Mm3216 milling tutorial

School of Mechanical andSchool of Mechanical andAeronautical EngineeringAeronautical Engineering

CNC Milling

MM3216 Computer Aided Machining

Prepared by Mok Chai Pui


Unit 1

Essentials of CNC Milling


Machine Information

MAZAKTROL MATRIX NEXUS 410A-II Vertical Machining Centre

X560 mm, Y410 mm, Z 510 mm

30 tools tool magazine Maximum rotating speed is

12000 rpm


Machine Axes and Coordinates

3 mutually perpendicular axes

Table movement by X and Y axes.

Spindle movement on Z axis.

http://www.youtube.com/watch?v=AKwlzIJG5lo


Basics of Milling

Milling is performed with a rotating, multi-edged cutting tool which performs programmed feed movements against a workpiece in almost any direction.

Each of the cutting edges remove a certain amount of metal, with a limited in-cut engagement, making chip formation and evacuation a secondary concern.

Milling is applied to generate flat faces, in most cases. However, with 5-axis machines and form cutters, it can cut in many shapes and forms.


What is a machine Coordinates System, MCS?

It refers to the physical limits of the motion of the machine in each of its axes and the numerical coordinate which is assigned (by the machine tool builder) to each of these limits.

A reference point for the workpiece coordinate system to refer to.


What is a workpiece Coordinates System, WCS?

Used to define the geometrical shape and size of the workpiece with all dimensions referring to the zero point.

Defines the intersecting zero point, X=0, Y=0 and Z=0


Workpiece Coordinates System (WCS) Settings and Programming

6 workpiece coordinates systems (G54 to G59)

Every workpiece need to set its own WCS using the work offsets.

The work offsets registers the distance from the MCS to WCS for each axis.

http://www.youtube.com/watch?v=EI2inCb0Wfs


Relationship of Workpiece Coordinate System (WCS) to Machine Coordinate System (MCS) In CNC machining, it is necessary to register the

position of the workpiece zero point (X,Y,Z) with reference to the machine zero-point (X,Y,Z). This is to enable the machine to locate the position of the workpiece to the machining zone.


Tutorial

Standing in front of the machine, if you want to bring the workpiece TOWARDS YOU, you have to move the table in the

(a) +X direction (b) -X direction

(c) +Y direction (d) -Y direction


Tutorial

Standing in front of the machine, if you want to bring the tool TO THE LEFT, you have to move the table in the




Tutorial

Standing in front of the machine, if you want to bring the tool TOWARDS YOU, you have to move the table in the




Tutorial

Standing in front of the machine, if you want to bring the workpiece TO THE RIGHT, you have to move the table in the




Tutorial

Drilling of a hole involves only one axis, which is the _______ axis.

Profile milling involves two axes movement simultaneously; the two axes are ____ and _____ axis.

Z

X Y


Tutorial

How does the machine locate the workpiece when executing NC programs?

After established the relationship between the WCS and MCS, the workpiece is located using any one of the 6 coordinate systems, G54, G55 …… G59.


Tutorial

When executing a CNC program to cut a part on a 3-axis CNC milling machine, we can observe that the

(a) machine table moves according to the programmed path.

(b) machine spindle moves according to the programmed path.

(c) machine spindle and the machine table move simultaneously according to the programmed path.

(d) machine spindle and the machine table move sequentially according to the programmed path.


Unit 2

Face Milling & Profile Milling


Face Milling Process

Applied to generate flat faces.

One or more of the following cutting actions will be involved: Radial Peripheral Axial

Face milling is on the periphery with some extent on the tool.


Speeds

Cutting Speed, m/min The surface speed at which the cutting edges pass the

workpiece surface. It depends on tool and work material used. Parameters are available from catalogues and handbooks.

Spindle Speed, rpm The number of revolutions at which the milling cutter

rotates on the spindle per minute.

1000

DNVc


Tutorial

39.27 m/min

509 rpm


Feed

Feed/tooth, mm/tooth It depends on the recommended maximum chip

thickness value that the tool removes for a specific work material.

Number of teeth Available number of teeth on a milling cutter, it depends

on the diameter of the cutter. It is used to calculate table feed.

