CNC Machining Report – Premounting Base Abstract: This work involved the formulation and analysis of the machining process plan to complete

the fabrication of a ‘premounting base’ with Computer Numerical Controlled (CNC) machinery. This

is further augmented with a Numerical Control (NC) program written in ISO G-code, to machine the

part utilising CNC milling techniques.

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Table of Contents

1 INTRODUCTION 12 NC/CNCFUNDAMENTALS 23 PARTANALYSIS 33.1 GeometryFeaturesandDimensions 33.2 SurfaceRoughness(orFinish) 53.3 DimensionalMachiningTolerances 53.4 StockMaterial(Billet) 53.5 CompatibleMachiningComponents 64 PROCESSPLANNING 64.1 WorkHoldingStrategyandDatumPoint 64.2 MachiningOperations 75 CNCISOG‐CODEMACHININGPROGRAM 85.1 CoordinateSystemsChoice 85.2 CNCMachineSpecifications 95.3 CuttingToolParametersandAccuracy 95.4 G­CodeMachiningProgram 106 CONCLUSION 13A. SCHEMATICOFMACHINEDWORK‐PIECE. 15

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Table of Figures Figure1:Suppliedschematicofpremountingbase. 1Figure2:3Drenderofthefinalpart. 1Figure 3: Detail schematic (top) and sectioned­through 3D model (bottom) of the

premountbase. 3Figure4:WireframeCADmodelofthepart. 4Figure 5: Datum point on billet, first stage of milling (top­down view, above mounting

table). 6Figure6:Datumpointfordrillingtheholesinlowerlip(top­downview,abovemounting

table). 7Figure7:Schematicrepresentationofavertical(left)andhorizontalmillingcentre(right).

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1

1 INTRODUCTION

The primary objective of this report is the formulation and analysis of the machining

process plan to complete the fabrication of the premounting base, shown in Figure 1,

with Numerical Control (NC)/Computer Numerical Controlled (CNC) machinery.

Figure 1: Supplied schematic of premounting base.

Most of the engineering schematics and 3D modelling produced in this report have

been generated utilising The Parametric Technology Corporation’s Pro/Engineer

(Pro/E) Wildfire 4.0. A 3D rendering of the machined part is shown in Figure 2,

below.

Figure 2: 3D render of the final part.

The methodology included research into NC/CNC fundamentals, part analysis and

machining process planning. This study resulted in the formulation of a NC program

developed in ISO G-code to enable the machining of the above part.

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2 NC/CNC FUNDAMENTALS

The Massachusetts Institute of Technology (MIT) filed a patent application, on the

14th of August 1952, with the United States Patent office for a “Numerical Control

Servo-System”, and is considered by many as its actual date of conception. However,

NC systems required for the commands of metal work to be created by hand in the

form of punch tapes. John Runyon, had the notion of interfacing a computer with the

NC machine so that all the input was processed via the computer directly. This

resulted in the US Air Force funding the development of a generalized programming

language for NC, and thus the invention of Computer Numeric Control (Sing, 1996).

The basic tasks of metal cutting, in terms of NC, were broken down as follows:

1. Determining the location where the machining was to be performed.

2. Controlling the path during the motion of the tool or work-piece.

3. Controlling the rate at which the tool was fed into the work-piece (or vice

versa) in traversing the path.

In essence, (1) required the establishing of a coordinate system and in NC this is

known as the datum point, which is a point of reference where all Euclidean distances

are measured. With NC, (2) control over the path is primarily established via linear

or circular interpolation, whilst (3) was defined as the feed rate (Wikipedia, 2009).

NC blocks are formed of words that are split into 5 categories of commands:

1. Preparatory functions

2. Axis motion commands: X, Y, and Z commands.

3. Feed rate and speed commands: Address words F and S.

4. Identification commands: N or T word.

5. Miscellaneous commands: ‘M’ words (M8, M9)

Linear interpolation is achieved with the G1 command, whilst referencing the

intended destination as (XYZ). Rapid positioning of the tool is performed with the G0

and F commands (Sing, 1996).

