Problems drilling hard concrete and using screw anchors

[Jeff Layman]

I finally got round to putting up the handrail I mentioned in a post a week or two ago. The company I bought it from modified their standard design after I sent them photos where it was to be fitted. It came with eight M6 x 100mm Thunderbolt concrete screw anchors.

As many others here agree, Bosch multipurpose drill bits are great for drilling into hard material. I've used them for drilling into concrete lintels with rebars and they went through pretty easily. For the handrail posts I needed to drill a 6mm hole at least 100mm deep, so bought a couple of new drill bits, 6 x 150mm. They had hex shanks, so at best would do only 120mm, and the flutes didn't extend all the way up. They are /misdescribed/ here: . As the reviews note, they are not 200mm long, but only 150mm. That was ok for what I needed. The first four holes were in brick. The bits went through very quickly, and it was easy to screw in the concrete anchors.

But the Bosch bits met their match with the next four holes, which had to be made in 150mm thick C30 concrete. I could get only one hole 100mm deep. Two got to 90mm, and the last one only about 75mm. I had been drilling with an old 550W B&D hammer drill, so thought it best to get out the 850W SDS Plus drill. This made no difference; the drill bit remained stubbornly at 75mm depth. When I removed it from the hole I was amazed. Photo here for comparison with drill bit used once: End of blunt drill bit in close-up:

What in hell can do that to a tungsten carbide tip? I've seen them with the cutting edge blunted after extended use although the "V" shape is still there, but /never/ seen one where the carbide bit has been completely flattened back to the shank.

I bought a small pack of M6 x 75mm concrete screws and put two of those in with the 100mm one I could get in. For the other one I used a Dremel with a disc cutter to take it down to 50mm, and I'm sure it didn't go in properly and has smoothed some of the thread it cut. All were very difficult to screw in, even using a 300mm Tommy bar. They seem, however, to have fixed the handrail post pretty well.

Those concrete screws really are hard steel, and it took some effort with the Dremel to cut one off. However, this is what the cut-off end looks like after trying to cut a thread in that hard concrete (unused screw for comparison): I have my doubts that the smoothed thread grips as well as one might think. No doubt that some of the screw has all its thread complete near the top, and that grips better, but are they really suited for use in very hard concrete with lots of flints and stones?

[newshound]

My guess would be heat. As you dump energy into a small and relatively well insulated volume of material the temperature will rise. I've not infrequently "unbrazed" the tungsten carbide inserts from masonry drills by being over-enthusiastic. Even with Bosch drills you do need to clear them of debris and take them out to cool with deep holes. It's OK to dip a hot drill into water (the steam layer prevents it from quenching too fast). You can also squirt water down the hole using a garden sprayer on "water pistol" setting.

The important thing with concrete screws is to have a suitable sized pilot hole in hard concrete. I don't like them much, I much prefer the Multi Monti type (IIRC SF do an "own brand" version). With these it is also really important to pilot drill to the shank diameter, then they go in relatively easily and can be removed and replaced if necessary.

[No Name]

the TC tip is typically welded onto the drill. if the drill tip gets too hot, the weld melts and so does the drill bit and shank.

This is quite common when drilling hard tiles. I have had occasions where I pulled out the drill bit when progress slowed down only to find the tip was glowing orange-red!

Basically use a new drill bit. drill for short periods of time, withdraw drill bit use something like compressed air to cool it down and drill again

Also consider the drill speed and the amount of force you are applying to the drill tip.

S.

[charles]

Have you met a steel reinforcing rod/bar? Or does the concrete mix have flints in it?

[John Rumm]

The SDS used with normal bits, won't have that much advantage really (although the typical SDS rotation speed might be a better match). SDS with a SDS bit however should be a world of difference.

Yup, that does look shagged (to use the technical term!)

Looks like it was overheated... Combination of high rotation speed and not making fast enough cutting progress probably.

Lower speeds, more drill pressure (for hard stuff you may want 25kg of weight on the bit), and make sure you clear the dust by withdrawing from the hole frequently. Even a spray of water into the hole from time to time.

Judging by the insertion torque you used, I am guessing they will be well stuck in there!

[Jeff Layman]

Thanks for all the replies.

