TYPICAL MODELING HOW-TO DETAILS
DRILLING & TAPPING
From the RAILROADmodeler list: 06/13/02
Does anyone know of a good screw and bolt, tapping and threading
primer that's aimed at hobbyists? If there isn't one, someone should
write one and perhaps post it here -- this seems like a good list for
such a thing.
Thanks again --
Well, I'll give it a try, although I'm sure Larry Jackman will have some
Let's start with the commonly cited tapping procedure, and then I'll add
some comments on modifying it for different situations.
The proper size tap drill (as determined from one of many published charts)
is first used to drill a hole in the proper location, preferably a through
hole, as blind holes are more work to tap. A small countersink is then used
to chamfer the mouth of this hole to a diameter equal to the major
(outside) diameter of the thread to be tapped. This chamfer does two
things; it provides an easy lead in for the tap, and it prevents the first
thread from deforming upward above the surface of the metal.
The tap is then lubricated (more on this below) and started in the hole,
turning it while applying slight downward pressure until it begins to cut.
After it has started a thread (a half turn or so) it is then turned
backward a quarter turn or so to "break the chip", so that the chips of the
material being cut are small enough to slide up the flutes and out of the
hole. The tap is then turned a half turn forward, quarter turn back, as
many times as needed to get an adequate depth of fully formed threads. The
tap is then turned back out of the hole.
The most important thing about tapping is NOT TO BREAK THE TAP in the work.
Sounds silly, but a broken tap is sure to ruin your day, and in some
instances, will ruin a whole week of days. Taps are brittle, they have to
be as they need to be harder than the material to be tapped. There are two
common ways to break a tap; snap it off by twisting it with more force than
it can withstand, and snap it off by bending it. It is important, when
tapping by hand, not to drive the tap with a tool that either allows you to
apply too much twisting force, or is so heavy that the weight of the tool
alone will try to bend the tap and snap it. "T" handle tap holders of the
hardware store variety are really overkill for taps 2-56 and smaller. I
like to use a General pin vise, the type with the freely rotating finger
rest on the end. This allows me to support the weight of the pin vise at
all times with my finger, even as I reset my hand for the next twist. One
problem with the General pin vise is that many times it will not grip the
tap tightly enough to turn it, which brings me to my next comment.
The common hobbyist tap, a threaded cylinder with three flats ground on it
is a very poor tool for tapping metal. The cutting geometry formed by the
ground flats is really better suited to displacing the material than
cutting it. Indeed, when taping plastic, I tend to treat these like thread
forming taps rather than cutting taps, and simply turn them in one
direction until finished. There is not much sense in backing them off to
break the chip when they aren't forming chips anyway. In thin brass sheet,
say up to 1.5 times the hole diameter, they deform and scrape their way
through the material, and not enough of the tapered lead of the tap is ever
in the work at one time to cause trouble. In deep holes in dense work, like
brass or Zamac, the poor cutting action requires so much force to advance
the tap that they are very likely to wedge in tight and break.
A much better tool is the HAND TAP, available from industrial supply
houses. This is a threaded cylinder with a tapered end and either three or
four half round flutes ground along its length. The edges of the flutes are
nearly radial to the circumference of the tap, so a much better cutting
action results. The disadvantage is that the chips don't feed well, and the
"two steps forward, one step back" tuning motion is required to keep the
chips feeding up the flutes.
Given the opportunity, I buy SPIRAL POINT TAPS (not spiral flute, these are
a different animal). Spiral point taps are designed for power tapping in
machine tools, but work fine for hand tapping, too. They have but two
flutes ground along their length, and a tip geometry that pushes the chip
down the hole in front of the tap. Since the chips don't need to be broken,
these can be turned continuously in the same direction without stopping and
backing off to break the chip. Since the chips don't feed back up the
flutes, the flutes can be smaller, and thus there is more metal in this tap
for a given diameter, so it is less likely to break. Since four flutes
aren't needed for chip removal, the amount of thread form behind each of
the two cutting edges is greater too, making the cutting surfaces less
likely to chip off and jam, which inevitably breaks the tap.
So far, so good for through holes, but where does the chip go in a blind
hole? Well, so long as the hole can be appx. 10% deeper than the required
depth of thread plus the length of taper on the tap, it just curls up in
the bottom of the hole. If the treads need to come closer to the bottom of
the hole than this, then the procedure for tapping blind holes needs to be
used, but the spiral point tap can still be used for the first (taper) tap.
To tap BLIND HOLES, that is, to form threads close to the bottom of a
closed hole, a series of three taps is used, The first is the normal TAPER
TAP, either of the standard hand tap pattern, or a spiral point tap. This
is turned as deep in the hole as it can without hitting the bottom of the
hole (or the ball of chips when using a spiral point tap) which will either
strip the threads out of the hole, or break the tap. This is then followed
by a PLUG TAP, which has a shorter taper, which is then followed by a
BOTTOMING TAP, which has no taper at all. It is very common to chip the
leading cutting edge off a bottoming tap, thus jamming the tap and snapping
it off in the work, which is why in general it is best that holes to be
tapped be through holes, or as deep as they can be. Dedicated taper, plug,
and bottoming taps can be purchased, or are often made by grinding the end
off taper taps.
