Hobby-Level CNC Milling Machine Design

MIE243 - Design Project

Date: Wednesday December 3, 2014

Submitted By: James Chen - 1000008289
Gina Kim - 1000581587
Julian Lai - 1000478459
Gabriel Mangalindan - 1000478486
Christian Paniccia - 1000759761
Shaishav Shah - 0999511899
Submitted To: Dr. Matthew Mackay
Executive Summary:

CNC Milling is the computer automated process of milling a material. A user inputs information
into a G-Code program which then dictates to the computer program how to go about milling a
material. The purpose of this document is to detail the research and design process our team
embarked on in order to design a CNC Milling Machine of our own. Detailed in this document is
research our group has done, including benchmarks from other designs currently on the market.
From these designs and the constraints given to us in the project document, our team was able to
construct a set of engineering specifications which lists the quantitative and qualitative attributes
our final design should achieve. Most importantly, the final design should be for a hobby leveled
user, meaning that it should be lightweight, inexpensive, and materials used should not be
difficult to obtain.

After detailing our engineering specifications, our team brainstormed our candidate designs. The
first candidate design was a machine which operated on five axes. However, we deemed it too
complicated and too expensive.. We limited the freedom of motion to three axes (xyz) and
continued from there. Each of the subsequent candidate designs were three axes machines, with
the freedom of motion of each part either being constrained or allowed to move. The final design
was a milling machine with a fixed gantry, table which moves in the x direction, and spindle
which moves in the y & z direction. This design minimizes the amount of moving parts, reduces
the cost of our design, and ensures that minimal forces are acting on our machine, thus ensuring
a longer lifespan.

The final design consists of a spindle that is coupled directly to the motor using a direct drive,
powered by a three phase induction motor. The table and spindle are positioned by lead screws,
which are powered by separate stepper motors on each axis. Finally, the frame of the design is
made of aluminum to ensure that it is lightweight and resistant to external forces.

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Table of Contents:
1.0 Project requirements 5
1.1 Introduction 5
1.2 Benchmark Designs 6
1.3 Engineering Specifications 8
2.0 Candidate Designs 9
2.1 Candidate Design 1 9
2.2 Candidate Design 2 12
2.3 Candidate Design 3 14
2.4 Candidate Design 4 16
3.0 Research 19
3.1 Axis 19
3.2 Spindles 20
3.2.1 Spindle Bearings 20
3.2.2 Spindle Types 21
3.2.3 Drill Bits 21
3.2.4 Material of Drill Bit 22
3.2.5 Drill Shape 22
3.2.6 Spindle Motors 23
3.2.7 Spindle Accessories and Add-Ons 25
3.3 Bearings 25
3.4 Linear motion components 26
3.5 Motors 27
3.5.1 Stepper Motors 27
3.5.2 Servo Motors 28
3.6 Milling Machine Table and Vice Components 29
3.6.1 T-Slot Dimensions Selection 30
3.6.2 Vacuum Table 33
3.6.3 Choice of Material 34
3.6.4 Size Estimate 34

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4.0 Final Design Selection 35
4.1 Final Design Component Analysis 36
4.1.1 Linear Motion Components 36
4.1.2 Spindle 38
4.1.4 Table 40
4.1.5 Frame 40
4.2 Technical Drawings 41
4.3 Cost Analysis 44
5.0 Appendices 45
5.1 Table 45
5.2 Machine Base 46
5.2.1 Base Bars 47
5.3 Gantry 48
5.3.1 Gantry Bars 49
5.3.2 Gantry Holder 50
5.3.3 Spindle Holder 51
5.4 Motor Drawings 52
References 54

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1.0 Project requirements
This section of the report outlines the scope of the design project by describing the problem
presented and the design objectives specific to the project.

1.1 Introduction
The objective of this project is to design a hobby-level CNC milling machine. Hobby-level CNC
milling machines are rapidly growing in popularity, especially with the advent of related
machines such as 3D printers and rapid-prototyping suites.

The details of the design are outlined by Dr. Matthew Mackay for MIE243: Mechanical
Engineering Design. Our main goal is to propose a detailed conceptual design that has the
potential to be developed into a working prototype. In order to produce a hobby-level product,
the final design should not require an expensive or materials that are difficult to obtain. The final
design should also be sized accordingly in order to fit comfortably within a home workspace.

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1.2 Benchmark Designs
We started by analyzing a wide range of commercially available CNC machines. The following
four are ones we chose to use as benchmark designs.

Model Walter 2520L [1][3] Stepcraft 420 [2]

Overall Dimensions 61 x 46 x 14 cm 43.5 x 56 x 28 cm

Travel Range 61 x 46 x 8.9 cm 42 x 30 x 8 cm

Number of Axis 3 3

Cutting Rate 100 mm/sec 32 mm/sec

Travel Rate 150 mm/sec 50 mm/sec

Spindle Speed 0-10,000 rpm N/A

Precision 0.00635 mm 0.04 mm

Materials PCB, wood, plastic, aluminum wood, thermoplastics, aluminum,

brass, copper

Weight 40 lbs 26.5 lbs

Method of Holding Aluminum “T” slot extrusion Plastic Clamps

Material of Main Body Welded aluminum Aluminum

Cost about $4995 about $1000

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 Model 4 Axis CNC3020 [4] CNC Shark [5][6]

