Tap Drills are Strange

[Zonie]

Tap drills are strange? Well, at least the recommended sizes are strange. I mean, who ever heard of a number 3 drill, or a letter "I" drill for that matter?
Well, of course, those of us who have built a few of these muzzleloaders have heard of them, but from the look on the face of the guy at the Hardware store, he must not build guns. ::

Just to slow things down a little, some words about threads are in order:

American threads have been standardized for years, and the standard is called UN. That's why you see things like 1/4-20UNC, 1/4-28UNF, 5/16-18UNC, 5/16-24UNF and so fourth.

Breaking down the code, the first number, whether it is 1/4 or .250 is the maximum major (or outside) diameter of the external thread.
The number after the dash is the number of threads per inch, and the letters UNC stands for Unified National Course, or UNF stands for Unified National Fine.
That basically means the shape of the thread is a 60 degree included angle and the size has been standardized as a course or fine pitched thread.

Because the major diameter of the external thread is a standard size, and the pitch of the thread is a standard distance between threads and the form of the thread is set at a 60 degree included angle, the minor diameter (or root diameter) of the thread usually ends up being a bastard size.
Adding to the confusion, the minor diameter of the external thread may be sharp, but it is allowed to have a fillet radius down in the bottom of the V. For this reason, the Thread Standard controls the maximum size that the external threads minor diameter may be.
Of course, the internal thread must not have a minor diameter which is smaller than the largest external minor diameter or there will be interference when they are assembled.
This results in the minor diameter of the internal thread being a strange size and this too is controled by the Thread Standard.

This is where the tap drill comes into the picture.
Recommended Tap Drills are sized to meet the minor diameter of the internal thread. That way the tap doesn't have to cut it.
That is also where those strange sizes come from.
For a 1/4-28UNF thread for example, a #3 drill (.213) is recommended and for a 5/16-24UNF, a letter I drill (.272) is recommended.
The actual size requirements for the internal threads minor diameter are:
1/4-28UNF =.211-.220
5/16-24UNF = .267-.277

Now that your dozing off, I will mention a thread series I'm sure you never heard of. It is called a UNJF thread.

This thread is controlled by Mil-S-8879.
These threads are for the high stress applications that Aerospace requires.
The main difference between these UNJF threads and the UNF threads is the size of the fillet radius at the bottom of the V on the external threads.
It seems, the weakest part of a threaded assembly is the minor diameter of the external thread. This is espically true with a standard UN thread where there could be a sharp corner instead of a radius at the bottom of the V. Cracks almost always start at the sharpest corners. (That's why the bag of potato chips has all of those little sharp corners at the top of the bag. So the bag material can fail there and rip open. Well, most of the time, anyway.)

By requiring a controlled radius at the minor diameter, the possibility of failure is greatly minimized.

Now, if the external UNJF threads fillet radius at the minor diameter is large, the internal thread must also have a minor diameter that is larger than the UNF thread to provide clearance for it.

What the hell does all of this have to do with my gun building?? you ask. :curse: Well, keep reading, your almost there.

Remember, I said the UNJF threads are for high strength applications. It doesn't take too much thought to realize that if a slightly larger internal minor diameter works for airplanes, it might also work for guns.

Looking at the minor diameter of the internal UNJF thread we find that the sizes are:
.250-28UNJF = .215-.223
.312-24UNJF = .272-.280

Check out the .250-28UNJF thread minor diameter again. Why, a 7/32 drill is .2187 and that fits right in there!!

And, a 9/32 diameter is .2812 and that's only .0012 larger than the maximum minor diameter of the fancy Military Spec. .312-24UNJF thread!

The bottom line of this is, if you have the recommended number or letter drill bit to use for the tap drill for these larger size threads, by all means, use it, but if the guy at the hardware store gets a glassy eyed look when you ask for a #3 drill, snap him out of it by saying "I meant, do you have a 7/32 drill bit? ::

Note that I do not recommend departing from the recommended tap drill for threads with finer pitches than 28 because there just isn't that much material in these small threads, but for the larger sizes I know my suggestion will work.

