Taps & Dies

Taps & Dies

  • Threads may be produced quickly and easily with standard taps and dies.
    • Taps cut internal threads. (Tapping)
    • Dies cut external threads. (Chasing)
  • Most taps are made from H.S.S. or carbide.

Parts of a Tap

Parts of a Tap

Standard Taps

  • Most commonly used are hand taps; they are supplied in a sets of three.
    • Starter or Taper tap
    • Intermediate or Plug tap
    • Finishing or Bottoming tap
  • Use them in the correct order.

Markings on a Tap

  • 5/8 – 11 – NC
  • G – Ground
  • H3 (Use oversize taps for heat treated pieces)
Markings on a Tap

Markings on a Tap

Using Hand Taps

  • Tapping, particularly small diameters, can be a problem: Drill the correct size hole before tapping.  Use thread charts.  Thread strength is not much affected by a small increase in tap drill size.  50% thread depth may be all that is necessary in some steels.

Formula for Tap Drill Size

  • Hole Size = Basic MD – (1.08253 x % thread / TPI)
  • Example: 1/2 – 13 UNC, 50% thread depth = 0.5-  1.08253 x 0.50 / 13 = 0.4584

Doing a Professional Job!

  • Always countersink the hole before tapping, not after.
  • On certain special occasions we may protect surfaces from protruding threads.
The procedure of a tap

The procedure of a tap

Driving the Tap

  • When using hand taps rotate ¼ turn forward, one-half turn back to break chip.
  • Use a ‘T’ handle tap wrench for work in confined spaces; use a standard tap wrench for all other work.
  • Use tapping fluid to lubricate cutting operation.
  • Keep flutes clear of chips.
  • Use drill chuck or spring-loaded centre to start tap if possible.

Special Situations

  • Serial taps may be used for tough materials.  There are normally 3 to a set, each one has a slightly larger pitch diameter.  Normally they have rings around the shank.
  • Interrupted thread taps. are used for gummy materials and sometimes on stainless steel.

Machine Taps

  • There are special taps designed specifically for machine tapping.
    • Spiral pointed (gun taps) – Note chips are pushed ahead of the tap.
    • Spiral fluted taps.  Note chips are pulled up through the tap and out of the hole.
Machine Taps

Machine Taps

Using “Gun” Taps

  • Be careful when using spiral point (gun) taps to machine blind holes.  Drill extra depth or use spiral-fluted taps.
  • Do not turn machine taps forward and backwards; cut all the way through or to depth before reversing.

Driving Machine Taps

  • Machine taps are normally held in a tapping head. Set the clutch on the tapping head carefully.

Thread Forming Taps

  • Some taps do not cut.  Thread forming taps generate threads by displacing metal rather than cutting it.
  • Thread forming taps are used on soft alloys and leaded steels.
  • Thread forming taps cold work the metal and produce good surface finishes.
  • Note size of original hole is different than for normal tapping.

Tapered Threads

  • Tapered pipe threads may be cut with taper pipe taps.

Special Taps

  • Pulley and nut taps can get into tight areas.
  • Special taps may be ordered. For example, ACME thread taps.

Some Useful Hints

  • Never use a tap for cleaning out a hardened work piece, use a SHCS instead.
  • Select the right tap.  Consider hook and rake angles.
  • Taps may be surface treated to reduce friction.  Titanium nitride is a popular coating for taps.

Removing Broken Taps

  • Broken taps may be removed with tap extractors.
  • You may have to use the EDM process, which is expensive

Threading Dies

  • Used to cut external threads
  • Not always the best choice when compared with single point tool method.
  • Most common dies are refereed to as split adjustable or button dies. Adjustments may be made, but only a couple of thousandths.  Die will break.
  • Use Hexagon re-threading dies to repair and clean existing threads.
  • Always chamfer bar before threading.
  • Use cutting oil and go forward ¼ turn and back ½ turn.
  • Start die square with help of lathe or other equipment.

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