Overall Dimensions 41 x 54 x 44 cm 61 x 85 x 57 cm

Travel Dimensions 20 x 30 x 9 cm 33 x 61 x 11 cm

Number of Axis 4 3

Cutting Rate 51 mm/sec 75 mm/sec

Travel Rate 60 mm/sec 85 mm/sec

Spindle Speed 0 - 13,500 rpm 16,000 - 35,000 rpm

Precision 0.01 mm 0.0254 mm

Materials wood, plastic, aluminum wood

Weight 62 lbs N/A

Method of Holding Aluminum “T” slot extrusion Plastic Clamps and two hold-down
slots

Material of Main Aluminum Alloy 6061/ 6063 Steel, high-density polyethylene

Body

Cost about $945.25 about $2899.99

In-depth research is covered in section 3.0

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1.3 Engineering Specifications
The following engineering specifications were determined based on the market research that was
conducted. The metrics for the objectives of the design are based off the highest values for
commercially available, hobby-level CNC milling machines. This was purposely done to avoid
limiting possible design solutions.The final design must be a hobby-level product meaning it
should be a high performance CNC machine, but should not require unreasonably expensive
materials to operate.

1. Should be simple to assemble; must be a hobby-level product:

● Must not require access to exotic or expensive materials to build
● Must have maximum cost of: $3000
● Should require minimum maintenance over lifetime
● Should use standardized, easy to obtain replacement parts
2. Must have cutting precision of at least 0.04mm
3. Must be able to produce cuts in 3-dimensional space; must operate using at least 3 axis of
movement
4. Must have a maximum size of: 80cm x 50cm x 50cm (x, y, z)
5. Must not exceed maximum weight of 100 lbs
6. Should have quiet operation

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2.0 Candidate Designs
This section defines the 4 possible design solutions that accomplish the function of a hobby-level
CNC machine. All candidate designs adhere to the previously highlighted engineering
specifications.

2.1 Candidate Design 1

5-axis milling machine with a fixed gantry; table moving in x&y direction, gantry moves in
a, b, z direction.

In this candidate design, the work table moves in the X and Y direction in a plane, actuated by
two leadscrews powered by 2 stepper motors. The spindle motor and the spindle, mounted
vertically, move in the Z direction(up and down) again actuated by a leadscrew powered by a
stepper motor. The table consists of two sets of motion conversion mechanisms and two motors
to achieve movement in the two axes.

There is also another set of motion conversion components for the spindle. Additionally, the
spindle also undergoes A-B motion, through 2 servo motors, resulting in two additional sets of
motion conversion mechanisms. The size is designed to be within our engineering specifications
limits. This design models many of the industrial cnc mills on a smaller scale, and will use
standardized parts to lower the cost and maximize component replaceability.

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Page 10 of 56
Linear motion component choice Lead screw with anti-backlash nut

Type of Motor Stepper motor for positioning, 2 for the table,

1 for gantry. A 3 phase AC synchronous
motor for the spindle motor. 2 servomotors for
A-B direction movement

Size (cm) 45 x 35 x 45

Gantry height (cm) 40

Strength ● Compact design

● More detailed and precise resolution
possible due to 5-axis
● More versatility and degrees of
freedom.
● Less time required to shape material

Weaknesses ● More wear due to more moving parts

● More expensive to construct and
maintain, due to additional parts.
● Larger motors and more moving parts
● Congested motor spacing in the gantry
with 2 servo motors and 1 stepper
motor in close proximity
● Heavier gantry due to additional
motors
● Table needs strong support to be stable
while in cantilevered position
● Needs extensive bracing to cut metal.

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2.2 Candidate Design 2
Milling Machine with a moving gantry in x&y direction, and spindle in z direction

In this design, the table is fixed and the gantry is responsible for movement in the x, y, and z
directions of the spindle through the use of stepper motor. The same applies to motion of the
spindle along the z-axis.

Specific to this candidate design is that the gantry will move along the x-axis. Motion will be
actuated by two ball screws powered a stepper motor. A larger motor will be required to actuate
the moving gantry along the x-axis, as more power will be required to move a heavier object (ie
the entire gantry). This will also result in the speed of motion of the moving gantry to be
significantly slower.
Motion of the spindle along the y and z-axis will be actuated by a ball screw powered by a
stepper motor.

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Movement for gantry Ballscrew

Type of Motor 3 stepper motors for 3 axis

induction motor for spindle

Machine Size (cm) 45x45x45

Table Size & travel (cm) 35x35

Gantry Height (cm) 50

Others Only gantry moves

Strengths ● Relatively inexpensive compared to table motion in x & y as well as 5-

axis
● Due to fewer moving parts compared to the 5-axis, there will be less
wear
● Design of table is relatively simple due to its motion being completely
fixed
● Fixed table means more stable cutting of material

Weaknesses ● Requires suitable bracing in order to safely cut metal

● The three axes limits the versatility and freedom of movement of the
overall CNC machine
● Less cutting precision than the 5-axis design (resolution)
● Gantry limits the size of material that can be cut in two dimensions, the
y and z axes
● Requires more bracing to accommodate for high axial forces created by
ball screws on both sides of gantry
● Large motor will be required to move the weight of the gantry along
the x-axis with precision
● Weight of gantry will result in slower movement along x-axis
● Larger motor results in more initial costs and maintenance

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2.3 Candidate Design 3
Milling machine with a fixed gantry; table moving in x&y direction, and spindle in z
direction

In this candidate design, the work table moves in the X and Y direction in a plane. The spindle
motor and the spindle, mounted vertically, move in the Z direction(up and down). The table
consists of two sets of motion conversion mechanisms and two motors to achieve movement in
the two axes. There is also another set of motion conversion components for the spindle. The size
is designed to be within our engineering specifications limits. This design models many of the
market available CNC mills and will use standardized parts to lower the cost and maximize
component replaceability.

