Impact driver

How it works

An impact driver uses the same hammer-and-anvil principle as the impact wrench. A motor turns a hammer that is coupled to an anvil. While the load is light the anvil turns with the hammer. When the fastener resists, the hammer slips, gathers energy and strikes the anvil, delivering a burst of torque, and the cycle repeats many times a second. Pott’s 1935 patent for the impact tool describes the mechanism in these terms, with energy momentarily stored and released as the hammer strikes the driven part, and Fitch’s 1940 patent for Ingersoll-Rand describes subjecting the work to a series of impacts when the torsional force of the tool proves ineffective.

The output is a hex bit holder in place of the square drive of the impact wrench. Milwaukee’s manual describes its 18 volt impact driver (Cat. No. 2665-20) as intended for drill and driver bits with a 7/16 inch hex shank and ball detent recess, held by pulling out a ring and inserting the shank. The tool speed is set by the trigger: the farther the trigger is pulled, the greater the speed.

Makita’s three-speed 18 volt brushless model reviewed by the Journal of Light Construction in 2009 adds a selector with soft, medium and hard impact settings, from woodworking hardware to drywall screws to lag bolts, and gives its maximum torque on the hard setting as 1,420 inch-pounds at up to 3,400 impacts per minute.

Anatomy

Motor and battery: On a cordless model a battery pack slides into the base of the grip; Milwaukee’s manual describes an 18 volt pack released by buttons on both sides.
Hammer and anvil: The impact mechanism between the motor and the output. The principle is the one in Pott’s 1930s patents.
Hex bit holder: A socket for hex-shank bits with a ball detent; on the Milwaukee model a pull-ring holds the bit.
Trigger: Speed increases with pressure on the trigger.
Direction and lock switch: The Milwaukee manual describes a three-position control switch for forward, reverse and lock, adjustable only when the trigger is not pressed.
Impact mode selector: The Makita model in the 2009 review has a three-speed power-selection switch for soft, medium or hard impact.

Variants

Hex-drive impact driver: Takes hex-shank bits; Milwaukee Cat. No. 2665-20 uses a 7/16 inch hex shank.
Square-drive impact wrench: The same mechanism with a 3/8, 1/2 or 3/4 inch square drive for sockets. The Milwaukee manual covers three of these with the one driver; see the impact-wrench entry.
Brushless, multi-speed impact driver: Makita’s BTD144 of 2009 is described in the Journal of Light Construction as its first brushless-motor 18 volt cordless impact driver, with three power settings.
Pneumatic and corded impact tools: The earlier power forms of the mechanism, shown in Ingersoll-Rand’s catalogs.

Specifications

Voltage · V: 18 in both sources readMilwaukee Cat. No. 2665-20 and the Makita BTD144 are both 18 volt tools; other classes exist.
Impacts per minute · ipm: 0–2,500 (Milwaukee 2665-20); 0–3,400 on the hard setting (Makita BTD144)Manufacturers’ published figures for two models.
No-load speed · rpm: 0–2,000 (Milwaukee 2665-20)Manufacturer’s specification table.
Maximum torque · in-lb: 1,420 on the hard setting (Makita BTD144)Figure given in the Journal of Light Construction’s product report, from the maker.
Drive shank · in: 7/16 hex (Milwaukee 2665-20)Other drivers take a 1/4 inch hex bit; that size has not been sourced here.

Made to

IEC 62841-1
IEC 62841-2-2

Safety and care

Safety

Milwaukee’s manual states that only accessories designed for impact wrenches and drivers should be used, because other accessories might shatter or break and cause injury.
The manual instructs the user to remove the battery pack before changing or removing accessories and any time the tool is not in use, and to wear safety goggles or glasses with side shields.
The manual instructs the user to wear ear protectors and states that exposure to noise can cause hearing loss.

Care

Milwaukee’s manual says to return the tool, battery pack and charger to a service facility after six months to a year, depending on use, for lubrication, cleaning and mechanical and electrical inspection.
If the tool does not run at full power with a charged pack, the manual says to clean the contacts on the battery pack first.
Clean dust and debris from the tool and charger vents, and never immerse the tool, pack or charger in liquid (Milwaukee).

