Technical Information Tapping Information
Lubricants
Use:
Use of a suitable lubricant or cutting compound is necessary on most tapping operations. The type of lubricant as well as the method of application is
often of extreme importance and can be responsible for the success or failure of a tapping operation.
Recommendation:
Better results can sometimes be obtained by the use of one of the many modified or specialised lubricants recommended by cutting oil specialists.
The general principle is to have more EP (Extreme Pressure) additives added with the degree of difficulty, usually hardness increase. Oils stick, and
improve frictional properties essential in tapping tough applications.
Application:
Proper application of the lubricant is just as important as the type used. To be effective, ample quantities of lubricant must reach the chamfer or cutting
portion of the tap during the entire tapping operation. In many cases, the lubricant must also aid in controlling or disposing of the chips.
Flow:
The flow of lubricant should be directed into the hole rather than at the tap and should have sufficient pressure to wash the chips away from the hole as
much as possible. Also, if the flow is not continuous, it should start before the tap enters the hole and continue until the tap is completely reversed out
of the hole. In this way, ample oil is provided at the start of the cut and loose chips will be suspended in the oil so that they do not interfere with the tap
backing out of the hole. On machines where the work revolves and the tap is stationary, it is desirable to use several streams of lubricant on opposite
sides of the tap, especially on horizontal tapping.
Cleanliness:
Tapping lubricants must always be clean. If filter equipment is not used, the lubricant must be replaced periodically to eliminate fine chips, grit and foreign
matter that accumulate in the tank. Also, it is very important that the piping and tank are thoroughly flushed and cleaned before filling with new lubricant.
The dilution of lubricants often changes during use so that additions may be necessary to maintain the recommended proportion of active materials.
Tapping drill
The tapping drill hole diameter should be drilled as large as possible, within the respective fitting just under the upper permissible dimension of the tolerance.
If the tapping drill hole diameter is too small, then this will cause the thread root diameter (minor diameter) to cut the material. This should be avoided,
because the small chips which derive from the root of thread, clog the normal chip flow and rip pieces of material out of the finished thread. Consequently,
the tap is overloaded and often breaks because of the high torque.
Another problem which occurs in certain materials due to thread root diameter cutting, is when a chip-bulge has been formed around the root radius.
The minor diameter of the tap is clogged with small chips, which leads to a clamping of the tool teeth are ripped out, which leads to tool breakage. It
is therefore, necessary that the material which is to be tapped, be taken into account when determining the tap hole diameter. Typical materials which
do not squeeze or clamp are iron, brass and bronze and materials which squeeze are steels, steel castings and malleable steels. The tap cuts more
economically, when the tap drill hole diameter is within the upper range of the permissible tolerance.
Warning: When drilling holes in materials which tend to work harden, care is needed to ensure the drills are sharp otherwise tap life is decreased.
Tapping drill formula
Thread Type Formula Example
Metric (ISO) Drill Size = Nom, Tap Dia, in mm – Pitch M6 × 1 = 5.00mm drill
1/4 BSW 75% thread required:
Whitworth Form Threads
(inch calculation)
1.28
Drill Size = Nom, Tap Dia, – ––– × % of thread depth
TPI
1.28
75
Drill Size = 0.250 – ––– × ––– = 0.250 – 0.048
20
100
Therefore Drill Size = 0.202
Nearest Standard Drill = 5.1mm = 0.2007 inch
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The correct size of drill to give the desired percentage of thread can be calculated by using the following formula:
1/4 UNC 75% thread required:
Unified Form Threads
(inch calculation)
1.30
Drill Size = Nom, Tap Dia, – ––– × % of thread depth
TPI
1.30 75
Drill Size = 0.250 – ––– × ––– = 0.250 – 0.049
20 100
Therefore Drill Size = 0.201
Nearest Standard Drill = 5.1mm = 0.2007 inch
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