Due to the inherent design of this CNC machine, the complexity of finished parts this machine is
able to achieve is not as high as the 5-axis set up.We believe that since this is meant to be a
hobby level CNC machine it is justified to stay within these limits, as designing a machine that

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produces highly complicated and intricate parts would incur in heavy price penalties.

Linear motion component choice Lead screw with anti-backlash nut

Type of Motor Stepper motor for positioning, 2 for the table,

1 for gantry. A 3 phase AC synchronous
motor for the spindle motor

Size (cm) 45x35x30

Table Size & Travel (cm) 28x28

Strength ● Compact design

● lots of room to mount a motor, less
constrained compared to other designs

Weaknesses ● Tall, could be unstable or would need

to be secured to the table (or whatever
surface it’s placed on)
● Table will extend past the base of the
machine, potentially long overhangs
● Less space for x and y axes
● The additional manoeuvrability can
give the table additional stress
● More constrained space for the linear
motion systems
● Limited design freedom compared to 5
axis

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2.4 Candidate Design 4
Milling Machine with a moving table in x-direction, moving spindle in y&z direction, and
fixed gantry

This candidate design utilizes a moving table in the X direction and a fixed gantry with a spindle
that can move in both Y and Z direction. There will a pair of motion conversion mechanisms and
motors to move the table, and two more on the spindle. The benefit of transferring the Y component
of motion to the spindle from the table is that the stress in the system is more evenly distributed
throughout the components and therefore, will wear at a slower rate than other designs. Another
benefit is that the motor required to move the spindle in the Y direction as opposed to the table
would be much smaller which will result in a lighter and more compact machine.

A potential flaw with this design lies in the positioning of the gantry supports on the cutting table.
Since this component is stationary, it prohibits the dimensions of the table to exceed the width of
the gantry supports. This issue is not necessarily fixable, but larger dimensions for the entire CNC
machine can be chosen to compensate for this fallback. Additionally, this design does not have
the same drill head flexibility as a 5-axis design. As a result, it will not be possible to drill diagonal
holes without changing the mounting orientation of the material in question.

Addressed briefly in the pros and cons, the cost for this machine should be relatively low compared
to all of the other candidate designs. Other than a few wearing parts, this machine optimizes the
amount of material required to support the drilling mechanism. By using the fewest number of
motors compared to the other candidate designs, the cost can be kept to a minimum since the
motors will likely be the one of most expensive components of the machine.

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Linear motion component choice Lead screw with anti-backlash nut

Type of Motor 3 stepper motors for 3 axis

induction motor for spindle

Machine Size (cm) 45x45x45

Table Size (cm) 25x25

Others Table moves in x and spindle in y & z

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Strength ● Relatively inexpensive compared to table motion in x & y as well as 5-
axis
● Due to fewer moving parts compared to the 5-axis, there will be less
wear in comparison
● The fixed gantry allows for greater structural integrity and as a result,
higher precision
● Easy to scale relative to the 5-axis and the moving gantry
● Is better suited to cut into harder materials than with a moving gantry
● Easy to maneuver the moving table component as compared to the
entire gantry
● Smaller motor required to move the spindle on the y-axis than the
entire table

Weaknesses ● Requires suitable bracing in order to safely cut metal

● The three axes limits the versatility and freedom of movement of the
overall CNC machine
● Less cutting precision than the 5-axis design (resolution)
● Gantry limits the size of material that can be cut in two dimensions,
the Y and Z axes

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3.0 Research
Research was conducted on the following components in order to determine the different types
of of parts and designs which currently exist on the market today. The quantitative and
qualitative specifications of these parts were researched in order to fulfill our engineering
specifications. Varying judging criteria were used to decide which type of part to use for our
candidate designs. For some components price was the biggest issue, and a cheaper part which
could still fulfill a function would be deemed the best; however, for other components, a more
expensive part may have been chosen due to the quality being an essential design aspect for our
machine.

3.1 Axis
The amount of freedom for axis allows for more precise and complicated objects to be milled.
Milling machines have 3, 4, or 5+ axis of rotation.

3-Axis: This is the simplest design for milling machines. Movement occurs in the x,y,z direction
and is accomplished by either the table moving, or gantry, or both. This design is the cheapest,
and easiest to maintain due to the smaller number of moving parts.

5-axis: The five axis of rotation is similar to the 3-axis, in that it moves in the x,y,z direction,
however it also provides axis of rotation of the table. This increases the complexity of object
designs which can be milled.

The benefits of a 5-axis system is that less time is required to cut the material to the specified
shape. This includes rough cuts and surface finishing [7].
The drawbacks to a 5-axis system are that more components are involved to achieve additional
rotational motion. This will add considerably to overall price including initial cost and cost of
maintenance.

