TOPIC 1
TREAD CUTTING
Learning OutcomeYou will be able to use taps and dies to cut threads
The student will be able to:
_ Discuss the use of taps and dies to cut threads.
_ Use taps and dies to cut threads
Bolts and Studs
Damaged threads on bolts and studs may be reclaimed. If the thread is not completely
stripped and only flattened, it may be cleaned using a thread die.
There are two types of die:
Fixed nut type
Nut type
The fixed nut type is operated by an open end or ring spanner.
Adjustable Button Type
Adjustable type
The adjustable button type die requires a special tool called a Stock to hold and turn it.
The button die must be placed and adjusted in the Stock prior to cutting or cleaning threads on bolts or studs.Make sure the conical ends of the adjusting screw bear evenly on each side of the split
when the die is inserted.
lock onto the bolt or stud when turned. Make sure the bolt or stud is securely held
during the thread cleaning process
Keep the stock and die square with the bolt or stud.Make sure the tapered portion the die is leading the cut.
Turn the handle slowly in aclockwise direction .Back off in an anticlockwise direction every two or these turns to break the chips and clean the die .Keep the die well lubricated.
Note.
Two types of tread:
Left hand tread and Right hand thread , Clean the die and stock after use.
Nut and Thread Holes
A set of thread cutting and cleaning taps are used to clean and reclaim damaged internal threads.
Bar-type wrenches have a flat centre section holding jaws shaped to grip the squared end of the tap. One sliding jaw is adjusted by a screw operated by rotating one of the hands about its axis. The ends of the two handles are knurled.
The smallest capacity bar-type tap wrench holds taps ranging from 3 mm to 13 mm in
size. There is a range of different capacity bartype wrenches.
Select the smallest capacity wrench that will hold the tap firmly. Check that the handles of the tap wrench can be turned without striking any obstruction.
There are different capacity tee-type tap wrenches. The largest capacity takes taps up to 13 mm size.
Tee-tap wrenches are suitable for work where restrictions in space would prevent you turning a bar-type wrench.
Pin chucks are for delicate tapping of small threads in instruments. Pin chucks have a very small adjustable two-jaw chuck mounted on the end of a knurled rod. They are used for threads smaller than 2.8 mm diameter. The pin chuck is twisted between finger and thumb. This operation requires a delicate sense of feel
Type of Taps
Taps are normally used in sets of three to allow progressive cutting of the threads. There are many types of taps. These are described to indicate some of the major features
Type of taps
Additional terms must be known before proceeding with the discussion of taps. The more important terms are explained below.
Axis of tap : An imaginary line (B) passing through the exact centre along the full
length of the tap
Shank : The part (C) behind the threaded and fluted section (D) of the tap.
Chamfer : The tapered outside diameter (E) at the front end of the threaded
section.
Flute : The four grooves or flutes (G) provided for the cutting faces (H) of the threads
for chip passage and lubrication
External (Male)
Centre : The cone-shaped end (I) of the tap. It is used for manufacturing purposes in holding the tap and is usually found at the threaded end of small taps only.
Internal (Female) Centre : A small drilled, countersunk hole (J) at the end of the tap
necessary for manufacturing purposes.
Body the threaded and fluted section (D) of
the tap.
Use of Taps
or bottoming taps should never be used to start a thread.
The intermediate tap (L) is used when one end of the hole is closed and a full thread is not required at the bottom of the hole.
The taper tap (M) is used to start all threads, and may be used to finish the tapping operation when the tap can be run entirely through the material.
The machine screw tap (O) is used for fine thread, small-diameter tapping operations. The cone-shaped end (N) of the machine screw tap is made that way for manufacturing purposes and is usually found at the threaded end of small taps only. The machine screw tap also comes in the bottoming tap, intermediate tap and taper tap.
TOPIC 1
Written Activity 1Answer all of the following questions.
