Date: Sat, 21 Dec 1996 15:30:18 -0800
From: Mike West <mwest@CDSNET.NET>
Subject: Tapping holes by hand

How to tap a hole with threads. By hand under durance vile.

First, as they say in the recipe for rabbit stew, you must catch the
rabbit.

So, in order to tap a hole, first you must have a hole.

In a drill press, or some other stable form of holding machinery,
it's a science.
 When you do one by hand in your hand drill, it's an art-form.

Also, like any other hand function, you need to practice it.

If it's really an important job, you'll get a scrap piece of metal
and try it out first.

Even if you're practiced, you'll find that the recommended tap drill
in some cases, is too small, or too big. This is why you check first.

You will never do as well as the simplest drill press.

The tap drill is a drill that just makes a hole the size of the "root"
of the threads. That's at the bottom of the thread form.

That's the scientific size.

You get out there with your hand-held drill and "wallow" a hole into
something, and suddenly your tap drill is too big.

The threads are too loose for the fastener because of this.

It might be more accurate to say your "hole" is too big.

In most cases that will be the case when hand drilling and tapping.

In some cases, the drill will not cut a hole big enough.

How will you know? Well if the hole is too small,you'll break a tap
off in it. And if it's too big, it will be loose the whole way in.

Only experience can give you better data than this.

Still we're gonna' tap a damn hole!

For our purposes, we'll use aluminum castings .

Exhaust studs in the VW heads is our goal here:

It's a blind hole, meaning it doesn't go all the way thru.

For this story, we had it boogered so bad we had it welded up and
there's just a flat spot there.

There's two ways to get the right tap and tap drill and for this one
it's not at the "Builders Emporium". We aren't doing lawn furniture.

WE go to a machine tool supply house carrying our stud at least and if
you want it all, carry the head too.

You tell him you want a real tight thread in there and where you want
to put it. You are looking for a "hand tap" and a tap drill.

Based on the information he gets he can fix you up.

Do not be shy about carrying the head in with you. It's done every
day in their business.

He will look at the head and see the material, see how deep you can
drill and how far the stud has to go in there.

If you just have to know, we need an 8mm X 1.25 pitch Plug tap with a
pitch dia limit of "D3", and a tap drill to match.

The tap drill will be in the 6.38mm range. Don't be outraged if he gives
you a drill that is American size.
 There are probably three that will do the job and most houses don't carry
metric for that reason.

I suppose you want to know what all this stuff means too.

The taps come in several varieties of which I will name three.

A "Leading" or "Tapered tap has a taper on the end of 5 to 9 threads.
 This is for starting a hole and is used for thru holes.

You will see the incomplete threads where they ground a taper on the end.

The "Plug" tap has 3 threads taper and is for getting in close to the
bottom of a hole. So it's a compromise between a "tapered" and a
"bottoming" tap .

The "Bottoming tap has 1-1/2 thd. taper and will tap all the way to the
bottom of the hole. Less the 1-1/2 thds.

The bottoming tap requires the tapped hole be started with a tapered
tap and then it can be put in and finish the tapping to the bottom.

We are using the compromise of the "plug" tap. We don't want two.

Flutes: Taps have "flutes" down thru the threads. In the big world of
machining, it's a function of how big a chip is cut and the type of
material being cut.

 In our case it amounts to how many surfaces there are to guide the tap.

I prefer a 4 flute for hand tapping but often can only get a 3 flute.

The "D" limit: is like the "H" limit on American taps.

Looking at a 1/4" american tap .2500 -thd the "H" is in graduations
of .0005". Half a thousandth. So an H3 is .2515 O.D. H7 is .2535"

By virtue of the "thread form" this also controls the "pitch dia".

I've walked around the "D" limit because my book doesn't want to tell
me the graduations so I can't tell you.

I have to assume they tap holes in the "Old Country" like we, do so it
probably works out to millimeter equivalent to .0005".

While you are there, get a small can of "Alumitap", it's a lubricant and can
mean the difference between failure and success.

Don't quote me here but I think the next best lubricant is "animal
tallow". Which means if things are tight, you go grab the bacon grease.

You really need a tap handle. You're already handicapped so make an effort.
It doesn't have to be expensive, maybe you have a used tool store etc.

Now that we have our tap drill, we better locate where we want the hole.

If you want to measure it, go round up some measuring stuff.

Did I say you also need a scribe and a center punch? And a hammer to hit the
punch.

This isn't cheap is it? Not the first time anyway.

Fortunately the 8mm X 1.25 tap is used in a lot of places on the engine.

While he's off getting his measuring equipment, we'll get the gasket and see
how it fits the manifold, then we'll put it on the one stud header and
tranfer punch it.

No tranfer punch huh? Well we'll make do.

Line up the gasket square and concentric to the exhaust port there and
then holding it firmly, scribe the hole thru the gasket.

If you don't have a gasket, you can use the flange on the header itself. Bit
awkward that.

Now put the gasket away and find the center of that circle with your good eye.

If you use a small steel scale to lay across it, you can get it close to
perfect. Particularly if it has millimeter marks on it.

Half of 8 is 4, if you need a clue here.  :-)

Now we have a criss-cross on the circle, we can center punch it.

