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I have an 01' Cobra and when working on the rear suspension had removed the emergency brake cable mounting bracket and small bolt that is attached to the bottom of the lower control arm. Well the bolt snapped off, and all my attempts using bolt extracting techniques failed, and so I decided to just drill out the remaining bolt and through the metal to use a heli-coil or similar to rethread.

During this process I broke 2 drill bits off in the hole and there is still some of the bolt in the hole as well. I bought a quality cobalt drill bit and have drilled for 20-30 minutes and the hole is like the hardest, solid metal and I'm making zero progress it seems. I've tried low torque/low speed, high speed, everything and I'm not making progress. It seems that between the metal from the control arm itself, broken drill bit pieces, old bolt, etc. the heat has molded them all inside that hole to the hardest metal combination ever which I can't drill through.

I'm using a handheld drill and I can't get the control arm off where the hole is to use a drill press. I do have a 1/2" hammer drill as well I haven't tried and not sure if it's more powerful than the handheld drill. I also believe a lubricant is needed when drilling metal to keep it cool. I was using penetrating oil and spraying in the hole while drilling.

Again at this point all the metal in the hole has seemed to weld together and I'm way past any bolt extracting methods. I need to drill about another 3/4" to 1" through to have a hole all the way through. The hole size is roughly 3/8".

What in the world do I need to do, or technique should I use to drill out this hole? All the YT videos I watch make it like using the cobalt bit should chew up the metal and drill through, but that's not working for me at all. Maybe I'm not using the right torque and speed setting, but I tried multiple settings and none provided success. Maybe I'm not using the proper type of drill lubricant either and need to use a different kind. Please let me know a better method for drilling out this hole so I can repair it.

Edit: Added stock photo of IRS control arm and hole location for reference

enter image description here

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    Welcome to Motor Vehicle Maintenance & Repair!
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Jun 16, 2020 at 14:54
  • Is the bolt just needed to support the brake cable?
    – Martin
    Jun 16, 2020 at 14:55
  • @Martin - yes. I already tried placing a small bolt in the part of the hole drilled out to hold it in place, but it won't bite and secure well enough (not long enough); I need to drill it through completely and rethread. That cable bounces around while driving and makes all sorts of noise at the e-brake in the car.
    – atconway
    Jun 16, 2020 at 14:57
  • Can you add a photo?
    – HandyHowie
    Jun 16, 2020 at 16:54
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    Cobalt drill bits are just coated, they are crap, buy a drill bit made of M2 steel, it will get the job done. In a pinch I have used masonry drill bits as they have carbide cutting tips, this will drill through a file, super hard material.
    – Moab
    Jun 16, 2020 at 19:13

2 Answers 2

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Given the mess so far, I think you will have to remove the arm and set it up in a vice so the it can be drilled with a sharp bit at low speed and at a slow advance rate.

If that fails then you may well have to take it to a (good) machine shop which has a Spark Erosion Machine which will be able to remove the broken drill bits you have left in there.

Once you have a hole then either tapping it for a helicoil or welding a plug that you then drill and tap to the original size are possibilities.

The other option is to replace the arm... either with new or get one second-hand from a scrap yard.

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  • I second the words of Solar Mike. Also, it's difficult to put enough force on a hand drill to be useful. You might try using some sort of long bar clamp which can use to clamp the drill to the metal you are drilling, thereby exerting far more force than you can just pushing on it. That way the drill will cut into the metal instead of sliding over it.
    – the_storyteller
    Jun 16, 2020 at 15:27
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If the bolt is just needed to keep the brake cable from dangling around and has absolutely no critical function then I'd try to clamp the bracket (perhaps by some U-bolt) to the control arm and forget about the broken bolt.

Update:

Regarding your request for drilling: Your stated scenario indicates that you experienced something called "work hardening". When one drills with low pressure, in worst case with an unsuited dull bit, on high alloy metal (especially stainless steel) the material hardens, making future drill attempts, even with the right tools, more difficult.

While work hardening could be cured with heat I'd strongly recommend against, as your control arm would be seriously weakened by such rash measures.

If drilling is a must then I'd try to drill with a small diameter first. There are specialty drill bits around for hard/tough materials (carbide or at least coated, special form for hard materials). Don't go to the usual do-it yourself shops, an industrial supply shop might be the better choice.

Drill with low velocity but high pressure (as much as possible without breaking the drill bit).

Update #2:

If there are still broken pieces of the drill bits in the hole: Drilling them is futile, the route from Solar Mike might be the better choice (either a spark erosion machine or a new control arm is needed).

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  • Unfortunately the lateral movement (forward and back + swinging) was more impactful that thought when driving and it appears the stationary mount is key to proper usage. I was just going to use a heavy duty zipstrap to hold in place or similar and it wasn't effective. This needs to be mounted solid. I appreciate the 'think outside the box' suggestion, but I'd like to get some advice specifically to drilling the metal if possible please (as I need to know how to do this anyway for future scenarios possibly as well).
    – atconway
    Jun 16, 2020 at 15:08
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    "Cobalt" mostly means it is a high speed steel containing 5 to 15 % cobalt, usually with tungsten and/or moly. For this application it does not make much difference which high speed steel you are using. As noted , you have work hardened the hole and your bits are dull, Use a new small bit, low speed and HIGH pressure.
    – blacksmith37
    Jun 16, 2020 at 22:06

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