Taps & Dies - Geometric Threading - Tapping Heads - Gages

[widgitmaster]

Types of Taps

Spiral Point Taps— Also known as gun taps, they have an angular point that shears and shoots chips out ahead of the tap. This keeps chips from clogging the flutes, thus reducing tap breakage. Spiral point taps are generally stronger than hand taps and can be run faster with less power. Use them in through-holes and holes two diameters deeper than the required thread length.

Hand Taps— Have straight flutes for hand and machine threading. Although most are used in machine operations today, the traditional name "hand tap" lives on.

Spiral Flute Taps— Also called helical flute taps, the spiral flute uses a lifting action to remove stringy chips that form when tapping soft materials such as aluminum, brass, copper, and magnesium. Use in deep holes, blind holes, and holes with a keyway or gap.

Pipe Taps— Use for internal threading of pipe, pipe fittings, and holes in which you'll be assembling threaded pipe.

Pipe Tap Types

Standard: Use these taps in a variety of pipe tapping applications.
High-Hook: High hook pipe taps are a good choice for materials such as aluminum and free-machining stainless and mild steels that produce long, continuous chips.
Interrupted-Thread: Interrupted-thread taps are recommended for use on soft or stringy metals and those that tend to load the teeth with chips.
Short Projection: Short projection taps are great for shallow holes and when taping depth is limited. Shorter projection and less thread chamfer allows full threads close to the bottom of the hole.
Long-Shank: Long-shank taps are for deep holes that can't be reached with regular taps.

Roll Forming Taps
Roll form taps have neither flutes or cutting edges and therefore do not produce chips and cannot create a chip problem. They are forming tools, displacing metal without removing it in a manner comparable to external thread rolling. The resulting thread is burnished and toughened by the cold forming action. Thread forming offers several distinct advantages over thread cutting for many applications. Materials particularly well suited for thread forming include aluminum, brass, copper, lead, stainless steel, carbon steel, cast steel, leaded steel and zinc. In general, any material which produces a stringy chip is a good candidate for forming. If the chips are powdery, the material may be too brittle.

Specialty Taps— Include thread forming, extension, nut, ACME tandem, and combined taps and drills.

Materials
Most taps are made of high-speed steel in grades M-1 and M-7, which are for use in a variety of applications on ferrous and nonferrous materials. High performance taps are made of premium high-speed steels and work well on stainless steel, nickel-based alloys, and other exotic metals. Premium steels add abrasion and heat resistance for longer tap life and better tap performance. These taps also have a special cutting design for lower machine power requirements, better chip removal and coolant flow, better thread finishes, and closer tolerance pitch diameter size. Use carbide taps for highly abrasive materials such as cast iron and aluminum. Also excellent for use on cast brass, cobalt chrome alloys, copper and copper alloys, fiberglass, and soft plastics. Carbon steel taps are for jobs that don't require the accuracy of high-speed steel taps and don't need resistance to heat and abrasion (such as in some hand-tapping jobs).

Surface Coatings Coatings improve tap life by as much as 300 to 800 percent over uncoated (bright finish) taps. They also improve the part's surface finish and let you use faster cutting speeds.
TiN (Titanium Nitride)— This gold-colored coating works with a variety of ferrous and nonferrous materials. Not for wrought aluminum and titanium alloys. TiCN (Titanium Carbonitride)— Harder, tougher, and more wear resistant than TiN, TiCN is becoming increasingly popular. Works in a variety of ferrous and nonferrous materials. Color is blue-gray violet.

Surface Treatments
Chrome Plating— Adds hardness to tap threads, improves lubricity, reduces loading, and offers cool cutting. Use on a wide variety of ferrous and nonferrous materials. Silver color.
Oxide— Prevents buildup or welding of workpiece materials on tap surfaces in ferrous materials. Reduces friction between tap and workpiece while providing a porous layer to hold lubricants. Avoid use with nonferrous materials. Blue-black color.
Oxide Over Nitride— Use on ferrous materials, high-temperature alloys, titanium and titanium alloys, free-machining steels, iron, high-tensile steels, and stainless steels. Blue-black color.
Nitride— For use on both ferrous and nonferrous abrasive materials that dull taps. Improves surface toughness of the tap. Use on aluminum, cast iron, brass, bronze, die castings, magnesium, zinc, and copper. Colorless.

Tapping Fluids
The use of tapping fluids is highly recommended for best tapping results. Using tapping fluids increases tap life and production, provides more efficient control of chips, and reduces the frequency of tap resharpening. Fluids are more effective if applied under pressure. Using a top and bottom stream of fluid is desirable when tapping horizontally where the tap is stationary and the part revolves. Do not use a lubricant with hard plastics. Use compressed air to remove chips.

