SUB MODULE 7.3: TWIST DRILLS

EASA PART 66
MODULE 7 MAINTENANCE PRACTICES
SUB MODULE 7.3

Twist Drills:

Twist drill is a cutting tool comprised of cutting point at tip of a shaft with helical cutting edge.

While the range of tools, designed to create holes in metals and other materials is vast, the Morse-type (named after its inventor, an American engineer) of Twist Drill (refer to Fig. 3.13) is the one most commonly used in aircraft (and in general) engineering.

The shank is the part of the twist drill that is gripped and driven by the chuck of the drilling machine and it is on the shank that the details of the type (grade) and diameter of the drill can usually be found printed or engraved.

On drills up to 12.5 mm (½”) diameter, the shank is parallel and placed into the jaws of a self-centring chuck. On drills above 12.5 mm the shank is usually tapered (to a Morse Taper) of 1:20. The tapered shank fits directly into a matching tapered housing in the drilling machine spindle.

The tapered shank usually ends in a tang and this arrangement
provides a more positive drive, which is necessary to overcome
the higher forces when drilling with the larger diameter drills.

Elements of Twist drill

 

 

The various elements of twisting drills are described below.

Axis: An imaginary center line

Body: It is the main parts of the tool, it extend from the drill shank to the tip of tool.

Shank: It is the cylindrical portion of the drill that held in holding device. The shank may be straight or tapered. Hex shank drill bits are also available. The parallel shanks are provided in small tool bits. It is held in the machine by chucks. The larger size drills are provided tapered shank. It carries a tang at the end of the shank to ensure proper grip between the drill bit and driving machine.

Tang: It is the flattened end of the tapered shank, which fit into the driving slot in a socket. It ensures the positive drive from the drill spindle.

Neck: The portion of reduced diameter in between the body and shank

Margin: The part of the body which do not cut away. This narrow extension through the entire body provides clearance between drill and work material. A double margin drill bit have two margin instead of one.

Dead center or chisel edge: It is the point where two cutting edge meets at the extreme tip. It should always be lie in the axis of twist drill.

Flutes: It is the grooves in twist drill, which provides lip or cutting edge.

Flank: surface of drill, which extends behind the lip to the flute.

Lip: It is the cutting edge formed at the intersection of flank and flute.

 

 Twist Drill Angles:

 



Rake angle/helix angle

It is the angle of the flute in relation to the work surface. The value of rake angle varies between 18° to 45°; its typical value is 30°.

The rake angle governs the tightness with which chip curls and hence the amount of space occupied by the chip. The higher rake angle cause tightly rolled chip while the small rake angle tends to chip curl into more loosely rolled helix. Large rake angle causes excessive vibration and chatter of drill.

Lips clearance angle

It is the angle formed by the flank and a plane perpendicular to the axis. Lip clearance is the relief given to the cutting edges that allow drill to enter into the work piece without any hindrance.   In general purpose drills, the clearance angle behind the cutting edge is 12°. Too much clearance angle cause the breaking of the lip of drill due to insufficient support. Clearance angle should be kept minimum in order to provide enough strength and rigidity to cutting edge. Too little clearance results drills having little or no cutting edge.

Cutting angle/ Point angle

The cutting angle is the angle between two lips when it is projected on a plane parallel to the axis. Simply it is the angle between the lips in the side view of a twist drill. Normal of value of Point angle is 118°.  Both cutting edges are sharpened at the same angle, so each makes angle of 59° to the center axis

Chisel edge angle

It is the angle included between the chisel edge and lip of the twist drill when measuring on a plane normal to the axis

 

 


 

 

 EASA PART 66 MODULE 7 ESSAY:

A 6mm internal threaded hole is to be made in a thick metal sheet. Write down the complete procedure mentioning all tools that you will use.

(Note: Write your answer in the comment section below)

Comments

  1. We will use centre punch, pilot drill, finishing drill, reamers, taper tap,second tap and plug tap tools are used for making 6mm internal thread.

