Broken Tap Removal: The CNC Way

Having successfully drilled and tapped eight 4-40 holes for the MOSFETs and two 8-32 holes for the heatsink clamps, I needed four more holes for the 6-32 standoffs that will mount the heat spreader to the base. As is always the case, the tap broke in the next-to-last hole…

Broken tap
Broken tap

This is a three-flute tap, the break is recessed below the surface, and it looks like it’s cracked along one of the flutes. Bleh! I don’t have any tap extractors, mostly because I don’t do that much tapping, and I doubt the extractors work all that well on tiny taps.

I tried something I’d never done before: slit the top of the tap with an abrasive wheel and unscrew it. That didn’t work, of course, but it’s a useful trick to keep in mind. I think the tap was cracked lengthwise and, in any event, a three-flute tap doesn’t have the proper symmetry for a slot. Better luck with larger four-flute taps.

Slotted tap
Slotted tap

So I must dig the mumble thing out…

Starting the moat
Starting the moat

The overall plan:

  • Clamp the heat spreader to the Sherline tooling plate
  • Helix-mill a trench around the tap
  • Grab the stub with Vise-Grips
  • Unscrew it
  • Repair the damage

The clearance hole for a 6-32 screw is 0.1405 inch and that’s a 3/16-inch end mill: 70 + 93 = 163 mil radius, call it 0.170 inch. You really don’t want to kiss the tap flutes with the end mill, so you could make that the ID a bit larger.

Manual CNC, feeding commands into Axis and using the history list to chew downward 20 mils on each pass. With the origin in the middle of the broken tap and the cutter starting at (-0.170,0), the code looks like:

G2 I+0.170 Z=-0.020
G2 I+0.170 Z=-0.040
... and so on ...

About 3000 rpm and 2 inches per minute feed; the feed was too slow, because the aluminum chips were much too fine. I actually used cutting lube for this job: the heat spreader got nice and warm.


I stopped at Z=-0.100 and made a final pass around the bottom of the hole to clean out the ramp. Then, try unscrewing the tap…

Tap stub - first attempt
Tap stub - first attempt

Of course, the stub broke off more or less flush with the bottom of the hole, so I continued milling downward to Z=-0.260, a bit more than halfway through the plate. This time, the needle-nose Vise-Grips got a good grip on an uncracked section and the remains twisted out with very little effort.

Grabbing the stub
Grabbing the stub

Although the central pillar is outside the tap’s OD, leaving a solid aluminum shell, there’s not much meat to it. The shell broke off with the first twist and came out with the tap.

Those are not, by the way, gold-plated Vise-Grips. It’s a flash picture and the worklight is a warm-white compact fluorescent: the color correction that makes the aluminum look neutral gray turns the reflected CFL into gold.

Aligning replacement nuts
Aligning replacement nuts

I milled off the remains of the shell around the tapped hole, leaving a more-or-less flat bottom. If I cared enough, I’d machine a snug-fitting replacement aluminum plug, epoxy it into place, then (attempt to) drill-and-tap the hole again.

Instead, because the hole was deep enough for a pair of 6-32 nuts and a washer, I simply aligned those on a screw and filled the hole with JB Weld epoxy.

It doesn’t show in the picture, but the screw is well-lubricated with silicone grease to prevent it from becoming one with the nuts.

I eased epoxy into the recess, chasing out the inevitable air bubbles, and then scraped off most of the excess.

Epoxy fill
Epoxy fill

Let it cure overnight, scrub it on some sandpaper atop the sacrificial side of the surface plate, and it’s all good again…

Sanded flat
Sanded flat

The little finger of epoxy sticking out to the front fills the end of the slit I carved into the top of the tap, which is visible in the other pictures if you look closely. The area around the hole isn’t stained; that’s smooth epoxy.

Of course, the thermal conductivity of epoxy is a lot less than that of solid aluminum. I’m not really pushing the limits of TO-220 packages, so this kludge will work fine in this application. It’s also nice that the repair is on the bottom of the heat spreader, where nobody will ever know I screwed up…

Now, to return to the project at hand, with even more motivation to avoid tapping holes in the future!

