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I am planning to install a 1.5-inch conduit through a poured concrete wall. I am planning on drilling a two-inch hole to accommodate the size of the conduit body. I have been searching up and down, but I don't know what drill bit to use. Bi-metal? Diamond? Carbide?

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    If you buy or hire a core drill bit never use it with the hammer on, use normal drill mode or you will damage the bit. Dec 17, 2019 at 1:48
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    Note - make sure whatever drill you use with a core bit has an internal clutch that works, otherwise you'll break your wrist if the drill jams.
    – james
    Dec 17, 2019 at 9:54
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    You also need to consider the iron reinforcement in the concrete, which you can cut through with a diamond core driller and which will defeat and possibly snag a conventional hammer drill. That aside, is it structurally safe to cut through the reinforcement?
    – nigel222
    Dec 17, 2019 at 10:36
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    Only half right Paul Skillcorn. It is hammer off for diamond core bits because they cut their way through via diamonds. But, it's hammer on for carbide boring bits. Plus, some bits require water to keep them cool,although on smaller holes you might get away with it if it's easy going. All about the heat affecting the way the 'teeth' are bonded to the steel core.
    – handyman
    Dec 17, 2019 at 20:45
  • @nigel222 - iron reinforcement? iron? What era are your concrete walls from?! I'm pretty sure DIYers are only going to come across steel reinforcement...
    – AndyT
    Dec 19, 2019 at 11:11

5 Answers 5

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Hire a core driller. Trust me and do it. You will regret doing this yourself and the cost will seem like nothing in comparison to your struggles with drilling a hole that size in solid concrete.

probably cost you 200 bucks.

You can rent one from a tool rental company, or even from home depot.... https://www.homedepot.com/tool-truck-rental/Small-Core-Drill/3510639/index.html

But trust me, as someone who has do this five or six times a year, just hire someone for this job. By the time I calculate all the time and cost of renting a drill rig and diamond bit and figuring out how to use it and then returning it .... its never cheaper.

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    Is your link dead or geographically locked? I got: You don't have permission to access "homedepot.com/tool-truck-rental/Small-Core-Drill/3510639/…" on this server. Reference #18.3d645e68.1576596367.5b359114
    – Mołot
    Dec 17, 2019 at 15:27
  • @Mołot, maybe a region issue. It's a good link. It is to rent a tool like this and it costs about $115/day to rent. It's like a drill press that attaches to the wall/floor to drill a hole.
    – JPhi1618
    Dec 17, 2019 at 15:33
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    Tangent question: that's an awesome tool, but why does it list a larger (6") maximum diameter for dry drilling over wet drilling (5")? I would think wet drilling would be easier on the machine. Dec 17, 2019 at 16:43
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    @user2647513 Because the wet shroud takes up an inch on the diameter and won't fit a 6" coring bit through it, from the looks of it. If you're not using the wet shroud then you can fit a 6" bit before your tool starts cutting into itself at the base.
    – J...
    Dec 17, 2019 at 21:26
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    @nathancahill a good reason not to link to it on Stack Exchange - as SE claims that it is not USA-centric.
    – Mołot
    Dec 18, 2019 at 17:38
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Use a diamond core drill to get a nice clean hole. If you use a hammer drill and carbide drill bit the concrete on the opposite side is going to splay and look like like a beaver was chewing on the wall, not professional at all. You can rent the core drills at most commercial rental places and you will be through the wall in a matter of minutes with a nice clean hole.

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    W.r.t. the beaver that left chew marks on my concrete wall: Just leave him the hell alone. You do not want to f*** with him!
    – davidbak
    Dec 17, 2019 at 19:58
  • I'm not recommending a hammer drill because concrete is too hard, but it is possible to avoid ugliness by drilling a pilot hole. Then enlarge from both ends to meet in the middle. Have done this for a neat 1.5 inch hole through brick. Also means you don't have to buy or rent a long wide drill.
    – nigel222
    Dec 18, 2019 at 9:41
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For a bore hole that large, your best bet is a water-cooled diamond bore. There are many rental centers that can rent you the tool and the bore.

