Drilling rock with a rotary hammer for blasting?

[BleedingOrange]

Thanks everyone for the advice, I'm gonna have to think this one through a bit more it sounds like.

If the other guys are retiring, why not try and buy the best equipment that they are going to be selling off, that will properly do the drilling job ?

 

[fried1765]

I've never drilled granite. I've drilled plenty of concrete. Really hard concrete is what, 6000psi? Granite is listed as 19,000psi. I'd say rent a rotary hammer if you can and test it out on some granite. That's about the only way you'll know yourself how it works in your conditions.

Found this interesting video on hammer drill VS rotary hammer with 1/2" bit in concrete. Really illustrates the difference. But again, this is concrete not granite, and 1/2" VS 1".

"rent a rotary hammer if you can"?

That is the operative question.
The OP is in rural Costa Rica.
Local rental centers there are very problematic.

 

[Egon]

Concrete will vary as to the type of rock used.

Granite is a general term. There are many different types with different mineral composition.

For smaller rock feathers and wedges may be the way to go. Around here all the old granite foundation blocks show signs of drilled holes and feathers and wedges. I have had no problem using an Ordinary electric SDS rotary hammer drill for half inch six inch deep holes in granite.

 

[Jim Nelson]

Back in the good old days, when men were men, All rock drilling was done with drill steel with graduated sized bits and sledge hammers. Men had either a helper or 2 man team where one held the steel straight and steady while the other swung the hammer then traded off. When I lived in Co. they had Miner days every summer in the town of Leadville, the 2 mile high city, and hand drilling was one of the contests. Fun to watch but not for me. The Leadville area was where I had my mining experience as a young buck and though it was good money I soon figured out there were a lot of better ways to put food on the table.

 

[Captain Dirty]

Hola everyone,

I work primarily doing excavations with an old backhoe in Rocky mountainous terrain. . . Anyways, the question is, would I be able to drill hard granite and the like with a medium to large electric rotary hammer drill? Not a regular size hammer drill like one of the big ones closer to jackhammer size? It has to make like 1" holes for the charges. . . . I'll have to make a lot of big holes in hard rock. Would a rotary hammer be an adequate tool for this? The manufacturers only talk about concrete . . . Any advice experiences insight? Thanks!

In the old days John Henry swung a two-handed 8 - 12 pound sledge hitting a star drill held by a "jack", often a boy, who gave the bit a partial turn between hammer blows--slow rotation with slow, powerful impacts. The operation was backed up by a person with a grindstone and a blacksmith who sharpened and reconditioned worn drills. Lost skills.

A hammer drill is a small to medium sized electric drill (6 -8 amps, SDS or SDS+ bits usually < 1" diameter); cost is one to a few hundred dollars. The impact is provided by a rotating cam--relatively fast rotation with quick, small impacts--OK for cinder or concrete blocks or awkward positions where the smaller size outweighs its ineffectiveness. Typically, the operator gets frustrated with the slow progress and increases the pressure (and maybe the speed) causing heat, dulling the bit quickly, and possibly even getting the bit hot enough to de-solder the carbide tip making the bit useless.

A rotary hammer is a larger, usually electric but sometimes gas-powered drill (around 13 amps, SDS Max or Spline bits 1/2" - 2" diameter); cost is several hundred to a couple of thousand dollars. The impact is provided by a piston--relatively slower rotation and slower, heavier impacts. It is usually used vertically downward and drills faster under its own power with operator guidance. If the operator applies additional down-pressure the greater energy yields greater heat and quick destruction of the bit.