When drilling the holes I regularly cleared them of dust using a plastic tube from a window spray pushed down the hole with blowing at the same time (goggles definitely required!). So it was never allowed built up. It was also noted in the anchor information that the hole must be clear of dust when screwing in.

Although the drill bit got very hot, would it have mattered? Tungsten Carbide retains its hardness even at red heat. Looking at the close-up of the tip, you can see the remaining TC at the 2 and 8 o'clock positions, so it doesn't look like the weld melted and the TC bit fell out. Newshound mentioned water. Out of interest, what would happen if water was used as a cooler/lubricant for drilling holes in concrete? It wouldn't last long, needing constant replacement, but would it have any useful effect? I wondered if by adding water it might form a "grinding paste" with the retained concrete dust which might clog the flutes.

The 6mm drill size used was that recommended for the supplied screws. I wondered if a 6.5 mm drill would have made the thread-cutting and insertion a tad easier, but maybe it would have been a bit too big.

If I ever need to hold something to concrete again, I'll probably return to a standard rawlbolt or shield anchor fixing, and certainly not do the work when it is 30+deg C! :-)

[Nick Cat]

Maybe there's a diamond at the bottom of your hole!

More seriously, TC is vulnerable to wearing away fast from inadequate hammering, and SDSes can certainly be operated such that the bit does more scraping than hammering.

NT

[williamwright]

snip

Very interesting post, with pics!

Bill

[williamwright]

snip

A real slow speed and efficient SDS hammer action would have helped. Don't press on. Let the drill do the work.

Bill

[newshound]

You might still have some tungsten carbide there, but you have still lost a lot of material from the drill. Where do you think it has gone? I think you have gone beyond red heat locally. One possibility perhaps is that having flattened the end of the drill, you then met a piece of rebar. The Bosch drills are good, but they still rely on the classic "cutting" geometry to get through steel. (Look up Count Rumford and canon boring machines).

Newshound mentioned water. Out of interest, what would happen if

Clogged flutes can be a problem with water but a hot drill will mean they dry out quickly. A wire brush will clear them out.

If you ever get to see substantial industrial diamond disks being used to cut through metres of concrete, you'll see they are normally flooded with water.

That's definitely a risk with that type of concrete screw; but it sounds like you have stronger than average concrete.

[Chris Green]

Yes, but as somoene else pointed out the TC tip can become detached from the rest of the bit if it gets very hot. I think most of my SDS bit failures have been due to the TC bits at the end 'falling off'/

[Jeff Layman]

Pleased you enjoyed it.

I think the score is now only 655 : 1 in your favour. I'm catching up!

[Jeff Layman]

That thought did cross my mind! :-)

See Bill's second reply. It may be that drilling hard masonry is more an art than a science!

[Jeff Layman]

I can only assume that if it hadn't disintegrated completely and been ejected with the concrete dust via the flutes, it was left as a small lump at the bottom of the hole. That would, of course, probably stop any further drilling even with a new bit.

I was thinking more about clogging "in situ" which might mean more "grinding paste" remaining at the drill tip and damaging it.

Good point.

[Dave Liquorice]

And isn't the Bosch drill a "normal" type one not an SDS one? An SDS whacks the drill far harder than the vibration plate in a "hammer" drill.

The OP says he changed drilling machine but not if he used and kanckered an 6 mm SDS bit or used the same Bosch drill in an ordinary chuck fitted to the SDS machine. Not that you are suposed to use SDS in hammer mode with such a chuck fitted.

[newshound]

+1. Throws up some interesting problems.

[Jeff Layman]

I used an ordinary B&D hammer drill before using an Energer SDS Plus drill. That drill comes with an "ordinary" chuck as well as an SDS one, except that the ordinary chuck has the same SDS Plus shaft as the SDS chuck has. So I assume it is designed to take ordinary drill bits to use in SDS hammer mode. I think I would have noticed if the drill bit was blunt before using it with the SDS drill, as I had to remove it from the B&D chuck first.

[Jeff Layman]

Thanks both for the thumbs up. I guess there have been far too many OT posts here for some time, and a real DIY query is a tonic for all. It's what this group /should/ be for.

[williamwright]

The mass of the chuck inhibits the percussive action. The bit slips. It just doesn't work.

Bill

[williamwright]

The ordinary chuck is so you can use wood bits, hole saws, etc, with the hammer turned off. It won't work for hammer action.

Bill