Download 1 Numerical Control - UNT College of Engineering

Numerical Control
It might be said that numerical control is control by
numbers.
Numerical control uses any of the basic machine tools.
In a broad sense, numerical control converts numerical
values into physical values, such as quantities and
dimensions.

Electronics plays an important part in numerical
control.

Electronic circuits have been designed to
"remember" a number.

They have been designed to turn a switch on and
/or off when a particular number is reached.

This means that if a control unit can be designed
to measure and "remember" numbers, the unit
then has the ability to run a machine.
BASIC COMPONENTS OF AN N/C SYSTEM

1. The control unit, which interprets the coded
instructions (the tape) and directs the machine through the
operations.

2. The tape, which is punched full of holes, with each hole
providing an electronic pulse. This tape holds the coded
instructions.

3. The motor, which supplies the power to move the tool
or table of the machine.

4. The electronic feedback device (transducer), which
tells how much movement has taken place.
Vertical Milling Machine Equipped with N/C control
MOOG N/C
HOW NUMERICAL CONTROL WORKS
 As the prepared tape passes through the
tape reader head, silicon photo diodes sense
light as it passes through holes in the tape.

This causes signals to be sent to the
electronic control.

The number and location of the holes in
each row across the tape conform to a code.

The code is interpreted by the control
unit. This unit stores all information until a
complete block of information is obtained.

The control then causes the motors to
make the required number of steps.

During the tool cycle, the next tape block
will be read and stored.
TYPES OF NUMERICAL CONTROL
There are actually two forms of numerical
control:

1. Discrete positioning. This is commonly
known as point-to-point.

2. Continuous path, or contouring
Tape
PREPARATORY FUNCTION




4 peck drill;
Table moves to selected location. Peck drill cycle
initiated upon arrival at location. Cycle time set by
timer on console.
5 DRILL:
Table moves to selected location. Spindle moves
down rapidly to preset feed point, proceeds at set
feed rate to final depth and retracts rapidly.
6 MILL:
Spindle moves down rapidly to set feed point,
proceeds at set feed rate to final depth and then
at set mill feed rate to selected location.

7 BORE:
Table moves to selected location. Spindle moves down rapidly to
preset feed point, proceeds through work at set feed rate. When
final depth is reached, spindle retracts from work at set feed rate to
feed point and then retracts rapidly.

8 TAP:
Table moves to preset location. Spindle moves down rapidly to
preset feed point. Tap feeds into work at rate determined by lead
screw. When final depth is reached, the spindle reverses itself and
retracts from hole to feed point. The spindle then rapidly retracts.

9 READ Z BLOCK.
This preparatory function does not initiate any motion of the table
or spindle. It supplies feed point and final depth information into the
machine. This depth information is then used in the next operation
of the machine. This information will be read while machine is
positioning for next operation.
MISCELLANEOUS FUNCTIONS

00 PROGRAM STOP:
Program is interrupted by placing the system in its manual mode of
operation after completion of all commands in the block. Coolant flow
and spindle feed are interrupted after the spindle has been fully
retracted from the workpiece. It is necessary for the operator to push
a button in order to continue with the remainder of the program.

02 END OF program:
The function signifies the completion of the workpiece. After
completion of all commands in the block, the system is reset to its
manual mode. Coolant flow and spindle feed are stopped after the
spindle has been fully retracted from the workpiece.

06 TOOL CHANGE
This is a command to execute the change of tool(s) manually, not to include tool
selection. After completion of all commands in the block, the system is placed in its
manual mode of operation. Coolant flow and spindle feed are interrupted after the
spindle has been fully retracted from the workpiece.

07 COOLANT ON : This is a command to turn on flood coolant.

08 COOLANT ON : This is a command to turn on mist coolant

09 COOLANT OFF: This is a command to shut off all coolant.

80 NO CHANGE IN MISCELLANEOUS FUNCTION:
All miscellaneous functions remain in previously commanded operation. This applies
to all miscellaneous functions with the exception of PROGRAM STOP, 02, and TOOL
CHANGE, 06, which merely interrupt the sequence of automatic operations.
MEASURING BASIS FOR N/C CONTROL
SYSTEMS
The Cartesian
(rectangular) coordinate
system is the basis for
measuring N/C machine
tools.
 In this system, all point
positions are described in
terms of distances from
the origin .These are
called the X, Y, and Z
axes.

The Cartesian Coordinate System


The Cartesian Coordinate System
A. Quadrants (4)
1. 1st quadrant (+,+)
2. 2nd quadrant (-,+)
3. 3rd quadrant (-,-)
4. 4th quadrant (+,-)

B.
Origin (0,0)

C.
Moog N/C Vise Location #1 Zero Offsets
1.
zero offsets are always (2, -2)
2.
Moog N/C is always programmed in 4th quadrant
3.
Machine control does not recognize negative
numbers so Y values are always programmed as
positive even though in the 4th quadrant they are
negative.
4.
The tape coding values for the zero offset are
X = 02000, Y = 02000
Vise Location
Coordinate/Conventional Dimensioning
Dimensioning
Program Sheet
Work piece Coordinates
Tooling Sheet
Tape
Components of a Typical Block

