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Tap Drill Size
In the thread charts within the technical section the correct tapping size is given. The tapping drill sizes given generally afford a thread depth of 75% or more - which is recommended for short hole tapping. For long length threads, fine pitches and small diameters a reduced amount of thread depth is sometimes desirable.
For deep hole tapping or average commercial work a threaded depth of 60%-70% will usually provide greater strength than that of the mating screw.
First class tapping can only be done with a copious supply of proper lubricant. Use of the correct lubricants is as important as the decision to use it, it must be kept clean and carefully directed into the hole being tapped, an ample supply is needed on the cutting edges, not only to disperse heat, but to aid in the formation and removal of chips.
When starting a tap do not force or retard the tap, or a bell mouthed hole will be produced with thin threads. Allow the tap to establish its own pitch. During tapping of a deep hole, avoid the flutes becoming clogged with chips - breakage is inevitable in these circumstances, consider a fluteless tap for very deep holes. Chip disposal is a large problem on taps of 12mm and smaller where flute space is restricted see description on spiral point and spiral flute taps below.
These are also hand taps. The TAPER tap is takes out half the thread area.
The INTERMEDIATE will take out to 75% of the area of the thread.
Only the PLUG/Finishing tap cuts SIZE.
For hand tapping, the conventional set of 3 taps with straight flutes is recommended. It is essential that the tap is presented squarely to the work and that the taps are correctly aligned. When taps are used in a machine it is usual to use only the Intermediate/second lead. Using the bottom tap only, can cause problems and will significantly reduce the life of the tool.
Spiral Point Taps
These taps are sometimes called GUN-Nosed, are made with a special leading flute ground at an angle to the tap axis. This left hand flute at the lead pushes the swarf ahead of the tap threads thus allowing the use of smaller flutes since chip clearance is not required. The result is therefore stronger taps which are suitable for through hole tapping in most materials. Blind hole tapping should only be attempted where there is sufficient room at the bottom of the hole to accommodate the swarf.
Spiral Flute Taps
These taps have a continuous spiral flute the same hand as the thread, thus forcing the swarf up the hole. The most suitable applications are on blind holes in ductile materials with long continuous chips.
Fluteless Roll (Cold Forming) Taps
Fluteless or cold forming taps can be used in a wide variety of materials from steel, aluminium, copper and soft ductile brasses but will also cut stainless's and titanium and nickel materials. As these materials may tear rather than cut cleanly, they tend to clog the flutes of conventional taps. The fluteless tap offers an alternative forming by displacing the material instead of cutting. It should be noted that the biggest change is the requirement of a larger tapping drill, the fluteless tap also can tap faster than a standard tap. Lubrication is of the utmost importance in thread forming, with an extreme pressure additive is advisable in soluble.