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3 PART ANALYSIS

3.1 GEOMETRY FEATURES AND DIMENSIONS

The supplied part schematic is detailed in Figure 1 and in Appendix A. However, this

alone is not easy to visualise, and as such the part was modelled to specification in

Pro/Engineer to produce Figure 3 (Toogood, 2007), shown below.

Figure 3: Detail schematic (top) and sectioned-through 3D model (bottom) of the premount base.

The geometry of the part above, described quite simply, is rectangular in nature. All

the varying surfaces are, essentially, formed in layers of material that has been

“removed” from a solid rectangular block of the material.

The other striking features include the drilled holes in the upper section of the part

and in the lower lip, and their dimensions are detailed in Figure 1.

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Hence, the part’s most noticeable features are:

• Overall dimensions of 170 x 47 x 21 mm (L x W x H) of its most extreme

surfaces.

• The upper, second and lower lip form a thickness of 13 mm from the base of

the part.

• The tallest surface, with a thickness of 21 mm, is 10 mm wide.

• The upper lip is 3 mm wide and the second lip is 4 mm wide.

• The lower lip is 5 mm in width and 13 mm thick from the base of the part.

• The centre channel has a thickness of 8 mm from the base of the part and

covers an Euclidean distance of 25 mm in width, formed between the second

and lower lip.

• The two holes of Ø7 mm, drilled in the raised surface of 21 mm thickness,

have utilised core drilling.

• The four smaller holes (two in the raised surface of 21 mm thickness and two

in the lower lip) have a diameter of Ø4 mm.

The upper, second and lower lip surfaces as well as the drilled holes are clearly

depicted in the Pro/Engineer generated wireframe model, in Figure 4.

Figure 4: Wireframe CAD model of the part.

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3.2 SURFACE ROUGHNESS (OR FINISH)

The most commonly specified roughness specification, Ra, is the arithmetic mean of

absolute values of vertical deviations of the surface (Kalpakjian and Schmid, 2005),

where it is mathematically defined as,

€

Ra =1n

yii=1

n

∑ (1)

The Ra parameter is a determination of the granularity in the smoothness of the

surface (or lack thereof). Since the dimensions of the supplied schematic (Appendix

A) are stated in metric units, Ra is quoted in micrometres (µm).

In Figure 1, the following drafting symbols indicate the required smoothness of the

surfaces the pointed tip touches, and should be finished as follows,

Ra = 0.8 µm, equivalent smoothness of polishing to ~ 320 UK Grit.

Ra = 1.6 µm, equivalent smoothness of polishing to ~ 200 UK Grit.

3.3 DIMENSIONAL MACHINING TOLERANCES

The required tolerances for all dimensions are taken as ± 0.1 mm. For example, the

final part length of 170 mm will be machined to 170 ± 0.1 mm and therefore will vary

between 170.1 mm and 169.9 mm. As the supplied specification has indicated most

of the dimensions accurate to 2 significant figures, this machining tolerance would be

suitable for those particular dimensions.

3.4 STOCK MATERIAL (BILLET)

As per the supplied schematic (Appendix A), the stock material has the following

dimensions: 175 x 50 x 25 mm (L x W x H). The material used is high-grade

structural carbon steel (ISO: R683/IC45e, BS: 060A47, AISI 1045). The AISI 1045

specification states that typical hot or cold drawn variations of carbon steel have a

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Rockwell hardness of “B” and is therefore in the same (hardness) grade as most

Aluminium alloys (Gere and Goodno, 2008).

3.5 COMPATIBLE MACHINING COMPONENTS

From the above part analysis, it is clear that this work-piece will need to be machined

via milling techniques on a suitable milling centre. It is also possible to infer that four

particular tools will be required to perform the machining of this part: (i) first tool for

rough profiling the surface features, (ii) second tool for finish profiling of the same

features, (iii) third tool for core drilling (Kalpakjian and Schmid, 2005) and (iv) a

fourth tool for drilling the holes with a smaller diameter.

4 PROCESS PLANNING

Process Planning is a means for systematically identifying and specifying the needed

machining operations to machine the final part from the billet and is dependant upon

the results of Part Analysis (Lynch, 1991). Therefore, this planning will focus on

machining the work-piece in a milling centre.

4.1 WORK HOLDING STRATEGY AND DATUM POINT

The work holding strategy will include two stages, of which the first stage is shown

below in Figure 5,

Figure 5: Datum point on billet, first stage of milling (top-down view, above mounting table).

