[TowerTalk] Hammer Drills- Rotary Hammers- Demolition Hammers

Kimo Chun kimo at lava.net
Wed Aug 30 23:10:41 EDT 2006

Hammer Drills, Rotary Hammers and Demolition Hammers are
different devices. My understanding is as follows (I own
and use all three types):

Hammer Drills are small and typically don’t exceed a chuck / shaft
size capacity of one-half inch. They will typically
use plain round shaft bits or ones with one or more flat faces
on it and uses compression pressure of the jaw to hold the bit
(and in your case
 the cutting in of the jaw edge into the rod).

A rotary hammer is designed differently. The hammer action
mechanism is more robust and will withstand heavy use whereas
a hammer drill will wear out sooner. Thus the different name
to distinguish how it works. A rotary hammer generally starts
at over one-half inch bit capacity and up to typically two inches.
They may go higher but perhaps that’s the domain of pneumatic
tools and /or fixed devices (not hand held).

Also, the rotary hammer uses a different method of holding
the bit and use special bits. The most common are Spline
(an older? standard but still currently used)and the SDS series.
I don’t know the difference between SDS, SDS plus and SDS max.
I believe they have their own range of both shaft size and
drilled hole range. Meaning, you probably won’t find a
quarter-inch hole bit for an SDS-Max range hammer. Study any
power tool online catalog of bits and it will become apparent.
Some rotary hammers have optional standard chucks that can
be placed into its special chuck to allow use of plain bits.
However, it is not recommended that this be used in any hammer
mode as the hardware and bits are often not up to the task.

On any hammer drill or rotary hammer the multi-position switch
allows you to adjust the “aggressiveness” of the hammer action
or to remove it altogether. Bosch (and others will typically
mark that position with the universal label “0” meaning
“no hammer”. Then it typically goes to 1,2 or more hash marks
(like Roman numerals) to indicate the hammer and more aggressive
hammer settings.

A pure “Demolition Hammer” has no rotary action at all.
It will also often have a multi-position switch to adjust the
hammer action.

I’ve used my demolition hammer for driving ground rods.
Mine is an old Hilti TP800 medium duty (for them). It weighs
something like 20 pounds. Bits are available to drive the
different standard sizes of ground rod. I’ve had no problem
with this setup but we don’t have granite or other hard rocks
here. We have Basalt rocks that are like hard compressed
powder. They are pretty tough. Hilti uses yet another bit /
chuck standard; Probably their own proprietary one.

I had someone purchase a bit for their SDS-Max rotary hammer
which did a nice job of putting rods through coral earth.
These can be had online for less than $35 or so. They just
needed a ladder to stand high enough to start the process.

As you usually install 5/8 inch or 3 /4 inch diameter ground
rods either 8 or 10 feet long; I wouldn’t count on any
compression-grip chuck to hold that rod end and keep it
rotating in many situations. You can imagine the friction
generated over the surface of a mostly buried rod versus
your small grip area in the chuck. No way. The spline is
like a gear. The large surface area at a near right angle
to the rotation allows for much pressure to be applied.
The SDS uses flat surfaces and steel bearings or perhaps
a fitted ridge or key held in depressions in the bit to
lock onto it and apply pressure.

A straight machine-tool (hard) steel bit fits over the end
of the rod to keep it from “mushrooming” and just pounds
it in. I don’t believe the bit for a rotary hammer is any
different. It doesn’t try to rotate the rod. Any rotation
is incidental and it just uses the rapid hammering to
gradually drive the hard point of the rod through whatever
rock or soil is in its way. At worst it may bend the rod
from the “vertical” to go around a rock – if possible.

You notice in your situation that the “grip” of the
chuck on the bit became a problem. I suppose a cap bit
may develop a “grab” on a rod that is “mushrooming” to
whatever extent possible but it won’t “dig in” to the
rod like a sharp-edged drill chuck jaw may. Not to
mention the danger of an out of control drill “stuck”
to the end of suddenly-frozen ground rod.

Also, hammer blows to the side of the tool steel bit
to loosen it will be tolerated a lot better than by
the chuck of a hammer(or plain) drill! :-)

True, there may be situations (hard rock) where the
rotary hammer or demolition hammer and bit may not be
able to penetrate. It will be a combination of the
tip of the (often) copper-clad steel rod not being
tough enough or sharp enough to break through and /
or the friction of the soil and lack of weight and
hammer force to overcome it.

Look for other equipment (more industrial) rental
places to get these. Home Depot, arguably, is not
much more than a “Pro-sumer” arena. Likewise, some
of their employees may not have the experience to
advise you properly. Go to a specialty tool dealer
for that. Ask the local tradesmen where they go to
buy heavy-duty tools and supplies (other than
Home Depot).

I hope I got most of this right and it clears some
of this up. Good luck.

73, Aloha,

Kimo Chun  KH7U

Delta Communications, Inc.
Honolulu, HI


Message: 6
Date: Wed, 30 Aug 2006 16:33:49 -0400
From: "J A Ritter" <w0uce at nc.rr.com>
Subject: [TowerTalk] Dismal Ground Rod / Hammer Drill Results Today
To: <towertalk at contesting.com>
Message-ID: <001801c6cc73$9b1137a0$0201a8c0 at OFFICE>
Content-Type: text/plain;     charset="us-ascii"

With twelve rods to put in I rented a large Hammer Drill from Home Depot
today.  They did not have any attachment to use with ground rods but the
clerk said I could simply insert the end of the rod into the chuck and
hammer away.

Being an old timer... I enlisted the help of a strappin tall, in shape local
ham and we set out to "hammer" in 12 ground rods.  What a frustrating
experience it turned out to be...

The Hammer Drill did not hammer -  all it did was turn like a normal heavy
duty drill.  No matter which setting the drill was set on the results were
(1) the chuck turned clockwise (2) the chuck turned counterclockwise or (3)
the drill ran but the chuck did not turn.  There was no hammer or vibration
anything other than right or left torque.

The best we could achieve was getting three rods "twisted/pushed" in then
the drill slowed to a crawl with 3' of rod remaining out of the ground on
all 3 attempts.  Every attempt the chuck grabbed the rod and it was all we
could do to get the darn drill off the rod, we finally gave up.

Either we didn't have a clue what we were doing or the drill did not work or
both.  $36.00 rental fee later, plus two trips to get and return the drill
and the results were dismal.   Hammer drills may work in other locations but
certainly did not in NC clay covered by topsoil at my QTH...

Can anyone tell us from experience what the three position selector lever is
supposed to do and do hammer drills really work if a ground rod attachment
is used and the operators know what they are doing?

Jack W0UCE
Raleigh, NC

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