitch of the thread will also be marked.
Markings are also madeto indicate the type of tap i.e. first, second or plug.
Types of taps in a set
Hand taps for a particular thread are available as a set consisting of three pieces. (Fig 2)
These are
- first tap or taper tap
- second tap or intermediate tap
- plug or bottoming tap.
These taps are identical in all features except in the taper lead.
The taper tap is to start the thread. It is possible to form full threads by the taper tap in through holes which are not deep.
The bottoming tap (plug) is used to finish the threads of a blind hole to the correct depth.
For identifying the type of taps quickly - the taps are either numbered as 1,2 and 3 or rings are marked on the shank.
The taper tap has one ring, the intermediate tap has two rings and the bottoming tap has three rings. (Fig 2)
Tap wrenches
Tap wrenches are used to align and drive the hand taps correctly into the hole to be threaded.
Tap wrenches are of different types.
Double ended adjustable wrench, T-handle tap wrench, solid type tap wrench.
Double-ended adjustable tap wrench or bar type tap wrench (Fig 3)
This is the most commonly used type of tap wrench. It is available in various sizes. These tap wrenches are more suitable for large diameter taps, and can be used in open places where there is no obstruction to turn the tap. It is important to select the correct size of wrench.
T-handle tap wrench (Fig 4)
These are small adjustable chucks with two jaws and a handle to turn the wrench.
This tap wrench is useful to work in restricted places, and is turned with one hand only.
This wrench is not available for holding large diameter taps.
Solid type tap wrench (Fig 5)
These wrenches are not adjustable.
They can take only certain sizes of taps. This eliminates the use of a wrong length of the tap wrenches, and thus prevents damage to the taps.
TAP DRILL SIZE
What is a tap drill Size?
Before a tap is used for cutting internal threads, a hole is to be drilled. The diameter of the hole should be such that it should have sufficient material in the hole for the tap to cut the thread.
Tap drill sizes for different threads
ISO Metric thread
Tapping drill size for M10x 1.5 thread
Minor diameter = Major diameter - 2 x depth
depth of thread = 0.6134 x pitch of a screw
2 depth of thread =.0.6134 x 2 x pitch
=1.226 x 1.5 mm = 1.839 mm.
Minor dia (D1)=10 mm - 1.839 mm
=8.161 mm or 8.2 mm.
This tap drill will produce 100% thread because this is equal to the minor diameter of the thread. For most fastening purposes a 100% formed thread is not required.
A standard nut with 60% thread is strong enough to be tightened until the bolt breaks without stripping the thread. Further it also requires a greater force for turning the tap if a higher percentage formation of thread is required.
Considering this aspect, a more practical approach for determining the tap drill sizes is
Tap drill size = Major diameter - pitch
=10 mm- 1.5 mm
=8.5 mm.
Compare this with the table of tap drill sizes for ISO metric threads.
ISO Inch (Unified) threads formula
1
Tap drill size =
Number of threads per inch
For calculating the tap drill size for 5/8" UNO thread
Tap drill size =5/8"-1/11"
=0.625"-0.091"
= 0.534"
The next drill size is 17/32" (0.531 inches).
Compare this with the table of drill sizes for unified inch threads.
What will be the tap size for the following threads?
(a) M 20
(b) UNC3/8
REMOVING BROKEN TAPS
A tap broken above the surface of the work piece can be removed using gripping tools like pliers.
Taps broken below the surface pose a problem for removing. Any one of the several methods given below can be used.
Use of tap extractor (Fig 1)
This is- a very delicate tool and needs very careful handling.
This extractor has fingers, which can be inserted on the flutes of the broken tap. The sliding collar is then brought to the surface of the work and the extractor turned anticlockwise to take out the broken tap.
A light blow on the broken tap with a punch will help to relieve the tap if it is jammed inside the hole.
Use of punch (Fig 2)
In this method the point of the punch is placed in the flute of the broken tap in an inclination and struck with a hammer. The positioning of the punch should be such that the broken tap is rotated anticlockwise when struck.
Annealing and drilling the tap
This is a method adopted when other methods fail. In this process the broken tap is heated by flame or by other methods for annealing. A hole is then drilled on the annealed tap. The remaining piece can be removed either by using a drift or using an EZY-OUT (extractor). This method is not suitable for work pieces with low melting temperatures such as aluminum, copper etc. (Fig3)
Use of arc welding
This is a suitable method when a small tap is broken at the bottom of materials like copper, aluminum etc. In this method the electrode is brought in contact with the broken tap and stuck so that it is attached with the broken tap. The tap may be removed by rotating the electrode.
Use of nitric acid
In this method nitric acid diluted in a proportion of about one part acid to five parts of water is injected inside. The action of the acid loosens the tap and then it is removed with an extractor or with a nose plier. The work piece should be thoroughly cleaned for preventing further action of the acid.
Use of spark erosion
For salvaging certain precision components damaged due to breakage of taps, spark erosion can be used. In this process, the metal (broken tap) is removed by means of repetitive spark discharges. The electrical discharge occurs between an electrode and the electro-conductive work piece (tap) and the minute panicles are eroded both from the electrode and the work piece. In many cases it may not be necessary to remove the broken tap completely. (After a small portion has been eroded, a screwdriver or punch can be used to remove the remaining portion of the tap.) The shape of the electrode also need not be round. It can be square or in the form of a slot 'pa on the work piece for assisting the tools for rotating the broken tap.
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