Topic: Drilling and Tapping

ArtMetal
Bramblebush

Posted by Chris Ray on September 01, 1997 at 17:37:20:

Tapping is a simple machining operation that many people overlook either because we have broken a tap in a hole once and couldn't remove it (happens to all of us at one time or other), or because it's simply a mystery. Well, it isn't a mystery and if you've had a broken tap remain in a hole and couldn't remove it, well it's just a tough apples kind of thing and we have to move on with our lives. Some folks have had good luck with tap removal tools though, but others haven't. If we have had a bad experience with this operation, then consider it an opportunity to learn how to do it right rather than giving up on it.

Anyway don't let bad luck or the fear of mystery drive you away, come back and take another look at this operation. First let's see why we should even consider tapping holes for our fasteners in the first place. Why not through bolt everything? Well, because it isn't always feasible, that's why. Also drilling and through bolting may only be a quick and dirty way to acomplish something on a work that you've spent many a caring hour on and the result could very well be quite nasty looking in relation to the finish on the rest of the work. Often the look of a work is ruined not by an attractive bolt head but rather by the nut and protruding stem of the bolt on the backside of the work.

There are times when a part or piece that is either removable or needs to be secured in place at a later time requires a tapped receiver hole. Maybe there isn't room to add a nut behind the work or there needs to be a clean way to remove and/or replace a part, especially if there are adjustments or repairs to be made later. There are a number of good reasons for tapping including into blind holes and it's really obvious why to the most rudimentary of metal workers.

Okay, we now are convinced, what next. Each tap and bolt size usually has two distinct threads, a fine thread and a coarse thread. The most common type that we use is the coarse configuration and without going into endless detail about the why's and wherefore's of each type let's simply move on. What's good for the goose is good for the gander, same with tapped holes. Most likely you will be tapping for coarse threads most of the time, if not always, unless you do a bit of machining work. If you do that kind of thing then you already know everything here and should move on to read about forging stuff or something like that.

Each tap has a recommended drill size for predrilling the receiver hole. Sometimes you may find this size stamped on a tap itself, or you may have to look up the mating drill size on either a chart or somewhere else. What you may find is that the drill sizes are often number/letter sizes and not fractional sizes which we normally keep around the shop. What to do? Well you don't have to run out to the neighborhood hardware store (chances are they don't have what you're looking for anyway) so you will then have to use the next larger drill bit size that closely resembles the recommended number/letter drill size. Again refer to a chart for the decimal sizes/fractional size conversion chart.

It probably would help to put up a metric/number/letter/fractional drill size chart here in the Bramblebush sometime and if there is a demand for it, I may do that, but in the meantime let your fingers do the walking and find your own chart. Chances are however, that if you bought a matched set of drills and taps then everything is already set up for you in the package.

Okay, I'm going to get to the tapping part in a minute but first I want to say something about drilling holes, then we'll move on.

Drilling is a simple operation, however there may be a couple of things that could be helpful in improving your drilling techniques. One thing is predrilling and an application here is when you may be drilling into heavy stock or using a large drill, (1/2" or larger). It helps enormously to predrill a smaller hole first and that hole could be about the size of the two flats on the point of the large drill, or a little larger. Doing this aids in spotting the drill for accuracy to begin with and also speeds up the drilling operation for the larger drill bit considerably. After predrilling, less pressure is also required for a larger drill bit, a handy thing to know when you may have to use a hand drill rather than a drill press. Life is good. Oh, don't forget to punch mark your drill location, but I probably didn't have to remind you of that though.

Alright, now a word about breaking through the drilled hole while drilling. This is kind of important, especially if breaking wrists or thumbs and getting bruised by spinning unclamped metal smarts too much. Whenever a drill is beginning to break through the far side of a piece of metal, always back off the pressure a bit. Many folks do the opposite and find themselves in serious trouble.

What the hole looks like from the other side is something more like a ragged tear and easily grabs the bit with heavy pressure bearing down on it. If the final pass is lightly taken, then whatever metal is trying to grab the bit is only a few thousands thick and is easily defeated, even if you are not clamping the work down on the press. This is especially good to know whenever you are manhandling a powerful 3/4 hp hand drill instead of a drill press. That drill can spin itself around and your wrist with it if you aren't careful. Oh, by the way, it always helps to use a lubricant when using a large drill bit except into cast iron, then you drill that dry.

Ahhh.....now we're getting back to the tapping thing again. Well, tapping is an operation that requires one to be very sensitive with their fingertips. When starting to tap a hole, the taper on a plug tap easily works itself into the hole then begins to resist. What you need to do is to back off the tap every time you advance it about an eighth turn or so in order to release the curled chips that are forming inside the hole. Otherwise the tap is going to get jammed, and boink! it breaks. Also use a lubricant every time you tap, and for each new hole started.

Taps are made of a very hard and brittle steel so they are not very forgiving when over stressed. This is where fingertip sensitivity comes into play here and it's important that you develop the feel for taps and how much they may be turned when tight in a hole before backing off. You have to be aware of just how limited the twisting can be and yes, a hardened tap can twist a little bit before breaking. Remember that the tapping wrench usually applies quite a bit of torque to the tapping tool.

While we're on topic here let me clue you in on another tip. When you are through tapping a hole that is equal or less than the diameter of the tap, then using the matched drill bit for the hole is fine and offers the most strength to the threads. However, whenever you are taping deep into a hole, especially blind holes or heavy bar stock then be advised to use the next larger drill bit for the starter hole. This relieves the tremendous pressure that builds up after a tap is working it's way into that hole.

How about the strength of the threads then, when we use a drill bit larger than the recommended size? Well, if 75% of the threads match the bolt threads then you only lose about 5% (five percent) of the total strength. Interesting isn't it? I still can't understand that one but it's a general rule, not absolute though.

How do we tap into a deep blind hole? Well here's how I do it, but of course I usually have complete sets of each kind and type of tap which most folks might not have around. If you do then here is what you might do.

First start with the long taper tap, which by the way helps you to align your tap much better than the plug tap. Tap to the bottom of the hole, then next, insert your plug tap and do the same. Finally, use your bottoming tap which has practically no taper execpt for the first thread, and finish up the job. A little tedious but it's much better than drilling way past the bolt length and then tapping with a plug tap which needs to extend past the point of the final bolt thread in order for the bolt to seat itself well.

I probably should wrap this up with a pithy saying of some sort or other but I can't think of anything clever to say right now.

Chris Ray

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