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Tap Guide.
By Brian, MW0GKX.

With this page I hope to give a grounding in taps, the tools that cut threads in holes not the water dispensing kind!

Why? Well the winter construction season is coming and you need to hold your project components together somehow. Also it gives you something to read on those cold, wet nights while thinking of the summer antenna projects.



Basics.

There are many types of tap and I'm not just talking thread type (metric, imperial, UNC etc) or TPI (threads per inch or how course or fine the thread is). There are various designs of flute (the grooves that allow the chips of cut material to clear the hole), differing chamfers (more about that below), shank sizes and even the material the taps are made from.

In the basic Tap and Die (the dies cut the thread on bar stock to make the bolts) set you will get taps of differing diameters in either imperial or, more common now here in the UK, metric sizes - if it is a larger set, possibly a mixture of both - the corresponding dies, a tap wrench and a die stock. The tap wrench and die stock are the "handles" that the respective tool fits into so you have the leverage to turn them and cut the threads. These are fine for general purpose work but be aware that there are "proper" taps for different jobs.

Chamfer.

The tap chamfer is the tapering of the threads at the end to distribute cutting action over several teeth. The type of hole to be tapped has a lot to do with the chamfer style of that tap that's best suited to that hole. Some holes go all the way through; some, while not through-holes, are quite deep; some are quite shallow (little deeper than the diameter of the hole to be threaded). Each of these three kinds of holes - through, deep-bottoming blind, and shallow bottoming - has a tap chamfer best suited to threading requirements:

  1. Taper Taps Taper Tap Image
    Taper taps have a 8-10 thread chamfer, the longest chamfer of the three types, to distribute action over the maximum number of teeth. The taper also acts as a guide when starting the cutting action in the hole, they start the thread square with the workpiece. Taper taps are commonly used in shallow through holes and in materials where a tapered guide is necessary.
  2. Seconding TapsPlug Tap Image
    This style, with a 4-6 thread chamfer, is most widely used in deep through holes and where there is sufficient room at the bottom in blind holes. This is the type of tap that is commonly supplied with basic Tap and Die sets.
  3. Bottoming, Plug or Finishing TapsBottom Tap Image
    This style, with a 1-2 thread chamfer, is made with just enough chamfer for starting in the hole; as the name implies, it is designed to thread blind holes to the bottom. If the blind hole is deep enough it is good practice (well Ok, easier) to start with a taper tap to get the square alignment and then, once several threads have been cut, change to the bottoming tap of the same size to complete the job.

Holes.

Possibly the biggest consideration when selecting a tap, or the drill to make the hole you wish to tap, is "What Size Hole do I need for this Tap?" Obviously you will not be able to tap a 6mm diameter hole with an M6 tap (M6 screws and, therefore, taps are 6mm outside diameter). Well help is at hand! The Drill Holes and Tapping page will give you information for determining the size of metric or imperial tap you need for that hole or vice-versa. A handy chart for metric and a calculator (java based) for the imperial (inch) sizes are included on that page.



Different Types of Taps

Below are the commoner types of tap that you may come across in the rallies, boot sales etc., with a short description of what benefits that particular type has.
When purchasing taps at such places be aware of the fact that second hand taps may be blunt or worn. Also check on the thread type and diameter. It's no good getting an M6 tap for a bargin price only to discover later that the screws you have are M6×1 (metric course) and the tap is M6×0.5 (metric fine)!

HAND TAP Hand Tap

These standard taps have a number of straight flutes for chip clearence.
Hand taps are for general purpose use.
Good for general purpose use on blind and through holes.



SPIRAL POINT TAP Spiral Point Tap

General dimensions are similar to the standard hand tap of the same size but the spiral point tap has the cutting face of the first few threads, cut at an angle relative to the tap's axis angle, to force the evacuation of chips ahead of the cutting action. This feature, plus the excellent shearing action of the flute, make spiral pointed taps ideal for tapping of through holes. This type of tap has a shallower flute passage than conventional taps which gives the spiral point tap greater strength and requires less power to drive.



SPIRAL FLUTED TAP Regular or Slow Spiral Tap Fast Spiral Tap

These taps, as the name implies, are made with spiral instead of straight flutes. This spiral fluting feature aids in drawing chips out of a hole and also serves to bridge a gap inside the hole such as a keyway or cross-hole.
Commonly available in slow, or regular, spiral (above left) or fast spiral (above right).




INTERRUPTED THREAD TAP Interrupted Thread Tap

These taps have an odd number of lands with alternate teeth in the thread helix removed. This removal of every other tooth helps to break the chip and allows a greater supply of lubrication to reach the cutting teeth, reducing the incidence of torn threads.
Ideal for tapping non ferrous metals and low carbon steel; as well as use in titanium and high hardness alloys.



COMBINED DRILL & TAP Combined Drill & Tap

Known as the "Cost Cutter" this tap is usually only found in production use.
It is specially designed to drill and tap in one pass only. This value-added tool reduces machining operations and subsequent parts handling. The drill end features a self-centering helical point.
Suitable for machine use on through holes only.



EXTENSION TAP Extension Tap

These taps are made to conventional tap dimensions, except that they have an extended shank to reach inaccessible holes. Extension taps are available in both hand and spiral point styles, and in small shank style.



NUT TAP Nut Tap

Nut taps feature a long chamfer which assists in entering the drilled hole, and distributes the cutting action over several teeth. These taps were initially designed for tapping nuts and have a long thread length. The shank diameter is smaller than the tap's minor diameter to allow the accumulation of several nuts after tapping. Nut taps also feature an extended square length.
Not to be confused with the Extension Tap, above.

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