Feed/minute, mm/min Also known as table feed, Feed/min=feed/tooth x no. of teeth x rpm


Tutorial

A 45 mm diameter end mill with 4 flutes is used in a face milling operation. If the recommended spindle speed is 2450 rpm and the feed rate is 0.05 mm/tooth. What is the feed rate in mm/min?

A 16 mm wide slot is milled using an end mill of the same diameter. The length of the slot is 86.5 mm. If the feed rate is 200 mm/min, what is the required machining time to finish the slot?

Feed (mm/min)=rpm *feed/tooth * no. of teethFeed = 2450 * 0.05 * 4Feed = 490 mm/min

Time required to cut = 86.5*60/200 = 25.95 seconds.


What are some of the considerations when performing face milling operation?

Include a small radial corner when changing direction. Keep the tool in full contact with the workpiece while

cutting. As the tool enters and exits cutting edges, it may break

the inserts. Avoid milling over holes. Machine holes after the facing operation. Reduce the recommended feed rate by 50% when

cutting workpieces with holes. Avoid dwell and chatter.


Contour Milling (Profile Milling)

Cutting along the contour of a workpiece which may consists of lines and arcs.

Depth of cut remains constant in 2D contours.


What is approach and retract?

Approach and retract profile parameters are also called engage and disengage or lead-in and lead-out.

The value selected is crucial and usually set tangential to the first point of cut and last point of cut.

The approach parameter is used to specify the line length and arc radius for the tool to enter the machining boundary.

The retract parameter is used to specify the line length and arc radius for the tool to leave the machining boundary.

It is important to plan how the cutter will make initial contact with the workpiece.


How many types of approach and retract?

There are three common ways for the cutter to enter and leave the workpiece: without approach or retract. with approach and retract using

arcs modes. with approach and retract using

arcs and lengths modes.


What could be the problem when there is no retract and approach?

The cutter will start plunging directly at the start point of the contour cut.

End Mill will break if there is no cutting edge at the centre.

Cutter mark will be left on contour as there is a momentarily stop at the start/end point of the contour. The tool has to stop the movement in the axial direction before picking up the feed rate of the contour cut.


Circular Approach & Retract

Normally starts and ends at the mid-point of an entity if it is a closed contour

Circular arc is usually of 90o movement Depends on the radius of

approach/retract, the tool will plunge outside the job

There is a smooth transition when approach and exit from the contour. A good surface finish could be attained.

Distinct positions for approach and retract


Approach and Retract with Circular arc and straight line moves

Similar benefits as in the circular approach and retract method

Start and Exit can be at the same position. A pre-drilled hole could be made to facilitate the cutter plunging if necessary.


(Straight Line) Approach and Retract for open profile

Tool starts and exits away from the contour Straight line tangential to the entry/exit geometry.

This avoids cutter mark on the workpiece


What is Cutter Compensation?

Cutter compensation is also called tool position or offset. Important when performing contour milling or profile

milling. The cutter centre is offset to the specified side of the

programmed path with a value that is entered into the control.

With this, the programmer is able to program the profile of the workpiece without considering the diameter of the tool used.

Different diameters of tools can be used for the machining operation without affecting the program.


Example of cutter compensation

To produce a part 80mm square.

Using tool of 25 mm dia. Toolpath running along an

80 mm square.

http://www.youtube.com/watch?v=EVlm8aOtk6I


Example of cutter compensation

The actual tool will never be exactly 25mm dia. The part will therefore end up with different dimensions.

The programmer does not know what exactly is the tool size

The tool size is dynamically changing due to cutter wear.

Nominal size 25mm tool may not be available at the shop floor. It may have been signed out by other users


Cutter compensation G41

Tool left, (G41): the cutting edge of the tool to the left side of the workpiece contour.

G41G40

Normal Programmed Path

Compensated Cutter path


Cutter compensation G42

Tool right (G42): the cutting edge of the tool to the right side of the workpiece contour.

Tool on, (G40): the cutter centre moves on the workpiece contour. This will result in undersize workpiece produced. Application example include: slot cutting, engraving.

G42

Normal Programmed Path

Compensated Cutter path

G40


Compensated tool path

Cutter dia 25mm, offset value is 12.5mm

Reference to the workpiece zero point, w/p corner is 27.5 mm away.

