Frequently asked questions (FAQ)

Frequently Asked Questions especially

Concerning taps

Dies

Relation between tap and screw threads

Tap standard and accuracy.

Roles of each part of taps.

Kinds of taps depending on application

Standard of Screw Threads

Answers to question

Is there any difference between tapping screw and threads made by taps ?

Difference is whether the accuracy is needed or not.

Tapping screws are used in such application as the accuracy is not so much needed.They are used the rear lids of washing machines which are not opened or shut so often.

Screws made by taps are for such usages as require precision and accuracy. They are used for Shinkansen railway and for aircrafts. One of their advantages is that they can be disassembled whenever is needed.

Why the flute of taps is needed ?

Tap without flute cannot have cutting edge and cannot cut threads. Flutes are also needed as a space to accommodate chips. If the flute is too small, chips being packed in the flutes can destroy the internal threads and can break the taps.

Is the grinding wheel same with the grinding stone being used in the kitchen ?

No, they are completely different. A grinding wheel has the shape of saucer and has a hole in its center. By placing the center hole in grinding machine's main spindle and turning the wheel at quite high speed, we grind the periphery of an object.

They have many different diameters up to more than 60 centimeter. The width ranges from 1 mm to 30cm. We choose various grinding wheels of different grain size and different grain hardness to meet with different application.

Materials of taps

To avoid the complex explanation, we state a brief explanation as follows:

The materials taps cut thread in are very wide such as iron, casts, light alloys, stainless steel, titanium alloys, plastics and so on.

Along with the change of such materials, there has been a wide change in the tap materials as well. In the beginning period, the alloy tool steels among special tool steels were the main material for the tap. From about 1945, HSS including such superior chemicals as chromium, tungsten, molybdenum, vanadium, cobalt and others, started becoming popular, and there was little use in alloy tool steels.

Today the materials in tapping are gradually getting higher. Tap materials are required to be tough enough for such materials in tapping. And HSS materials including vanadium which feature is its high toughness is now the main material for taps. Generally the hard materials are very difficult to wear, but are easy to break. On the other hand, the soft materials are difficult to break, but easy to wear. As the tap materials, such features as difficult to wear and difficult to break are required at the same time.

Yamawa has solved this difficult theme by introducing HSS including vanadium. Taps of vanadium HSS have both features, such as high wear resistance and toughness.

There also exist the carbide taps which are using carbide material, however, they are not popular yet. Lets explain briefly. Carbide is made by hardening the powder of carbide tungsten ( looks like powdered lead of pencils ) and baking it. It is very hard like china, but has such disadvantage as too sensitive against shock and easy to break.

Characteristics of special purpose taps besides the difference of tap material

They have other specialties in such elements: Shape of taps ( flute width, flute depth, chamfer angle and so on), most preferable condition in heat treatment, special surface treatment.

Surface treatment

It is used to extend the tool life by putting the hard layer over the tool surface. In older days, chromium clad was used in some tools. Today, the surface treatment is getting popular in not only taps, but in almost all cutting tools.

No chips in thread forming taps.

Usually, when taps cut threads, chips come out. But thread forming taps ( also called as roll taps and fluteless taps ) make the threads by inserting themselves into the materials while turning, and do not produce any chips. Thread forming taps have neither flutes nor cutting edge. Since thread forming taps make threads by forcing themselves into the materials, the materials to be tapped are limited to rather soft materials.

Different from the regular threads cut by regular taps, the threads made by the thread forming taps are very tough because the metal fiber is not cut in the threads of forming taps. Further since we can avoid the trouble of removing chips, we can expect the increase of demand for these thread forming taps.

Holes in dies

They are chip holes. Like the flutes of taps, they are for ejecting chips.

How is the thread inside of dies made ?

We make Hob Taps similer to Hand Taps first. By inserting the Hob Taps into the center hole, we make internal threads in the dies.

Adjustable dies

They are so prepared as we can minutely adjust the size of threads. They are suitable for cutting high precision threads and for when the cutting condition may change.

Relationship between EXTERNAL PIPE THREADS and PIPE DIES.

External pipe threads and Pipe dies
The relation between EXTERNAL PIPE THREADS and PIPE DIES is same with the relation
between INTERNAL PIPE THREADS and PIPE DIES.
We will explain one theme by one theme from the beginning for the customers who may
have just started reading from this sheet.

