Re: Help needed with G code

With Steve Rose
8 Tooling & Production/March 2002 www.toolingandproduction.com
SHOP TALK
Milling arc shapes – part 1
Use of I & J in milling 90 degree arcs
To mill an arc shape, you must first program the arc shape.
There are many programming methods available. The use of
I and J commands is a traditional method to program arc moves.
This month, we discuss 90 degree arcs and the I and
J commands. In the coming months we will cover use of the
R command.
Keep in mind that programming methods vary between CNC
types. Check your control manuals and dry-run all new programs
to ensure safe operation of the machine.
Use of I and J
Many controls
use a variety of
program codes to
machine arcs. One
choice is to use the
I and J commands.
The choice and direction
of these
codes is based on
the plane in which
the arc is machined.
I code: The I code
is used when the
arc is machined in
the plane of the
X-axis.
J code: The J code is used when the arc is machined in the
plane of the Y-axis.
Milling in the normal G17 X-Y axis plane, the program uses
the I command for an X-axis move and the J command for a Yaxis
move.
Let's look at the program code for these two convex arcs.
These programs use a climb milling direction and a 1.0" Ø end
mill. Note: The program values are to the center of the cutter.
Example #1: Machine 2.0" arc
G1 X-.5 Y-0.5 Start adjacent to datum.
G1 Y4.3 Y-axis move.
G1 X2.7 Start arc position.
G2 X5.2 Y1.8 J-2.5 End arc position.
G1 Y-0.5 Y-axis move.
G1 X-.5 End adjacent to datum.
As shown in this sketch, the tool is in position to begin the cut
to machine the arc. The values of the I and J commands are
determined by the distance from the arc center to the center of
the cutter in the corresponding axis plane.
The I value is the incremental distance from the arc center, to
the center of the end mill cutter in the X-axis. In this case the
distance is zero and use of the I code is not required.
The J value is the incremental distance from the arc center, to
the center of the cutter in the Y-axis. This distance is 2.5",
resulting in a J value of
2.5. Note, in this program
the J value is
negative. The determination
of positive and
negative values will be
discussed in next
month's Shop Talk.
Review the next example.
The incremental
distance from the arc
center to the center of
the cutter in the Y-axis
is zero. The J command
is zero, and not used.
Example #2: Machine 2.0" arc
G1 X-.5 Y-0.5 Start adjacent to datum.
G1 X5.2 X-axis move.
G1 Y-1.8 Start arc position.
G2 X2.7 Y-4.3 I-2.5 End arc position.
G1 X-0.5 X-axis move.
G1 Y.5 End adjacent to datum.
The distance from the arc center to the center of the cutter in
the X-axis is 2.5", so the I command is I-2.5.
There is still much more to discuss regarding the programming
method. Next month: determining the sign of the I and
J codes.
Steve Rose is a professional trainer and president of RTSI,
Solon, OH, which also offers Internet web site development. Mr.
Rose can be reached by phone 440.542.3066;fax 440.542.3006;
e-mail srose@...; or on the web at www.cnctraining.
com.
Circle 23 or go to www.rsleads.com/203tp-023
Copyright Rose Training Systems Inc., 2002

With Steve Rose
1 0 Tooling & Production/April 2002 www.toolingandproduction.com
SHOP TALK
Milling arc shapes – part 2
Use of I & J in milling 90º arcs
Last month we began the complex topic of using I and J codes
to mill arc shapes. Recall, I is used when the arc is machined in
the plane of the X-axis and J is used when the arc is machined
in the plane of the Y-axis.
Follow the tool path around the arc. At the arc start point,
the cutter center is 2.5" from the arc center in the Y-axis and
0.00" from the arc center in the X-axis. The J code corresponds
to the Y-axis, so the J value is 2.5.
Is this J value positive or negative?
When using the J code, if the Y-axis movement increases the
Y program value, the J code is positive. If the Y-axis movement
decreases the Y value, the J code is negative.
The general rule for determining the sign of the I and J codes is:
If the Y-axis value is decreasing, the I/J code is negative.
If the Y-axis value is increasing, the I/J code is positive.
If the X-axis value is decreasing, the I/J code is positive.
If the X-axis value is increasing, the I/J code is negative.
Increasing numbers are those going from a smaller value to a
larger value. Decreasing number are those going from a larger
value to a smaller value. Remember, -7 is a smaller number
then -2.
In this example, the tool sits at the arc start point. At this
point, the Y value is Y4.3. When the tool moves to the end
point of the arc, in the Y-axis, the tool moves to a smaller value
(Y1.8). The Y-axis value decreases, so the J code is a negative
value.
Example #1: Machine 2.0" Arc
G1X-.5 Y-0.5 Start adjacent to datum.
G1 Y4.3 Y-axis move.
G1X2.7 Start arc position.
G2X5.2 Y1.8 J-2.5 End arc position.
G1 Y-0.5 Y-axis move.
G1X-.5 End adjacent to datum.
In the next example, the tool sits at the arc start point. The
distance from the arc center to the center of the cutter is 2.5" in
the X-axis, so the I code is used.
As the cutter moves from the start point to the end point the
X-axis value increases (from -5.2 to -2.7), so the I code value is
positive.
Example #2: Machine 2.0" Arc
G1X0.5 Y-0.5 Start adjacent to datum.
G1X-5.2 X-axis move.
G1 Y1.8 Start arc position.
G2X-2.7 Y4.3 I2.5 End arc position.
G1X0.5 X-axis move.
G1 Y-0.5 End adjacent to datum.
Draw your own part
or visit our web site
(www.cnc-training.com)
for a complete rectangular
part. Use a pencil to
sketch the cutter at each
arc start point and calculate
the values needed to
machine the shape.
In the next Shop Talk,
we continue programming
arcs using the
R code.
Circle 15 or go to www.rsleads.com/204tp-015
Copyright Rose Training Systems Inc., 2002
Steve Rose is a professional trainer and president of RTSI,
Solon, OH, which also offers Internet web site development. Mr.
Rose can be reached by phone 440.542.3066;fax 440.542.3006;
e-mail srose@...; or on the web at www.cnctraining.
com.
Example # 1
Example # 2