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3.2 Spindles

The type of spindle required depends on the type of materials that will be milled. For our design,
wood, light metal, and polymer materials will be milled. Each type of material requires a spindle
with a certain torque, speed, and horsepower. However, these specs are all related to the different
aspects of the spindle.
The spindle bearings are what allow the spindle to rotate at high rpms. There are a variety of
bearing systems and types of materials for bearings which affect the temperature, vibration
levels, and life of the spindle

3.2.1 Spindle Bearings

The two types of bearing systems used are angular systems and roller systems. The angular
contact ball bearings are more suited for supporting the cutting metal due to their precision and
axial and radial load carrying capacity. The cylindrical bearings offer a higher load capacity and
greater stiffness.

There are mainly two types of materials used for the bearings:

Steel Bearings These are standard bearings. Cheap, and

heavier than ceramic bearings.

Ceramic Hybrid Bearings These bearings are preferred in spindle

motors. There are approximately 60% lighter
than metal bearings. Ceramic bearings do not
suffer from cold welding which occurs with
metal bearings. Also, due to their near perfect
roundness, ceramic bearings are able to work
at much lower temperatures than metal
bearings, which in turn is better for the
spindle lubrication. Additionally, ceramic
bearings are more rigid, which reduces the
amount of vibration in the spindle [8].

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Lubrication of Bearings: There are a number of lubricants available for bearing systems. These
include oil-mist, oil-air, oil-jet and pulsed oil-air. Some of these lubricants are essential for
spindle speeds exceeding 18,000. However, they are mostly expensive and cause degradation of
spindle systems. As a result, because our spindle will not be operating at rpms of that calibre, a
self lubricating system will be used. Lubrication should last for the approximate lifespan of each
bearing, ensuring that bearings will be lubricated throughout their lifetime and maintenance costs
will be reduced.

3.2.2 Spindle Types

Types of spindles vary by their driving force system. The different types include belt driven, gear
train, and inline spindles.
Belt Driven: These spindles are easy to maintain, quiet, and low cost [32].
Gear Train: Much more expensive than belt driven, noisier, requires more maintenance.
Direct Motorized Spindle: These spindles are coupled to the motor allowing the drill bit to
achieve a higher speed. They are less expensive than gear trains and have quiet operation [32].

3.2.3 Drill Bits

The drill bit of the spindle is important because different spindles are used to cut different
materials, to varying degrees of precision. Additionally, some spindle drill bits are made of
materials which are more resilient and have a longer lifespan, but as a trade off are more
expensive. The three things to look for with spindle drill bits are the type of drill bit, the shape of
it, and the material from which it is made of.

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3.2.4 Material of Drill Bit

Material Pros (All are able to cut Cons

through wood and polymers)

High Speed Steel - costs the least The only cons of the
- cuts through most metals materials of the spindles is
the increasing cost as
Carbide - harder than HSS hardness increases. As a
- can handle a faster feed rate result, High Speed Steel
- lasts longer than HSS would most likely be the
best material to choose.
Titanium Nitride - harder than carbide

Diamond - hardest material in the world

3.2.5 Drill Shape

The are two common shapes of spindles being the V-shaped end mill and the roughing end mill.
The V-shaped end mill is useful for engraving and doing detailed work on more delicate
materials such as wood, while the roughing end mill is used for milling materials at high speeds
like metal. As a result of their different applications, both drill bits will be compatible with our
design.

V-Shaped End Mills Roughing End Mill

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Type of Drill Bit: The type of drill bit refers to the way the spindle mills a material [9].
Some of the questions we considered are:
● Are the chips ejected down or up?
● The difference between the two could decide whether a material splinters or not.

Type of Spindle Pros Cons

Up Cut ● Chips ejected upward ● Possible surface

and bottom of material splintering
is smooth

Down Cut ● Chips ejected ● Possible surface

downward and top of splintering
material is smooth

Compressed Cut ● Smooth surface on top ● N/A

and bottom
● Perfect for pre
laminated woods

3.2.6 Spindle Motors

The spindle motor is perhaps the most important part of the spindle as it allows the device to
spins at RPMs necessary to cut certain materials. There are three main types of motors for the
spindle, all of which are electric.

These include the single phase induction motor, the three phase induction motor, and a brushless
DC motor.

Single Phase Induction Motor: This is a very simple motor comprised of gears and pulleys. It
is the cheapest of the motors and only has one variable speed. CNC mills typically need a
variance of speeds in order to mill the material. As a result, the single phase induction motor is
not suited for our design.

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Three Phase Induction Motor: The three phase induction motor is considered one of the most
reliant electric motors types. Unlike the single phase induction motor, three phase induction
motors are compatible with a variable frequency drive (VFD). The VFD allows the motor to run
at a variety of speeds, with the max speed being a result of the type of VFD used. The
disadvantages to the motor are few, and it accomplishes our engineering specifications of
providings enough RPMs to cut all materials (wood, metal, polymers).

Variable Frequency Drive: A variable frequency drive is a controller which is able to vary the
frequency provided to the motor. In turn, the frequency received is translated into a certain motor
speed, thus providing varying speeds. This device is useful as it allows our spindle to operate at
varying speeds when milling material.