Recently you may often receive instructions to manufacture EXTERNAL PIPE THREADS
with such symbols as G and R.
But there are few dies with such symbols in the market.
Don't you have any difficulty in choosing the dies and in getting dies ?
Please read following explanation one by one, then, we believe you can solve all your questions.

  1. Relation between EXTERNAL PIPE THREADS, G and R, and EXTERNAL PIPE THREADS,
    PF and PT.
    First, please understand the relation between EXTERNAL PIPE THREADS, G and R, and
    EXTERNAL PIPE THREADS, PF and PT.

    EXTERNAL PIPE THREADS, G = EXTERNAL PIPE THREADS, PF
    EXTERNAL PIPE THREADS, R = EXTERNAL PIPE THREADS, PT

  2. Dies to manufacture EXTERNAL PIPE THREADS, G and R.
    Once you understand the relation stated in article 1, you can easily choose Dies to cut
    your External Pipe Threads.
    To cut External pipe threads G, you can use PF dies.
    To cut External pipe threads R, you can use PT dies.
    Now you are ready to correctly cut your EXTERNAL PIPE THREADS.

  3. More detailed explanation
    Following is more detailed explanation. If you have interests, please proceed your reading.

    G and R are the symbols for EXTERNAL PIPE THREADS specified by ISO ( International Standardization Organization).
    PF and PT are the symbols for EXTERNAL PIPE THREADS specified by JIS(old Japanese
    Industrial Standards Committee, abbreviated JISC).
    More than 10 years ago, JISC adopted ISO specification to follow the direction of ISO.
    Due to this, the symbols for EXTERNAL PIPE THREADS of current JIS are G and R.
    That is, PF and PT are not the formal symbols in current JIS.
    But the market is still using or is sticking to old symbols such as PT and PF.
    Because of the current market situation, YAMAWA continues to use old symbols, PT and PF.

  4. Above explanation is shown in following table. The table shows the relation between EXTERNAL THREADS and corresponding Dies. We believe you have already come to understand.

    YAMAWA's standard PF dies are the dies to cut Class A of PF External Threads and can cut both Class A and Class B of External Thread PF (=G).

    Now, we believe you have come to understand EXTERNAL PIPE THREADS and how to use PIPE DIES.

  5. Another detailed explanation
    Taper threads shown in the table are used for pressure joints. Pressure joints are used mainly for pressure tight feature and are used for the connecting pipes for gas and liquid. Parallel threads are used for mechanical joints only for cover.

  6. The meaning of each symbol is as follows:
    G= Gas Gewinde ( Gewinde is the German word standing for thread.)
    R= Rohr Gewinde ( Rohr is the German word standing for pipe.)
    PF= Pipe Fastening
    PT= Pipe Taper

Pitch diameter

When mating external threads with internal threads, there usually is a space between major diameter of external threads and major diameter of internal threads, and between minor diameter of internal threads and minor diameter of external threads, and the contact is between the inclined faces, that is, thread flanks of each thread. It is the pitch diameter that decide such movements as power transfer and so on. Pitch diameter is an imaginary diameter at a position where width of threads and that of flute become equal. When the difference between the pitch diameter of external threads and that of internal threads is small, such mating is good. But when the difference is large, the mating can become too loose and will not be good. This means the pitch diameter is one of the most important factors in deciding reliable screw threads as well as the pitch in threads.

We usually use thread plug gauges and thread ring gauges to confirm whether the internal threads and the external threads are finished as good as specified. In both cases, we are to check the pitch diameter of threads.

More explanation of multiple threads.

When the lead of a screw proceeds twice its pitch, the screw is double thread. And when the lead of a screw proceeds three times its pitch, the screw is triple thread.

Single thread has one starting point. Double thread has two starting points. Triple thread has three starting points. In normal single thread, if you follow the thread by using a needle, you can confirm the thread proceed one pitch at one turn. If you follow double thread by using the same method, you will see the needle point will proceed two pitches at one turn. In triple thread, the needle point will proceed three pitches at one turn. In single thread, one thread is cut around a cylinder. In triple thread, three different threads are cut around a cylinder so that the thread proceeds 3 pitched by one turn.

As an example: Is such thread available as M 6 x3 ( 3 times of regular pitch of 1 )

Such thread is not workable and not available. The bigger the pitch becomes, the larger the thread becomes. The larger thread means the smaller minor diameter. In external threads, smaller minor diameter results in the weakness of threads. In some screw threads, it is impossible to manufacture such threads.

