Pilot Holes and Field Matching Drilling Aluminum and Steel

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Pilot Holes and Field Matching Drilling Aluminum and Steel

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Drilling Capability Tolerance Chart

Pilot Holes and Field Matching Drilling Aluminum and Steel

To make a fastener hole of the correct size, first drill a hole slightly undersize. This is known as predrilling, and the hole is called a pilot hole. Ream the pilot hole with a twist drill of the correct size to get the required dimension. General pilot and reaming drill sizes for aluminum rivets are shown in the table below and discussed in the notes.

When drilling hard metals the twist drill should have an included angle of 118 ° and should be operated at low speeds; but for soft metals, use a twist drill with an included angle of 90° and it should be operated at higher speeds. Thin sheets of aluminum alloy are drilled with greater accuracy by a drill having an included angle of 118° because the large angle of the drill has less tendency to tear or elongate the hole.

Center punch locations for fastener holes before beginning the actual drilling. The center punch mark acts as a guide and lets the drill grip or bite into the metal with greater ease. Make the center punch mark large enough to prevent the drill from slipping out of position, but punch lightly enough not to dent the surrounding material. Hold a hard, smooth, wooden backing block securely in position behind the hole locations when drilling.

Drilling is usually done with a hand drill or with a light power drill. Hold the power drill firmly with both hands. Extend the index and middle fingers of the left hand against the metal to act as a guide in starting a hole, and as a snubber or brake when the drill goes through the material. Before beginning to drill, always test the inserted twist drill for trueness and vibration by spinning the hand drill or running the motor freely and watching the drill end. If the drill wobbles, it may be becauSe of burrs on its shank or because the drill is bent or incorrectly chucked. A drill that wobbles or is slightly bent must not be used because it causes enlarged holes.

Always hold the drill at right angles to the work, regardless of the position of the hole or the curvature of the material. Use an angle drill or drill extensions and adapters when access is difficult with a straight drill. Never tip the drill sideways when drilling or when withdrawing from the material because this causes elongation of the hole.

When holes are drilled through sheet metal, small burrs are formed around the edge of the hole. This is especially true when using a hand drill since the drill speed is slow and there is a tendency to apply more pressure per drill revolution. Remove all burrs with a burr remover before riveting.

Pilot Holes General:

Typical pilot holes when drilling through 2 to 3 plate thicknesses not exceeding .375 inches should be 6 to 10% smaller than the intended match drill size. For holes that will require reaming for accuracy, and pilot hole should be produced .006" to .010" undersized.

Pilot and reaming twist drill sizes for Aluminum Rivets.

Rivet Diameter	Pilot Hole	Ream Size
3/32	3/32 ( .0937)*	40 (.098)
1/8	1/8 ( .125)	30 (.1285)
5/32	5/32 ( .1562)	21 (.159)
3/16	3/16 ( .1875)	11 ( .191)
1/4	1/4 (.250)	F ( .257)
5/16	5/16 ( .3125)	0 (.316)
3/8	3/8 (.375)	v (.377)
Note that ream size exceeds the maximum tolerance of .004 inch. This is permissible only if the next larger drill size happens to be so much larger than the tolerance of .004 inch.