 

[bob1961]

if ya look at them big metal box set of drills in the gray boxes they will have a couple of different drill sizes fer listed fer the same drill bit....they will have the inch size, the letter and a number listed fer one bit................bob

 

[Walker]

JIm This was very helpful for someone like me who has never used taps but now has to. Thanks.

 

[Zonie]

If you haven't worked with taps before, here are a few pointers that can keep you out of all of the trouble.

Buy, beg or steal some thread cutting oil and use it liberally on the tap and in the hole. This is a special oil that has sulphur added to help keep the metal from sticking to the taps cutting edges.
Regular oil can be used in a pinch, but without the sulphur, the tap can sieze in the hole or break because of the extra drag on it.

When starting the tap, keep it as close to perpendicular to the surface as possible. Start the tap in about 1 1/2 to 2 turns and eyeball it to see that it is going in straight. If not, back it out and try again.

When the tap is into the hole about 2 to 3 turns, reverse direction about 1/2 turn, stop and turn it back in until you feel it start to work again. Then proceed. Do not turn it in more than one turn without backing it out 1/2 turn.
What your doing here is to break the "chip" with the backside of the taps teeth and allowing it to fall into the flute.
Failure to do this may cause the tap to break.

If the tap suddenly has a lot of resistance, do not force it.
Back it out and clear the chips out of the teeth, relubricate it with the thread cutting oil and then proceed.
Never force it further than it wants to go. It will almost always break the tap.

A broken tap is holy H to get out of the part.

Run the taps working threads completely thru the hole to assure that all of the threads are fully cut.

Most of the taps you buy at the hardware store are called Taper Taps. This is because they have a lot of taper ground on the first threads. This helps the tap pick up the drilled hole for ease of alignment.
Because of the taper, this kind of tap cannot fully tap a blind hole like the nipple hole in a drum or breech block.
To tap a blind hole you must have a "bottoming tap". These have almost no lead or taper, and for that reason they only work well in a hole that has been pre-tapped with a taper tap.
Most common hardware stores do not sell bottoming taps so most of us buy a taper tap and carefully grind off the tapered nose. Doing this, takes a lot of time because you have to stop and cool the tap off in water every couple of seconds. If you don't keep the tap cool, it will loose it's temper and become dull rapidly.

Speaking of dull, always buy the very best taps you can get.
The cheap pieces of manure that are black and made in China aren't worth the powder to blow them across the back yard, let alone to hell.

 

[jh45gun]

The numbered sizes are called wire sizes for instance the wire numbered drill for a scope screw is #31. and the tap is a 6/48 Usually the local hardware store does not carry these but if you have a tool company that sells drill bits they will carry them and they are not that bad in price.

 

Zonie, I broke a tap off in my Jaeger lock plate and there wasn't enought sticking out to grab onto. I ended up going to a machine shop and they tig-welded a piece to the nub and turned it out for me. That was a $30 lesson in patience

 

[TANSTAAFL]

If possible, I use three taps (not to be confused with using serial taps) for blind holes, a taper, a plug and a bottom.

Differences are in the length of bevel angle, which is determined by the number of undeveloped threads, taper has 8-10, plug has 3-5, bottom has 1-1 1/2. The plug tap is probably the most common for all around use.

I would not recommend going directly from a taper to the bottom tap, puts a lot of stress on it, that dreaded "SNICK," can make a Baptist preacher whistle.

 

[Blacksmith]

Zonie, welding suppliers often sell thread tapping/drilling fluids. Buttercut used to be sold in Tractor stores too but think it's gone now. Carbon tet was good stuff for cleaning and thread cutting. Some of the new latex stuff works excellent. Ballistol may work but never tried it. I save it for the firearm cleaning/put to sleep work. La-Co brand cutting fluid (sold in Farm Stores) is excellent stuff and I've been using it for quite a while here. Good drilling fluid and not too pricey. thanks for the tech stuff on threads. Never hurts to have a refresher. Numbered and lettered bits is hard to find individually out here in the stumps (to replace busted ones from the set). got 'nuther question on "class of fit" on threads (by the drill size used for the hole to thread) but will save that for nother post.

