Hammer-Drill Technique?

Hi- You may have hit some reinforcing rod. You should not have to push hard. Less force than trying to drill a 1/4 inch hole in steel. If you can beg , buy or borrow a Hilte hammer drill, the type with the axial hammer , and not rotary impact, your troubles will be over. It will drill a 1/2 inch hole in four inches in concrete in 15 sec or so. Jim

"Doug White" wrote: (clip)Any suggestions would be really appreciated (clip) ^^^^^^^^^^^^^^^^^ You might try a 3 or 4# sledge hammer and a star drill. I think what is happening is you are hitting hard rocks embedded as part of the aggregate. Several sharp blow of the sledge hammer ought to break them down.

I have run into this and found that putting a concrete nail in the hole and hitting it a good whack with a hammer will break the aggregate and allow the hole to be finished. This, of course is assuming that the wall does not contain re-bar. Gerry :-)} London, Canada

One thing to keep in mind is that there is a big difference between the carbide tipped masonry bits intended for us in regular rotary drills and those intended for hammer drills. If you get the cheaper ones the hammer drill will destroy them in short order.
For a regular hammer drill as opposed to a rotary hammer, more pressure is generally a good thing, but let up periodically to allow the bit to cool. If you let too much heat build up it will damage the bit. With a real rotary hammer just a moderate pressure is best since it is not as reliant on operating pressure for it's impacts.
If you really want to save time get a good Hilti powder actuated fastener gun and get a box of the threaded studs for concrete. Just shoot them in, put the tracks over and install a lock washer and nut. They have threaded studs in lengths for both concrete and steel. Very handy little items.
Pete C.

I think he was referring to the difference between a hammer drill and a rotary hammer, where the rotary hammer has the center (axial) transfer bar that transmits the impact mechanism farther back in the unit. Hilti of course makes some of the best units on the market, including some like the TE5 that can do the rotary hammer thing around a corner.
Pete C.

What the others said, altho I am curious as to what Jim L. meant by Hilte's "axial hammer".
If you indeed hit rebar, a regular metal bit (or a magnet!) would reveal this.
One of the diffs among "hammer drills" is the bpm (beats per minute). Good Bosch's/Porter Cables have about 5,000, while crap has 20,000, which is *much less* effective in tough concrete. My old Skill "extra tool" is stopped dead by hard aggregate; while my small Bosch (SDS) whizzes thru it. Pistol grip hammer drills tend to have higher bpm's than D-handle type hammer drills, altho a Bosch pistol grip key-chuck type I've seen has similar stats to the smaller D-handle SDS type.
If you are able to plug in a bosch or a hilti at a store, just by pressing the chuck up against wood and listening to the percussion, by comparison you will be able to tell if your drill is a 5,000 or 20,000 bpm type--totally different sound/feel.
*Rotary Hammers* have even fewer bpm's than good hammer drills and quite a bit more energy per hit (sometimes available on the sales tag for comparison). Some rotary hammers have an energy per hit approaching that of a pure demolition hammer.
Altho in principle not relevant, I suspect the SDS bit systems offer much better performance as well--. It may be that the carbide on SDS is better than on twist-drill styles, as well--there is real shitty carbide out there! I've seen carbide on lathe tools no better than HSS!! You can also *sharpen* your carbide bits!! Makes a diff!!
I have both a small Bosch D-handle SDS type, and their biggest rotary hammer, and one muhfugguh of a concrete wall (9" thick, 1920's, no rebar, thank gawd). Either will do 1/2" holes a couple inches deep in seconds, aggregate or not. The Skil Drill will take goddamm *forever*. The rotary hammer will do 1.5" holes all the way thru sed 9" wall, almost like butter--well, hard butter, at any rate. :)
Ito of "technique", I have not found that it much matters, esp. w/ low bpm styles: just lean in hard! Water would probably help bit life, but that's an ergonometric challenge. ---------------------------- Mr. P.V.'d formerly Droll Troll

Oh yeah,
20,000 bpm, while mebbe impressive as a raw number, just tends to "buzz" hard rock--OK for cinder block/mortar, but not for tough stuff. 5,000 bpm and lower *breaks* rock--also why leaning in hard on the drill helps, as more momentum is transferred-- ALTHO, now, as I recall, having drilled *hundreds* of 3/4" holes 5" deep (door opening, donchaknow--goodgawd....), there *is* in fact a sort of a "sweet spot" ito of pressure on a bigger rotary hammer. I suspect, tho, that this is not much of an issue with the higher bpm's of smaller hammer drills. ---------------------------- Mr. P.V.'d formerly Droll Troll