The last thing to mention is LUBRICATION. Lubrication helps the tap slide
through the work, and also helps the chips slide by the tap. For tapping
plastic, my favorite tap lubricate is spit, although plain water will work,
and dish soap works better. These will not attack the plastic, and clean up
easily with water. For tapping brass or Zamac any light machine oil such as
WD-40 works OK , although I use a dedicated tapping fluid such as Tap
Magic, mainly because I have it on hand. A proper tapping fluid is required
for tapping steel, because it contains not only oil but also chemicals that
aid the cutting action, but tapping steel is beyond what most hobbyists are
going to do.
In conclusion, your best course of action would be to pick one or two
standard threads to use on your projects and buy quality tap drills and
taps. A good selection can be seen at www.mscdirect.com I'm not familiar
with who would be a good Canadian supplier, although I'm sure MSC will ship
to Canada. I suppose your only real choices for your project are 0-80 and
00-90, and unfortunately, 00-90 taps are expensive, about $25US each. This
opposed to $5 - $10 US for quality 0-80 taps. Take a scrap boiler or other
piece of material that's similar to what you will be working with, and
drill and tap some practice holes to get the feel for the tools.
Admittedly, breaking a $25 tap is going to hurt, but it's going to hurt
even worse if you do it in a critical place on your model.
And oh, yeah, my secret to keep from breaking taps. I always buy two. If I
cheap out and only buy one, I'll break it for sure. If I pay the extra
money for a spare, it seems I never need it :)
Hopes this helps.
To which Larry Jackman replied: 06/13/02
Almost every thing you have said is on the "money".
Bottom taps do have taper on then 1 1/2 to three threads are tapered.
Plug taps have 3 to 5 threads taper and taper taps have 5 to 7 threads
taper. We did grind taper taps to plug and bottom taps but they do not
work very well, so did not do that except when we had to. Also when a
tap gets dull throw it a way and do not sharpen it. By sharpening you
make it under size and will cause a lot trouble when you screw a bolt in
to the thread.
In the machine shop we very seldom used a bottoming tap. When we had to
bottom we used a plug.
How deep should your hole be and how deep should you tap?????
You do not need more threads than the dia of the bolt plus one thread.
So if you were doing a 1/4-20 bolt you need 1/4 inch which would = 5
threads plus one more thread to make a total of 6 full threads. Any more
than that is a waste of time. If any pressure against the bolt will
break the bolt before it strips the thread. This applies to the National
Course standard bolts only. The National Fine standard ( old SAE) you
can not have enough threads in the hole because you will strip the
threads before you break the bolt.
For lube on the tap. In aluminum ALWAYS use Kerosene. Any other metal
you can use what ever you want including water. When tap magic came out
we switched to that and never used anything else after that. It makes
the tap so slick that the tap may try turn itself into the hole. 8?)
There are other brands of the same material. Write to one of the makers
or go to a machine shop show and get a sample can. For most of you it
will last the rest of your life and maybe the life of your children. It
just takes a drop into the hole.
Power taping on a drill press or milling machine is a lot of fun but
that is another story.
O yes, one more thing I almost forgot. NEVER NEVER NEVER use a four
flute tap. They are pure Trouble with a capital T. They clog and break
most of the time. Use a two flute tap. They cut and move the chip a head
of itself. If you have a through hole you can pull the chip out the
Also he mentioned a spiral tap. Have seen then but never used them. Only
place I have seen them in use is on a screw machine or automatic lathe.
Can not advise on their use.
What you always wanted to know but were afraid to ask. WOW 8>)
With a second thought added:
One thing if you do use a four flute tap use a little larger tap drill.
Most recommended tap drills are for 75% full thread. Shoot for about 60%
full thread. That will protect the tap from breaking. Also 1/2 turn a
head and 1/4 turn back. You do not need to do that with a two flute tap.
Turn it straight in with no backing. We just never used s four flute in
the machine shop. I hate them. WOW 8>) 6>) 8>)
Then came this post:
I am a neanderthal (sp) when it comes to tapping. I drill a hole a size
under the screw size and use a screw, working it in and out with some "3
in one" oil to tap the hole. I'm sure an actual tap would work faster
and better but this is the time honored way I do it. I do not bother to
tap holes in plastic, I simply screw the screw into the hole drilled
into the plastic. I have broken a couple "taps" off in the hole, and
drilled them out and begun again. After 1150 cars of my own and a couple
hundred for others, I maybe have 5 broken screws to show for it.
So there you have it, how to drill & tap on models...
© S.A. McCall