 

[bob1961]

Buy, beg or steal some thread cutting oil and use it liberally on the tap and in the hole.

i have some sticks i got while working fer a welder makeing custom iron railings with the fancy scroll work and leaves and flowers and when we did any tapping we had these kinda wax sticks that are marked fer tapping and drilling and they work pretty good....don't know what there made of though.............bob

 

[leslie]

Hardware stores ain't what they used to be.Most of the help are lost when you get a little off the standard stuff everybody buys.Fortunately we have shops that serve heavy industry and its easy to get all kinds of "strange tools".

 

[riarcher]

All the "Water Soluable Oils" that I know of are really cutting / milling / grinding oils (as opposed to "lubricating oils") If you happen to be one of those folks that use water soluble in your patch lube you could dilute it less and use it for taping. In a pinch, Hydraulic Fluid is not a lube oil and couldbe used for taping (the tap will just dull sooner is all).
Not to start anything, but only "hand taps" should be periodicly backed out as mentioned above. Machine and gun taps are not designed to be backed out for chip breaking. By design, these are made to run through or to the bottom. They have the back side of the flute relieved to minimize preasure when machining and will bind up with the chip if backed out. "Plug taps" and the such are designed to drive the chip ahead of the point to keep the cutting clear.
Taps are highly specialized to do specific jobs. Like everything else, you need the right tool for the job. There are taps for Plastic, stainless, cast iron, steels, steels to be heat treated, steels that are H.T. to name a few. A new tap for the wrong application will tear the threads, stick, and or break.
The least desirable of all the taps is the old standard 4 flute hand taps. They are made of low carbon steel, soft, dull quickly and break much more often. And you definately get what you pay for. For 3/8 ths and under, the 4 flute hand tap is the worst in reguards to breakage in a hole. Go with a 2 flute if possible. And it's better to use a tap dry than to use a lube oil, except for few exceptions, like in aluminum, Kerosene/fuel oil works perfectly, plastic likes water better than oil. Believe it or not, in mild steel, chicken grease or lard is better than a lubricating oil. :m2c:
PS; Zonie, the "GH in the tap number indicates how much oversized the tap actually is in comparison to indicated size by 1/2 thousandths. Example; 1/4-28 GH-3, 3X.0005=.0015 meaning the tap is designed to cut a .250 (indicated size) to a .2515 maj. dia. This allows calculating "theoretical" thread clearance to allow for not galling the threads or to allow room for lubricating threads. Some materials are more elastic than others and allows for this or if the material srinks during heat treating you can compensate for the swrinkage with the appropriate GH value. Long ago, before ISO, the industry standardized the male thread and decided to allow the female thread to compensate for needs. Made things easier! (now if I can just get the wife to see the wizdom of the world I'd be ok!) :hmm:
All this is making me "tappy". ::

 

[riarcher]

Bob, #1 ingredient,,,,, beeswax.

 

[Zonie]

riarcher: To each his own I guess. You may be right about lubricating oil when you say " And it's better to use a tap dry than to use a lube oil, except for few exceptions,...". (notice, he said "lubricating oil" not thread cutting oil.) I still feel for doing most tapping on low carbon steel, using the sulphated cutting oil specificially made for tapping mild steel is the only way to go.
The last thing I want to do is tell a person to use a dry tap to thread his $130 lockplate or $220 barrel and have his tap break off in the hole.

Yup, I know about the GH stuff, and your right. Oversize taps used most often when tapping steels which will be plated or shrink during heat treatment or aluminum parts which will be anodized. (Anodizing builds up on the surface so the holes have to be oversized prior to the process so they will be the right size after the process.)
It's not often that hobby gun builders will run into this though.

As for thread Classes, which was mentioned above, they control the tolerances and therefore the fits between the internal and external threads.
The classes are the third set of numbers and letters on a tap.
Typically the full tap or die callout is .250-28UNF-3B or .250-28UNF-2A.
The 1, 2 or 3 in the last group of numbers tells you how precision the thread is. Class 1 is the loosest fit with the greatest tolerance. Class 2 is a good average class of threads and is the most common. Class 3 threads are the closest fitting threads and are used in high stress applications or where certain locking devices are required.
Aerospace and some critical Military applications use class 3 threads almost exclusively.
The A stands for external threads so you would see this on a die. A B stands for internal threads so you would see this on a tap.