Relationship between INTERNAL PIPE THREADS and PIPE TAPS.

Recently you may often receive requirement to manufacture such INTERNAL PIPE THREADS as G, Rp and Rc.
But in the market there are few pipe taps available with such symbols.
We suppose you are worried about which taps to choose for cutting such pipe threads
and how to find out the suitable taps in the market.
By understanding the following explanations from the beginning, one by one
you can answer all of your questions.

  1. Relation between INTERNAL PIPE THREADS, G, Rp and Rc, and INTERNAL PIPE
    THREADS, PF, PS and PT.
    First of all, please understand the relation between INTERNAL PIPE THREADS, G, Rp and Rc, and INTERNAL PIPE THREADS, PF, PS and PT.
    Internal thread G = Internal thread PF
    Internal thread Rp = Internal thread PS
    Internal thread Rc = Internal thread PT.

  2. Taps to manufacture INTERNAL PIPE THREADS, G, Rp and Rc.
    Once you understand the relation stated in article 1, you can easily choose the taps
    necessary to manufacture your PIPE THREADS.
    The answer is
    PF tap is for cutting G internal thread
    PS tap is for cutting Rp internal thread
    PT tap is for cutting Rc internal thread.

    Now you are ready to correctly manufacture your PIPE THREADS.

  3. More detailed explanation
    Following is more detailed explanation. If you have interests, please proceed reading.

    G, Rp and Rc are the symbols for internal pipe threads specified by ISO (InternationalStandardization Organization).
    And PF, PS and PT are the symbols for internal pipe threads specified by JIS( former
    Japanese Industrial Standards Committee, abbreviated JISC).
    More than 10 years ago, JISC adopted the specification of ISO to follow the directionof ISO.
    Due to this, the current symbols in JIS are G, Rp and Rc.
    Today, PF, PS and PT are not the formal symbols of JIS.
    But the market (both end users and distributors) still uses or sticks to such old symbols as PT, PS and PF.
    Because of this market situation, we, YAMAWA, are leaving the pipe thread symbols, PT,
    PS and PF unchanged.
    We are very sorry that this may have become the cause of your confusion.

  4. Another more detailed explanation
    In ISO specification, Class B of PF internal threads ( lower standard than Class A) ceased to exist.
    Class B used to be listed in old JIS.
    This means G internal threads are all Class A, PF internal threads ( Class A is the higher standard than Class B).
    There is no trouble because Yamawa's standard PF taps are those for Class A, PF internal threads.
    Above story is stated in following table:

    We believe you have come to understand the relation between INTERNAL PIPE THREADS
    and how to use PIPE TAPS.

  5. Another detailed explanation
    In the above table, there show 2 kinds of Parallel threads.
    They are Parallel thread (*5) of Taper thread (*2) and Parallel thread(*6) of Parallel thread (*3).
    These 2 Parallel threads have same designation and threads per inch, but are only different
    in their symbols, Rp and G ( or PS and PF).
    Rp and PS are the symbols for Parallel thread(*5) of Taper thread(*2).
    G and PF are the symbols for Parallel thread(*6) of Parallel thread (*3).

    The difference of these 2 Parallel threads lies in the way to use INTERNAL PIPE THREADS
    manufactured.
    The Parallel thread (*5) of Taper thread (*2) is used for pressure joints. Pressure joints are mainly for pressure tight and their threads are used in connecting
    pipes for gas and liquid.
    Of course Taper thread (*4) of Taper thread (*2) is used for pressure joints as well.

    Next, where is the Parallel thread (*6) of Parallel thread (*3) used ?
    Parallel threads(*6) of Parallel thread(*3) is used for mechanical joints such
    as connecting pipes only for cover.
    That is, Parallel thread (*6) of Parallel thread (*3) is the threads used in connecting
    pipes not ensuring pressure tight feature.

  6. Finally we explain the meaning of symbols.

    G= Gas Gewinde ( Gewinde is the German word standing for thread.)
    Rp= Rohr Gewinde, parallel
    Rc= Rohr Gewinde, cone or conical
    PF= Pipe Fastening
    PS= Pipe Straight
    PT= Pipe Taper

Different standard of screw threads in the world

Each country made his own industrial standard indispensable for the machine industry along with its development. As the result, there were born many different standards in the world.