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In Figure 5, the datum point of the work-piece has been indicated on the mounted

billet, observing from a top-down perspective, as it would be clamped in place on the

machining table. It can be seen that the bottom-left most vertex (of its upper surface)

has been chosen as the datum point (also shown in Figure 7).

If the entire sequence of machining were to be performed on the work-piece, as

clamped in Figure 5, the drilling of the Ø 4 mm holes in the lower lip would require a

5-axis vertical milling centre. To allow for this part to be machined on a simpler 3-

axis vertical milling centre, the work-piece would need to be clamped into the

position on the machining table as shown in Figure 6, so as to drill the two Ø4 mm

holes located at 40 and 130 mm, along the X-axis, from the newly positioned datum

point (with their horizontal centre located 2.5 mm below the lower lip’s surface).

Figure 6: Datum point for drilling the holes in lower lip (top-down view, above mounting table).

4.2 MACHINING OPERATIONS

As per the features of the part in Figure 3, the machining techniques involved will

require the following milling operations:

• RECOMMENDED MACHINE TOOL: vertical milling centre.

• CUTTING TOOLS: milling cutter (1) for the rough profile,

(2) for the finish profiling, (3) for core

drilling two holes of Ø7 mm and (4) for

drilling four holes of Ø4 mm.

• FIXTURES: clamping work-piece to machining table.

• CUTTING PARAMETERS: rough and finishing profiles.

• NC/CNC MACHINE: CNC vertical milling centre.

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The choice of cutting tools have immediately dictated that the entire machining

process will be performed with only two milling techniques: (i) repeated passes of

linear interpolation with a milling cutter and (ii) canned drilling at select points. The

CNC vertical milling centre has been chosen as the machine tool to complete the

machining task, as CNC is inherently far more flexible than NC machines due to their

integration with computer and software control rather than that of simple

Microcontroller control (Morton, 2005).

5 CNC ISO G-CODE MACHINING PROGRAM

5.1 COORDINATE SYSTEMS CHOICE

The chosen coordinate system depends on the type of chosen milling machine, and

can be divided into three categories (Smid, 2007):

• THE NUMBER OF AXES: typically three or more.

• THE ORIENTATION OF THE AXES: vertical or horizontal.

• TOOL CHANGING FACILITY: automated changing of cutting tool.

Image Source: (Smid, 2007)

Figure 7: Schematic representation of a vertical (left) and horizontal milling centre (right).

The most common form of milling centre has a minimum of three axes, comprising of

the three orthogonal Cartesian axes, with a vertical orientation. As such, this is the

chosen coordinate system.

As per the process planning phase, the datum point is also shown in the left image of

Figure 7, and as such the (X, Y, Z) axes are zeroed to this particular vertex, and is

therefore given the coordinate (0, 0, 0).

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5.2 CNC MACHINE SPECIFICATIONS

Part Analysis and Machining Process Planning have determined that a 3-axis vertical

milling centre is required. As such, its operational characteristics and requirements

(Krar, Gill and Smid, 2000) have been defined accordingly, as follows,

1. MACHINE THROUGHPUT: ≤ 2 parts per hour.

2. MATERIAL HARDNESS: Rockwell “B”.

3. SURFACE FINISH: Standard to performance (as per specification).

Considering typical specifications of a vertical milling centre, the following criteria

for selection are further imposed,

• MACHINE-TOOL TYPE: CNC vertical milling centre.

• AXIS DRIVE: minimum of three (3) orthogonal axes (X, Y, Z).

• MAX. TRAVEL – AVAXIS: ≥ 300 mm.

• MAX. TRAVEL – AVAXIS: ≥ 300 mm.

• MAX. TRAVEL – AVAXIS: ≥ 300 mm.

• ACCURACY: within ± 0.005 mm (as per specification).

• NUMBER OF TOOLS: minimum of four (4).

• MAX. TOOL DIAMETER: ≥ 20 mm.

• I/O FUNCTIONS: spindle speed, feed rate and coolant control.

• FEED RATE: typically, 2 – 500 mm/min.

• SPINDLE SPEED: typically, 60 – 5,000 rev/min.