C. Components of a Typical Block
1. track numbers (1-8)
2. row numbers (1-20)
3. track values (1,2,4, and 8)
4. coded row data
a. block number - rows 1-3
b. preparatory function - row 4
c. X dimension - rows 5-9
d. Y dimension - rows 10-14
e. reserved row (not used) - row 15
f. tool number - rows 16-17
g. miscellaneous function - rows 18-19
h. end of block (EOB) - row 20
Coded Numeric Data


D. Coded Numeric Data
0
no track punched
1
track 1 punched
2
track 2 punched
3
tracks 1 & 2 punched
4
track 3 punched
5
tracks 1 & 3 punched
6
tracks 2 & 3 punched
7
tracks 1, 2, & 3 punched
8
track 4 punched
9
tracks 1 & 4 punched
EOB
track 8 only punched
The Fixed Decimal System


The Fixed Decimal System
A.
Options
1. Full floating decimals (many N/Cs, all CNCs)
2. Fixed decimals (many N/Cs including ours)



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B. Programming Implications:
All X and Y coordinate information assumes the
fixed decimal point is two digits from the left of a
five digit dimension and a decimal point is never
used.
Absolute vs Incremental Programming
A. Absolute: all dimensions are derived from their
respective distances from the machine tool's
origin.
B. Incremental: subsequent dimensions derive their value
based on their distance from the previous dimension.
Part Dimensioning for N/C
A. Dimensioning Convention Nomenclature
1.
baseline dimensioning
2.
absolute dimensioning
3.
datum line dimensioning
4.
coordinate dimensioning
 B. Eliminates "Tolerance Stack"
 C. Facilitates Programming
 D. Facilitates Inspection



Simple Point-To-Point Drilling Program
A.
Conversion of Conventional Drawing to Coordinate
Dimensioned Drawing
1. zero offsets: X = 02000, y = 02000
2. X and Y dimensions given from datums
3. sequence of operations provides for most
economical tool movemen
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C.

D.
Programming Sheet
1.
review data in heading
2.
review data for block rows (N,G,X,Y,T,M)
Tooling
1.
2.
3.
4.
5.
Sheet
data in heading
operation description
spindle feed and RPM
X and Y axis feed
tool description
Tool Presetting

A. Purposes

1. Quick and efficient replacement of broken or dull
tools

2. close tolerances on depth dimensions
(interchangeability of tools during the production
cycle mandates exact uniformity of length for a
given type and size of tool; this is crucial for N/C
production and flexible automation (FMS/FMC))
Variance in Tool Length

1. Fresh drills from the manufacturer come in
varying lengths.

2. Milling cutters, counterbores, spot drills,
countersinks, reamers, etc., display the same
characteristic

3. Routine sharpening amplifies discrepancies in
length
Setting Tool Lengths

A. Procedure
Tools are loaded into their respective tool holders and
clamped such that their preset length corresponds to
that of the tool setting drawing within a tolerance of +
or - .001" (+ or - .02mm).

This typically requires special fixtures such as vernier
height gages, high-precision dial indicators .

For the purposes of standardization , the preset length
for all drills destined for use on the Moog N/C will be
5.125".

Collets held cutters and very small drills will necessarily
have a much shorter preset length.

N/C Exercise #2
Introduction
This numerical control program is designed to systematically drill holes in a part for production. A
Bridgeport N/C machine will use a drill cycle geometry code to first center drill the holes then
return to the same location to drill the trough holes then once more counter boring the same
holes. For this process we will use a point-to-point method. Reading further |pjti will be informed
of the sequence of operations, the
tools used and the references used.

Sequence of Operations
Included is a tooling sheet which states the tools needed and what order they are used. For this
operation a center drill will/be used to eliminate the tendency for the drill bit to walk on the parts
surface. The machine continues through the all the hole locations with each tool to eliminate
excessive tool changes. Once the center drill is done the tool is changed to the drill bit
to/systematically drill the through holes. Once the drill has created all the through holes needed
the tool is changed to the counter bore. Once the holes are counter bored the machining process
is done. The tool life is optimized using calculation of speeds and feeds with numbers from The
Machinery's Handbook*. These numbers are based on a forty five minute tool life for drilling
operations.

Tooling Used
The order of tooling has been set and should be numbered accordingly. There will be three tools
used in this exercise and also three turret stop settings. The first tool listed is the number three
center drill and is numbered 01. The second is the 1/8 diameter drill bit and is numbered 02.
The third tool is the one quarter inch counter bore and is numbered 03. The tools have there
own respective turret stop settings. No tooling material was specified nor a manufacturer so
specifications for the tools would be speculation. I used tools made of high speed steel and used
stock made of 4130 steel with a Brinell hardness of 200. Following is the RPM and feed rate
calculations:
Cutting speed(CS)

The most important considerations for any type of
machining process are cutting speed, feed and coolants.

Cutting speed(CS): Means the cutting distance in one
minute. CS is normally based on surface feed per minute
(SFPM).

Machine ‘s spindle speed is given in (rpm), so cutting speed
should be changed to rpm.

RPM = 12 * CS/ π * d
FEED

Feed: Cutting tool should be fed into the work-piece; the
bigger the drill, the slower the drill should be fed into the
work-piece.

Feed is calculated in inches/rev.
IPR= cpt (in/min) * # teeth
IPM = RPM * IPR