1. What tool do you use to reclaim an external thread (bolt or stud)?
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2. What are two types of dies available?
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3. What would you use to turn an adjustable die
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4. What tool do you use to reclaim a damaged internal thread?
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5. What are the names of the three types of taps in a set?
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6. What type of tap would you use for the following jobs?
To start a thread _______________________
To cut a thread to the bottom of closed hole? _________
If a new hole has to be threaded or a hole with a damaged thread needs oversizing and rethreading the correct procedures must be followed.
Holes of the correct size and condition are essential for successful tapping. Note these facts about threads;
* The major diameter of a thread is measured from crest to crest.
* The minor diameter of a thread is measured from root to root.
* Theoretically, a tap would produce a 100% thread in an accurate hole of the same diameter as the minor diameter of its thread.
* The tap would produce no thread in a hole of the same diameter as its major diameter. and it would produce a 50% thread in a hole with a diameter midway between its major and minor diameters
* A standard nut with only 60% thread engagement is strong enough to be tightened up until the bolt breaks without stripping the thread.
* The torque required to turn a tap to cut a 72% thread is double that required to cut a 60% thread. The required torque to cut a larger percentage of thread is very much greater.
How to Tap Through a Hole
* Check the size of thread required.
* Select the correct taps and check that they are sharp and in good condition.
* Determine the correct size of tapping drill.
* Select the correct sharpened drill of this size and drill the tapping hole carefully.
* Hold the work firmly in the bench vice with the hole axis vertical.
* Use soft jaws to protect finished surfaces.
* Fit the taper tap firmly in a matching size bar-type wrench.
* Hold the tap wrench with both hands close to the centre.
* Place the end of the tap in the hole.
* Sight up that the tap is vertical to the surface of the work.
* Complete two turns, keeping the wrench handles level and applying even downward pressure.
* Release and remove the tap wrench without disturbing the tap.
* Place a small try square on the work to check that the tap is vertical to the work
surface.
Test again in a second position about 90o from the first.* The square end of a steel rule may be used for this check.
* If the tap is not square to the surface note the correction required.
Replace and tighten the tap wrench on the tap without disturbing it.
Hold the wrench as before and begin turning.
* To correct a tap that is out of square, apply slight side pressure in the direction required as the wrench is turned.
* Never apply side pressure to a stationar
Complete two turns and check, as before, that the tap is square.
* The tap must be made square within first few turns.
Out-of-square situation cannot be corrected after this.
If the tap is square with the work, apply a suitable cutting fluid.
Hold the tap wrench at the end of the handles with the fingers to allow sensitive ‘feel’ of the torque applied to the tap
* No downward pressure is required once the tap begins to feed itself into the hole.
* Use a closed hand grip on the handles when larger taps are used.
As you turn the wrench, try to feel the degree of resistance being offered to the tap. If you feel it is increasing, ease the wrench back in the reverse direction
Some tap manufactures recommend that taps should not be reversed except when the flutes become clogged or when the thread is completed.
If the flutes are clogged, reverse the tap carefully and remove it from the hole.
Use a brush to remove the chips from the tap and the hole
While you are developing the ability to judge the ‘feel’ of tapping, adopt the following routine:
* Frequently ease the wrench back in the reverse direction for about a quarter of a turn if you feel resistance increasing.
* Complete another turn or so.
When the tap seems to bind, ease it with very short backwards and forwards movements rather than a continuous movement.
Aim at learning to produce smooth well-formed threads with the minimum number reversals of the tapping direction Brush the chips from the work and check the fit of the thread with a matching screw or bolt
How to Tap a Blind Hole
Blind holes cannot be tapped right to the bottom. As you drill the tapping hole, use the depth gauge on the drilling machine. Make sure that the depth of hole is greater than the depth of thread required.
Screw a nut on the tap to act as a depth stop.
Use normal tapping procedure but clear out chips frequently.
Clear chips from the job by inverting a small job.
