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January 18, 2010

Greenfield Threading Tech Tip: Determining the Minimum Tap Drill Depth

Determining the minimum tap drill depth by Greenfield Tap and Die along with Cutting Tools Chicago aka General Cutting Tools.  The tap drill depth for tapped holes should be deep enough to allow for the minimum thread depth, the tap’s chamfer, any chips that may accumulate in the bottom of the hole, and any over-travel of the spindle as it slows to a stop before reversing.

To determine the ideal minimum drill depth, start with the basic rule: the tap drill depth should be at least one thread or one pitch beyond the chamfer of the tap. In other words, for a 1/4-20 tapped hole requiring a 3/8″ (.375″) thread depth, using a two-thread bottoming chamfer with a length of .100″, and adding a minimum of one additional thread or .050″, the minimum drill depth would be .525″ (.375 + .100 + .050 = .525″).

It is important to note that using the “one-pitch rule” does not always prevent the tap from hitting the bottom of the hole. In this case, consider the following three factors:

  • Understand the machine’s capability to stop the spindle rotation at the bottom. Many newer machines are able to stop and reverse in one-half turn or less. If this is the case, the “one-pitch rule” may be sufficient. However, an older machine may allow the spindle to over-travel one, two, or even three pitches. Here, the additional drill depth should be at least one pitch beyond the over-travel amount.
  • Consider how the chips are produced. Spiral taps extract chips, preventing them from accumulating in the bottom of the hole. Chips produced from straight flute hand taps are likely to be flushed to the bottom by the coolant. The front of the tap may bottom on these chips, chipping or breaking the tap. In this case, one diameter of additional depth is recommended.
  • Understand the tap design. If using a plug or taper chamfer, note the tap sizes through 3/8″ diameter are manufactured with 90′ external centers that could hit the bottom. A general rule is to add one-half the diameter to the length for sufficient clearance.

Contact General Cutting Tools, your authorized Greenfield Tap and Die distributor for more information.

We ship to Illinois, Iowa, Indiana, Wisconsin, Michigan, Minnesota and the reset of the US.

January 16, 2010

Greenfield Threading Tech Tip: Pipe Taps… How Deep to Thread?

Greenfield Taps with Cutting Tools Chicago aka General Cutting Tools answers the question: Do you know how deep to run a pipe tap into a drilled hole before it stops? In other words, do you know when the pipe thread is either too small to accept the L-1 thread plug gage, or too large to be flush with the surface of the hole being tapped within ± 1 turn on the gage?

Pipe threads are sized differently than standard straight threads. They are based on the basic size  at the top of the threaded hole produced by the pipe tap. The basic size, commonly called the 12-thread count, occurs approximately 12 threads from the front of the taper pipe tap. This means you can run any standard projection pipe tap into a drilled hole approximately 12 threads from the front of the tap, which should be the approximate basic size for that pipe.

General purpose pipe taps are appropriate for threading a wide variety of materials, both ferrous and non-ferrous.

Ground thread pipe taps are standard in American Standard Pipe Form (NPT) and American Standard Dryseal Pipe Form (NPFT). NPT threads require the use of a sealer, like Teflon® tape or pipe compound. Dryseal taps are used to tap fittings which will give a pressure-tight joint without the use of a sealer.

The nominal size of a pipe tap is that of the pipe fitting to be tapped, not the actual size of the tap. The thread tapers 3/4″ per foot.

All pipe taps are furnished with 2 1/2 to 3 1/2 thread chamfer.

Short projection pipe taps are made with a projection shorter than standard for taper pipe tapping where the depth of tapping is limited.

Special short projection taper pipe taps can be furnished with American National Standard Taper Pipe thread (ANPT), or Dryseal American National Standard Taper Pipe thread (NPTF, PTF-SAE Short, or PTF-SPL Extra Short).

For information on short projection pipe taps and hole preparation for NPT, NPTF, and ANPT internal pipe threads, consult Greenfield’s Technical Bulletins.

Special short projection pipe taps and left hand pipe taps are available through Lightning’ Service.

Contact General Cutting Tools, your authorized Greenfield Tap and Die distributor for more information.

We ship to Illinois, Iowa, Indiana, Wisconsin, Michigan, Minnesota and the reset of the US.