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I did this once. All of the previous answers are valid, but if you're trying to do this through an 8" or thinner concrete wall "on the cheap", don't mind investing a significant chunk of time, and have some DIY skills and access to, or funding for, some modest tools, you can do the following:

  1. Get a 1/2" variable-speed hammer drill, a 1/4" carbide-tipped (masonry) drill bit long enough to go all the way through the wall, a 2-lb. hammer, a long (8") masonry chisel that's 1/4" wide, a diamond masonry hole-saw bit with a 1/4" pilot drill, a 1/2" drill bit extension (6" to 8" long), and a spray bottle with a 20% solution of dish soap.
  2. Hammer-drill a 1/4" pilot hole all the way through the wall with the carbide-tipped masonry drill bit. Go easy and mark the shank of the drill bit with masking tape so that you will know when you are about to break through the other side. The far side may spall some, but the break-through probably won't be bigger than the 2" diameter hole that you are making. If you're careful, it will be much smaller than 2".
  3. Drill in from both sides of the wall with the 2" diamond hole-saw bit and the drill (now in non-hammer mode) using the 1/4" pilot hole that you drilled to guide the 1/4" pilot bit of the diamond hole-saw. Use very light pressure and go very slowly (<300 RPM) to avoid burning up the diamond hole-saw bit, and stop several times per minute to remove the hole saw and spray the soapy water into the circular groove cut by the hole-saw to serve as a flush, lubricant, and coolant. It helps to put a small stripe of masking tape on the chuck of the drill and remember that 300 RPM is 5 revolutions per second. If the stripe of tape goes by more than 5 times each second when you're drilling, or is just a blur, then you're spinning it much too fast. Be patient. The diamonds are "glued" to the front end of a hole saw using nickel plating, and if you overheat the metal or knock the diamonds off (one usually follows the other), you may have to buy a new diamond hole saw, so be gentle.
  4. Use the hammer and chisel to remove the "core" left in the hole by the diamond hole-saw from both sides of the wall. (Use vise-grips or a chisel holder to save your fingers from the hammer.) Repeat this operation with the hole-saw from both sides of the wall as necessary, each time following up with the hammer and chisel to remove the core. Use the drill bit extension when needed, and remember that a headlamp is very handy for this kind of work.
  5. Clean up the inner walls of the hole with the hammer and chisel, and make a final pass with the hole saw and lots of soapy water coolant to smooth it out. The inner part of the hole will have a rougher surface finish, but that's okay as long as the hole is big enough to accommodate what you're putting through it.

Note: If this is a concrete block wall, then the first inch or so on each side will go pretty easily. After that, any concrete fill and rebar that may be inside of the concrete blocks will be harder. Don't worry, you'll get better with the chisel as you go - just let the diamond hole-saw do the hard work. The outside will look great, and who cares if the inside surface of the hole gets a little rough when you're more than an inch or two below the wall surfaces on each side? You're probably going to seal around the conduit at the outer surfaces of the wall with RTV silicone or some other caulk anyways, right?

With this technique, the bottom line is that it may take more than one kind of drill bit or hole saw, but a carbide-tipped masonry bit or diamond-grit hole saw should be your choices. I used a Starrett brand hole saw, and not the off-brand import cheapies. (Buy once - cry once.)

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We did a 3.5 inch hole through an eight inch retaining wall to install a drain.

One thing about the hammer drill is you actually don’t need to and absolutely shouldn’t lean into it! You can cause it to bind up and that could break your arm. A gentle grip with your stabilizing hand so you can let go quickly and lightly press in short pulses. You should hear a Jack hammering sound.

Spritz with soap water solution after every 5 or so pulses and rinse out the hole and borer bit every once in a while. This will also help you see if you are pressing too hard and knocking the teeth off your bit. We lost 1.5 teeth out of 8 on our borer but still made it through! It helps to have a friend doing the spritzing for you. Also good in case you need someone to drive you to the hospital.

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