Twist drill bit

How it works

The drill is turned about its axis and pressed into the work. The cutting is done by the two cutting lips, which are formed where the flutes meet the cone-shaped point. The flutes curl the chip tightly and let lubricant flow down to the cutting edge, according to the Army, Air Force, Navy and Marine Corps manual TM 9-243.

Morse’s 1863 patent states the aim in terms of the cutting edge: a sharp, straight edge presented to the metal at a suitable angle, in place of the scraping action he attributes to the common drill. The same patent makes the spiral twist increase toward the shank so that the space for the chips enlarges as they travel out of the hole.

Anatomy

Point: The cone-shaped end that does the cutting; TM 9-243 gives 31 degrees as normal for the point it describes.
Cutting lips: The edges formed by the intersection of the flutes and the conical point.
Dead center: The edge at the extreme tip of the point, which should lie on the axis of the drill.
Flutes: The cut-out spiral grooves of the body that curl the chip and admit lubricant.
Margin: The narrow strip at the edge of each blade, part of a cylinder interrupted by the flutes; the drill diameter is measured from margin to margin.
Shank: The end that fits the drill chuck.

Variants

Fractional-size drill: TM 9-243 lists sizes from 1/32 to 1/2 inch in steps of 1/64 inch.
Numbered-size drill: TM 9-243 gives No. 1 (0.2280 inch) to No. 80 (0.0135 inch); letter sizes also exist and are measured with a drill gage.
Carbon steel and alloy steel drill: TM 9-243 says carbon steel drills are used for general drilling and alloy steel drills for hard metals such as stainless steel.

Specifications

Point angle · degrees: TM 9-243 describes the point as normally 31 degrees; this is the manual’s figure, and no standard has been sourced.
Diameter · in: Fractional 1/32 to 1/2 inch in 1/64 steps; numbered No. 1 (0.2280) to No. 80 (0.0135).

Made to

ASME B94.11m
DIN 338
ISO 235

Safety and care

Safety

TM 9-243 lists these precautions: use a drill of proper size, free of rust, with clean flutes; keep it tight in the chuck; never use a bent drill; be certain the cutting edges and point are not dulled; secure the stock; wear safety glasses; turn the drill motor off before laying it down.

Tips

Center punch the hole location
TM 9-243 says the location should be center punched, which helps the drill start cutting and keeps it from wandering. For a large hole it calls for a small pilot hole first to guide the larger drill.1
Keep the drill cutting and ease off at breakthrough
The manual says to apply steady, uniform pressure, because a drill turning on the metal without cutting becomes excessively hot. It says to ease the pressure as the point is about to break through and to withdraw the drill at once when the hole is complete.1
Read the size with a micrometer or drill gage
If the size number etched on the shank has worn off, TM 9-243 says to measure from margin to margin near the shank end. A drill gage, a plate of sized holes, gives the number, letter or decimal size and the correct drill for a given tap.1
Check that the dead center is on the axis
The manual warns that a reground drill may have its dead center off center, so that one lip does most of the cutting and the drill is strained.1

History

Earliest documented

before 1863

United States

Morse’s patent of 7 April 1863 describes the “common twist-drill” as already existing and claims an improvement in the form of its groove. The patent does not say who made the earlier twist drills or when.1

The patent shows that the twist drill was not new in 1863. Morse describes a “common twist-drill” and names its fault: it scrapes the metal that is to be drilled, where his groove is shaped to present a cutting edge. His claims are therefore limited to the form of the groove and the increasing twist, not to the spiral drill as such.

Morse’s stated design aims are a straight, acute-angled lip, strength in the metal behind the edge, and free discharge of chips. The patent adds that the shape of the groove is kept throughout its length, so the cutting edge keeps its form as the drill is shortened by regrinding.

The company that grew from the patent is documented by its own records. The Whaling Museum’s finding aid gives Morse’s invention as 1861, the patent as 1863, and the organization of a company in New Bedford with $30,000 capital stock in 1864. It records that Morse left the company shortly thereafter, and holds company records from 1864 into the twentieth century.

The drill still in manual use has the parts Morse’s patent describes: spiral flutes, a conical point and cutting lips. TM 9-243 sets out sizes by fraction and by number and treats the drill as the standard tool for metal. O*NET’s Tools Used table lists drill bit sets for four of the occupations mapped to this archive’s trades.

Timeline

1861
Stephen Ambrose Morse invents what the New Bedford Whaling Museum’s finding aid calls the “Morse Patent Straight-Lip Increase Twist Drill.”1
1863
Morse of East Bridgewater, Massachusetts, is granted US Patent 38,119, dated 7 April 1863, for a twist drill with a straight cutting lip and a spiral that increases in twist toward the shank.1
1864
Investors in New Bedford organize a company with $30,000 in capital stock to make the drill; the finding aid says Morse left the company shortly after.1
1864–1915
The Morse Twist Drill & Machine Company’s administrative, financial, patent, advertising and employee records survive for these years in the Whaling Museum collection.1
1990
The Whaling Museum acquires company papers taken, with permission, from the Morse building; further papers are acquired in 2005.1
2016
The U.S. Departments of the Air Force, Army, Navy and Marine Corps manual TM 9-243 (Change 10) describes the twist drill, its sizes and its safe use.1

Gaps in the record (4)

The earlier twist drills that Morse’s patent calls “common” are not identified; no pre-1863 maker, date or museum object has been sourced.
The company’s later history (acquisitions, the end of New Bedford operations) rests on sources not yet fetched and is not stated here.
No current standard for point angle or tolerances has been read; the standards slugs are guesses.
Care has no sourced entries yet.