    PROCEDURE:
    First we have to make a mark on the surface with the centre punch so that we can easily identify where the internal thread has to be made.When this has been done then we have to make a small hole with the pilot drill so that the drilling for 6mm hole will easily be done and the drill will fit easily while making hole. After doing pilot drilling we will make a hole with the finish drilling like twist drill etc. When we have made the hole with the finish drilling then we should use reamers in that hole so that we can make its dimension perfect to 6mm. When we are done with the reamers then we will use to make internal therads with the internal threads taps.These taps are of three types First we will use taper tap then we will use second tap and at the end we will use plug tap. In the start we will use taper tap through which it will make a starting thread od 2/3 threads tapered.After the use of taper taps then we will use second tap in which it will make the threads till end but we should give great care while doing tapering because if we are not cleaning the swarf and the hole is blind then it will he a problem at the end. We should clean the holes after making 2 or 3 threads. We have made the holes then we will do finishing of the threads so we will use plug tap for finishing.

    If there is an event of tap breaking in the whole then we should consult special prodecures for its removal without causing further and expensive damage.

    Hammad Malik

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  2. Specific tools are used for for drilling a 6mm internal threaded hole.

    Centre punch is used to mark the centre point then pilot drill makes a hole in the metal sheet.

    After the hole has been made, reamers are used to enlarge the drill hole to the accurate dimensions of 6mm and provide a smooth internal finish.

    After the hole has been accurately dimensioned, cutting taps are used for making internal threads. Taper tap is used to start the cutting thread. Second tap is used to deepen the thread. Plug tap is used for finishing. Second tap is used only if it is not BA thread.

    All these step should be done according to the relevant maintenance manual.

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  3. When a hole is to be drilled in a metal, the drill that is being used and all the other tools involved in the process should be appropriate and in a serviceable condition.
    When drilling a hole in a thick metal, Center of that metal should be first marked with a Center punch because the hole to be drilled has a diameter of 6mm. Care must be taken when indenting the surface and avoid using large force that can cause the metal surface to distort. Then a hand drill will be used to drill the hole of an appropriate size because it is able to drill a hole of a diameter up to 6.5”. The size of the drill to be used should be equal to the minimum diameter of the thread to be drilled. The cutting angle should be appropriate and equal on both sides, normally 59 degree. Unequal sides can cause an unequal hole to be drilled.
    Then a reamer will be used to enlarge the hole to an appropriate dimension. A spiral parallel reamer is used to enlarge the hole that contain grooves or threads inside, that’s why this will be used.
    Then internal grooves will be formed by using taps. First taper tap will be used for the correct alignment. It’s tapered from a point about two third of the entire threaded length. Then a second tap or intermediate tap will be used and it’s tapered only to the first two or three threads and where possible it will penetrate through the entire length of a hole and will from full threads.
    For finishing plug tap will be used. Because Plug tap is not tapered and is used to provide finish to the threads in a hole or to cut threads in the center of a blind hole. First the center tap will be used for the correct alignment after using appropriate lubricant. Before rotating clock wise with an appropriate spanner, it should be slowly rotated to form initial threads. When initial threads are formed then they should be not be continually rotated clockwise because it won’t break the threads and can cause the tap to stuck ( jam ) in the hole. This is countered by moving the tap 1/2” or 3/8” backward after few turns. This rotation and breaking of rotation is continued until full threads are formed.
    This is then followed by a second tap and if recommended bottom ( plug ) tap. The swarf should be removed because if not it can cause the tap to stuck in the bottom.
    When a tap is stuck appropriate methods should be used ( that are recommended in the relevant manual ). All the information will be provided in an AMM and it should be concerned.

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  5. If we have to make an internal threaded hole of 6mm on a metal surface, then we must follow the following procedure.

    First of all we start from using centre punches to make an indent on the metal. The point may be grounded at an angles between 60 to 90 degree.
    Then we will use the bench vice to grip the metal firmly and it should be tight enough to hold the metal in a fix position.
    Then to make a hole in the metal sheet we will use a twist drill.
    After that we will use straight fluted reamers to enlarge drilled hole to accurate dimensions and to provide a smooth internal finish.
    And lastly we will use plug tap to finish the threads in deep through holes or to cut threads to the bottom of blind holes.