The Metric threads by the way also have a 60 degree included angle thread.
Like the UN threads, the first number is the major diameter of the external thread. The second number is the distance between the centers of the thread.
Using this guideline, a 6mm-1mm thread is .2385 inches in diameter and the threads are spaced .0397 apart. In American sizes that would be a .238-25! A 6mm-.75 would be a .2385-33. No wonder that 1/4-28 nipple didn't seem to fit quite right!! ::

Of course there are dozens of other threads in use. Square threads, Buttress threads, Round threads (lightbulb sockets) and many more, but the Unified National is what most folks will be using for their guns.

 

[RussB]

Good post...... Just Jim :applause:.... :front:

Russ

 

I would also suggest buying a copy of Machinery's Handbook - even the old editions are perfectly adequate for threading info - and there is a lot of useful stuff contained in those pages.

 

[more powder]

Get you a can of stuff called rapid tap i have taped 100s of hole with it in all kinds of metal

 

[flashpanner]

Hint. Need a easy way to figure which tap drill is needed for the tap?? You either memorize a list that is available from sources like "Machinery's Handbook" or remember this simple formula: Take the major dia of the tap (use a micrometer to measure) from this subtract the reciprocal of the TPI (threads per inch). this will get you within a couple of thousands of the tap drill needed for a 75% engagement of threads.
Example: 1/4-28 tap. 1/4" = major dia = .250
28 TPI = 1/28 = .035
.250-.035 = .215
.215 is close to the .213 or #3 drill recommended

I like TapMagic brand for tapping fluid works better than the cutting oils and is easier to clean up!

Another use for the "Machinery's Handbook", it makes a dandy bench rest pad/riser for the barrel!! :crackup:

 

[JOHN L. HINNANT]

Greetings Professor Zonie,

Once again an excellant lecture. Your information is without fault, and the responses are also quite good.

I would add to this discussion by mentioning another family of threads called "Special Threads". These threads have the same diameters as the taps you mentioned, but have different threads per inch (TPI). This where the special gunsmiths sizes such as 6-48, 8-40, etc., are found.

MSC (Manhattan Supply Company) is a great source for these taps and tools in general. Their catalog is an excellant reference source and an education in what tools and equipment is available to us. These people have always been first class to work with, and their prices are hard to beat.

Another great supply source for gunsmith tools is Brownell's located in Montezuma, Iowa. I have dealt with these people since 1957, and cannot say enough good things about these folks. This company caters to gunsmiths and specializes in gunsmithing tools and supplies.

A very good tap cutting fluid is one called Tap-Matic. A special formula is available for aluminum.

A simple little tapping accessory known as a tap guide really helps in starting a tap straight, keeping it going in straight, and reducing tap breakage.

These guides can be purchased or turned out on a lathe. They usually are turned on the outside with two different diameters, looking somewhat like a formal top hat. After turning, they are drilled length wise through the center to accept a paticular size tap.

When I taught High School Machine Shop, I had a student turn one of these guides whenever we needed a size we did not have: finally had a complete set.

Incidently, for those who do not know, it seems that the T-C Hawken, Renegade, etc., do not use the standard 3/4"-16 UNF thread on the breech plug. The size is 11/16"-16.

Over the years, I have replaced a number of T-C factory barrels with higher quality barrels. Learned about that breech thread spec. on the first job.

Hope this information will prove useful.

Best regards, John L. Hinnant

 

[riarcher]

I wouldn't go "dry" either, just the point being, (to me) ludricating oil is a no-no. Seen too many "tachinks". Actually, bar hand soap (Not Lave with pumace!)works in mild steel pretty well in a must do situation.
I think MSC has an on-line catalog now and as mentioned worth a look.
EXCELLENT POST! :thumbsup: A lot of information that I think willbe usefull to many.
May I suggest anouther on why there are a gazillion different types of files? After all, a file is just a file,,, right? :winking:
:hatsoff: eace:

 

[Zonie]

riarcher: For a little something on FILING, follow this link. FILING 101.1