G81

Introduction to G81

When making a part it is crucial to remove all excess material from the raw material so that there is only a little bit of extra material to remove for the fine contour on the part. The G81 cycle is used both for internal and external contours and is thus hugely beneficial for any machinist to know.

I want to stress that before using a roughing cycle with G81 it is common to make a facing so that the part is somewhat smooth on the face, for more on that visit the page on G82.

G81 for roughing

Removing excess material

In my experience the most common application of the G81 command is to remove the external excess material on a part. It can also be used to remove internal excess material but I will focus on the external and then move over to internal.

G97 S1100 M08 M03
G00 X45 Z2 T040404(WNMG 0.8)
G96 S150
G85 NEXT D4 F0.25 U0.3 W0.08
NEXT G81
G00 X27
G01 Z0 G42
G75 X30 L0.5
G1 Z-18
G75 X42.2 L0.5
G1 Z-24.2
G1 X45
G0 Z2
G40
G80
M05 M09
G00 X999 Z999

Explanation

setting up and G85

- First with G97 we set the spindle speed and on the same line I start the spindle and the coolant.

G97 S1100 M08 M03

- Then I select the tool and approach the part, in this case the tool has position 04 in the turret. On the line after I use G96 to set the cutting speed to 150mm/min.

G00 X45 Z2 T040404(WNMG 0.8)
G96 S150

- On the third line I use the G85 cycle were I define the parameters of the roughing such as depth of cut, feed, material to leave etc.

G85 NEXT D4 F0.25 U0.3 W0.08

I want to stress the importance of choosing the right X-values were the tool is positioned before the G85 cycle is run. The value of X aforementioned in relation to the final X-value in the G81 cycle has an impact on how the tool approaches and exits the part while it is running the cycle.

The contour with G81

- Now I have defined the parameters of the roughing and set the feeds and speeds, all that is left for the roughing is the contour, which I define with G81. I could have defined the contour before the G85, but I like to order it in this way.

Before I write G81 I write NEXT on the same line, N is the letter denominating the line and has to be followed by either letters or numbers. In this case I call it NEXT as in N - EXTernal. I would not recommend using a number after N in this case.

NEXT G81
G00 X27
G01 Z0 G42
G75 X30 L0.5
G1 Z-18
G75 X42.2 L0.5
G1 Z-24.2
G1 X45
G0 Z2
G40
G80

- After writing NEXT G81 I start the contour with the point closest to the axis and then move into the part with a G01 Z0 G42. This is because the direction of the tool is Right, if it would be Left I yould be using G41 instead. For more on this see the page on radial compensation.

Then I just make the external contour and end it with G40 and to end the G81 cycle I write G80.

M05 M09
G00 X999 Z999

Then I simple turn of the coolant and stop the spindle and then exit the part and return the machine to the ending position.

G81 for internal roughing

Removing internal excess material

The other way of using G81 is for internal roughing which work almost in the same way as external but with a few minor differences.

G97 S1700 M03 M08
G00 X30 Z25 T060606 ( INT ROUGH WNMG 0.8 DIA 25 )
G00 Z2
G85 NINT D4 F0.25 U0.3 W0.08
NINT G81
G00 X69
G01 Z0 G41
N163 G76 X68 L0.15
N164 G76 X66.05 A225 L0.15
N165 G1 Z-22
N168 G1 X27
N169 G0 Z2
N170 G40
N171 G80
N141 VLMON[1]=0
N142 G0 Z50
N143 G0 X999 Z999 M9 M5

Code breakdown

First I start the spindle and load the tool now placed on position 6 and approach the part.

G97 S1700 M03 M08
G00 X30 Z25 T060606 ( INT ROUGH WNMG 0.8 DIA 25 )
G00 Z2

Then I set the G85 cycle as in the external case with the same parameters as in the external case. For the internal case I give the line the name NINT for N-INTernal, it is very important that it does not have the same name as any other line in the programme.

G85 NINT D4 F0.25 U0.3 W0.08

Then I do as before and name the line where the contour is starting and write the contour. Note that the internal contour uses left compensation whereas the external contour used right compensation. More on this on the page on radial conpensation(G40, G41, G42).

NINT G81
G00 X69
G01 Z0 G41
G75 X66 L0.5
G1 Z-22
X27
G0 Z2
G40
G80

I end the contour with G80 and then have a line were I exit the part followed by a line were I go to the home position and turn of the coolant and spindle.

G0 Z50
G0 X999 Z999 M9 M5

It is possible to make internal roughing by turning radially instead of axially as shown above. The difference is that the contour has to be written in the other direction and the G82 cycle has to be used instead.