    ReplyDelete
  6. The following tools should be used to start making 6mm internal threaded hole.
    Use centre punch, Twist drill , reamer, internal screw cutting tape
    .For making hole first we use centre punch to make indent point to start the drill.Second we use morse type twist drill to make thehole in the metal sheet.After the twist drill,Reamer is used to accurate dimensions of the hole and provide a smooth internal finish.internal thread taps is used to make internal thread.Internal thread making taps are of three sets but in BS came only in two sets Both can be use to make internal thread. For the holding the metal sheet we use bench vice placed on surface metal.The size of the drill should Be appropriate,The size of the drill should be equal to minimum diameter of the thread being cut,use appropriate length of a spanner to avoid wobbling and overtieghting of a tap.To avoid the tap to jam in a hole it should be rotated back approximately 1/2 aur 3/4inch backwards

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    1. Taper tap is used to cut the length 2/3 of diameter to start the cutting the threads.
      Second tap is used to make middle threads of the hole this tap is not used in BA taps and can be used to make the threads in all hole if hole is not closed at one end. Plug tap is used to make the ending threads of a hole.

      Delete
  7. Muhammad Hamza Ahmed

    If we have to make an internal threaded hole of 6mm on a metal surface, then we must follow the following procedure.

    First of all we start from using centre punches to make an indent on the metal. The point may be grounded at an angles between 60 to 90 degree.
    Then we will use the bench vice to grip the metal firmly and it should be tight enough to hold the metal in a fix position.
    Then to make a hole in the metal sheet we will use a twist drill.
    After that we will use straight fluted reamers to enlarge drilled hole to accurate dimensions and to provide a smooth internal finish.
    And lastly we will use plug tap to finish the threads in deep through holes or to cut threads to the bottom of blind holes.

    ReplyDelete
  8. Multiple tools are to be used on the making of a hole which are to be discussed below.

    USING SURFACE TABLES
    Firstly we will use surface tables. Surface tables will provide a smooth and leveled surface. A balanced environment will be provided to us by the use of surface tables which will help us to do our work easily and efficiently.

    MAKING INDENTATIONS USING CENTER PUNCHES
    Center punches would be used after placing the thick metal sheet on the top of the surface tables, center punches will make an indentation in the hole to provide us with the starting drill bit to be easiy setteled into the hole. Center punch should be deep enough to help the drill bit to sit but not too much force should be used during punching as to damage the sheet.

    DRILLING A HOLE USING TWIST DRILL
    Usually we'll use pilot drill to be used after center punches if we are to make a hole larger than 6mm, but in this case we will not be using the pilot drill as we are making a hole that has a diameter less than that of 6mm, Twist drill will most probably be used as the point of twist drill will be small enough to make good contact, A twist drill having a smaller diameter than 6mm will be used to make a hole. We will not be using the hand brace/brest brace here as will require more force to be used then an electrically powered tool.

    REAMING THE HOLE
    Reamers are used to enlarge holes that have already been drilled.
    After making the required hole having a diameter of less than 6mm we will use reamers which will help us to maintain a diameter of 6mm (the required diameter) and give the required enlargement required. A hand parallel reamer is to be used because we'll need a standard size hole rather than expanding or tapered reamer.

    TAPPING THE HOLE
    The last tool to the completion of making our hole would be the tap.
    Tap is a device used in making internal threads of an already drilled hole. If we are using the american based taps we'll usually have 3 types of taps being used the taper tap, second tap and plug tap, Meanwhile in the BS system we usually dont have the intermediate tap (second tap). The taper tap is used to make the threads in the hole of about two thirds of the threaded length, which helps in correct alignment of the tap. After taper tap then second tap should be used it will help in making threads in the middle of the hole this can be used to thread the whole length of the hole if there is an open end to the hole if the hole is closed we'll use the plug tap which helps in making the threads in the end of the hole. When using taps we'll carefully rotate the tap for about 1/2 and 3/4 length and rotate them backwards to remove the swarf if the swarf is not removed our tap or the screw will be held inside the hole and will not be able to come out. The tap can jam and snap off in a hole its removal can cause serious difficulties.

