[TowerTalk] SDS Hammer Drills - Difference?

Ian White GM3SEK gm3sek at ifwtech.co.uk
Thu Aug 31 04:48:41 EDT 2006

Jim Miller wrote:
>OK, this is probably off topic, but seems it has been carried right up 
>to this question.  Sorry if not.  Just tell me where to go.  I've been 
>told before so I know the way.
>I can't imagine a "normal" hammer drill  (I have a Milwaukee) working 
>very well at all for installing ground rods.  Not nearly enough 
>"hammering" (pretty miserable, actually) and can't turn off the rotation.
>WHAT is the difference with the SDS drills besides the type of "Chuck" 
>on it?????
>Does it have a mode where it only hammers?
>Does it do a better job of hammering?
>What is the big deal with SDS anyway?

Right... sorry this has turned out so long, but I didn't have the time 
to make it shorter  :-)

Every home handyman probably has a compact electric drill with a keyed 
chuck, and an optional "hammer" action which makes it much better for 
drilling brick or concrete. The hammer action is produced by a rotating 
ridged plate... but if you think about it, this mechanism is wasting a 
lot of impact energy by moving the whole chuck, which has much more mass 
than the drill bit. Also, the ridged plate produces more of a vibration 
than a true hammer-blow.

Way up-market are the large industrial electric breaker/drills that have 
a powerful hammer action and also can rotate. The hammer action in these 
tools is quite different. It is actually electro-pneumatic: the electric 
motor drives an air cylinder, which propels a striker like a captive 
bullet. The striker delivers a very sharp hammer-blow directly onto the 
end of the drill bit - and that's what makes these drills so effective 
at hammering and breaking.

To avoid wasting impact energy in the chuck, the drill bit must be free 
to slide, so the impact goes right on through to the sharp end of the 
drill. That is why all these big hammers use some kind of splined or hex 
chuck with a snap lock.  Unless it's rotating, the chuck doesn't move at 

There was an obvious gap in the market between these two classes of 
tools. Into that gap came the Bosch SDS system. This puts the best 
features of the big electro-pneumatic breaker/drills into a compact 
one-hand power tool.

The system you see in compact drills is "SDS Plus" (often called just 
"SDS"; SDS Max is its big brother, used in the heavier breaker/drills). 
SDS+ has a splined chuck that takes bits with about a 3/8in shank, and 
inside is the genuine pneumatic "captive bullet" striker system.

The difference from the old-style "hammer" drill is unbelievable! You 
really do have to try it to know, because you just don't expect that 
kind of drilling power from a tool you can pick up in one hand.

Spiral drill bits are available in a wide range of sizes and lengths, 
out to 1.0 metres which are very good for making pilot holes for ground 
rods. The surprising thing is that the SDS+ system delivers a very 
effective hammer action down to the tip, even from such a long distance.

SDS+ started out with Bosch in Germany, who creamed off the market for a 
few years and then started to license the technology to other 
manufacturers. In Europe there is now a very wide choice of models, 
broadly classified by weight.  Drills in the "2kg" (5-pound) class are 
one-handed, in-line, pistol-grip and look like a slightly stretched 
normal electric drill. The "4kg" (10-pound) class are two-handed, with 
the motor at right-angles to the drill bit.

Both weight classes have exactly the same SDS+ mechanism. The dead 
weight of the drill has very little effect on the hammer action so most 
people prefer the lighter drill. SDS+ is the one you'd want to own for 
yourself and use regularly. For the times when you need a more powerful 
hammer action, then by all means rent a big industrial breaker/drill... 
but don't underestimate what SDS+ can do until you've tried it. (We live 
on rock here, but I haven't needed a biggie yet.)

SDS+ seems to have been available in the USA for a long time, but well 
hidden in the general fog of confusion about "hammers". The key feature 
to look for is the 3/8in snap-lock chuck - if it doesn't have that, it 
isn't any kind of SDS.

You are now beginning to see some real competition in the USA from Far 
Eastern own-brand imports driving the prices down... but there are some 
real dogs out there, which have the SDS+ mechanism but may be lacking 
important features like hammer stop, rotation stop, good speed control 
and a safety clutch. Watch out for dumping of models that haven't sold 
well in Europe.

For the rest of the story about choosing an SDS+ drill, see:

73 from Ian GM3SEK         'In Practice' columnist for RadCom (RSGB)

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