Do I need impact rated drill bits?
I've run into this problem dozens of times. I read article after article, review after review touting impact rated drills as a scam, worthless, no better than standard drills, etc. Customer reviews have blasted these accessories as unnecessary expenses developed by manufacturers to make more money. We even published an article on impact drills and found that Bosch's standard drills outperformed all impact drills we tested according to our methodology. Read our original article Are Impact-Rated Bits Worth Your Money? So why bother revisiting this idea?
I admit, I am a very loyal person. I don't know everyone from every tool manufacturer, but I know the ones I've had a chance to know about that don't generate crap and put it on store shelves. So I set out to discover what an impact-rated drill really is. Knowing the depth of Milwaukee Tool's R&D and engineering teams, I enlisted their help to unravel this mystery. They kindly took the time to answer my questions and walk me through the technical aspects of these accessories.
Milwaukee manufactures the Shockwave line of impact rated drill bits and accessories. In addition to getting feedback from their technicians, I also got access to a Shockwave drill, nut driver, extended drill holder, and 30 degree steering knuckle.
How do impact rated drills differ from standard drills?
When looking at rated impact drills versus standard drills, the basic idea is to look at the differences in design. It starts with the material itself. Milwaukee blends a custom steel that is more malleable than its standard drill bits. If you think back to high school chemistry, ductility is the ability of a solid to deform under tension. Most of us know this through the example of metal being stretched into wire. For impact-rated bits, this ductility allows them to twist and bend under the stress of impact driver torque.
In addition to the steel mix itself, Milwaukee also includes "Shockzone" in their drills. This area is very noticeable as a narrower area in the middle of the bit. It allows for extra flex to cushion the bit from torque while still giving the tip a chance to rotate and drive the fastener further.
If impact rated drill bits are designed to withstand torque, why do they still fail?
There are several reasons why an affected rated drill can still fail. The first is shared with the standard bits. When the bit slides in the fastener head (affectionately known as camming out), the fastener head becomes visibly damaged. Less obvious is that the drill bit is also starting to fail. Each time the cam fails, the tip gets rounder and rounder until it can no longer drive efficiently. Two ways to avoid this are to make sure you choose the correct size drill bit to match the fastener, and to make sure you apply enough force to the fastener to ensure the bit stays engaged.
In these two pictures, see the difference between a low speed/low torque cam out and a high speed/high torque cam out. With less force than a new drill, the drill transfers most of the damage to the fastener head. In the high-speed test, the drill bit suffered much more damage, while the fastener suffered much less damage. This is an issue with both impact rated drills and standard drills.
The next discussion is about metal fatigue. Take a paper clip and bend it. Now bend it back. Metal is malleable enough to reshape under the pressure you apply. But it comes at a price. Continue the same back and forth motion. Eventually, the metal will weaken and break. Touch the failed area immediately and you will find it warm. This is where metal fatigue comes into play. Exercise can absorb stress for a certain period of time, but not indefinitely. As impact rated drills bend, they experience metal fatigue, which is not something that breaks will fix. Eventually it will break.
Now grab a pencil, preferably one you don't plan to use again. Bend it slightly, but not too much. You'll notice it bends, but not like a paper clip. Come on, give it more power. The fact that it didn't bend and break like a paper clip suggests it's less malleable (if it were metal). It does not reshape easily, but breaks instantly under pressure.
While it's not perfect, a paper clip and pencil demonstrates the difference between a standard drill and an impact-rated drill. Standard bits can be made very strong, but at the cost of being more brittle. When they are subjected to the extreme torque of an impact driver, it snaps off quickly. Impact rated impact bits blend strength and ductility to keep going while absorbing torque. Eventually, it will succumb to metal fatigue, but it will last much longer under this stress.
The engineers who designed the Milwaukee Shockwave bit were smart — really smart. Extensive testing was carried out to develop a steel mixture that can absorb torque and withstand metal fatigue for extended periods of time. Finding the balance between absorbing flex and driving strength is no simple task, but the result is that Shockwave rated impact bits fail due to metal fatigue much later than standard bits break. In fact, at this pressure, Milwaukee Shockwave bits will last 10 times longer than standard bits.
Why do standard drills consistently outperform impact rated drills in testing?
This is the million dollar question – it has everything to do with the testing methodology. If we put a rated impact drill bit with a standard bit in pressure treated wood using a 4" deck screw, we're in the happy position of the standard bit. A standard drill will never come close to its point of failure, even with an impact driver. Most of the reviews I've read have the impact rating bits hit their point of failure first.
Let's gain some perspective.
The studies I've read rarely find bit- management to be a failure. Almost all of them list bit rounding as the point at which a bit is considered to have failed. Many people think the point of failure is three cams in a row. If we think about it, softer (more malleable) bits should round faster! Using this test method, all but the worst standard bits will outperform rated impact bits.
Milwaukee Shockwave Rated Impact Drills are designed to outperform standard drills when applying the highest torque drivers. We're talking the kind of torque that bites off a standard bit on the first try. Most fastening jobs will not come close to this force. To get to the point where an impact driver is delivering torque to its potential, we're talking metal-to-metal fastening, where high torque is required to break the friction between the metals. We're talking about hard stop tightening, when you get to the end, the driver gives all the torque to tighten it up. We're talking bag bolts, where the driver not only delivers high torque, but for a longer period of time, and does deliver a punch to the nut driver. These are the applications where standard drill bits and nut drivers can't stand and break off, usually at their weakest point or shaft.
So the bottom line is, do I really need an impact rated drill?
For most fastening applications, no, you don't. Unless you're doing the aforementioned application or something else that pushes your impact driver to its true potential, you'll actually get better performance from a standard drill. Still, there is room in your toolbox for these bits and accessories. Sooner or later you will find that you need to loosen a stuck nut, drive a screw into metal or screw in a lag bolt.
When it comes to driving screws and you need to make a decision, the general rule of thumb is to use a standard bit on wood and an impact rated bit on metal.
I can say openly that I think all professionals should have a set of shock-resistant bit and nut drivers, such as Milwaukee Shockwaves, in their toolbox, along with an extended bit holder or two. Most homeowners who keep an impact driver on hand will also benefit. Know performance expectations and costs. If your application doesn't need them, don't use them. Just because you're using an impact driver doesn't mean you need to use an impact-rated bit. Keep those more expensive accessories for a rainy day.
words of thanks
A big thank you to the engineering team, R&D team, project manager and Kharli Tyler at Milwaukee Tool for the work and research they did in developing the Shockwave rated line of impact drill bits and accessories and for satisfying my curiosity and desire to know the technical details of this work!