21 thoughts on “Broken Tap Removal: The CNC Way

  1. Someone once told me Carbon tetrachloride makes a great tapping lubricant. Sounded along the lines that you can use Coke to clean your battery terminals in your car… but one day I was hand tapping a 4-40 in aluminum and it started to jam. The more I went back and forth, the more stuck it became. Having nothing to loose, I tried the most similar stuff I had around (isopropanol) and it worked like a charm! Another thing to keep in the back of your mind, a broken tap in aluminum can be electrochemically removed by anodizing the part. I’ve anodized aluminum at home before (just need battery acid and a battery charger!) but have not done it at home to remove broken taps yet.

    I can’t wait to see what this new project is going to be! It’s got lots of things I like… could it be a smart universal battery charger/load/cycler? Some sort of solar inverter? Motor controller? LF ham radio beacon? Hmmm….

    – Steven Ciciora

    1. Carbon tetrachloride makes a great tapping lubricant

      We bought this house “with contents” and, amid a mighty pile of Real Crap, I found a genuine carbon-tet fire extinguisher, from back in the day when CCl4 wasn’t something to worry about. I suppose I should haul it to the town’s toxic-waste disposal site one of these days…

      Also up: I found a stash of 10-32 taps. Seems to me if I grind all but one tooth off a tap, it’s pretty close to being a thread mill without the $50 price tag. I want an LED ring light on my microscope and think that’d be a dandy place to try thread milling…

      removed by anodizing the part

      Or by soaking it in nitric acid for a while; evidently the steel vanishes and the aluminum remains. Now, if I only had a vat of nitric acid lying around…

      I do have a bunch of isopropyl alcohol, though, and I’ll give that a try on my next tapping exercise: better to not break the tap in the first place, methinks.

      a smart universal battery charger/load/cycler

      Close enough. Turns out that it’s an 8-cell NiMH tester for an upcoming CC column. I have a bunch of 9.6-V packs and several handfuls of loose cells that desperately need characterizing, but doing them one-at-a-time in the WMR CBAII will take about two days less than forever. So I’ll scratch an itch and get a column out of it.

      Applying an Arduino Mega is obviously absurd from a cost standpoint, but digital/microcontroller stuff isn’t really in my charter. Heat transfer & suchlike, yup, I can write about that… and I get a bunch of Quality Shop Time, too.

      1. I removed a tap last month from a chunk of aluminum, chemically. What I used is stuff called Sparex, used by jewelers as pickling compound. It’s sodium hydrogen sulfate: the sodium salt of sulfuric acid. The nice thing about it is you can put your hand in the stuff, either dry or when it’s been dissolved. I put it in a crock pot (the Danger Pot out in the shop) and put it on warm, and dropped in the aluminum with the tap, and left it overnight. The next day the aluminum was covered in black smut from the copper in the alloy, and I measured, with a micrometer, that it was almost two thousandths of an inch smaller across one face (meaning it lost a thousandth) and there was *no* trace of the tap. I couldn’t tell which hole it had been in. You can buy Sparex online pretty cheaply, and it’ll keep in dry form for *years*. Plus it does a great job of cleaning off silver, gold, or copper — its intended purpose — should you ever need that.

        1. there was *no* trace of the tap.

          Now I know why there’s a crock pot in the attic…

          Sounds like it’d be exactly the right stuff for stripping the gunk off PCBs before etching: wash it quickly and it’s clean. I’ve been using copper-cleaner scouring powder (intended for copper kitchenware) that does a pretty good job, but the can is getting on toward empty.

          Safer than nitric acid, fer shure!

          1. I found it in kilo lots on amazon for about $10 (though no telling what shipping would be.) If you should ever want to put anything nice in it, like silver, mix up a new batch because you get some interesting plating effects if you have silver, copper salts in solution, and iron salts in solution all in there at the same time. Also, when it’s actually hot, like 120F, and you put your hand in, you know it’s an acid. But at room temp, the stuff is almost totally unoffensive. It also lasts for months in solution.
            I’d like to find an online guide to chemical methods for removing X from Y. Brass in (aluminum/steel) — ammonia. Steel in aluminum: sparex. Aluminum in steel: NaOH (I believe.) But past that? dunno. I presume steel in most any passivating metal would etch out with sparex.
            Oh, I’ve read you can remove copper/brass/bronze from aluminum with mercury, *if* the aluminum is scratch-free. But if there’s even a tiny scratch, it’ll get eaten fast and hot, like occasionally catching fire. No idea if it’s true, and I’m probably not ever going to try, but it’s fun to think about.