    The proper procedure should be used and followed to make a proper hole.

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  9. The following tools required for making 6mm internal threads are Centre punch, twist drill, reamer, internal screw cutting tape bench vice surface plate table.
    For this process we have to take a metal and palace it on grade C surface plate table. in further procedure a piece of metal will hold in bench vice make a mark by using centre punch on metal. Then we will use twist drill and make a hole on metal after using twist drill clean the surface and check. For more finishing we will use straight reamer and make it more accurate. After completing this process of drilling and reaming. We take internal screw thread cutting taps for making internal threads in a hole. After that we have to use taper tap for starting internal threads it is tapered gradually from the point for about two thirds of the threaded length and the second tap is following taper tap for the first two or three threads only. For finishing of plug tap the finish the threads in deep, through holes or cut threads to the bottom of blind holes. we make amm for more accuracy according to required as manufacturer requirement .

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  10. The Following Procedure is followed to Make a 6mm Internal thread hole.
    Procedure:
    TO MARK AN INDENT PRIOR DRILL:
    The procedure is started by Holding the metal Sheet in the bench vice placed on Surface table (Grade C) for accurate work. A Center Punch is used prior to drilling process to mark an Indentation in the metal sheet large enough to for the drill bit to set in the indent.
    The indentation should not be that large which Distort the metal.
    DRILLING PROCESS:
    For the drilling process a Morse-type twist drill is used to drill the hole in the metal the diameter of which is slightly less then the Required diameter.
    REAMING PROCESS:
    To make the hole accurate to the required dimension the Reamer is used
    The hand Parallel Straight fluted type reamer is used in the pre-drilled hole
    The reamer only remove 0.2-0.3mm of the material so they should be used having this concept in mind.
    TAPPING THE INTERNAL THREAD:
    The final process is to tap the threads in the drilled hole
    Tap are normally made in the sets of three but in BS the taps come with two taps set.

    • Taper tap is used for the initial thread making process the tip of which is tapered gradually to the points of about two third to the thread length. So that it can easily slid into the pre-drilled hole

    • Second Tap is Used followed by the taper tape to depend the thread in the drilled hole the tip is slightly taper only two to three threads if the hole is open to both sides then the second tap can be used for over all internal threading

    • Plug Tap has no tapered portion and is used to finish the thread in deep.

    During the threading process the the tap should be turned backward after each full turn for about half turn to break the cutting off.

    Proper attention should be given to the procedure followed to make the hole Properly and accurately.
    Aimal Khan

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  11. We should be used Center punch , pilot drill, finishing drill , reamers taper tap,second tap and plug tools are used for 6mm long internal threading.

    Firstly we should use centre punches tools used to make an indentation in metal.Indentation is locating the centre of a hole. When this has been done than we have to make a small hole with the pilot drill , than we drilling 6mm hole easily done.After we have done pilot drilling we will make a hole with the help of twist drill.when we make a hole and finish drilling than we should use reamers to enlarge drilled holes to accurate dimensions and provide a smooth internal finish. When we done with the reamers than we will use internal screw threads.

    There are three types of internal threads:
    Taper tap,second tap and plug tap
    First we will use taper tap through which it will make a starting thread 2/3 threads tapered.After the use of taper taps then we will use second tap in which it will make the threads till end but we should give great care while doing tapering because if we are not cleaning the swarf and the hole is blind then it will he a problem at the end. We should clean the holes after making 2 or 3 threads. We have made the holes then we will do finishing of the threads so we will use plug tap for finishing.

    If there is an event of tap breaking in the whole then we should consult special prodecures for its removal without causing further and expensive damage.