Black box method of VFD and AC Motor. (Electrical input is converted into Mechanical
Power) [10]

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3.2.7 Spindle Accessories and Add-Ons
A spindle can have a number of accessories attached to it as well, which can improve the
performance of the machining, but also increase the cost. The two add-ons our group has
researched are cooling liquids and vacuums. These attachments, however, raise the price of the
design, and since the design is a hobby level machine, they are deemed unnecessary. If the
material being milled needs to be cooled, the user may cool their material manually with a
coolant spray, and if the workstation becomes messy, he can easily clean it off.

Standard Spindle Interior [7]

3.3 Bearings
For the linear support systems we will employ cylindrical linear bearings that slide along
cylindrical shafts. These rails will resist the vertical forces applied to them caused by the weight
of the parts they support. They also provide support for secondary loads such as torque or lateral
loads should any be generated during operation.
There will also be bearings at the end of the leadscrews acting as support. Since there are no
significant axial forces at play, regular ball bearings will be used.

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3.4 Linear motion components
The linear motion system refers to the components used to convert the rotational input from the
motors to linear movement required in our CNC machine. The proper selection of components is
crucial in ensuring the movement in the axes are smooth and accurate. The following table lists
the possible component options available on the market and their pros and cons.

Type Pros Cons

Lead screw ● Common. Relatively cheap ● Backlash, but can be

● Self-locking, do not require a eliminated with an anti-
braking system backlash nut.
● Good power transfer ● Potentially high friction
● Could possibly consider using a and heat in continued use
regular threaded rod for cheap ● Shorter lifespan vs. ball
price screw
● Better suited for vertical ● Requires greater torque,
operations lower efficiency vs. ball
[12] [13] [14] screw [19]
[11]

Ball screw ● Very little friction and wear ● Very expensive

● Good anti-backlash properties ● Ball nuts must be greased
● Require low torque, high and sealed
efficiency [19] ● Not self-locking, requires
● Offer incredible speed while braking systems [19]
maintaining good power transfer
[15]
[13]

Rack and pinion ● Can provide high speed ● Lower strength and
● Has auto-tensioning properties. stability compared to
● Easy to maintain lead/ball screw
● For routers cutting soft parts ● Multiple parallel racks
(plastic and woods), no need to might be hard to align and
gear down for torque = fast speed tune to match each other
● Geared more aggressively to ● Moving motor requires
better utilize the low-end torque more cable management
[17] of stepper motors, and is also ● Trade-off for strength
more mechanically efficient than
ACME screw systems [15] [16]

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 Direct drive (belt or ● High speed capability ● Bad at taking high loads
chain) ● Inexpensive (used for laser cutters
● Low maintenance usually)
● Limited metal cutting
abilities
● Belt can behave somewhat
elastically when moving at
high speeds
● Belt/chain can whip and
[18] resonate at high speeds
● Requires tensioning [15]

After analyzing the pros and cons of all the options, we narrowed our choices down to lead
screws and ball screws. We feel that these two offer better fit with our design requirements
compared to the direct drive and the rack and pinion systems.

While ball screws provide higher efficiency and lower friction compared to lead screws, they
cost significantly more and lack self-locking properties. The lack of self-locking properties
requires us to include a braking mechanism in our design, increasing complexity and cost.
On the other hand, lead screws’ backlash issue can be easily countered with anti-backlash nuts.
Given that our machine is considered a hobby level design, the other cons of the lead screws’ are
not as critical, making the lead screws the ultimate choice.

3.5 Motors
The type of motor plays an important role in not only the movement of the components, but also
in the positioning and accuracy of the CNC machine itself. There are two leading motors that
deal with positioning, the Servo Motor and the Stepper Motor.

3.5.1 Stepper Motors

Unlike the well-known DC motors which simply spin when voltage is applied, the stepper motor
divides one rotation of 360° into numerous smaller steps to achieve precision in positioning. One
of the biggest benefits of this motor is that they are controlled electrically, in an open loop
system and therefore do not require expensive feedback devices [20]. A drawback with open

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loop systems is that if the motor is not properly sized for the load there is a possibility of it
missing a step [21].

In the case of a hobby level CNC the operator will most likely be using a computer to control the
stepper motor and its motion. The indexer will tell the motor exactly what to do in terms of how
many pulses it sends out to change the frequency, acceleration and speed of the motor. The rest
of the steps are the actual motor driver [22].

3.5.2 Servo Motors

Similarly to the Stepper Motor, Servo Motors use electrical energy to produce precise motion
from increments of rotation. The main difference between the two lie in the closed loop system
of the Servo Motor. In closed loop systems encoders count the number of steps taken by the
motor and match them to the number of step commands given to ensure that the position is
correct. This leads to a more accurate and precise result however the addition of encoders and
other components vastly increases the price of the motor [22].

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Table 1. Comparison of Servo and Stepper Motors

Motor Type Pros Cons

Servo ● Higher accuracy and precision due ● Higher complexity requires

to closed-loop system more components
● Good for rpm > 2000 ● Overall higher cost
● Can withstand high torque ● Larger size

Stepper ● Easier maintenance and ● Lower accuracy and precision

commission due to open-loop system;
● Open-loop system is more simple chance of missing a step if not
and cheaper properly sized for load
● Overall lower cost ● Generally not used for rpm >
● Low rotor inertia allows for 2000
medium to high acceleration ● Torque degradation at higher
● Can withstand high torque speeds (2000 < rpm)
● Stable at rest and holds position
without fluctuation

3.6 Milling Machine Table and Vice Components

Most CNC machines will be equipped with a milling

table with characteristic “t-slots” that run along its
length.