With no offset, the cutter will move to X-27.5, the size of the workpiece will be reduced.

With offset, the cutter will move to X-40 to produce correct size of the workpiece.


Explain what is stepover distance?

Each roughing method uses a stepover value. The stepover specifies the amount the tool moves over for each cut in the roughing toolpath.

You can enter a stepover value as either a percentage or a distance.

When you set a stepover percentage, a good starting point is 50% to 75% of the tool tip (or minor) diameter.

The stepover distance shows how large a step is being taken.


Effect of stepover distance

A wide stepover distance, there will be lesser passes and machining time will be shorter. To an extent, there may be uncut material left behind.

A narrower stepover distance, there will be more passes and machining time will be longer.


Tutorial

A student has to carry out a Face milling and Contour Milling on a workpiece of size about 100 x 100mm. T03 (Ø16 endmill) is to be used in contour milling. He claims that he can achieve time saving by using the same tool to do the face milling as there is no tool change is required. Comment on the student’s claim.

With a 60mm Face Mill, 4 passes will be required to cover the whole area with default setting on MasterCam.

With a 16mm Endmill, 10 passes will be required to cover the whole area with default setting on MasterCam. This leads to a much longer machining time for the facing operation.


Tutorial

The Aluminum part shown has to be machined from a rectangular stock. Student ‘A’ suggests to mill the steps prior to drilling. Student ‘B’ says that it is better to drill all the holes first followed by milling of steps. Evaluate the pros and cons of the two approaches.


Solution

Student ‘B’ – Drill all the holes prior to milling of steps It could be a little easier for programming as drilling all start at

the same plane. There is no worry that the drill will hit the shoulder on the stepped block. However, the drilling time will be much longer as all 3 holes are equally deep. When the milling cutter cuts over the holes, interrupted cuts occur. This leads to chattering and shorten tool life.

Student ‘A’ – Mill steps prior to drilling This is the preferred planning. It leads to shorter processing

time and better tool life in both drilling and milling operations. The student has to make sure that there is sufficient retraction distance to avoid the drill collision with the stepped block.


Tutorial

An experienced process engineer said that it is no good to use Ø16mm endmill to do the contour cut on the following alumimum part shown below. Explain what could be his reasons. What is the recommended tool diameter to be used?


Solution

The smallest radius on the profile is R8. This means that the Ø16 endmill will be in full contact with the part when performing contour cut at that position. An excessive contact area between the tool and the workpiece leads to vibration and poor surface finish. It will also be difficult to control the dimension.

It is recommended the cutter to be used has to be slightly smaller than the radius that it is going to cut. Possible suggestion could be Ø12.


Tutorial

Refer to the question above, an endmill is registered on the machine with Nominal Ø12 and Actual Ø12mm. After the finishing cut, a measurement of 90.05mm is noted across the contour. The part is oversized. What could be the main reason? What has to be done to achieve the desirable dimension 90mm?


2 main possible reasons:

1. Tool may be deflected in the cutting process due to the cutting forces and machine rigidity. As the tool bends away from the work material, it cuts less and ends up with oversized parts.

2. Tool may have worn off. It is smaller than 12mm. The difference 90 – 90.05 = -0.05mm The Tool Offset, Actual diameter

should be input as 12 – 0.05 = 11.95mm Carry out another cutting pass to

yield the desirable dimension.


Tutorial

When the cutter diameter increases, what will happen to the table feed (mm per minute)?

a) The table feed increases if the cutting speed and feed per rev remians unchanged.

b) The table feed decreases if the cutting speed and feed per rev remians unchanged.

c) The table feed remains unchanged if the cutting speed and feed per rev remains unchanged.

d) The table feed increases if the cutting speed and feed per rev also increase.


Unit 3

Engraving and Pocketing Operations


What is engraving?

Metal engraving is the process of cutting a series of lines and arcs into the surface of a metal object or plate to form a design, image or words.

CNC machines are commonly used in this process as the controller can follow the contour of the word or the image closely.


Engraving tools

Engraving tools are usually small in diameter and they are made from solid carbide for maximum tool strength.

As the tip is relatively weak, the plunge feed rate has to be lower than an endmill.

