Drilling depths for easy tapping.

Author:Rose, Steve
Position:T&P shop talk

A major challenge when creating threads is to avoid breaking the tap. A tap breaks when the drilled hole is not deep enough. Broken taps cause lost time, broken tools, and scrapped parts. With some foresight and a few simple calculations, we can reduce the risk of breaking taps.

Take a look at this example. The full drill depth is 1.4" and the full tap thread depth is 1.0". The difference between the two is only 0.400".


The limited amount of space means there is little room for error. If the tap depth exceeds the drill depth, the hole will not be deep enough and the tap breaks.

Drill depth

Program the drill deep enough to allow for the tap's point. The example shows the full drill depth. To calculate the programmed value, determine the drill point allowance and add it to the full drill depth.

The point allowance depends on the type of drill and its diameter. Each drill type has a rule for calculating the drill point:

Rules for drill point allowance:

* For HSS drills with a 118deg included angle the formula is: drill diameter x 0.3 = drill point allowance

* For Cobalt drills with a 135deg included angle the formula is: drill diameter x 0.2 = drill point allowance

When programming the hole (5/8 - 11 TPI), consult a tap drill chart and use a 17/32dia HSS drill (0.531"). Calculate the point allowance: 0.531x 0.3 = 0.159.

Add the point allowance to the required drill depth to achieve the programmed depth. 1.4 + 0.159 = 1.559 (Z-1.559)

Tap chamfer clearance

The hole requires 1.0" of full threads. You must allow for the chamfer on the lead of the tap when programming. The programmed depth of the tap must be larger than the 1.0" listed on the print; the difference is the tap chamfer.

To calculate the length of the chamfer, use the appropriate formula. Tap...

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