As depicted in the image to the left, depending on the

size of the milling machine, the table can be quite
narrow and might not be very deep. In general, the
depth of the table block will be above four times the
depth of the t-slots. [23]

This milling machine is fitted with manual cranks that allow the movement of the table in the x
and y axes, but will not be included in our design since this will all be automated.

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The image to the right provides a better understanding
at the geometry of the t-slots used for mounting external
vices. In any cases, “t-nuts” will be custom
manufactured for the best fit within the slots, but it is
possible to use any other flat nut that also fits in this
space.

This sketch shows the rough measurements of the cross

section of a t-slot for a specific machine. In general, the
size of these slots will not vary by a great degree between
machines since their only function is to provide stability to
vice attachments.

These slots may be made smaller to accommodate size

constraints of small machines, or larger allowing for larger
bolts to be used in order to accommodate for heavy loads.

3.6.1 T-Slot Dimensions Selection

The diagram on the right pertains to the measurements
displayed in the chart below. These are standard
dimensions for t-slots used in most milling machines.
In practice, it is possible to use custom dimensions for
these slots, but since many retailers sell standardized t-
slot nuts, it would be best to avoid custom sizing for
the sake of convenience.

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The sizing chart below should be referenced when manufacturing a milling table outfitted with t-
slots [24].

Vices are mounted to the milling table via bolts, nuts, and t-
slots. The vice to the left is an example of a standard vice used
to hold the majority of cutting materials. It will often be
mounted onto a circular base that allows for freedom of
rotation, leading to better cutting angles for the material. When
using this type of vice, it is advisable to follow the proper
mounting procedures in order to prevent cutting errors due to
slipping [25].

The machine table shown on the right is a variation of

the ones mentioned above. This table is able to swivel
on its semi-circular base, adding an additional pseudo
cutting-axis. This table can be used in conjunction with
the vice in the image above to allow for numerous
mounting configurations that could assist the automated
drill head in making difficult cuts.

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 The vice on the left is called a universal angle milling vice
and is basically a combination of the vice and table mentioned
above [26].

The benefit of using this vice is that it provides multiple axes

of freedom in one apparatus. However, the main drawback is
that this vice might not be able to handle heavier items or
higher cutting loads since there are so many moving parts.
This vice can be mounted on most milling tables as the cut-
away spots in the base allow for t-slot mounting [27].

Other stabilization items may also

be mounted to the table using the t-
slots. These may include strap
clamps (the apparatus on the centre
and right), and v-blocks for keeping
cylindrical items in stable
equilibrium [28].

The final design should incorporate

enough flat space on the table to
allow for most additional support
items to be mounted.

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3.6.2 Vacuum Table
One other type of milling machine table that exists is the vacuum table. This table uses pneumatic
power to secure items to the table by taking in air through many small holes in its surface. The
benefits of this form of ‘clamping’ is that there actually is not any clamping involved. The catch
and release transition times are drastically minimized from that of the vices, since this mechanism
only requires the user to hit a switch to turn the air pump on or off rather than turning a crank.
Despite its benefits, this table will not be used in the final CNC milling machine design due to a
variety of drawbacks. The main issue with a vacuum table is that it does not allow much versatility
with
the

shapes of materials that can be cut. In general, only large, flat surfaces can be reliably secured to
the table surface using this method, which denies the user the ability to work with curved or small
objects. Additionally, any impurities on the surface of the material can greatly affect the ‘grip
strength’ of the table on the item in question, posing a threat to the overall quality of the cut and
the safety of the user. The final drawback is that an air pump is required for the operation of this
table, which is inconsistent with the ‘hobby-level’ specifications of cost effectiveness, and size for
the CNC milling machine [29].

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3.6.3 Choice of Material
Using the Cambridge Engineering Selector and other online resources, aluminum was chosen to
be the primary material used in the construction of the milling table. Aluminum is resistant to
corrosion, has high tensile strength, and is able to withstand large compressive forces. Although
this material can be heavy due to its high density, it is much cheaper than most other materials
per unit mass. Additionally, the final CNC design will not be designed for portability, but rather
for cost effectiveness and durability.

3.6.4 Size Estimate

Since the entire machine is estimated to have a footprint of roughly 45 x 45cm (17.7 x 17.7
inches), the table should have a length of no longer than 38cm (15 inches), a width of no wider
than 25cm (10 inches), and a depth of about 5cm (2 inches) to accommodate for the t-slots with
which it will be fitted. The weight of the table will be approximately 12.8kg.

Any additional movement in the direction of its width can be addressed by using various vices
and the moving spindle. The table can also adequately deal with most movement in the direction
of its length, as it will be mounted to a linear-motion device.