ISO standard

Activity trying to make ISO the common standard in the world is very strong. However, it will be difficult that a standard is recognized as only one standard in the whole world. Every one has to recognize a fact that now there is the metric world in Europe and the inch world in US

Since the current world situation can include political interests, it is difficult to improve our current situation.

Basic size

Including machinery tools, all industrial products cannot be completed fully to the specified size. So, we specify the basic size first and specify later the eccentricity such as tolerance or allowance.

The allowance of screw threads and taps is specified by using such small unit as micrometer (1/1000millimeter). Actual sizes are expressed by adding the allowance of + or - or +/- marks to Basic size.

Same size to manufacture internal threads and external threads ?

No. It is same with a phenomena that a cylinder of just 6 mm dia. never enters into a hole of just 6 mm dia. Although the size is same, we have to manufacture the internal thread and the external threads by using different sizes.

For example, in the case of screw thread M6 x 1, following is the actual specification ( Accuracy is JIS 2nd class.) . Unit is millimeter.

Basic size:
Major diameter 6.000, Pitch diameter 5.350, Minor diameter 4.917.
Size of external threads:
Major diameter 5.970 ~ 5.820, Pitch diameter 5.320 ~ 5.220,
Minor diameter 4.743 ~
Size of internal threads:
Major diameter not specified (but for larger than 6),
Pitch diameter 5.470 ~ 5.350, Minor diameter 5.153 ~ 4.917

Major diameter of external thread meets with that of internal thread, and minor diameter of external thread meets with that of internal thread. So the sizes of internal threads prove to be slightly larger than those of external threads. Usually external threads are smaller than the basic sizes and internal threads are larger than the basic sizes. Accordingly, external threads and internal threads, although they are manufactured at different manufacturers, can mate each other at all.

Size of threads and that of taps

Tapping is the operation to cut threads by inserting a rotating tap into a hole which is bored beforehand to the specified size. The size of internal threads is not always the same size of the tap. Usually the size of internal threads becomes larger than that of the tap. Because of this tapping tendency, tap's pitch diameter is specified to be close to the basic size within the range of internal thread size and the allowance is also specified rather smaller.

Coarse threads and fine threads

This may be an exceptional case. If we are going to cut the thread of 8 mm dia in iron sheet of 1.5 mm thick and if we use the coarse thread of 1.25mm pitch, the number of threads produced in the sheet is only one. This would bring about very unstable screw threads. If we use the fine threads of 0.75 mm pitch, we can cut 2 threads in the sheet.

Unless there is special cutting condition like this, usually, the screw thread of coarse pitch is normally used.

Threads ground off in an inclined manner at tap end

When we cut internal threads with tap, we have to avoid the chipping of cutting edges. Inclined ground off threads at tap end protect the cutting edges. If we are going to cut full threads in the material from the very beginning, it will bring about abnormal load over the tap edge and may cause the breakage of cutting edges. The inclined ground off threads can cut threads little by little and can lessen the load of cutting.

Plug taps and bottoming taps

For through holes, we can cut threads only with taper tap. But for blind holes, we cannot cut threads in deep hole end with taper tap. We have to use plug taps or bottoming taps in order to complete the threads in the hole end.

The number of full threads

Theoretically, we need only one thread to cut threads because other threads are there only to follow the previous threads. But if we use a tap which has one full thread, the tapping becomes very unstable and threads already cut can be damaged or destroyed. In the case of through hole tapping, once the tap goes through the hole, it becomes very difficult or impossible to return back the tap through the hole.

One operation with plug tap or bottoming tap

Due to the increase of tap's performance and the improvement of machine tools, we do not use 3 piece set tap so often as before. The reason why we don't use only taper tap to cut threads is that we cannot cut the threads to the very bottom of hole with only taper tap with remaining unfinished threads.

Trouble related to horizontal tapping

Such troubles as chip jamming and misalignment between tap and bored hole are caused by gravity. So in tapping, vertical direction is recommended. There are not such taps as are specially suitable for horizontal tapping in the market.

But because of the reason of operation, horizontal tapping sometimes cannot be avoided. In such cases, we will design such special taps as improve chip ejection or as have oil holes to avoid chip jamming with its volume oil pressure.

Taps for insert coil

In insert coil thread, the spring type coil is inserted. The coil corresponds with the pitch of the thread cut.

The insert coil threads are used to strengthen internal threads cut in very soft materials and worn through many time of attaching and detaching. They are also used to amend the thread cut too large or to try to make the thread of larger diameter.