Types of Dies

Acorn Die
A threading die used in screw machines whose cutting portion has a shape similar to that of an acorn.

Hex
Used for rethreading and repairing damaged and rusty threads. No special die holder is necessary; a wrench will do! A thread file is also useful in the restoration of damaged screw threads.

Round
Used or threading bolts, studs, rods and other parts that require external threads. Use with any conventional die stock that has corresponding outside diameter dimensions. Includes adjustable and nonadjustable versions.

Two Piece
Two-piece dies that allow a wide range of adjustment. Each piece has two cutting edges that are easily sharpened. Ideal for threading and thread repair work. One side has 2 to 3 thread chamfer for regular threading, while the other side has 1 to 1 1/2 thread chamfer for close to shoulder.

Thread Form Symbols

ACME-C Acme Thread-Centralizing
ACME-G Acme Thread-General Purpose
AMO American Standard Microscope Objective Thread
ANPT Aeronautical National Form Taper Pipe Thread
(Ground thread tap marked NPT)
BA British Association Standard Thread
BSF British Standard Fine Thread Series
BSPP British Standard Pipe (Parallel) Thread
BSPT British Standard Taper Pipe Thread
BSW British Standard Whitworth Coarse Thread Series
M Metric ScrewThread Series
N American National 8, 12 and 16 Thread Series (8N, 12N, 16N)
N BUTT American Buttress Screw Thread
NC American National Coarse Thread Series
NEF American National Extra Fine Thread Series
NF American National Fine Thread Series
NGO National Gas Outlet Thread
NGT National Gas Taper Thread (see "SGT")
NH American National Hose Coupling and Fire Hose Coupling Threads
NPS For tap marking only (See NPSC, NPSM)
NPSC American National Standard Straight Pipe Thread in Pipe Couplings (Tap marked NPS)
NPSF Dryseal American National Standard Fuel Internal Straight Pipe Thread
NPSH American National Standard Straight Pipe Thread for Hose Couplings
NPSI Dryseal American National Standard Intermediate Internal Straight Pipe Thread
NPSL American National Standard Straight Pipe Thread for Loose Fitting Mechanical Joints with Locknuts
NPSM American National Standard Straight Pipe Threads for Free-Fitting Mechanical Joints for Fixtures (Tap marked NPS)
NPT American National Standard Taper Pipe Thread (See ANPT, NPTR)
NPTF Dryseal American National Standard Taper Pipe Thread
NPTR American National Standard Taper Pipe Thread for Railing Joints (Tap marked NPT)
NS American National Thread-Special
PTF Dryseal SAE Short Taper Pipe Thread
SGT Special Gas Taper Thread
SPL-PTF Dryseal Special Taper Pipe Thread
STI Special Thread for Helical Coil Wire Screw Thread Inserts
Stub Acme Stub Acme Thread
*UN Unified Constant Pitch Thread Series
*UNC Unified Coarse Thread Series
*UNEF Unified Extra Fine Thread Series
*UNF Unified Fine Thread Series
UNJ Unified Thread Series with a 0.15011P to 0.18042P Controlled Root Radius on External Thread only.
UNJC Unified Coarse Thread Series with a 0.15011P to 0.18042P Controlled Root Radius on External Thread only.
UNJF Unified Fine Thread Series with a 0.15011P to 0.18042P Controlled Root Radius on External Thread only.
UNM Unified Miniature Thread Series
UNR Unified Constant Pitch Thread Series with a 0.108P to 0.144P Controlled Root Radius; Ext. thread only.
UNRC Unified Coarse Thread Series with a 0.108P to 0.144P Controlled Root Radius; Ext. thread only.
UNRF Unified Fine Thread Series with a 0.108P to 0.144P Controlled Root Radius; External thread only.
*UNS Unified Thread-Special
V A 60¡ "V" thread with Truncated Crest and Root. The theoretical "V" Form is usually flatted to the user's specifications.
WHIT British Standard Whitworth Special Thread
*Taps are not marked with "U" but with the symbol for the corresponding American Standard thread form with which it is compatible.

Thread Rolling
Thread rolling is widely accepted as the fastest and preferred method of economically producing uniform smooth, precise threads of superior physical qualities.
This process is also capable of performing non-threading operations such as burnishing, knurling and rolling of helical and annular grooves of various forms.
Thread rolling utilizes hardened steel rolls to produce external threads. The working surfaces of the rolls have a thread form which is the reverse of the thread to be produced. In penetrating the surface of the blank, the rolls displace material to form the thread roots, and force the displaced material radically outward to form the thread crests.