Page 34 of 56
4.0 Final Design Selection
To choose our final design we used an iterative process demonstrated in our candidate designs.
We started off with the 5-axis design as it allows for the machine to make complex cuts
alongside the ability to cut the material more efficiently and precisely. Ultimately, the 5-axis
candidate was too complex. Additionally, this design did not meet the specifications of being
hobby level where simplicity of assembly and cost effectiveness are emphasized. Despite the fact
that the five axis CNC machine provides a more precise resolution, it was deemed unnecessary
since another model would also be able to meet our specifications.

To improve on the 5-axis design, we simplified the amount of moving axes to three (X,Y and Z
directions). In this design the gantry moves in the X and Y direction, while the table is fixed,
with the spindle moving in the z direction. While this design is not as precise as the five axis
machine, it improves on the reliability of the machine as it has fewer moving parts, thus less
wear. Additionally, the cost of this design would be much lower than that of the five axis design.

The third candidate design is one in which the gantry is fixed, with the table moving in the x
direction, with the spindle again moving in the y & z direction. This improves on the second
design as in this case the table is moving, while the gantry is fixed. In the previous design with a
moving gantry there were greater axial loads placed on the milling machine. This is due to the
gantry having an external moment on the machine due to being a distance away from the
machines centre of gravity. Fixing the ganty also increases precision along the x axis. The table
moving in the third candidate design accommodates for this as the table could easily move
without providing any additional external moments. Thus, this would address the engineering
specification of the design being reliable and having a long lifespan.

In our final candidate design our gantry is again fixed, however we constrained our table in along
the y-axis, allowing it to only move in the x direction. This then allows us to move our spindle in
the y&z direction. The reason for this change stems directly from the spindle being lighter and
smaller than the table. The spindle is easier to move, thus it should be doing most of the motion
in our design, with the table only being responsible for one direction of motion to ensure that the

Page 35 of 56
most complicated part, the gantry, remains fixed. This design is deemed most reliable as it
minimizes external stresses. Also, as a result of a fixed gantry, there is more bracing available for
cutting materials such as metal. Additionally, due to the smaller spindle being the component
with the most freedom of motion, our design can be more compact and best fits the engineering
specification of being hobby level and within a certain dimensions/weight.

4.1 Final Design Component Analysis

This section explains each component recommended for the final conceptual design. The
justification for selecting each component is based on the above research and the estimated cost
is provided.

4.1.1 Linear Motion Components

The following are the list of selected components to allow for motion in the x, y, and z direction.

Motion Support

Component Quantity Justification

Medium Carbon 6 Steel shafts support motion along x, y, and z axis. 2 shafts
Steel Shafts per axis provide extra support and rigidity for paths of
motion. Steel was selected as the material is long lasting,
and relatively cheap and easy to maintain.

Linear Bearings 10 Linear bearings constrain motion along the shafts and
allow ease of linear motion. The use of two bearings per
shaft provides proper support in all directions and
distributes the load evenly.

[31]

Page 36 of 56
 Ball Bearings 3 Ball bearings are used to fix position of lead screws to
frame and support their rotational movements. Since there
should be no significant axial forces applied to these
bearings, we opted to use regular ball bearings as they are
cheap, common, easy to maintain, and fit the function of
the design. Without the ball bearings the lead screws would
not be able to spin freely and smoothly.
[31]

Motion Facilitation

Component Quantity Justification

Steel Lead Screw 3 Lead screws actuate motion along x, y, and z axis. After
analyzing the pros and cons of all the options, we narrowed
our choices down to lead screws and ball screws. While ball
screws provide higher efficiency and lower friction
compared to lead screws, they cost significantly more and
lack self-locking properties.
The backlash issue experienced by lead screws can be
countered with anti-backlash nuts. Given that our machine
is considered a hobby level design, the cost and function
both meet the highlighted engineering specifications.

[31]

Stepper Motor 3 Stepper motors are used to power the lead screws. These
motors are used to position the table and spindle. The
stepper motor allows for a discrete rotation of the lead
screw. The size of this discrete rotation is directly
responsible for the precision of linear steps experienced by
[31] the table and spindle.
Both stepper and servo motors have their respective pros
and cons; however in regards to our specifications, the
stepper motor is a better choice. This decision was based on
the design of a hobby level CNC machine, therefore
minimizing cost, complexity and maintenance is of the
highest priority.

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4.1.2 Spindle
The following is the list of components that facilitate the cutting of material.

Spindle Type

Component Quantity Justification

Three Phase 1 A three phase induction motor will be used as the motor
Induction Motor / which powers the spindle rotation. A three phase induction
Variable motor was chosen instead of a single phase induction motor
Frequency Drive
due to a three phase induction motor being capable of being
compatible with a Variable Frequency Drive. This
combination of motor and VFD allows a AC asynchronous
motor to operate a variable speeds. A single phase induction
motor operates at a single fixed speed which is directly
related to the torque of the motor. Thus, it is not preferable to
[31] use this type of motor as milling a variety of materials
requires a variety of speeds.

In the end, a three phase induction motor coupled with a

variable frequency drive allows our spindle to operate at a
variety of speeds which allows the spindle to spin at various
RPMs. It is also capable of producing a high enough RPM
that meets our engineering specifications.

BT 30 Direct 1 The direct drive spindle was selected as it was more cost-
Drive Spindle effective than a gear train at amplifying the AC motor speed.
Higher speeds allow for a greater variety of material to be
cut, and less vibration and wearing on the bracing.