Applications of engraving can be easily found in plastic injection moulds, jewellery, plagues, medals....


What is a pocketing process?

Pocketing process is to remove a volume of material in a cavity.

It may involve the drilling first and then opening up through long-edge milling.


What tools can be used for pocketing?

Slot drill - made from HSS or solid carbide. The longer cutting edge extends right to the centre which allows centre cutting or plunge milling.

Inserted Carbide Endmill – both inserts are of the same size. There is no cutting action at the centre. It does not support plunge milling


What are the 3 basic methods in machining a cavity? Discuss the factors governing your choice.

a. Pre-drill a starting hole Applicable in most cases, disadvantage is that additional

processes are required prior to cavity milling.

b. 2 axis ramping Applicable in machining cavities which are rectangular in

nature.

c. Helical ramping Applicable in machining cavities which are circular in

nature.


Methods 1: pre-drill a starting hole

A starting hole is drilled prior to milling. The endmill is always fed into the cavity

at the starting hole position. The tool does not require centre cutting

capability. The endmill machines the cavity from the

start hole spiralling from inside out. Applicable in most cases. Disadvantage

is that additional processes are required prior to cavity milling.


Method 2: 2 axis ramping

By linear ramping with X and Z axis, an insert endmill is able to feed into the cavity at a small angle, typically 3 to 5o.

After reaching the cut depth, it will cut in the XY plane. Upon completion, it will ramp to the 2nd cut depth.

Applicable in machining cavities which are rectangular in nature.


Method 3: Helical ramping

The tool is fed in a helical path in the axial direction of the spindle.

It is very useful if the cavity is too small for 2 axis ramping.

Typical application is to machine holes with large diameters.

It is recommended that the diameter of the hole is about twice the diameter of the cutter.

Applicable in machining cavities which are circular in nature.


Name some strategies for removing material in a cavity.

Depends on the shape of cavity, different roughing patterns can be applied to clear stock material in a cavity.


What is an island in a pocket? What is a gouge?

Islands are areas inside the pocket boundary that are not intended to be cut during pocketing.

In machining a pocket with islands, the size of the tool may not machine the space between two islands or between an island and the pocket wall. This is known as a gouge.

A gouge happens when the gap between the islands is smaller than the cutter diameter.


Tutorial

An island is a part of the material remaining ____________ after the machining operation.

A gouge happens when the cutter is ________ than the gap between the island and the wall of the contour.

uncut

bigger


What are the differences among Slot Drill, End Mill and Inserted Endmill? State the respective applications. Slot Drills are usually of 2 cutting flutes. They are rather short in length and

rigid to cut in the axial direction. They are good to start machining slots or cavities without pre-drilled holes.

End Mills are usually having 4 or 6 cutting flutes, depending on the tool diameter. They are relatively longer in length.

Some of them could perform cutting in the axial direction. One of the cutting edges is longer than others so as to cut material up to the centre of the tool.

Some of the End Mills have a cavity at the centre of the tool which does not allow axial feeding.

Inserted Endmills have usually 2 to 4 Indexable Carbide inserts mounted onto the Endmill.

The number of cutting edges depends on the tool diameter. The Inserted endmill usually does not support axial cutting. Ramping at an angle of 5 degree is normally used in machining cavities.

Inserted Endmill usually start from Ø10 onwards.


Question 10

After milling a cavity, you noticed that there is a slight error in the width of the cavity. Which of the following is a possible solution?

changing the tool offset register G41 should be changed to G42 G42 should be changed to G41 can use only G40


Unit 4

Drilling


What are the considerations when drilling?

HSS twist drills can be TiN coated or uncoated. Common size ranges from 0.2 to 20mm.

Important to maintain the temperature on the tool tip. When the drill is cutting deep into the work material, coolant may not be able to reach the cutting zone. Thus the temperature arises which causes the drill to lose its cutting edge. Rubbing will start rather than cutting which leads to subsequent drill breakage.

Swarf removal is also becoming more difficult when a deep hole is drilled.

To overcome the problem, Chip breaking or Peck drilling technique could be employed.


Spot drilling (G81)

90o NC Centre drill is used for spot drilling.

