Lesson topics CNC Machining Center Setup and Operation Lesson 1: Basic machining practices required for machining centers Copyright 2009 Shop safetyShop

Lesson topics

CNC Machining Center Setup and OperationLesson 1:

Basic machining practices required for machining centers

Copyright 2009

Shop safety Shop math Introduction to blueprint reading Tolerance interpretation Measuring devices

Lesson Topics

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Copyright 2009

A machine shop is a dangerous place! We cannot prepare you for every dangerous situation You must always be alert

Safety equipment Protective eyewear Clothing Hearing protection Safety shoes Helmets Gloves Respirators and facemasks First aid kits Fire extinguishers Cranes and hoists Warning signs

Shop safety

Safety practices When in doubt, ask! Raw material handling

Heavy, sharp, greasy, slippery, dirty, sharp edges, chip residue, awkward shape

Finished workpiece handling In addition to raw material handling:

Hot Spring

Tightening and loosening fasteners Getting around in the shop Behave in a professional manner

No horseplay! Machining-center-specific safety issues

Safety interlocks, guarding, signs, safe operating procedures

Next topic: Shop math

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Calculator recommendations Keep it simple Watch out for trick functions Avoid solar powered calculators Big buttons-big display Clear entry button

Next topic: Blueprint reading

Shop math

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Arithmetic operations

Most CNC operator work can be done with the four basic arithmetic operators:

Add (+): Subtract (-):

Multiply (X or *): Divide (/):

3 + 4 = 77 - 4 = 32 * 9 = 18 20 / 2 = 10

Entries into CNC machines are done in decimal format (fractions are never used)

1/8 = 0.1253/16 = 0.1875

You may be able to do some calculations in your head

20 / 2 = ? (20 divided by 2)

2.5 + 0.5 = ?

3.0 - 0.25 = ?

5.25 + 8.75 = ?

109.75 - 3.5 = ?

But don’t hesitate to use your calculator if you’re in doubt

125.352 - 13.837 = ?

Shop math


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Arithmetic expressions

An expression is a math question to be answered…

3 + 4 = 7

Question

Answer

4 + 6 – 2 = ?

7 – 3 + 5 = ?

10 * 2 / 5 = ? (10 times 2 divided by 5)

Shop math


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Priority of arithmetic operations

Consider this expression:

4 + 12 / 4 = ?

What answer did you come up with?

16

16 divided by 4 is 4, right?

But wait a minute – there is another way to do this…

4 + 12 / 4 = ?

3

4 plus 3 is 7

Which answer is correct?

Shop math


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With no other math “punctuation”… … division has a higher priority than addition

4 + 12 / 4 = ?

3

4 plus 3 is 7

The correct answer is 7

Shop math


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Parentheses can be used to specify operation order

( )

(4 + 12) / 4 = ?

This makes it clear that 4 must be added to 12 first. Then the result (16) is divided by 4. The answer is 4.

Order of arithmetic priorities:

1) Anything in parentheses

2) Functions (like square root, sine, cosine, tangent)

3) Multiplication

4) Division

5) Addition

6) Subtraction

3 * (4 + 2) = ?5 + 0.5 / 2 = ?(8 + 2) / 5 = ?

7 + 9 / 2 = ?4 + 1 / 2 - 2 = ?8 * (2 + 3) / 2 = ?

Shop math


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Using a formula

A formula is an arithmetic expression that contains variables

rpm = 3.82 * sfm / tool diameter

rpm represents spindle revolutions-per-minute

sfm represents speed in surface-feet-per-minute

tool diameter represents the diameter of the cutting tool

With this formula, consider this situation:

You need to drill a 0.5 in diameter hole and the cutting tool manufacturer recommends 80 surface feet per minute.

rpm = 3.82 * 80 / 0.5

What rpm will you use?

Shop math


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Measurement systems

Some companies in the United States use the Imperial system

Other companies use the Metric system (the Metric system is used almost exclusively in other countries)

The inch is the most basic unit of distance in the machine shop

The millimeter is the most basic unit of distance in the machine shop

How big is an inch?

A deck of playing cards is about ½ an inch

How big is a millimeter?

The thickness of a quarter-dollar is about one millimeter

One inch = 25.4 millimeters One millimeter = 0.0397 inches (about one-twenty-fifth of an inch)

Shop math


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Measurement systems

To convert millimeters to inches: To convert inches to millimeters:

inches = millimeters / 25.4 millimeters = inches * 25.4

How many inches is 350 millimeters? How many millimeters is 3.5 inches?