• SPINDLE POWER: typically, 2 – 5 HP.

5.3 CUTTING TOOL PARAMETERS AND ACCURACY

The chosen milling cutters should be of high performance Cobalt/HSS, as they will be

able to easily machine through Rockwell “B” grade material and stay sharp for quite

sometime. For the rough profile a Ø18 mm cutter is to be used, and a Ø20 mm cutter

for the finish profiling.

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The rough profile is to be performed with a slower spindle speed and faster feed rate

of the cutting tool. In contrast, the finishing profile will run the spindle at a higher

RPM, but with a much slower feed rate, thereby ensuring an extremely smooth cut.

Other factors also come into consideration, such as availability of coolant during

cutting to not only provide lubrication but also to remove any shavings in the nearby

vicinity of the cutting (Bray, 2003).

Whilst the emphasis of accuracy is a factor of the capability of the particular CNC

centre being used, a skilled operator should be able to make adjustments to minor

deviations in cutting tolerances. The CNC machining program has been designed

with cutter compensation for a Ø20 mm cutting tool, and as such high quality tools

are recommended.

5.4 G-CODE MACHINING PROGRAM

To machine the final part the sequencing of operations are defined, in metric units of

millimetres, relative to the datum point of the co-ordinate system of a CNC vertical

milling centre with a minimum 3-axes of control (X, Y, Z).

DESCRIPTION OF TOOLS: Tool 1: Ø18 mm cutter, Tool 2: Ø20 mm cutter, Tool 4: Ø4

mm drilling bit and Tool 7: Ø7 mm core drilling bit.

% % ROUGH PROFILE WITH 18 MM DIA MILLING CUTTER N000 G90 G21 N010 G97 G94 M06 T01 %T01 18MM DIA N020 M03 S200 F150 M08 %REDUCE BILLET LENGTH TO 170 MM N030 G00 X180 Y50 F0 N040 G00 Z-25 F0 N050 G01 Y-5 F150 N060 G00 Z50 F0 N070 G00 X-10 Y0 F0 %REDUCE BILLET WIDTH TO 47 MM N080 G00 X-10 Y57 Z50 F0 N090 G00 Z-25 F0 N100 G01 X200 F150 N110 G00 Z50 F0 N120 G00 X-10 F0

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%MILL BILLET THICKNESS TO 21 MM (25-4=21) N130 G00 Z-4 Y47 F0 N140 G01 X200 F150 N150 G01 Y27 F150 N160 G01 X-10 F150 %MILL LOWER LIP TO 13 MM THICKNESS (25-12=13) N170 G00 Z-12 Y0 N180 G01 X200 F150 N190 G00 Z50 N200 G00 X-10 Y0 F0 %MILL 25 MM CHANNEL OF 8MM THICKNESS (25-17=8) N210 G00 Y20 Z-17 F0 N220 G01 X200 F150 N230 G01 Y15 F150 N240 G01 X-10 F150 %MILL LIP OF 4 MM WIDTH (13 MM THICKNESS) N250 G00 Z50 F0 N260 G00 X-10 Y24 F0 N270 G00 Z-12 F0 N280 G01 X200 F150 N290 G00 Z50 F0 %MILL UPPER LIP OF 3 MM WIDTH (13 MM THICKNESS) N300 G00 X-10 Y54 F0 N310 G00 Z-12 F0 N320 G01 X200 F150 N330 G00 Z50 F0 % % SMOOTH PROFILE WITH 20 MM DIA MILLING CUTTER N340 M06 S500 F50 T02 %T02 20MM DIA %REDUCE BILLET LENGTH TO 170 MM N350 G00 X180 Y50 F0 N360 G00 Z-25 F0 N370 G01 Y-5 F50 N380 G00 Z50 F0 N390 G00 X-10 Y0 F0 %REDUCE BILLET WIDTH TO 47 MM N400 G00 X-10 Y57 Z50 F0 N410 G00 Z-25 F0 N420 G01 X200 F50 N430 G00 Z50 F0 N440 G00 X-10 F0 %MILL BILLET THICKNESS TO 21 MM (25-4=21) N450 G00 Z-4 Y47 F0 N460 G01 X200 F50 N470 G01 Y27 F50 N480 G01 X-10 F50

