What an impact driver does
An impact driver looks like a short, stubby drill, but it works differently. When the resistance on a fastener gets high, an internal mechanism begins to deliver rapid rotational blows in addition to steady turning. Each blow adds a burst of twisting force. You hear this as a rapid hammering or buzzing sound once the screw gets hard to turn.
The key point is that the hammering is rotational, meaning it adds to the turning force around the axis of the bit. It does not push the bit forward the way a hammer drill does. Because the force is delivered in short pulses, the driver itself transfers far less twisting kick-back to your hand and wrist than a drill would at the same level of force. That is the central reason people like them.
Reduced wrist strain
When a standard drill meets high resistance, it either stalls, slips its clutch, or twists the tool in your hand. Hold a drill at its full force driving a long screw into dense wood, and your wrist and forearm absorb the reaction. Over a long day of repeated driving, that adds up to fatigue.
An impact driver reduces this. The pulses do the heavy work, so you can hold the tool with a lighter grip and a modest amount of forward pressure. Many people who drive hundreds of screws, such as when building a deck or fence, find it less tiring. This is one practical aspect of the repetitive strain discussion in tool design: the tool absorbs more of the load so your body absorbs less.
Driving long screws and lag bolts
Long fasteners meet high resistance as they travel deep into wood. Examples include:
- Long construction screws for joining thick timber.
- Structural screws that replace some larger bolts in certain applications, following the maker’s instructions.
- Lag screws and lag bolts, which are heavy fasteners with hex heads, driven with a socket adapter.
- Ledger and post fasteners in outdoor building.
Driving long screws is where an impact driver stands out. A drill may stall or force the bit to cam out, which means the bit rides up out of the screw recess and chews the head. The impact mechanism helps keep the bit engaged because each pulse is a brief burst of force. Even so, good technique matters: push straight along the screw, keep the bit fully seated, and let the tool do the work.
For lag bolts, use a hex socket adapter made for impact use, and drill a pilot hole sized for the lag screw as the maker specifies. Starting without a pilot hole in dense wood is likely to split the wood or stall the tool, and a snapped fastener is a bigger repair than a drilled hole.
Other advantages
Compact size. Impact drivers are generally shorter than drills, so they fit between joists, inside cabinets, and in other tight spots.
Quick bit changes. Most have a quick-release hex collet. You pull the collar and push in a one-quarter inch hex-shank bit. This is convenient, though it limits you to hex-shank accessories.
Less cam-out. The pulsing action tends to hold bits in the screw head better, which saves both screws and bits.
Efficient with small and large fasteners. An impact driver does not only work with large screws. With a light touch on the trigger, it can drive medium screws without trouble, although very small ones are easier to control with a drill.
Limits and trade-offs
An impact driver is not a replacement for a drill.
- Drilling holes. Hex-shank drill bits made for impact drivers exist, but an impact driver gives you less control for drilling, and large or precise holes are better done with a drill. The comparison of drill versus driver explains the split of duties.
- No clutch. Because there is no clutch, you control depth with your trigger finger. It is easy to drive a screw too far or snap a small screw. Some models have multiple power modes, which help.
- Noise. The hammering is loud. Hearing protection is a sensible precaution on longer jobs.
- Delicate work. Soft materials, thin trim, brittle plastics and small screws are at higher risk of damage. A drill with a clutch is generally safer for them.
- Bit choice. Use impact-rated driver bits, which are made to withstand the shock. Ordinary bits, especially cheap ones, may wear quickly or snap. The accessories you add make a difference, and the article on impact driver accessories covers them in more depth.
Getting good results
- Match the bit to the screw drive and make sure it fits snugly. Screw drive types vary, and a poor fit causes slipping.
- Seat the bit fully in the recess before squeezing the trigger.
- Start slowly, then increase speed once the screw bites.
- Stop when the screw is seated. Continuing to hammer can strip the threads in the wood or snap the screw.
- Use pilot holes in hardwood, near board edges, and for large fasteners.
- Keep the driver square to the work.
If a screw strips, avoid running the driver faster. Instead, back it out, change the bit, and consult tips for removing stripped screws.
Safety
Wear eye protection. Keep fingers away from the bit and the fastener, because impact drivers can start with considerable force. Remove the battery before changing accessories, or at least use the trigger lock. Do not use damaged bits, and do not hold small pieces of work by hand while driving.
Who benefits most
If you build decks, fences, sheds, raised beds or frames, or you regularly drive screws longer than a couple of inches, an impact driver is probably worth having next to your drill. For occasional light household jobs, a drill may be all you need. Many people find that owning both, one set up for drilling and one for driving, saves time and effort.