Shop math


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Decimal places

A whole number is called an integer Consider this real number:

4, 5, 8, 44, 103, 3,373, and 26,252 are examples of integers

5.436

3.274 is an example of a real number

Shop math

A number containing a portion of a whole number is called a real number

In a real number, the values to the right of the decimal point are called decimal places

There are three decimal places to the right of the decimal point

With most CNC machines…

Metric system allows up to three decimal places

Imperial system allows up to four decimal places


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Fractional format

When using the Imperial measurement system (not the Metric system)…

Consider this fraction:

But a CNC machine cannot accept fractional format

Fractions must be converted to decimal format

Shop math

Some values may be specified with fractions

To convert a fraction to decimal format, divide the numerator (top or left number) by the denominator (bottom or right number)

13/16Pronounced as “thirteen-sixteenths”

Divide 13 by 16

0.8125 (the result) is the decimal format of 13/16


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Saying numbers out loud in a machine shop

In the Imperial measurement system: 1.0 : “one inch”

0.1 : “one-hundred-thousandths of an inch”

0.01 : “ten-thousandths of an inch”

0.001 : “one-thousandth of an inch”

0.0001 : “one tenth” (even though this value is really one ten-thousandth of an inch)

Shop math

More examples:0.047 : “forty-seven thousandths”0.250 : “two-hundred-fifty thousandths”0.684 : “six-hundred-eighty-four thousandths”1.455 : “one inch, four-hundred-fifty-five thousandths”4.3723 : “four inches, three-hundred-seventy-two thousandths, and three tenths”


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Saying numbers out loud in a machine shop

In the Metric measurement system:

1.0 : “one millimeter”

0.1 : “one-hundred microns”

0.01 : “ten microns”

0.001 : “one micron”

Shop math

More examples:0.047 : “forty-seven microns”0.250 : “two-hundred-fifty microns”0.684 : “six-hundred-eighty-four microns”1.455 : “one millimeter, four-hundred-fifty-five microns”


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Polarity

All values have a polarity (plus or minus)

Shop math

With CNC machines, plus is always assumed

If no polarity sign is specified (+ or -), the value is assumed to be positive

All values shown to this point have been positive

To specify a negative value, the minus sign (-) is used:

-0.003 (negative three thousandths of an inch)

When you subtract a number from a smaller number, the result will be negative

5.002 - 5.006 = -0.004


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Summary

Most calculations required of setup people and operators are pretty simple

Shop math

The trick lies in doing simple calculations – over and over again – with out making any mistakes!


Mistakes will, of course, result in scrap parts – or worse

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Blueprint reading

Orthographic projection

Blueprint reading involves visualizing a three-dimensional workpiece from a series of two-dimensional views.

The manner in which the two-dimensional views line up is called orthographic projection.

With orthographic projection, there can be as many as six standard views.

Consider one of a pair of dice…

This shows all six views the six standard views used with orthographic projection

Notice how each view is lined up with other views

Next topic: Tolerance interpretation

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The way that views line up is key to visualizing what the workpiece looks like.

In addition, line appearance is also important:

Visible line – Surface is visible when looking at the workpiece from the view:

Hidden line – Surface is not visible when looking at the workpiece from the view:

Center line – Represents the center of an object (like a hole):

Notice that hidden lines and center lines are thinner than visible lines

Blueprint reading


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Blueprint reading

Visible line

Hidden line

Center line

Visible line

Hidden line

Center line

Front RightLeftBack

Top

Bottom


Here is another example:

All six views are still being shown:1) Front view (plan view)2) Top view3) Bottom view4) Left view5) Right view6) Back view

Notice the three line types


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Blueprint reading

Visible line

Hidden line

Center line

Visible line

Hidden line

Center line

Front RightLeftBack

Top

Bottom


Design engineers will rarely show all six views

They will only show enough views to adequately define the workpiece

These three view show enough to allow visualization of the workpiece

Unfortunately, design engineers vary when it comes to how much they show


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Section lines are used to show what the workpiece would look like when cut along the section line

Blueprint reading


Section line

Sectional view

They help clarify what a complex workpiece looks like


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Blueprint reading


An isometric view is almost like a photograph of the workpiece

Though rarely used, isometric views make it much easier to visualize the workpiece

Isometric view


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Sample workpiece

It really helps to have an actual workpiece when you are studying a blueprint

CNC operators will have an actual workpiece because the setup person will have run at least one during setup

But setup people and programmers will not have one – they must be able to visualize the workpiece from the blueprint alone

Blueprint reading


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Dimensioning

Dimensions provide information about the size of each workpiece attribute

1.0

2.5

1.5

4.25

4.25

2.12

3.0

3.75

.5

.62

1.0

.25

Blueprint reading

While blueprints are drawn to scale, they are rarely full-scale

If you take a ruler to this distance, it will not be 1.0 inch!

