What is the difference between an Impact Driver and a Hammer Drill?
Makita makes both and the Hammer drill weighs 42% more than the Impact Driver.
Other manufacturers seem to have about the same weight difference.
What is the difference between the two, how do they function and what they do?
Answers:
Huge difference. An impact driver, as stated above, was designed to primarily drive nuts, bolts and other fasteners, usually by compressed air. But in order to make the product more attractive to the DIY homeowner, different attachments were designed to make the impact driver a "multi-tool" (along with making electric versions). Although the attachments work to varying degees, they are not as efficient or effective as the tool they were designed to replace.
A hammer-drill has one primary function, and that is to drill through masonry, concrete, rock, etc. Most hammer-drills do have the option to switch between hammer and non-hammer modes, allowing you to use a hammer-drill for wood and other softer materials, however, you can't use a hammer-drill to secure your lug-nuts (i.e. impact driver).
A proffessional contractor who finds the need to drill through concrete occasionally will opt to buy the tool made for the job; a hammer-drill. Whereas a DIY homeowner, that would barely ever use the hammer-drill feature, but use the tool as a drill, sander, wrench, etc. would probably opt for the impact driver multi-tool.
Neither option is the wrong option, just determine what and how often this tool is to be used, and buy accordingly.
Other answers:
An impact driver's main use is to "drive" nuts and screws. Although they have many uses, you see these used allot my mechanics, especially when removing and installing tires. A Hammer drill drills, and also, when set correctly, drills while "punching" or hammering the bit up and down. Its best use, while set on Hammer, is when drilling concrete.
An impact driver's main use is to "drive" nuts and screws. Although they have many uses, you see these used allot my mechanics, especially when removing and installing tires. A Hammer drill drills, and also, when set correctly, drills while "punching" or hammering the bit up and down. Its best use, while set on Hammer, is when drilling concrete.