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%MILL LOWER LIP TO 13 MM THICKNESS (25-12=13) N490 G00 Z-12 Y0 N500 G01 X200 F50 N510 G00 Z50 N520 G00 X-10 Y0 F0 %MILL 25 MM CHANNEL OF 8MM THICKNESS (25-17=8) N530 G00 Y20 Z-17 F0 N540 G01 X200 F50 N550 G01 Y15 F50 N560 G01 X-10 F50 %MILL LIP OF 4 MM WIDTH (13 MM THICKNESS) N570 G00 Z50 F0 N580 G00 X-10 Y24 F0 N590 G00 Z-12 F0 N610 G01 X200 F50 N620 G00 Z50 F0 %MILL UPPER LIP OF 3 MM WIDTH (13 MM THICKNESS) N6300 G00 X-10 Y54 F0 N640 G00 Z-12 F0 N650 G01 X200 F50 N660 G00 Z50 F0 %CANNED DRILLING AT X35 (7 MM DIA & 4 MM DIA) N670 G00 X35 Y39 F0 N680 M06 T07 N690 G98 G81 Z-18 N700 M06 T04 N710 G98 G81 Z-25 N720 G80 %CANNED DRILLING AT X135 (7 MM DIA & 4 MM DIA) N730 G00 X135 Y39 F0 N740 M06 T07 N750 G98 G81 Z-18 N760 M06 T04 N770 G98 G81 Z-25 N780 G80 N790 G00 X-10 Y0 Z50 F0 %STOP CUTTER AND COOLANT N800 M09 M05 % %MANUALLY REPOSITION WORKPIECE AND RECALIBRATE DATUM POINT %CANNED DRILLING AT X40 AND X130 (4 MM DIA) N810 M06 T04 N820 M03 S500 F50 M08 N830 G00 X40 Y10.5 Z20 F0 N840 G98 G81 Z-10 N850 X130 N860 G98 G81 Z-10 N870 G80 %RETURN TOOL TO SAFE POSITION AND SHUTDOWN N880 G00 X-40 Y-10.5 Z50 F0 M09 M05 M30

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6 CONCLUSION

The NC program could be improved in terms of the overall Euclidean distances

travelled in completely machining the part, as well as increasing the feed rates

without compromising in quality. A further improvement to the NC program would

be the inclusion of cutter radius compensation via the G41, G42 and G40 commands.

As the developed program is for a 3-axis CNC vertical milling centre, time will be

needed to re-mount the work-piece and re-align the datum point. Therefore, this

system would only be able to produce approximately 2 – 3 single part sets per hour, or

less.

Adapting the program for use on a 5-axis CNC milling centre will remove the need

for manual interference during program execution, and therefore increase production

throughput.

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REFERENCES i Bray, S. (2003) Metalworking Tools and Techniques, Wiltshire, UK: Crowood

Press.

ii Gere, J.M. and Goodno, B.J. (2008) Mechanics of Materials, 7th edition, Toronto, Canada: Cengage Learning, Inc.

iii Kalpakjian, S. and Schmid, S. (2005) Manufacturing, Engineering and Technology, 5th edition, Upper Saddle River, NJ: Prentice Hall.

iv Krar, S., Gill, A. and Smid, P. (2000) Computer Numerical Control Simplified, New York, NY: Industrial Press, Inc.

v Lynch, M. (1991) Computer Numerical Control for Machining, New York, NY: McGraw-Hill.

vi Morton, J. (2005) The PIC Microcontroller: Your Personal Introductory Course, 3rd edition, Oxford, UK: Newnes.

vii Singh, N. (1996) Systems Approach to Computer-Integrated Design and

Manufacturing, Somerset, NJ: John Wiley & Sons, Inc. viii Smid, P. (2007) CNC Programming Handbook, 3rd edition, New York, NY:

Industrial Press, Inc.

ix Toogood, R. (2007) Pro/ENGINEER Tutorial Wildfire 4.0, Mission, KS: Schroff Development Corp.

x Wikipedia (2009) Numerical Control, http://en.wikipedia.org/wiki/CNC, Date

accessed 24 March 2009.

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A. Schematic of Machined Work-piece.

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