            1. A bit of searching says it’s $5 plus $11 shipping from a Reputable Source or $18 with Free Shipping from the usual eBay suppliers. I’ll check the local hobby stores, but I don’t hold out much hope, as they seem to concentrate on the RC and modeling end of the biz.

              remove copper/brass/bronze from aluminum with mercury

              I vaguely recall an airplane accident caused by mercury eating away the main spar. Apparently it’s more of a catalyst than a reactant and there’s basically no way to decontaminate the structural member. Nasty stuff in the wrong places…

              A long time ago, in a universe far away, I grew up with a little plastic maze containing a generous drop of mercury. Nowadays, such mazes have steel balls, but back then mercury was fine… and it was great fun to play with!

      2. Kerosene works very well, most machine shops use it for taps into aluminum. I use any light oil that I can reach out to!

        1. I was actually using a specialty lube (Anchorlube, it says on what’s left of the mostly rubbed off label) that’s supposed to be good stuff.

          So the lube has lost its mojo, small taps just break sometimes, or my technique is faulty… I’m betting on a combination of the last two.

          On the other paw, I did tap the rest of the dozen-odd holes with no problem!

  2. I have at times removed broken taps down to 3mm using the broken shank of a 2 or 1.5mm drill bit.You tap the drill bit shank against the edge of the broken tap forcing it to rotate back out. You could use some other hard material rod to do the same.

    1. forcing it to rotate back out

      With my luck, I’d just break off the tops of all three flutes… and then I’d break out the chemicals!

  3. What we usually do at work in the case of a broken tap is machine it out with a ball end mill. You have to be slow and work it in, but the ball end mill will usually break the tap into pieces.

    1. Sounds like a plan for larger taps, but I suspect that requires a carbide ball and a HSS tap: a teeny ball wouldn’t get much traction on a 6-32 tap.

      A friend once snapped a Zerk grease fitting in a brake cylinder; the stub sneered at ordinary drills. I suggested a carbide-tipped masonry bit in a drill press… and it worked! So there’s evidence that simply gnawing down through the offending chunk of metal can have a happy outcome.

      I must build an EDM gadget sometime soon: that’d be even more fun!

      1. Once upon a time Home Shop Machinist had an article on a homebuilt EDM, and I built one. The clever bit was they used 100W light bulbs as the power resistors for current-limiting. It worked pretty well, although my physical implementation was shoddy: if it were hooked to an electrode held in an arbor press or drill press, something with excellent locating ability, it’d do a much better job. And while it was running both lights would be flashing in the tens of Hz range, which made it look like the most mad-science thing I’ve ever built.
        (and a bit of research later, HSM had a series running in 2002 about how to make a fairly good one, including a wire-feed add-on; I’m probably thinking about an article in Machinist Workshop, sometime in the late 1980’s. I might still have the schematic in my filing cabinet.)

        1. HSM had a series running in 2002

          That’s the one I remember, but IIRC the electronics produced several issues of corrections and caveats. I’d use it more as inspiration than blueprints.

          The EMC2 CNC project has some EDM capabilities, so I think I could convert my Sherline mill into an EDM sinker by building the EDM drive circuitry, a leakproof tank, and a chunk of HAL code. That’s Yet Another Project, alas…

  4. I have also buried a tap in aluminum, and came to the same solution of milling around it to get it out:

  5. If you have access to a TIG welder, you can carefully butter up some weld on the broken edge of the tap and then drop a larger nut over the weld. Then weld the inside of the nut to the buttered-up portion and cross your fingers while turning the nut with a wrench.

    1. I’ve read good articles about doing that and I still don’t believe it… [grin]

      1. Well, I’m sure around Xmas time you can find a way to squeeze a TIG welder into the basement laboratory to find out :)

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