Drill taps

Drill taps are composite tools having drills at their end. Number of turns and feed of Drills are different from those of taps.

So it is not good to try to concentrate the two different operations into one tool. It may sound good to try to complete threading with one tool by inserting the drill tap into bored hole, but once either drill portion or tap portion breaks, we cannot use this tool any further.

Carbide tap

The biggest point where a tap is different from other cutting tools is that the tap can be removed from the hole only after reversing it. Because of this specialty, the tapping speed becomes very slow and the advantage of the use of carbide material which is only available in high speed cutting is rarely realized in taps. Actually many carbide taps have been developed for high speed cutting. But the recommended cutting speed for those taps are still very slow compared with that of other cutting tools.

Coated taps.

Coated taps are the taps on which surface the hard layer is coated to extend tool life. There are various kinds of coatings. Besides titanium coating as a general coating, further study about diamond coating is now proceeding. In some area, the diamond coating is already in the practical level.

Difference of flutes

Depending on the kinds of flutes, there are changes in the direction of chip ejection. In spiral fluted taps, chips are ejected upward along flutes toward shank portion ( opposite direction against the progressing direction of the tap). In Hand taps, a potion of chips is ejected upward, however, the flutes of spiral fluted taps can eject chips more smoothly. In Spiral pointed taps, chips are ejected downward out of tap end.

Direction of chip ejection, upward or downward

The direction is related to the kinds of holes, through holes and blind holes. In blind holes, internal threads are cut up to the middle of the holes. The holes don't go through. Through holes mean the holes which go through. If we use spiral pointed taps in blind holes, the taps may break due to no space for chips in the hole bottom. If we use the spiral pointed taps in through holes, we can get high efficiency. These are the reasons why we use the taps with different direction of flutes.

Relationship between INTERNAL PIPE THREADS and EXTERNAL PIPE THREADS.

Mating of INTERNAL PIPE THREADS and EXTERNAL PIPE THREADS. Here, mating of INTERNAL PIPE THREADS and EXTERNAL PIPE THREADS is specified. Formal mating is shown in following table. It is important to use PIPE THREADS according to this formal mating.

There is a difference between nominal size and actual outside diameter.

Notes: about Outside diameter of EXTERNAL PIPE THREADS ( OD of taps) Sizes of Pipe Taps are such as 1/8, 1/4 and 1/2. When we change them into decimals as they are, we may get 0.125inch (3.175mm), 0.25inch (6.35mm) and 0.5inch (12.7mm) respectively. But the actual sizes of PIPE threads are not these sizes.

1/8 9.728mm
1/4 13.157mm
1/2 20.955mm
1 33.249mm

Pipe size 1/8, pipe size 1/4 and pipe size 1/2 have come from the traditional way of calling to refer to pipes, and are different from actual sizes.

Difference between UN (UNC, UNF and UNEF) and UNJ (UNJC, UNJF and UNJEF).

Minor diameter of both external screw threads and internal screw threads of UNJ is larger than that of UN. This is to enhance bending strength of external threads as per diagrams below. The standard of UNJ screw threads (MIL-S-8879, AS 8879 and ISO 3161), one of the Unified screw threads, is established for fastening parts of aircrafts called "Air-fastener". It has only one combination of 3A class external threads and 3B class internal threads, both have the narrowest tolerance for Unified threads in order to make sure that air-fasteners are most strictly fastened. Furthermore, external screw threads of UNJ have rounded root. For internal screw threads, Yamawa (YMW) taps for UN threads can be used for threading UNJ as well by making larger holes than those recommended for UN threads within the tolerance of minor diameter for UNJ standards. At the same time, tapping condition must be highly accurate in order to make internal screw threads within 3B class. That's because tapping results can be affected by tapping condition such as feed mechanism of tapping machine, etc. Tapping machines with feed rate/rotation synchronized tapping attachment are strongly recommended.

  1. Larger minor diameter (larger core diameter) and rounded root make external screw threads of UNJ as strong as possible by making large larger holes than those recommended for UN threads within the tolerance of minor diameter for UNJ standards.

  2. Example

    Limit size for minor diameter of Unified internal threads
    1/4-20UNC (2B) Min: 0.1960" (or 4.979mm) - Max: 0.2070" (or 5.257mm)
    1/4-20UNJC (3B) Min: 0.2013" (or 5.114mm) - Max: 0.2121" (or 5.387mm)