Dealing with Broken Taps
by Kevin Ferguson
The man has a tap stuck in a BMW bike, and this is good general shop advice useful for anyone who repairs BMW machines.
1) Forget about drilling it. Even with carbide.
2) Big ones can be shattered with a center punch as others have noted. Wear safety glasses. I've not had much luck with this one.
3) Small ones might be turned out using a center punch at an angle on the flutes, use your smallest hammer (even a jeweler's hammer) and lots and lots (thousands?) of light blows. If it stops moving in the anti-clockwise direction, take it back ~1/4 turn clockwise, and then try loosening again. This will give the chips a chance to work loose. Again, wear safety glasses. Use compressed air to try to flush chips out frequently. Once it is out enough to grab with a pair of vice grips, DON'T try to use brute force, rather "worry" it out wiggle it back and forth and turn it out a little at a time. This advice applies to tap extractors as well. If the thing can be turned even a little, patience will usually see it out.
4) If it is in aluminum, use some alcohol (ethynol, methynol, or paint store denatured alcohol...NOT rubbing alcohol) as a "lubricant". This does not actually lubricate much, but it passivates (oxidizes) the chips and fresh cut threads so they are much less likely to gall (weld together) and bind the tap. In steel or stainless use tap-matic, rapid-tap, or just sulphanated cutting oil. Read the can for material compatibility.
5) A sinker EDM (as opposed to a wire EDM) is a solution that WILL work. There exists a very low-tech EDM device known as a "tap burner" made for just this purpose. If you call around local machine shops, you may find one with a tap burner and some sympathy for your plight. For years Popular Mechanics had an ad in the back for a DIY version (I think they called it a "Metal Disintegrater") For this use. electrodes of copper wire or brass rod work as well as graphite.
6) If in steel or stainless, consult a welder. Have him build up a "stalagmite" on the remains of the tap Grab this with vice grips and worry the tap out as described in point two.
7) Proprietary solvents: If in Aluminum, Nitric acid will dissolve the steel but not the aluminum. Use clay or similar to build a moat around the hole, fill with Nitric Acid, and let it soak overnight. Usual precautions with acid apply, and some Al alloys may not like the Nitric acid, so figure out a way to test first.
Taps seem like such simple tools that few appreciate their optimal use. Too late for this job, but here is how NOT to break a tap in the first place:
-Drill the correct size hole, consult a chart. Slightly oversized is OK, but never go smaller unless you know how to calculate the % thread depth etc. Metric is easy: Tap drill=Diameter-modulus, so a 10mm x 1.5 needs a 8.5mm tap drill.
-Use fine threads unless there is a good reason for coarse. The tap, as well as the bolt is stronger for a given diameter, and the tap will turn easier because it is taking a lighter cut. Aluminum and cast iron usually require coarse threads however. If you have to tap many holes in Aluminum, look into forming (rather than cutting) taps.
- #6-32 and 1/4"-20 taps have about the highest cutting force to cross section, so are easiest to break. Tapered (pipe) taps are also nasty. Where possible, consider other options.
-Buy good taps. Fewer flutes is better (stronger) Spiral points are good, but may not be suitable for shallow, blind holes. Good taps are expensive. Few can afford a set of first-rate taps. Buy individual taps in the sizes you need. I know of no over-the counter source of good taps in Albuquerque, so I mail order. MSC and McMaster-Carr are good sources. As with most tools, if not more so, good taps produce better results, are a joy to use, and last much longer than cheap ones. I have one Guerig spiral point tap that has tapped several hundred 1/4-20 holes in mild steel and some stainless as well, and still turns "like buttah".
-Even good taps eventually wear out. The cutting edges get dull, requiring more force to turn. Tiny cracks spread over time, so the older a tap is, the more likely it is to break. Replace worn taps; even if you have to wait for a replacement. Consider how much time a broken tap will cost. Large, very expensive taps may be worth sharpening, and there are shops that specialize in this.
-Lubricate the tap with a substance suitable for the material being tapped...see above.
-Don't try to run a tap with a power drill.
-Normal taps: Back up ~1/4 turn for every 1/2 - 3/4 turn forward. This breaks off the chips so they don't build up and jam the tap. Every 5-6 turns remove the tap completely and blow the chips out with compressed air. Yes it is slower. This is how experienced machinists do it, especially when they are in a hurry. Nothing slows a job down like breaking off a tap in part with tens of hours already invested in it.
-Spiral point taps: Ignore advice above and DON'T back up. The spiral point pushes a "bird's nest" of chips out ahead of the tap. The back edge of the flutes are often not designed to cut off the chips like a normal tap is. (So-called "eccentric relief") These work best in through holes, but can be used in blind holes if you are willing to work the chips out with a dental pick or tweezers.
-Apply NO side force to the tap. A "proper" tap wrench gives a tee shaped handle so that a pure couple can be applied. For difficult to access locations, Lysle makes a set of tap sockets in standard tap sizes so you can use standard drive accessories (ratchets, extensions, U-joints, etc).
-If the tap doesn't start parallel to the hole, it may break when the force of cutting into the deeper wall becomes too high. Where possible use some means to hold the tap in alignment with the hole for at least the first few threads. Using a tap guide from the same mill setup used to drill the hole is ideal. A tapping block is a thick metal or even wood block with a clearance hole for the tap. Set on the surface, it is a simple aid to starting the tap straight. Vee blocks work well too.
-Keep track of your mental state. I find that taps are much more likely to break when I am rushed, fatigued, and especially when I am angry. If all three, a broken tap is a foregone conclusion. In such cases I have learned to knock off and wait 'till morning to finish the job.
By Kevin Ferguson, [KFerguson@OPTOMEC.com]