Company Meeting Title

1
Relates to machining and turning centers
2
G20 G21 inch and metric modes
Commonly taught in basic CNC courses

• G20 selects inch, G21 selects metric
• Use the mode prints are dimensioned
• The mode you use is initialized

3
G20 G21 inch and metric modes
Commonly taught in basic CNC courses

• G20 selects inch, G21 selects metric
• Use the mode prints are dimensioned
• The mode you use is initialized

Not always taught in basic courses

• Ways to select inch/metric mode
• How to tell which mode the machine is in
• Advantages of metric mode

4
G20 G21 inch and metric modes
Commonly taught in basic CNC courses

• G20 selects inch, G21 selects metric
• Use the mode prints are dimensioned
• The mode you use is initialized

Not always taught in basic courses

• Ways to select inch/metric mode
• How to tell which mode the machine is in
• Advantages of metric mode

5
G20 G21 inch and metric modes
Ways to select inch or metric mode
Commonly taught in basic CNC courses
All current model CNC machines let you work in
either mode

• G20 selects inch, G21 selects metric
• Use the mode prints are dimensioned
• The mode you use is initialized

Most CNC users work exclusively in one mode or
the other
You can select the desired mode manually
once the measurement system modes is selected,
there is no need for further selection
or you can program mode selection
Not always taught in basic courses

• Ways to select inch/metric mode
• How to tell which mode the machine is in
• Advantages of metric mode

G20 inch mode G21 metric mode
6
G20 G21 inch and metric modes
Commonly taught in basic CNC courses

• G20 selects inch, G21 selects metric
• Use the mode prints are dimensioned
• The mode you use is initialized

Not always taught in basic courses

• Ways to select inch/metric mode
• How to tell which mode the machine is in
• Advantages of metric mode

7
G20 G21 inch and metric modes
Commonly taught in basic CNC courses

• G20 selects inch, G21 selects metric
• Use the mode prints are dimensioned
• The mode you use is initialized

Not always taught in basic courses

• Ways to select inch/metric mode
• How to tell which mode the machine is in
• Advantages of metric mode

8
G20 G21 inch and metric modes
How to tell which mode the machine is in
If the control is in the inch mode, youll see
four places to the right of the decimal point
You can easily tell which mode is selected
Commonly taught in basic CNC courses

• G20 selects inch, G21 selects metric
• Use the mode prints are dimensioned
• The mode you use is initialized

Not always taught in basic courses

• Ways to select inch/metric mode
• How to tell which mode the machine is in
• Advantages of metric mode

9
G20 G21 inch and metric modes
How to tell which mode the machine is in
If the control is in the metric mode, youll see
three places to the right of the decimal point
You can easily tell which mode is selected
Commonly taught in basic CNC courses

• G20 selects inch, G21 selects metric
• Use the mode prints are dimensioned
• The mode you use is initialized

Not always taught in basic courses

• Ways to select inch/metric mode
• How to tell which mode the machine is in
• Advantages of metric mode

10
How to tell which mode the machine is in
Most companies work exclusively in one mode
If prints are dimensioned in inch
If they happen across a metric print
Inches millimeters divided by 25.4
they convert it to inch and run the part in inch
They work exclusively in inch
11
How to tell which mode the machine is in
Most companies work exclusively in one mode
Some companies do program in the measurement mode
that the print is dimensioned (they work in both
modes)
If prints are dimensioned in metric
If they happen across a print dimensioned in inch
Some controls make it easier than others to use
both modes
Millimeters inches times 25.4
they convert it to metric and run the part in
metric
They work exclusively in metric
12
How to tell which mode the machine is in
When you switch modes on some controls
The decimal point simply moves one place
13
How to tell which mode the machine is in
When you switch modes on some controls
One inch is not equivalent to 10 millimeters!
14
How to tell which mode the machine is in
With other controls
A true conversion takes place
15
How to tell which mode the machine is in
With other controls
One inch is equal to 25.4 millimeters
16
G20 G21 inch and metric modes
Commonly taught in basic CNC courses

• G20 selects inch, G21 selects metric
• Use the mode prints are dimensioned
• The mode you use is initialized

Not always taught in basic courses

• Ways to select inch/metric mode
• How to tell which mode the machine is in
• Advantages of metric mode

17
G20 G21 inch and metric modes
Commonly taught in basic CNC courses

• G20 selects inch, G21 selects metric
• Use the mode prints are dimensioned
• The mode you use is initialized

Not always taught in basic courses

• Ways to select inch/metric mode
• How to tell which mode the machine is in
• Advantages of metric mode

18
G20 G21 inch and metric modes
Advantages of the metric mode
Commonly taught in basic CNC courses
There is a resolution advantage when you use
metric mode

• G20 selects inch, G21 selects metric
• Use the mode prints are dimensioned
• The mode you use is initialized

Least input increment
In inch mode 0.0001 inch In metric mode 0.001
millimeters
Not always taught in basic courses

• Ways to select inch/metric mode
• How to tell which mode the machine is in
• Advantages of metric mode

0.001 mm 0.000039 inch
19
Advantages of the metric mode
A ten inch long linear axis has 100,000 positions
in the inch mode
The same 10 inch long linear axis has 254,000
positions in the metric mode!
20
Advantages of the metric mode
Remember, were talking about resolution, not
accuracy
An analogy
A 5 degree indexer has 72 positions
A 1 degree indexer has 360 positions
The 1 degree indexer is not more accurate
it simply has better resolution
21
Advantages of the metric mode
When is this important?
How tight are your tolerances?
3.000 /- 0.0002 in
Even with this tight tolerance, programming the
mean value is still possible
Obviously, 3.0000
22
Advantages of the metric mode
When is this important?
But consider this tolerance
3.000 0.0001, - 0.0002
Mean value is 2.99995
You cant program to five places!
In metric, 2.99995 in is 76.19873 mm
When rounded to 76.199, it is within 0.00027 mm
of mean
Thats about 10 millionths of an inch, as opposed
to 50 millionths!
23
Advantages of the metric mode
When is this important?
Also consider offset adjustments
In inch, you must adjust in 0.0001 inch increments
In metric, you can adjust in much finer 0.001 mm
increments!
A 0.001 overall tolerance has less than ten
adjustment positions in the inch mode
in the metric mode this 0.001 tolerance has
twenty-five adjustment positions!
24
Advantages of the metric mode
When is this important?
With some machines, motion resolution is tied to
measurement systems
Actual motion if in inch mode (0.0001 in step)
25
Advantages of the metric mode
When is this important?
With some machines, motion resolution is tied to
measurement systems
Actual motion if in metric mode (0.001 mm steps)
26
G20 G21 inch and metric modes
Commonly taught in basic CNC courses

• G20 selects inch, G21 selects metric
• Use the mode prints are dimensioned
• The mode you use is initialized

Not always taught in basic courses

• Ways to select inch/metric mode
• How to tell which mode the machine is in
• Advantages of metric mode