As others have said, I think you have the wrong type of hammer drill. An SDS type drill will make short work of your problem. I suspect you have a conventional chuck and the Vermont American type masonry drill. I'm not being funny here, my dad assumed that the high speed hammer drill he has would punch holes in CMU with regular bits. There is a difference in carbide bits.
The Bosch Bulldog is an excellent drill.
(top posted for your convenience) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Keep the whole world singing . . . . DanG (remove the sevens) snipped-for-privacy@7cox.net

You probably need a better hammer drill. The inexpensive ones can't handle the aggriate inbedded in the concrete too well.
John

Assuming that you haven't hit rebar (test with a magnet), get a piece of silver steel or drill rod of the same diameter as the hole and grind a 70-90 degree chisel end on it. Heat the end 1 1/2" to orange red and quench in water, then immediately 0 deg C/300 deg F in a pre-heated oven.
Place it in the hole and hit it hard with a 2lb club hammer. Turn 60-90 degrees and repeat, if necessary un til you hear the hard aggregate crunch.
Worked for me recently in a similar situation.

Keywords: That's an interesting trick. I'll have to give it a try as soon as I can find a concrete nail...
Doug White

I _had_ good bits, but I think I wore them out. I went & bought some new ones & they worked MUCH better. The old one looked basically OK, but I think the sharp edge was just dull enough that it wasn't chipping the aggragate very well.
My Bosch is a rotary hammer, and I can't tell too much difference in applying varying amounts of pressure once I got some sharp drill bits.
I've got a Remington, but no threaded fasteners. They wouldn't work for these shelf brackets because the fasteners need to be countersunk to clear the shelf brackets. Otherwise you lose a lot of adjustable positions for the shelves. Something to keep in mind for the next project.
Doug White

My Bosch is a rotary hammer type, but it's almost 20 years old & works pretty well once I replaced the bit with a fresh one. It does have a conventional Rohm 3-jaw chuck. The bits I use are designed for hammer drilling, but I guess after 20 or so holes this one wasn't quite sharp enough to deal with really hard aggragate. A fresh one went right through stuff the previous one was only chewing through slowly. Same brand of bit, but just enough more edge to make the difference.
Doug White

I did lotsa sign installations with cheap hammer drills from HF. Sometimes ya just gotta muscle 'em in there. Hilti is the best in my opinion but if you are on a budget just grab one of the basicly disposable ones from Taiwan and a couple spare bits and drill to your hearts content. Oh and one thing before we get started... Safety first sorry Norm.

"Doug White" wrote
You can hit aggregate or rebar or wire. I drilled two half inch holes the other day, and got to a point where it wouldn't move. I went and got a 1/4" chisel and pounded on the bottom of the hole while rotating the chisel. It broke whatever was down there, and the hole proceeded.
A rotohammer will outdo a hammer drill, but hammer drills will do it with a little help. I used to keep those pieces of hardened steel that came with gate springs. They wouldn't bend. I'd grind them down to a chisel point, and use those when the drill hit aggregate. A small star works, too, but I like the round rod because it was free, and I know it is making some progress at the bottom of the hole.
Then, the drillbit can go on. Be real careful to keep drilling when it isn't going anywhere. A side effect is heat, and you can smoke a bit really easy.
Steve

"Pete C." wrote> Did a job once where we were drilling some half inch holes in some hard concrete. We were using a darn good Makita model hand drill. The foreman brought over a Hilti, and what was taking us 2-5 minutes took fifteen seconds.
I couldn't swing for the $$$ for a Hilti, and didn't like the proprietary $$$$ blades, so I got a Milwaukee that took SDS bits, and it worked really slick. I believe it was $145 ten years ago.
Steve

Need to sharpen your carbide bits on a green wheel on a grinder. Use air to blow the dust out as you drill. The dust acts as a lapping compound and quickly dulls a regular masonry drill bit.

How about drilling a smaller diameter hole and then increasing the bit for he larger.
All to often I see people use the size they want a hole and go for it. Remember the center of the drill doesn't have much force as it has very little torque. It hardly moves. The outside lips have a long path and lever distance from the center.
I do it on steel and other metals - wood also. Why not this ?
Martin Martin Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net NRA LOH, NRA Life NRA Second Amendment Task Force Charter Founder
SteveB wrote:

Keywords: Actually, that's exactly what I was doing. The shelf rails have holes that are smaller than the anchors I'm using. I made a guide on my lathe that aligns a smaller bit centered in the holes in the rail, and then I use the bigger bit to bore out the hole. You still have to be careful, because the larger bit can wander off the small hole if you try to drill too hard or fast & hit an off center piece of something hard.
Thanks to folks suggestions and mostly some fresh drill bits, I've got all 20 holes drilled and the rails & shelves are up. With very few exceptions, all the holes were exactly centered on the holes for the rails, and the ones that were off weren't out by enough to matter.
Doug White