Tips

Stop impacting when the fastener is tight
Milwaukee’s manual says the longer a bolt, screw or nut is impacted the tighter it becomes, and to avoid excessive impacting to prevent damage to the fastener or workpiece. Smaller fasteners reach their best torque sooner.1
Learn the time to torque on scrap
The manual advises practicing with various fasteners, noting how long it takes to reach the desired tightness, and checking with a hand torque wrench. If the fasteners are too tight it says to reduce the impacting time.1
Meter speed with the trigger
The tool can run at any speed from zero to full, set by trigger pressure. Starting a screw slowly and then pulling further is how the speed is controlled.1
Pick a lower impact setting for small hardware
The Makita model reviewed in 2009 offers soft, medium and hard impact for fasteners from woodworking hardware to drywall screws to lag bolts. The report gives its maximum torque only for the hard setting.1
Seat the hex bit past the detent
On the Milwaukee model the user pulls the ring out, inserts the shank and releases the ring. The manual notes it may be necessary to pull the bit out slightly to engage the holding mechanism.1
Expect to loosen at about three-quarters of the tightening torque
The manual says the torque needed to loosen a fastener averages 75% to 80% of the tightening torque, depending on the condition of the contacting surfaces.1

History

Earliest documented

30 January 1932 (Pott’s application for the impact tool mechanism)

Evansville, Indiana, United States

The rotary hammer-and-anvil impact mechanism used in the impact driver is described in Robert H. Pott’s US Patent 2,012,916, “Impact Tool,” filed 30 January 1932 and granted 27 August 1935, for “applying a torsional force.” The first cordless hex-drive impact driver has not been identified in a primary source.1

The impact driver is the impact wrench shrunk to fit a screw. Pott’s 1932 patent describes the hammer and spring mechanism for applying torsional force, and his patents were licensed to Ingersoll-Rand in 1934, whose 1940 catalog offers pneumatic impact wrenches. The reason for the mechanism, set out in the 1951 court opinion, was to relieve the operator of the recoil felt when a nut seats. The same reason applies to a screw: a drill-driver holds the full reaction torque in the user’s wrist, while a tool that delivers its torque in short blows does not.

The cordless impact driver depended on the battery tool. Makita’s corporate history lists the TD130D cordless impact driver as its first lithium-ion battery tool in February 2005, and lithium-ion cells had entered the market in 1991 according to the Royal Swedish Academy of Sciences. The Journal of Light Construction’s 2009 report of the Makita BTD144 shows the form that followed: an 18 volt brushless tool with selectable impact strength.

The date of the first cordless impact driver is not established here. Makita’s timelines read for this entry list the 2005 model as a first lithium-ion tool and do not claim that it was the first cordless impact driver. Any claim of an earlier cordless impact driver could not be matched to a primary record, so the question is left in gaps rather than answered.

Hex bits and square drives separate the two tools in use. The Milwaukee manual covers three square-drive wrenches and one hex-drive driver in one document, with the hex model taking 7/16 inch hex-shank bits, and warns that only accessories designed for impact tools should be used. An ordinary drill-driver with a chuck, clutch and gear ranges remains the tool for boring holes and for fine torque control, which the cordless-drill-driver entry treats.

Timeline

1932
Robert H. Pott files the impact-tool application, US 2,012,916, on 30 January.1
1935
Pott’s patent is granted on 27 August. The 1951 court opinion records that Pott and a partner built 24 or 25 wrenches of this kind, which did not succeed commercially.1 2
1940
Fitch’s US 2,220,711, assigned to Ingersoll-Rand, is granted on 5 November for a rotary impact tool with a hammer, an anvil and clutch members. The same year Ingersoll-Rand’s Air Tools catalog offers impact wrenches.1 2
1949
Black & Decker markets an automatic (impact) wrench in March.1
1991
Lithium-ion batteries enter the market.1
2004
Makita says it first developed brushless-motor fastening tools in 2004, for the defense and aerospace industries.1
2005
Makita launches the TD130D cordless impact driver in February, listed as its first lithium-ion battery tool.1 2
2009
The Journal of Light Construction reports Makita’s BTD144, called its first brushless-motor 18 volt cordless impact driver, with a three-speed power selector and 1,420 inch-pounds on the hard setting.1

Gaps in the record (5)

The first cordless impact driver, its maker and its year, are not established; Makita’s 2005 TD130D is the earliest such product read in a primary source, and it may not be the first.
The hand-struck impact driver (a tool driven by hammer blows) and its patents have not been researched.
Milwaukee’s manual is undated here and describes a model with a 7/16 inch hex shank; the 1/4 inch hex bit standard has not been sourced.
IEC 62841-2-2 has not been read; the standard slugs are inferred.
No independent test of torque, bit life or run time has been sourced, so buying is null.