    Ehtesham

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  12. Tools are the equipments as they are held in one hand and used for the particular functions. Following are the tool s used to make a internal thread of 6mm such as punches twist drills surface plates reamers or taps at the end .
    Firstly we have to take metal piece in which we have to make a internal thread of 6mm .
    Then we have to place the metal piece on the surface plate table of grade C as they are found in typical workshops and they are used to test for flatness.
    After placing the metal on the surface plate table then we should used bech vice to hold the metal.
    After that we have to punch the metal by center punch as it is use to indent to locate the centre of hole.
    After indenting a hole we have to use twist drill to make a hole as they are probably used to make holes about 6mm we can’t use pilot drill as we know that it is used for drilling holes larger than 6mm that’s why we preferred to use twist drills.
    If the holes aren’t that precise or enlarge we should use straight parallel handed Reamers to enlarge the hole and precise the hole Enough to to make proper internal threads as they are use for the general purposes .
    After enlarging the hole to specific size then we should use the internal taps to make the threads inside that hole. Firstly we have to use the taper tap to start the thread following with the second tap as we are using American thread system. After that we have to use the plug base tab to make hole fully threaded .

    Best preventive measures have to be taken my the qualified maintenance personnel’s for making internal thread of 6mm after consulting the relevant maintenance manual.

    ReplyDelete
  13. Tools are the basically equipment which are used for several purpose like cutting filing, drilling etc but while performing a task it is necessary to use tools with proper sequence and due to following instructions.

    First we have to take the sheet in which we have to make a hole then we have to place it on a table and fix it properly by bench vice to prevent from movement then we have to punch on a specific point to identify where the hole should be drilled then we drill a hole by using twist drill as we need a hole of 6mm so for the accurate length we use reamer to increase the dimension of the hole for the correct length 6 mm . After drilling holes properly now we have to made internal threads so now we use threads taps first of all we use tapper tap and made 2/3 thread then we use second tap and made thread at the end but while threading we have to clean it for accuracy and at the end we have to use plug tap for the finishing.
    We have to perform this whole procedure according to instructions

    Zohaib

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  14. To make a 6mm thick hole in a hard metal sheet we first required a surface table on which we can make an ident in a metal with the help of hammer. The punch must be struck hard enough to give an indentation large enough that we can place our dill point easily into it, whenever it is necessary to cut accurate and circular holes in a material the power operated drilling is used. These machines usually found in workshops.Hand brace drills or hand drills are also used to drill holes but it is only used to drill holes in soft materials. For hard materials we use se range of tools to create holes but the Morse type is most commonly used in aircraft engineering.Cobalt steel drills are normally used on very hard metals. When drilling the hole the points may be ground at angles between 60 degree to 90 degree depend upon the hardness of the metal.After punching the small drill (pilot drill) is used before finishing drill to dill the hole in the centre of the punch point so that a hole is properly make of the required size then the pilot drill is replaced by the finished drill which can be guided through the pilot hole to the appropriate size. When the hole is made we use hand parallel reamers which are used to enlarge drill holes to accurate dimensions and provide a smooth internal finish.After using reamers we use tapping to make the threads in the hole.Firstly we use tapper tapping in which thread cutting process of the hole will start, It is tapered about two third of the threaded length and can enter into the pre dill hole easily then we do second tapping it will follow the tapper tapping and deepened the thread. This tap is tapered for the first two or three threads only and also capable of fully threaded hole but it is not available in British standard.After second tapping the plug tapping can be done it’s only purpose is to finish the threads in deep and to cut the threads to the bottom of the blind hole.
    By following these steps we can drill the 6 mm thick hole in a hard metal sheet.The correct tapping and drilling size can be obtained from workshop charts and maintenance manuals.

    ReplyDelete
  15. Sometimes visual inspection is not enough to check the faults in the surfaces so we use other non destructive testing.
    To check the faults on the blade of engine we use eddy current flaw detection because it is more sensitive for the sub surfaces than any other testing moreover we donot require any removal of paint.
    To start test first we mount the coil on the probe and giving a alternating current of specific frequency when probe is attach with the testing material eddy current is flow in the part and produce magnetic field which oppose the original field on the coil and the surface which is in defect it show disturbance in flow.
    After that we compare it with the reference piece to check the size of fault.
    All procedure must be done according to the maintenance manuel.

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