NPT - National Pipe Thread
The taper on NPT threads allows them to form a seal when torqued as the flanks of the threads compress against each other, as opposed to straight thread fittings or compression fittings in which the threads merely hold the pieces together and do not provide the seal. However a clearance remains between the crests and roots of the threads, resulting in a leakage around this spiral. This means that NPT fittings must be made leak free with the aid of thread seal tape or a thread sealant compound. (The use of tape or sealant will also help to limit corrosion on the threads, which can make future disassembly nearly impossible.)

[widgitmaster]

Geometric Die Heads

CHASERS
Chasers for dieheads are supplied in sets of four for all heads except the 3.1/2” FT which has a set of 6. They are numbered and are fitted into the dieheads in numerical order in a clockwise direction facing the front of the head. This applies to both right and left threads, but for the latter the spindle is reversed.
Chasers are marked with the thread form, tpi or pitch, and diameter, LH if left hand, the grade, which applies to the material that they are ground for, and the gauge number which is required when regrinding.


BASIC OPERATION
a) Closing the diehead
The head should be mounted with the closing handle or auto die closer (ADC) in a convenient position, preferably near the top. The handle or ADC is then pushed away until the head locks closed. If operating manually, and the thread is to be cut in one pass, the detent handle should be left in the finish (-) position.

b) Adjusting the thread diameter Set the indicator line on the graduated scale opposite to the zero line by use of the adjusting screw. When doing this the pressure of the opening springs should be relieved, by pushing against the handle or ADC. This makes it easier to turn the adjusting screws, and makes the adjustment more sensitive. The nominal diameter should be obtained but further slight adjustment may be needed, so it is best to make a trial run.

c) Changing the chasers
Close the diehead to take the pressure off the chasers and remove the front plate. The new chasers must be fitted clockwise in numerical order, starting in any position.

d) Opening the diehead, dieheads open automatically. Types CH, CHS and DS open when the travel of the head along the work piece is halted by a suitable stop. Initially only the back half of the head stops, the chasers, in contact with the rotating work piece cause the front of the head to be pulled a little further forward until the detent pin disengages, opening the diehead.
DX dieheads can also be arranged to open in this way, but they possess an external plunger, which can trip the head open as it meets a stop suitably positioned. This is to overcome stripping of the thread if a short fine thread is being cut in soft material. Another diehead that operates in this way is the XT2, but this type cannot be made to ‘pull-off’. This can be an advantage on fast indexing autos where another type of head may be thrown open on indexing. If the need arises to open a diehead by hand, this can be done by holding the shank securely and pulling on the front of the head, for CH, CHS, DS, and DX types, or by pushing in the plunger on DX and XT2 types.

Operating Speed
This depends on many factors such as the material, the thread form and the finish required

Spindle speed = 12S
Pi.D

Where D = thread diameter, Pi = 3.142, S = feed speed (Ft/min) S varies with the material being threaded, a rough guide is:-

stainless steel and other tough materials 5 – 8 ft/min
mild steel 10 –20 ft/min
cast iron 8 –12 ft/min
brass and copper turning speed

If in doubt a rough rule of thumb is that threading speed should be around half turning speed.

Cutting the thread
The material should be turned to around 0.001” above the finished thread major diameter. A good flow of cutting fluid, ideally through the shank is recommended to improve the finish of the thread and the life of the dies and prevent the diehead clogging with chips.
The head should be offered up to the work piece with a steady gentle pressure, but without forcing, until the dies bite. The pressure should be maintained as the head travels along. For CHS dieheads the gap between the shank and backplate should be maintained.
When the required length of thread has been obtained the travel of the head must be arrested by a suitably set stop, or by the feed handle being firmly held. The dies will carry on cutting for about 2 – 3 mm until the detent pin disengages.

[widgitmaster]

Tapping Heads

There are many brands of Tapping Heads on the market, but they all do one thing very well - Tap threads!

They have a chuck or clamping device to hold and drive the tap, and a shank that is inserted into a machine spindle such as a drill press or Bridgeport mill.
The spindle rotates at a nominal RPM of around 500, and in the direction of the tap, either right or left hand.

They all require the hole to be pre drilled and countersink, and have adjustable clutches to set the torque required by the tap in use.

When a rotating tap is inserted into the hole, it starts cutting either thru the part or to a preset depth. When the tapping head reaches a preset depth, the tap feeds another revolution and disengages itself from the rotating force. Then by retracting the tapping head, it reengages the clutch but in a reverse rotation, causing the tap to unscrew from the hole.

Parts being tapped should be held securely in a vise or other clamping methods, to ensure they do not lift or spin which can damage the operator's hands!

The tapping head also has a drive arm approximately 4-6" long, which needs to be stationary during tapping, but this is also too dangerous to hold by hand! Most setups include a bar affixed to the quill or table for this purpose.

The newer tapping heads have a 1:4 ratio; whereas, the tap reverse RPM is 4 times greater than the initial feed RPM.