Dimensions can be specified in the Imperial (inch) or Metric system

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6.35 (typical)

19

100

63

19 63

100

13.7

19

13 R (4)

Drill 12.0 dia.

Blueprint reading


Example

Can you visualize what this part looks like?

Can you approximate how big it is?

This workpiece is dimensioned in Metric

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Blueprint reading

Example

Can you visualize what this part looks like?

Can you approximate how big it is?


This workpiece is dimensioned in inches

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Tolerance interpretation

Introduction to tolerances

A tolerance specifies the amount of allowable error for each workpiece attribute – and every workpiece attribute will have a tolerance

2.0

1.0

5.0

Say the tolerance is specified as plus or minus 0.005 inch

This means the overall length could be as short as 4.995 in or as long as 5.005 in and still be acceptable

Consider the 5.0 inch overall length of this workpiece

Next topic: Measuring devices

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Important tolerance terms

2.0

1.0

5.0

Consider the 5.0 inch overall length of this workpiece

Tolerance band – from the low limit to the high limit (4.995 – 5.005 is the tolerance band)

Mean value – Right in the middle of the tolerance band (5.0)

Low limit – The smallest acceptable value (4.995)

High limit – The largest acceptable value (5.005)

Overall tolerance – total amount of tolerance – high limit minus low limit (0.010)


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Judging acceptability

2.0

1.0

5.0

Consider the 5.0 inch overall length of this workpieceSay you measure the workpiece length and

find it to be 5.002

Is it acceptable?

Yes – 5.002 is between 4.995 and 5.005

But what if it comes out to 5.006? Is it acceptable now?

No – 5.006 is larger than the high limit

Or if it comes out to 4.994? Is it acceptable?

No – 4.994 is smaller than the low limit


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What are you shooting for?

2.0

1.0

5.0

Consider the 5.0 inch overall length of this workpieceWhen a workpiece attribute is not within its

tolerance band, an adjustment must be made

The attribute must be brought to its target value

In many companies, the target value is the mean value of the tolerance band (5.0 in our example)

The adjustment amount will be the difference between the measured value and the target value

If you measure the overall length and find it to be 5.007…

…the adjustment amount will probably be 0.007 (assuming target value is the mean value)


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Adjustment polarity

2.0

1.0

5.0

Consider the 5.0 inch overall length of this workpieceEach adjustment will have a polarity (plus or

minus)

Generally speaking:

If more material must be removed, the adjustment polarity will be negative

If less material must be removed, the adjustment polarity will be positive

If you measure the overall length and find it to be 5.007…

…more material must be removed – meaning the adjustment will be negative


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Types of tolerance specifications

Plus or minus tolerance:

5.0 +/- 0.005

Mean value

Attribute can be evenly larger or smaller than mean value by…

…this amount

• Mean value: 5.0• High limit: 5.005• Low limit: 4.995• Tolerance band: 4.995 – 5.005• Overall tolerance: 0.010


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High and low limit:

4.995 / 5.005

Low limit


High limit

Mean value = (high limit + low limit) / 2Mean value = (5.005 + 4.995) / 2Mean value = (10.0) / 2Mean value = 5.0


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Uneven plus and minus:


5.002 +0.003, - 0.007

Starting specificationAttribute can be larger than specified value by this amount

…or smaller than the specified value by this amount

High limit = Starting specification + 0.003 (5.005)

Low limit = Starting specification - 0.007 (4.995)

Mean value = (high limit + low limit) / 2 (5.0)


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Implied tolerances


Again, every workpiece attribute will have a tolerance

If it is not specified as part of the dimension, it is implied with decimal places

Somewhere on the blueprint, usually near the title block, you will see something like this…

x.x: +/-0.01x.xx: +/-0.004x.xxx: +/-0.002x.xxxx: +/-0.0005

An attribute dimensioned with:

4.75

has a tolerance of +/-0.004 (with this example)

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Measuring devices

General suggestion for getting started with any measuring device

Most measuring devices require that you master a certain “feel” to be proficient

Find something with a known size on which to practice

When the measuring device shows this size, you know you have applied the correct pressure to the device

When you measure an actual workpiece, you’ll know the correct pressure to exert

Gauge blocks or gauge pins have their sizes etched right into each component

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Measuring devices

Measured-value display methods

There are three common ways that measuring devices can display measured values

Vernier scale – Most difficult to read, but very common

Dial display – Much easier to read, and still quite common

Electronic digital display – Easiest to read, but more expensive

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All measuring devices have a limited range for measuring