Remove the tap, brush or wipe it clean and replace it in its correct storage place.
* If this is not possible, use a magnet or a bent and flattened piece of metal to scoop out the chips.
* Never try to blow out chips with compressed air.
When the taper tap reaches the required depth, change to the intermediate tap.
When the intermediate tap reaches the required depth, change to the bottoming tap.
If you feel excessive resistance before the measured depth is reached, clear out all chips and try again with the taper tap.
Repeat the procedure of changing taps until the correct depth is reached.
To start the thread and to maintain correct alignment of the tap in a blind shallow hole, proceed as follows.
Useful Reminders:
Be sure that the tap is the correct size for the hole. If too much metal is removed at one time, the strain will be so great that the tap will break.
Check the cutting edges of the tap to be sure they are not dull or damaged.
Be sure you are using the right kind of lubricant and enough of it.
Be sure to back off the tap a half turn counter clockwise after every two or three turns. This breaks the chips, allows them to drop down the flutes with the lubricant and prevents the chips from plugging up and tearing the new threads.
Threaded Inserts
When a thread in a hole is damaged beyond repair and requires replacing with the same thread, a thread insert can be used.
Thread inserts are formed coils of diamond-shaped stainless steel or phosphor bronze wire which have a driving tang (B diag.1) and a notch (C diag.1). The tang is easily removed from the insert after installation by breaking it off at the notch. This will be explained in more detail further on.
The insert is installed using the tool designed by the insert manufacturer. This, along with the size of drill to use, inserts and other details and specifications, is available as a complete repair kit by manufactures such as Heli-coil.
Written Activity #2
Answer all of the following questions.
1. What are two places in which a thread insert can be used?
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2. From what are thread inserts made?
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3. What are the four steps to follow to fit a thread in-sert?
a. -----------------
b. ------------------
c. -----------------
d. -----------------
Practical Activity #1
In this activity you are required to cut two internal and two external threads.
1. You will be supplied with a steel coupon measuring approximately 100mm x 25mm x 25mm and two pieces of mild steel rod. One is 12mm in diameter and the other is 1/4”
You will be required to drill and tap a thread into the coupon and then cut a thread on the round steel which will fit the thread you have tapped.
2. Locate a copy of; MU96C062 Practical Activity Manual 1. Use the threadcutting procedure described in the manual.
3. Draw the tools you will need from the toolstore.
4. Drill and tap the coupon
5. Cut the threads on both lengths of steel
Fit them all together
RECLAMATION OF NUTS, BOLTS AND STUDS
IntroductionBolts and studs sometimes break, leaving a broken piece of bolt or stud in a component. These broken pieces need to be removed in order to re-use the threaded hole or fit a new stud.
Cause of Failure
Bolts and studs break when in use for the following reasons:
* Incorrect tensioning
* Bottoming out
* Incorrect bolt or stud
* Aged, rusted or corroded
Incorrect Tensioning
Over tensioning of bolts and studs will stretch them to the extent they break or strip the threads.
Bottoming Out.
Incorrect Stud or Bolt
When incorrect bolts or studs are used they may break for the following reasons.
Bolts
If the unthreaded shank of the bolt extends into the threaded hole, stress on the bolt will cause vibration to fracture the bolt at the clamped surface. This leaves a broken piece of bolt to be removed
Studs
If threads run out at the jointing surface or above, it is likely when the stud is tensioned that thread material will be pulled up at the edge of the hole and prevent proper clamping of the mating surfaces
Age, Rusted or Corroded
As bolts or studs become old, or if they corrode their strength is changed and they are liable to break
Removal of Broken Bolts and Studs
Bolts and studs may break off either above, flush or below the surface of a component or piece of work.
There are a number of methods for removing these broken pieces.
A. Above surface:
* Two nuts locked together
* Stud remover
* Stillson wrench
* Cutting or slotting
* Welding.