These power tapping heads require the use of either spiral point or spiral flute taps, as well as the newer roll form taps. The old hand taps do not work and quickly break from chips packing in the flutes of the tap!

For best results, always ensure the holes to be tapped are free of chips and water based coolant, or your taps will quickly break! Production tapping of steel is easily accomplished with a flood of thread cutting oil or the application of such fluids with a small brush!

Many factories and machine shops have turret drill presses set up next to their CNC machines, so the operators can countersink & tap parts in between cycles.

Note that not all tapping heads will function in the horizontal position, and can not be used in a lathe!


Tapmatic VIDEO

[widgitmaster]

Thread Measurement

Here are a few pictures of thread setting and measurement gages


Three-Wire Method
The traditional three-wire method is the most accurate method of measuring the effective or pitch diameter of an external screw thread. Unfortunately in the past, holding and correctly positioning three wires against a thread while simultaneously taking an accurate measurement has been a very difficult task. Now The thread Check Measuring System provides as simple and precise way for determining the pitch diameter for threaded parts and thread plug gages The system enhances Repeatability and Reproducibility (R& R) and reduces measurement time to a fraction of the time normally taken using the traditional three-wire method.
The Thread Check Measuring System offers specially designed wire holders and wires, base assemblies, and vertically adjustable offset platforms that make thread measurement fast and accurate. Thread Check's holders are fitted with certified full length thread measuring wires that meet or exceed the requirements of ASME/ANSI B 1.2 B1 16M thread standards as well as Federal Spec. GGG-W-366B and ISO standards. All wire holder sets include the actual wire size, NIST traceable number, and the constant required for determining the pitch diameter.
Thread measuring holders are precision made to predetermined thread pitches. Thread measuring wires are held in predetermined position by light pressure clips. The holders rotate freely on the spindle/anvil of the measuring instrument so as to engage the lead angle of thread. Holders for fine pitches have wires positioned of center to allow for measurement at the back of the thread and closer to the shoulder of certain part. Holders can be purchased without wires for companies that have existing wires. Wires can be easily installed, Thread measuring holders are available n a full range of Standard, Metric, And Acme sizes. Our engineering department can design wire holders for multiple start threads, helical gears, worms and other special thread measurement applications.

[widgitmaster]

Thread Milling

Any three axis mill that is capable of helical interpolation can be used for thread milling. Helical interpolation involves three axes moving simultaneously. Two axes, 'X' and 'Y', move in a circular motion while the 'Z' axis moves in a linear motion. For example, the path from point A to point B (Fig 1) on the periphery of the cylinder combines a circular movement in the 'X-Y' plane with linear movement along the 'Z' axis. The 'X' and 'Y' circular motion will determine the diameter of the thread. The 'Z' axis linear motion will cut the pitch (or lead) of the thread.

[thkoutsidthebox]

Hi widgetmaster,
Great thread,

Question:

How do you measure threads on for example, a regular bolt, or perhaps a ball screw, without using a thread gauge. Do you measure from peak to peak, valley to valley, or peak to valley?

Thanks.

[widgitmaster]

Originally Posted by thkoutsidthebox Hi widgetmaster, Great thread, Question: How do you measure threads on for example, a regular bolt, or perhaps a ball screw, without using a thread gauge. Do you measure from peak to peak, valley to valley, or peak to valley? Thanks.

If you look at the 1st picture in post#4, it shows the pitch as measured from peak to peak!

The lead would be how far the screw advances in one revolution, on a single start thread, the lead is equal to the pitch!

Eric

[thkoutsidthebox]

Thankyou, I look at those photos but couldn't see my answer...I didnt know my question referred to the 'pitch'....Im still a noob!

[BobWarfield]

Well done, and very handy!

Best,

BW

[thkoutsidthebox]

So widgitmaster...sorry for asking this but....are you sure pitch and tpi are the same thing?

Reason I ask: I e-mailed a company asking about purchasing 18tpi threaded rod and they cam back with a quote for leadscrews. They didn't say the tpi but they said that the pitch is 3.

Can you shed any light on this? Would it make sense if they were talking in metric?
Thanks.

[BobWarfield]

Originally Posted by thkoutsidthebox So widgitmaster...sorry for asking this but....are you sure pitch and tpi are the same thing? Reason I ask: I e-mailed a company asking about purchasing 18tpi threaded rod and they cam back with a quote for leadscrews. They didn't say the tpi but they said that the pitch is 3. Can you shed any light on this? Would it make sense if they were talking in metric? Thanks.

You have indicated the annoying counter-example where pitch and tpi differ, which is on a metric thread. The precise definition of pitch is the distance between similar points on adjacent threads. Funny things can also happen with multi-start threads.

Best,

BW