It is much shorter than twist drills, thus provides the rigidity to achieve the positional accuracy of holes.

The tip angle could also serves as chamfer on drilled holes.

What should be the depth of spot drilling if a chamfer of 1mm is required on a Ø8 hole? What is the size of NC centre drill used?


Chip Break Drilling (G73)

Drilling starts at the top surface until the peck depth is reached, the drill will retracted for a short distance (Retract amount). This prevents formation of continuous chip which is dangerous, difficult to dispose and may obstruct coolant reaching the drill tip.


Peck Drilling (G83)

Similar to Chip Break Drilling, except that the drill is fully retracted to the clearance height which is above the work top surface. It gives sufficient time for the coolant to maintain the drill tip temperature and the swarf to be washed away from the drill flutes.


Rigid Tapping (G84)

Small sizes of threaded holes are done by machine tapping.

The tap is fed into the work with a feed rate (mm/rev) which is equal to the pitch of the thread.

Upon reaching the bottom, the tap is reversed at the same rate back to the start point.


Reaming (G85)

Simple drilling will not be able to produce very precise holes and surface finish.

In order to achieve holes of H7 dimensional accuracy and surface finish better than Ra0.8, reaming is to be done after drilling.

The reamer cuts with feed in and out of the hole while drilling has rapid retract.


Reaming (G85)

To produce a hole of Ø8H7 (+0.015/+0) to a depth of 20mm, the typical steps could be: NC Spot drill to depth 4.5mm, this will leave 0.5mm

chamfer on the reamed hole Peck Drill to a depth of 25mm with a twist drill Ø7.8mm Ream to a depth of 21mm, (1mm is the chamfer on the

end of the reamer)


Tutorial

A 9 mm dia spot drill is used to drill a centre hole with a depth of 3.5 mm, what is the dia of the hole when the spot drill is finished? 7 mm

What should be the depth of spot drilling if a chamfer of 1.5 mm is required on a Ø8 mm hole? 5.5 mm

Tapping an M8x1.5 threaded hole to a depth of 10 mm. How long does it take? (Given the spindle is rotating at 350rpm) Feed/min=1.5*350=525mm/min Time required =10/525*60=1.14seconds


Tutorial

Using a drill 10 mm in diameter to drill a through hole in a block of metal 35mm thick, the depth of the hole need to be programmed as _________ mm. (Given that the included angle of the drill is 118 deg, and answer in 2 decimal point.)

Depth=5/(tan59)=2.88 Allowance=0.5 Z=-38.38


Tutorial

You need to drill a 20 mm diameter hole in mild steel with a high speed steel drill. You look in a cutting conditions handbook and find the recommended speed is 100 m/min. What speed in rpm should you use?

N=(100*1000)/(3.1416*20)=1592rpm


Tutorial

A student working on FYP has to drill a series of Ø3 holes on an aluminium plate at a pitch of 20mm accurately. He makes use of a CNC Milling machine to carry out the task. However, he still could not achieve the pitch distance consistently. Explain what could be the problem and what will be your recommendation.

Possibly, the Ø3 mm twist drill wandered off from the desirable position. To drill holes at specific positions, he must do spot drilling first. The spots will lead the 3mm drill in the subsequent operation.


Tutorial

Identify the four different types of holes in the drawing.

Countersink hole Counterbore hole Threaded hole Reamed hole


Drill depths

Drill Depths

1. Centre drill

1. Peck drill

2. Centre drill

2. Peck drill

2. Counterbore

3. Centre drill

3. Peck drill

3. Tapping

4. Centre drill

4. Peck drill

4. Reaming

1 2 3 4


Drill depths

Drill Depths

Centre drill -1 -4

Peck drill -1 -43

Centre drill -2 -4 or ?

Peck drill -2 -14

Counterbore -2 -2

Centre drill -3 -5

Peck drill -3 -18

Tapping -3 -15

Centre drill -4 -6

Peck drill -4 -45

Reaming -4 -21

1 2 3 4


Tutorial

Match the command on the left to the description on the right.

G73 Rigid Tapping

G81 Peck Drilling

G83 Spot Drilling

G84 Reaming

G85 Chip Breaking

Mm3216 milling tutorial

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Transcript of Mm3216 milling tutorial