Component
B. Below surface:? Hammer and punch
? Ezy-Out extractor
? Drill and tap
Component
A. Above SurfaceTwo nuts locked together
If a stud is damaged and needs replacing, the use of two nuts locked together may work to remove the old stud as well as for installing a new one
When the two locked nuts method will not move a tight stud, stud removers must be used.
Stud removers are tools that will grip the threaded stud. They allow the stud to be removed by twisting the socket handle.
Note:
These tools are commonly used for extracting studs or broken bolts. These will always damage any thread they are used on. Usually this will not matter if the stud or bolt is damaged anyway. Due to their size and awkwardness, it maybe impractical to use this tool.
Cutting or Slotting
If the stud or bolt is too short to use either the two nuts, stud remover or Stillson method of removal, it may be possible to cut or slot the remaining piece and use a flat screwdriver to remove it. Filing two flat sides and use a spanner is another method.
Welding
In some situations it may be quicker and appropriate to weld a piece of iron bar onto the broken piece of bolt or stud. This bar can then be used as a lever to turn and screw the broken piece out.
Note:
This is not a good workshop practice and is generally only used on larger bolts and pieces of work such as body frames etc., where nothing is likely to be damaged.
Nuts
Sometimes nuts are impossible to remove. When this happens and all else fails (such as the use of penetrating oils, and rapid heating of the seized nut), the nut must be destroyed.
There are several methods of doing this with out destroying the stud or bolt to which the nut is attached.
One method is to drill through the nut walls (A). First, using a centre punch and hammer, mark the nut where you wish to drill. The hole positions should be marked so that the maximum diameter of the drill bit (B) will not break through and touch the threaded portion of the bolt or stud when drilling. The idea is to drill away as much material of the seized nut as possible, thus weakening it. Once the nut has been drilled at six points (C), the nut can be loosened with a chisel and hammer or the proper size wrench.
The second method, which is very effective, is to use a hacksaw to cut away two of the nut walls (D). To produce maximum weakening, the saw cuts must be made as close to the bolt or stud threads as possible without damaging the threads. When the two walls of the nut have been cut away, a wrench of the proper size can be used on the remaining two walls to remove the nut.
B. Flush Or Below Surface 2.4 Hammer and Punch
When a bolt or stud is broken off either flush or below the surface, it may be possible to remove it with a hammer and prick punch or centre punch, if the broken piece is not too tight.
The hammer and punch are used to turn the broken piece until it has screwed far enough out above the surface to be able to grip it with pliers or similar tool.
Ezy-Out Extractor
If the bolt or stud is too tight to use the hammer and punch, it may be necessary to drill and use an
Ezy-Out extractor.
A hole must be drilled down the centre of the broken bolt or stud
Drill and Tap
If the bolt or stud cannot be removed with the Ezy-Out extractor, it will be necessary to drill and tap the broken piece.
Centre punch the centre of the broken piece.
Starting with a small drill bit, drill a hole right through the broken piece. Using larger drill bits, increase the hole size until it is just smaller than the internal diameter of the threaded hole
This will remove the majority of the broken piece.
The remaining threads of the broken piece within the hole may be removed using a sharp pointed tool and pliers.
It may be necessary to use a thread tap to remove and clean the thread in the hole.
Note:
Care must be taken to drill the hole true and square.
Cause of Damage
Threads may be damaged due to:
? Being hit by a solid object.
? Being stripped due to overtightening.
? Being stripped due to ‘cross threading’.
? Being stripped due to nut or bolt not tensioned correctly and working loose.
Incompatibility of nut, bolt or threaded hole.
Being Hit by a Solid Object
If a thread on a bolt or stud is badly damaged due to being hit, the nut will bind and cause further damage to the threads in the nut and on the bolt or stud.
They may eventually bind until the nut will turn no further. If a damaged bolt is installed into a threaded hole the same binding may occur.
Stripped Due to Over Tightening
Threads on bolts, studs, nuts and threaded holes may strip due to overtightening.