You must know the starting point for measuring

Range of measurements

A six-inch caliper has a range of six inches and its starting point is zero

A “zero-to-one” micrometer has a range of one inch and its starting point is zero

A “two-to-three” micrometer has a range of one inch and its starting point is two inches

Measuring devices

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Many measuring devices have some kind of pointer

You must often add the reading you take to this pointer position

Pointer position

With a six inch caliper, the pointer may be on 4.2 inches, meaning you must add the read value to 4.2

With a “zero-to-one” micrometer, the pointer may be on 0.45, meaning you must add the read value to 0.45

Measuring devices

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Measuring devices

Vernier scale value display

This Vernier caliper allows measurements in both Imperial and Metric (few do)

Inch scale

Metric scale

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Measuring devices


Each Vernier scale will have a main scale…

Main scales

Vernier scale

Vernier scale

… and a Vernier scale

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Measuring devices


The origins for these main scales (not shown) are zero

0

Origin would be about here

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Measuring devices


The pointers for the Vernier scales are here

0

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Measuring devices


To read the displayed value…

0

The Metric pointer is pointing at just over 2.4 cm (24 mm)

The Vernier scale line that best lines up with the main scale is 0.062 cm (0.62 mm)

Add the two together – 2.4 cm (24 mm) plus 0.062 cm (0.62 mm) totals 2.462 cm (24.62 mm)

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Measuring devices

Dial display

Dial displays are usually much easier to read

First find the main scale pointer

It is pointing to just over 0.5 inch

Now find the dial reading…

It is centered between 0.010 and 0.011 (0.0105)

The reading is 0.5105

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Measuring devices

Electronic digital display

Digital displays are the easiest to read

With this 25-50 mm micrometer, the starting value (pointer) is 25 mm

The digital display directly shows the measured value (25.000 mm at the present time)

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Measuring devices

External workpiece attributes

Some measurements involve measuring over external surfaces

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Internal workpiece attributes

Other measurements involve measuring internal surfaces

Measuring devices

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General use: The knurled handle is turned to move the spindle face from the anvil face far enough to place the workpiece attribute between them. The knurled handle is then turned in the opposite direction to gently clamp the workpiece between the anvil face and the spindle face. For final clamping, the ratchet is used (turned in the same direction as the knurled handle) to apply the appropriate amount of force. Finally, the locknut can be tightened to lock the thimble in place – the micrometer can then be removed from the workpiece attribute (if necessary) for easier reading.

Locknut

Anvilface

Spindleface

Spindle

Sleeve

Thimble

RatchetFrame

Knurledhandle

Application: External workpiece attributesAccuracy expectation: +/- 0.0001 in or +/- 0.004 microns

Micrometer

Measuring devices

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Depth micrometer

Measuring devices

General use: The knurled handle is turned to retract the spindle until it is shorter than the depth to be measured. The base face is then placed on the upper surface for the measurement and held firmly in place. The knurled handle is turned in the opposite direction to bring the spindle face into contact with the lower surface and retracted slightly. For final contact, the ratchet is used (turned in the same direction as the knurled handle) to apply the appropriate amount of force. Finally, the locknut can be tightened to lock the thimble in place – the depth micrometer can then be removed from the workpiece attribute (if necessary) for easier reading.

Application: Internal depth attributes.Accuracy expectation: +/- 0.0001 in or +/- 0.004 microns.

Spindleface

Base face

Spindle

Locknut

Sleeve

ThimbleRatchet

Knurledhandle

Vernier display

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Stationary side

Internal calipers

External calipers

Sliding side

Scale indicator surface

Scale

Dial

Dial locknut

Slide

Knurled thumb knob

Slide locknut

End stylus

Slide end

Dial caliper

Measuring devices

Application: External and internal workpiece attributes.Accuracy expectation: +/- 0.0005 in or +/- 0.010 microns.

General use: For external measurements, the slider is pulled (with the knurled thumb knob) to widen the external calipers wider than the attribute to be measured. The external calipers are then placed over the attribute and the tightened to squeeze the attribute (care must be taken to confirm square ness). The reading can be taken at this point.

For internal measurements, the slider is pulled (with the knurled thumb knob) to widen the external calipers just narrower than the attribute to be measured. The internal calipers are then placed inside the attribute and the pulled apart to engage the attribute (care must be taken to confirm square ness). The reading can be taken at this point.

For end (depth) measurements. The slide end is butted against the upper surface (care must be taken to ensure square ness). The knurled thumb knob is then pulled to open the calipers until the stylus end contacts the other surface. A reading can be taken at this point.

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Lesson topics CNC Machining Center Setup and Operation Lesson 1: Basic machining practices required for machining centers Copyright 2009 Shop safetyShop