In which the difference between rotary hammers and hammer drills is expounded upon.
Sternly worded disclaimers
Fair Warning. If you want an expert opinion on rotary hammers, go elsewhere. I’ve been using cordless and corded rotary hammers very frequently for about a year. At this point, I have observations and opinions.
Other Fair Warning. Good luck finding those expert opinions! It seems like rotary hammers fall into that grey area of “simple” and powerful tools that don’t get a lot of discussion.
Rotary Hammers versus Hammer Drills
There’s a much bigger difference between hammer drills and rotary hammers than the guys at your local hardware store might let on. A hundred-dollar hammer drill will be adequate for drilling 1/4″ holes in soft, consistent materials like brick, cinder block, or mortar. If you’re putting bigger holes in rock or concrete, use the rotary hammer.
The difference is in the mechanism. A hammer drill is basically a drill with a buzz gear – it’s still spinning the bit pretty fast and wiggling it a bit. A rotary hammer is a piston driven hammer that rotates the bit slowly. Here, take a gander at this to get an idea of what’s going on:
That’s how holes are drilled by hand. The real work comes from the hammer striking the bit.
Here’s a comparison table, just so you can get an idea of RPM (rotations per minute) and BPM (beats per minute) of a random sample of hammer drills and rotary hammers:
|Tool||RPM (max)||BPM (max)||Impact Energy (ft lbf)|
|Cordless Hammer Drill||1,500||18,000||N.A.|
|Cordless Rotary Hammer||960||4,260||2.2|
|Corded Rotary Hammer||320||2,900||7.1|
With slower rotation and beats, but much higher impact, the corded rotary hammer goes through rock the fastest by far. That’s a key concept: The rotation of the bit serves only to clear debris and drill a round hole – the real impact is in the impact.
I’ll get into it in a subsequent post, but it’s also key to understanding how to save a fortune in bits. Just an opinion: the higher the RPM, the more friction generated, and the faster the temperature increase in the bit. I’m very careful with the bits for a corded rotary hammer, but I’m immaculate with the bits for a cordless rotary – for the cordless, I change them often, and clean them off as soon as they’re out of the hole. And another opinion: For a hammer drill, don’t waste your money on carbide tipped bits. Get the cheap straight steel four-pack, you’re just going to burn through them anyways.
My experience in the field so far jibes pretty well with this – I can put three or four 1″ x 8″ holes in rock with a corded rotary and can still put the bit in my back pocket. One 1/2″ x 3″ hole with a cordless rotary gets a bit pretty damn hot. Yes, I have put the smaller bit in my pocket, and have done the very funny dance as a result.
Tiny side note – buyer beware – I think manufacturers rate the torque for hammer drills in ft*lbs and the impact energy for rotary hammers in ft-lbf (an imperial version of Joules?). Apples and oranges.
Let the Tool do the Work.
Strangely, my trail crew says “let the tool do the work” when I actually do something. I’m not entirely sure what they mean.
If you really put a lot of force into a hammer drill, it may drill faster. I don’t know. But you’re probably beating up the mechanism that moves the bit in and out. You’ll wear the drill out faster.
This is key: If you put a lot of force into a rotary hammer, you’ll just diminish the amount of impact energy. You can especially hear it working with the cordless rotary – the drill bogs down big time. Put just enough force into the drill to engage the piston, but no more. I haven’t timed it out, but the bit feels cooler after drilling, the battery lasts longer, and the drill just sounds better to my opinionated ear if it’s allowed to do its job. With the corded rotary hammer, I haven’t been able to bog it down by pressing hard, but it makes for an exhausting day to try and force it.
For a hammer drill, you’ll either use a straight shank or a hex/triangle shank. ‘Nuff said, go back to hanging up ironic pictures on that brick wall in your fancy loft condominium.
For cordless rotary hammers, you’ll most likely use SDS or SDS+. ‘Nuff said, go back to grid-bolting that new sport route of yours.
For a corded rotary hammer, you can choose between SDS Max or spline bits. Look at the product specs when choosing between the two. For hammers that are exactly the same except for chuck, the spline option may deliver more impact energy.
Tags: Rotary Hammers