In this case the thread is torn from its parent material and the correct tension cannot be achieved.
This generally makes the bolt, stud, nut or threaded hole unusable unless repaired.
Stripped Due to ‘Cross Threading’
If a nut is threaded on at an angle and not true and parallel with the thread on a bolt or stud, the threads will be ‘crossed’. This results in the nut binding up on the threads and possible stripping.
The same damage will occur if a bolt is are wound in out of parallel with a threaded hole.
Stripped Due to incorrect Tensioning
When bolts, studs and nuts are not tensioned enough they work loose. When this happens movement may occur between the two threads resulting in wear of the crest and root of both threads. This process may result in the threads stripping completely.
Incompatibility of Threads
When two threads that are not the same pitch and length are forced together they will bind and become damaged. One or both threads will become unusable.
Written Activity #3
1. Give four (4) reasons why studs and bolts may break
1. ___________________________________
2. __________________________________
3. _________________________________
4. __________________________________
2. Shock and vibration of a stud or bolt can be caused by what?
__________________
__________________
___________________
3. If the hole into which a bolt is screwed is too short, the bolt will do what?
_________________________
4. What are age and corrosion is likely to cause?
_____________________
5. Name five (5) ways in which a bolt or stud which is broken above the surface could be removed.
1. ______________________
2. ______________________
3. ______________________
4. ______________________
5. ______________________
6. Name three ways in which a stud or bolt snapped below the surface could be
removed.
1. __________________
2. _________________
3. ________________
7. What are two ways in which a seized nut can be removed?
1. ____________________
2. ___________________
8. List five (5) ways in which a thread can be damaged.
1. ___________________________
2. __________________________
3. __________________________
4. __________________________
5. _________________________
Written Activity #1
1. What tool do you use to reclaim an external thread (bolt or stud)?
Thread die
2. What are two types of dies available?
Fixed nut
Adjustable button type
3. What would you use to turn an adjustable die?
A stock
4. What tool do you use to reclaim a damaged internal thread?
Tap
5. What are the names of the three types of taps in a set?
Taper, Intermediate, Bottoming
6. What type of tap would you use for the following jobs?
1. To start a thread Taper
2. To cut a thread to the bottom of closed hole Bottoming.
Written Activity #2
1. What are two places in which a thread insert can be used?
1. To replace a thread damaged beyond repair.
2. Provide a stronger thread in soft metal.
2. From what are thread inserts made?
Stainless steel or phosphor bronze
3. What are the four steps to follow to fit a thread insert?
1. Drill a hole to the proper size.
2. Tap the hole.
3. Install the thread insert
4. Break off the tang or hammer down the retaining pins.
Written Activity #3
1. Give four (4) reasons why studs and bolts may break
1. Incorrect tensioning.
2. Bottoming out.
3. Incorrect bolts used.
4. Aged (old) rusted or corroded.
2. Shock and vibration of a stud or bolt can be caused by?
Under tensioning
3. If the hole into which a bolt is screwed is too short, the bolt will?
Bottom out
4. Age and corrosion is likely to change the strength of a bolt.
5. Name five (5) ways in which a bolt or stud which is broken above the surface could be removed.
1. Two nuts locked together.
2. Stud remover.
3. Stillson wrench.
4. Cutting or slotting.
5. Welding.
6. Name three ways in which a stud or bolt snapped below the surface could be removed.
1. Hammer and punch.
2. Ezy-out extractor.
3. Drill and tap.
7. What are two ways in which a siezed nut can be removed?
1. By weakening it by drilling.
2. By weakening it by cutting with a Hacksaw
8. List five (5) ways in which a thread can be damaged.
1. By being hit.
2. Stripped by overtighting.
3. Cross threading.
4. Not tensioned correctly, becoming loose.
5. Incorrect thread, different thread.
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REPAIR AND RECLAMATION OF FASTENER
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