Currently, our Three Phase Induction motor operates at

roughly 1500 RPM. The designs which we benchmarked
operate at rpms exceeding 12000. Thus, in order to achieve
[31] these speeds with our current motor, a 10:1 gear ratio would
be needed. Due to the limitation of our space, a planetary
gear would be needed to achieve this aforementioned ratio. A
direct drive system however eliminates this costly alternative
while still achieving the required speed for our design.

Page 38 of 56
 ER 32 Precision 1 An Er 32 Collet allows for attachment of the drill bit to the
Collet spindle. The size is suitable for a variety of drill bits that will
be used and is standardized for use in conjunction with CNC
milling machines. The chuck is made out of hardened steel to
allow for stability and ease of maintenance.

[31]

Ceramic Hybrid 1 Ceramic hybrid bearings are approximately 60% lighter and
Bearing more rigid than metal bearings, reducing the amount of
vibration on the spindle. Also, due to their near perfect
roundness, ceramic bearings are able to work at much lower
temperatures than metal bearings, which in turn is better for
the spindle lubrication.
[31] This bearing operates inside the direct drive spindle.

HSS Roughing 1 The 3-flute flat end mill should be versatile enough to
end mill (3 flute perform a wide range of cuts. Due to the complex nature of
flat-end mill) end mill selection and the extremely large selection of end
mills to perform different functions, the operator is expected
to install the specific end mill that he deems best fit the
operation at hand.

[31]

The spindle will be driven by a process known as an inline spindle. An inline spindle is a spindle
which is driven directly by the motor. The way it accomplishes this is because the spindle is
coupled directly to the motor. This design is superior to a gear driven spindle as a gear train
which drives the spindle adds to the cost of the design and the amount of moving parts. A gear
driven spindle requires more maintenance due to the higher amount of moving parts, including
replacing gears and the addition of lubrication. Thus, it is preferable to use an inline spindle as
these requirements are a non-factor. Additionally, an inline spindle operates at a lower volume,
meeting one of our engineering specifications of having a quiet footprint.

Page 39 of 56
Two add-ons which our group has researched are cooling liquids and vacuums. These
attachments however raise the price of the design, and since the design is a hobby level machine,
they are deemed unnecessary. If the material being milled needs to be cooled, the user may cool
their material manually with a coolant spray, and if the workstation becomes messy, the operator
can easily clean it off.

4.1.4 Table

Component Quantity Justification

Aluminum [specify 1 Aluminum was selected as the material is light-weight and

dimension] T-slot cost-effective.
Table The T-slots are used to fix the material to the table, and
increase the return precision of the cut. The stepper motors
rely on incremental steps to achieve accurate positions, and
the less the material moves, the more accurate the cuts will
be.

4.1.5 Frame

Component Quantity Justification

Aluminum Base 1 Aluminum was selected as the material is light-weight and

cost-effective. Aluminum greatly increases the lifespan of
the machine as it is highly resistant to compressive and
tensile forces as well as corrosion-resistant.

Aluminum Gantry 1 Same justification of aluminum as stated above.

Cap Screws 6 Required to assemble and fix the frame of the CNC milling
machine.

Page 40 of 56
4.2 Technical Drawings

Page 41 of 56
Page 42 of 56
Page 43 of 56
4.3 Cost Analysis
The average density and cost per unit mass of aluminum are 2700kg/m3 and $2.465/kg respectively
[30]. To find the cost estimate for each design component, the density, volume, and cost per unit
mass should be multiplied. This method provides a rough final price without considering negative
space or manufacturing costs. All material costs were determined using the Cambridge
Engineering Selector [30] and the McMaster-Carr database [31].

Component Quantity Cost (USD)

Aluminum Base [Appendix 5.2] 1 $24

Aluminum Gantry [Appendix 5.3] 1 $13

Aluminum T-slot Table [Appendix 5.1] 1 $32

Cap Screws 6 $2.20 x 6 = $13.20

ER 32 Precision Collet 1 $33.74

BT 30 Direct Drive Spindle 1 ~$500

Ceramic Hybrid Bearing 1 ~$7.23

3 flute flat end mill 1 $25.61

Three-Phase AC Motor [Appendix 5.4] 1 ~$250

Lead Screws 3 ~$13 x 3 = $39

Stepper Motor [Appendix 5.4] 3 ~$65 x 3 = $195

Medium Carbon Steel Shaft [Appendix 5.2.1 & 6 $4

Appendix 5.3.1]

Linear Bearing 10 $17.57 x 10 = $175.70

Ball Bearings 2 $5.60 x 2 = $11.20

Ball Bearing 1 $3.17

Total Cost

$1326.85

Page 44 of 56
5.0 Appendices

5.1 Table

Page 45 of 56
5.2 Machine Base

Page 46 of 56
5.2.1 Base Bars

Page 47 of 56
5.3 Gantry

Page 48 of 56
5.3.1 Gantry Bars

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5.3.2 Gantry Holder

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5.3.3 Spindle Holder

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5.4 Motor Drawings

Shown here are 2-D drawings of the stepper motors and AC Induction Motors which are similar
to the types used in our final design. These drawings are the property of McMaster-Carr and are
renderings of devices found on their website.

Stepper Motor [31]

Page 52 of 56
Three Phase AC Motor [31]

Page 53 of 56
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