Drilling and Tapping (Dies, too)
How to avoid getting yourself in a hole
Text, photos and video by Tom Hintz
Posted – 3-8-2011
Taps come in three basic styles: bottoming, plug and taper. Plug taps are the most common and usually make up the bulk of consumer tap and die sets. The plug tap has a rather steep angle at its point that helps you get it started in a properly sized hole. After a few turns the tap should be cutting full-depth threads and if it was aligned correctly with the hole when it started it will continue on without further guidance. Remember that because of that tapered tip, the threads cut with this style of tap do not go all the way to the bottom of a hole. The un-threaded bit at the bottom of the hole will be around ¼” so it is not a big deal if you know that it will be there. I think leaving at least ¼” below where I expect the bolt to reach is just good form and leaves room for a little debris that might get into the hole.
Bottoming taps have straight sides with no taper and are meant to cut threads all the way to the bottom of a non-through hole, often for specialty needs. Because there is no taper at the nose of a bottoming tap it cannot be started on its own. Bottoming taps are designed to continue existing threads that have been cut by a plug or taper tap. Bottom taps are really a specialty item and for most home metalworkers, buying individual bottoming taps when (and if) the need arises makes the most money sense.
Taper taps are very much like plug taps except that the taper at the nose is longer and more gradual. The gradual taper makes it easier to get the tap started in a correctly sized hole but increases the distance from the bottom of the hole where full threads stop. Some tap and die sets have a few taper taps mixed in with plug styles. You have to pay attention when starting either tap style so this is not a big issue.
NPT (national pipe thread) taps are somewhat different in how they are cut. See the NPT section below for more on them below.
For the home metalworker dies are pretty much all the same. They have a tapered bottom hole to help get it started on the piece being threaded. Often the top and or bottom of the die are labeled to help avoid confusion. The difference in the tapered side from the non-tapered side is fairly easy to recognize visually if there are no markings.
Since taps do not make the hole themselves we have to do that with a drill. It is important to know that taps require very specific hole sizes for them to cut useful threads inside of it. Making that worse is that the correct hole sizes for a specific tap are often not included in smaller, consumer drill sets. Also, there can be more than one hole size for a given tap. At times different sized holes are used for different materials (say aluminum vs. steel) and the fit between the fastener and the threads in the hole can be tweaked with slightly different drill diameters. If you want to make your
eyes water, check out the chart for Drill and Tap Sizes by Thread and Screw that shows most common threads and hole diameter options. In an industrial setting all of these options can be important. In a home shop, we can stick with the standard hole size for the relatively small range of taps and dies that we are likely to use. I have two versions of the standard hole size charts in the Reference Section as well. One for SAE (society of automotive engineers) and another for metric because they are physically different sizes. I get the occasional email from people who believe that metric and SAE sizes really are the same, just described with different words. Not so.
The good news for the hobbyist is that most tap and die sets like the KD 40-piece Gearwrench Tap & Die Set that I use come with a card or printed chart with hole sizes for the included taps. If you are doing something that actually does require a different “clearance hole” you will probably know about it. The rest of us can stick with the “normal” hole and tap relationships and fewer headaches.
As you might suspect the alignment between the pilot hole and the tap have to be right on the money or bad things are going to happen. Whenever possible drilling the hole on a correctly set up drill press rather than by hand is the best bet. If you have to use a hand drill try guiding it with a square or something else that helps give you a visual alignment with the surface of the piece being drilled.
When starting the tap it is crucial to keep it square to the hole. This is another good reason to drill the hole on a drill press. It is much easier to keep a tap 90-degrees to the surface than correctly aligned with an angled hole. If you NEED an angled hole just take your time to keep the tap aligned correctly but those situations are rare in the home shop.
When starting a tap I find it easiest to give it small turns so I can focus on keeping the tap and tap handle aligned to the hole. This process is made easier with a ratcheting tap handle like the one in the KD 40-piece Gearwrench Tap & Die Set I use in my shop and reviewed earlier on this site. With the ratchet action you don’t have to change hands or go through large sweeps. Being able to move in comfortable arcs makes it far easier to keep the tap or die aligned correctly.
Once the tap starts to bite it is crucial to turn it backwards a half turn or so frequently to help break up and remove the chips of metal being cut by the tap or dies edges. Get in the habit of turning the tap or die ½ to 1 full turn forward and then back it up ¼ to ½ turn. Then turn it forward until it starts cutting again, continue forward a 1/2 to 1turn or so and then back it up to clear the chips. Keep repeating that sequence until the tap reaches the bottom of the hole.
If the tap starts to feel like it is binding up or like it is running into something back it out entirely and clean the hole. Look to see if the tap might be going into the hole at an angle. If not add a bit of oil to the tap and carefully run it back in and start cutting again. Never try to force a tap (or die) as that seldom leads to good threads but more often to a broken (and hopelessly stuck) tap.
I can tell you from experience that taking the time to use a tap correctly is much easier than trying to dig a broken off tap out of a hole. The super hard tap will resist virtually any drill you can find. Often the only way to get a broken tap out is to collapse it. You do that by putting a hard center punch at the center of the tap and whacking it very hard with a large hammer. Sometimes you will get lucky and the tap will shatter but more often it will just chip or crack the tap and the hammering must continue. It can be a slow and frustrating process that can also be very painful if you miss the punch with the hammer so take care!
I always use tap oil (any relatively light oil seems to work) applied at the cutting edges of the tap or die. That bit of lubrication seems to keep the cutting action smoother along with keeping the chips free. I am told that a little oil also keeps the temperatures down on the fine points on the tap or die that are actually doing the cutting.
Using NPT Taps
Many tap and dies sets include one or two NPT (national pipe thread) that are commonly used for things like the oil drain plug on lawnmower engines. Pipe threads are designed to help make a leak-proof seal, sometimes with the help of pipe thread compound or Teflon tape. The threads are actually tapered to help attain the seal and to prevent the plug or pipe that threads into it from going through. It would be a very bad thing to have the oil drain plug fall into the motors crankcase…..
NPT threads have their own drill sizes and we have a chart that shows the common NPT sizes in the Reference section of this site. Starting and tapping the hole begins just like cutting regular threads, using the same forward, back up and forward again sequence to break up the chips. However, a NPT tap should not go all of the way into the hole. Rather there should be between 3 and 5 threads on the tap visible above the edge of the hole. It is best to get close to this depth, remove the tap, clean the hole and try the plug. The plug should tighten up with a few of its threads above the surface as well.
Another technique some use is to pack the tap with grease to help capture the chips. This can be very helpful when cleaning up or re-tapping a hole in a motor that you can’t or don’t want to take apart. Remove the tap after every turn or two, clean it up and pack it with grease again. I know it is a little time consuming but not nearly as much as sucking a metal chip into a the moving parts can be.
One thing I have learned over the years is that after cutting all of the threads, clean the hole (or bolt) up well and then run the tap or die over the new threads another time or two. If nothing else it seems to burnish the threads a bit and makes them friendlier to the bolt or nut that interacts with them. When working with something with threads in it I often run a tap or die over the threads just to clean them up. A few minutes work can make torque settings more precise as well as making assembly that much easier because fasteners go in correctly.
Using taps and dies really is pretty easy. The most common errors come from rushing and getting too forceful when a tap or die binds up. Take the time to get taps and dies started correctly and remember to keep breaking the chips up with the backwards movements of your taps and dies. Then you can be confident that the threads you cut will work as expected.
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