My situation sounds different than the OP's, but my experience may be helpful. I live on a hill of glacial till--clay, sand, pebbles, cobbles, and boulders bulldozed by the glaciers. Mercifully, I have no ledge (bedrock). I continually encounter (typically granite) boulders too large to handle and have had success breaking them with feathers and wedges. I have over a dozen 1/2" (the size of the required hole) sets of feathers and wedges (wedges and shims); they are available in larger sizes that require bigger holes. I gave up on a hammer drill and now use a Makita rotary hammer rated for 1-3/4" bits. Conventional wisdom says the ratings are overstated. With a new bit the drill will make a 1/2" x 5" hole in the granite in about 30 seconds. I have a half-dozen 1/2" and 9/16" bits (For some reason the 9/16" are cheaper than the 1/2"; name brand such as Milwaukie and Bosch out-perform cheaper bits (I have not tried the even more expensive Hilti.)), and I rotate a fresh bit for each hole, letting the last bit cool. I drill holes approx. 6" apart along the line I hope to split, blow the dust out with compressed air (a baster-type bulb will work) and set the feathers and wedges. The process is relatively slow. The boulder must be excavated; if it remains in the ground the surrounding soil buttresses the rock and makes the wedges ineffective. The wedges should not be "mashed"; a heavy blow causes the surface around the hole to spall so there is less material for the feather to press against. Lighter blows suffice, and one should allow time for the forces in the rock to build. The rock will eventually split according to those forces and cleavage planes. There is great satisfaction when it splits as intended.

 

[jmc]

Interesting post, Captain Dirty.

Our family had a small-town Western Auto store during the mid 1950's thru 1976. We sold hand held "star drills" in smaller sizes and as a kid, I used one to make a tomahawk head out of a nicely shaped stone. I think my dad might have traded me that star drill to save his set of twist drills... Anyway, it's interesting to me that a small store in a small town would have enough demand to justify stocking a selection star drills. Things sure have changed.

 

[Captain Dirty]

Interesting post, Captain Dirty.
Our family had a small-town Western Auto store during the mid 1950's thru 1976. ... Anyway, it's interesting to me that a small store in a small town would have enough demand to justify stocking a selection star drills. Things sure have changed.

The older houses in my childhood neighborhood had stone foundations, possibly a root cellar, and crawl space under most of the house. The neighbor across the street excavated a full cellar by hand. He encountered some boulders too big to move. His sons were punished for various transgressions by having to spend time drilling those rocks with a star drill and 2-lb hammer. Before hammer drills I used a carbide tipped bit in an ordinary drill; a sharp star drill and 2-lb hammer was much faster.

 

[crazyal]

How much is your time worth? Here in the states I have to believe our time is worth more than down in Costa Rica. I'm not trying to brag or anything like that. Just trying to say why answers from here may not make sense for you. I have a couple rotary hammers, one SDS and a larger spline one. The SDS would be too small in my opinion but the spline one would do it. I have several bits that are larger than 1" that are about 2' feet long. I use it to punch holes in the granite around here. Now going back to my question "how much is your time worth?".

How many holes do you think you would need to drill for a typical job you plan on doing? How deep? I've never timed it but I would expect that it takes at least 5 to 10 minutes to drill a 1" 2' deep. If you have to drill dozens of holes it could take the better part of the day. It may make sense down there where it probably wouldn't up here.

The last time I had granite blasted the guy showed up with a tracked drill with an onboard compressor. He drilled about 100 holes at least 6' deep in less than an hour. It probably cost me less than $2000. When all the blasted rock was removed there was one small section that still needed removing. I had a 200 cfm compressor on site so he returned with an air drill and made a couple holes in a few minutes. If I was to try drilling those holes with my drill (not that I could have gone that deep) it would have taken me a week or more. For me the lost week of working my normal job would have cost more than what I paid. BTW my spline rotary drill weighs well over 60lbs. 8 hours of drilling holes with it would be a real workout.

 

[4570Man]

What kind of spline drill do you have? I’ve never seen an electric drill of that magnitude. There’s a lot of difference in a SDS drill and a SDS max but I’ve never seen one that weighs 60 pounds. My SDS max drills 1” holes in rock fairly decent but I’d never attempt drilling 20 holes 2’ deep or something like that. If you were going to attempt you’d need a vacuum or blower to clear dust and you’d either need a half dozen drills or a LONG time to wait for them to cool.

 

[nbryan]

I've run into granite here on the Canadian Shield that drills fine with my sds drill, and at other foundation jobs gave up after wrecking too many expensive bits and getting nowhere. Granite can be crumbly soft and easy drilling, or practically impenetrable. That last tough job we rented a trailered air compressor and heavy commercial drill and my crew and the owner spent 2 days getting nowhere with it! We drilled maybe 6 - 4" deep holes and gave up. The plans got adjusted and we carried on without those rocks getting moved.