Your first big test here: take the punch and slide it out to the center and
"feel" where those scribe lines are. Only way you can get it in the center.

Hold the punch square with the face and fimly against the part and whack it
gently once to set it,and check,then smartly with the hammer. If you hit
yourself, you aren't alone.  :-)

OK, you did the deed and the hole is not in the center. Damn! ! !

We will fix it.

Put the punch back up there in the punch mark you made and tilt it towards
where you want the punch mark to go. Whack it again. Easy.

You can see it has moved over as the punch mark got deeper.

Do this till it's centered and then square the punch up with the face again
and hit it one last blow square. It needs be deep enough to hold
the pilot drill well.

Now you have your center punched for the drill, not too ragged, the drill
will wipe out all evidence.  :-)

Now we whip out the hand drill and the pilot drill, which is say 3/16"
plus or minus what's in the box. It just should be somewhat smaller than the
tap drill. A small drill is more controllable for a first hole. Pilot drill
needs be big enough to hold the Tap Drill point well.

This is where we seperate the girls from the boys or maybe back a few
steps.
 Girls follow directions and the boys think they were born knowing
all this.   :-)

How deep do we want to go? Get the stud and the tap.

See where the threads are full size? Put the end of the stud there.

Where the stud threads are on the other end, the "depth" the stud will
be in the metal, is how deep the tap has to go.

Mark it on the tap or measure it.

Now take the "Tap Drill", see the point down there? That will not accept the
flat ended tap so pull it back to the full diameter of the drill.

Now measure that or mark the "tap depth" plus the point on the drill.

That is how deep the tap drill has to go. The "minimum" depth.

If you can go deeper without breaking thru your part then do it.

You need somewhere for all the tap shavings etc to go.

Go 1/2 dia. deeper if you can, as in 4mm this time.

Now you mark the "Pilot Drill" for that depth. A piece of tape or a
contrasting mark you can see while you're drilling.

God this is long!

Now you have it all "spocked" out you can line up the "Pilot Drill" and spin
it in there.

If you get that "pilot" in there straight, the rest is a piece of cake.

If you can, you get a friend to give you directions on square from
the side while you square it in your face.

Mark the "Tap Drill" for depth and get a little lube on the drill and
sink that one. Keep the thing square and don't wobble.

The "pilot hole" is like a center punch all the way thru the part.

Clean out the hole and you're ready to tap.

Any of you see that training film on getting Aluminum chips in your eye?
Better than a Stallone movie for gore.

 The thing is, if you get a steel piece in your eye, they take it out with a
magnet. You watch the whole thing in this film.

 If you get Aluminum or Stainless in your eye, it's non-magnetic so the
knife is the only route. Of course they have to search for it.

Just something to think about when you're blowing that hole out.  :-)

Tap in the tap handle, vise grips, crescent wrench, your choice,
tap marked for depth, put some lube on the teeth and set it in there.

Make like a drill press again and hold square and firm.

About taps: the tap you have is HSS, hi speed steel, and it is to
cut other steels so it is very hard. Hard means brittle.

It's somewhat better than glass.

Start your cutting, smoothly turning it in for a couple turns.

You'll feel it biting. Take a bite then turn backwards and clear chips.

If it is springing against your turn, you're going too fast.

After a couple turns it's into full tooth and you should reverse and
clear the teeth of chips. Just a turn of the wrist.

Now you only go an eighth of a turn in, then reverse, then another
eighth and back up to clear chips. Take a bite, then back to clear.

There really is no limit on the turn in, if it feels like it's working
too hard then do less with each forward turn.

Do not do more tho, over-confidence is what breaks taps.

After a few turns in, back it all the way out and clean it and the
hole. Put some more lube on and go back at it.

When you back it all the way out you have the same condition as if
you were "chasing" a thread. Meaning it's partly threaded or what.
and needs a tap run thru to clean it up again.

When you start your tap back in the hole you turn it backwards till
the tap "drops" in the "lead thread". You can feel it.

Then you start your turning in. If it starts hard, it's cross threaded. Back
out and try again.

I assume you do this with bolts when you put them in. Otherwise you
will cross-thread the thing. In this case you will end up cutting a
new thread and really screw things up.

Keep working it in till you have the depth you want.

If you did it right it will be a pretty fair threaded hole and your
stud will go in snuggly.

Threading Dies: I haven't used one in a long time. After you have the
dope on a tap, using a die for an external thread is a piece of cake.

Just follow the same procedures for cutting. Put a nice taper on the rod
you're threading, an even taper, so the die can sit down on it square.

Pipe Taps: there are two types, tapered and straight thread.
For the most part you will only see the tapered.

They make a tapered "Reamer", just what it says, it reams the straight hole
to the tapered "minor diameter" of the threads.

I have used maybe 2 in my life. Reamers that is.

It does make for less screw ups but they are very expensive.

I usually just drill it with the tap drill and let the pipe tap cut the
taper for me as I run it in.

The pipe thread is one where I prefer a smaller "tap drill" than specified.

.015-.032"(.3-.8mm) smaller depending on the size tap and material.

I would rather have a tight drill hole and have to work the tap slow than a
sloppy hole that I will have trouble getting to seal.

We are talking about my wobbly drill hole, not whether it's the right size
tap drill.

That's it. Go destroy some parts!  :-)

 west