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I was working on an old NC tape machine a long time ago when I decided to teach myself how to program an NC machine. I thought I had everything figured out.

I was very proud of myself. I wrote my code out on paper, double checked it 3 times, and loaded it in the machine. I looked around the shop with a sly smile hoping the boss would see my great accomplishment when I hit the start button.

Ah Good, he was coming my way.

So with overwhelming pride I quickly hit the start button just as he walked up to ask what I was doing.

And then as the machine whirred into action, I smugly turned around and said “hey what’s up boss?”

He watched for a while, smiled and then asked, “it’s 8:30 why is your machine just sitting there with the spindle running, not doing anything?’

WHAT? When I looked back at the machine I was mortified to see the tool was just sitting above the work piece doing nothing.

I learned an important lesson about Planes that day. I learned that NC machines are easy to confuse. I learned how easily I could embarrass myself.

So what went wrong?

Code Data

Code: G17, G18, G19
Modal: Yes
Group: 02
Action: Plane Selection
ANSI Std: Yes
Requirements: None
Options: Required for programming arcs and canned cycles
Conflicting G Codes Only one of the codes can be active at a time but any can be used as often as necessary in a program.

Description and How to Use G17, G18, G19

I failed to understand the importance of plane selection.

In order for an NC or CNC machine to properly interpolate an arc or use a canned cycle, it must know on which it is doing the work.

There are three planes use to program CNC machines. They are:
The XY or G17 Plane
The XZ or G18 Plane
The YZ or G19 Plane

Remembering these planes is pretty simple.

• Take a small box and set it in front of you.
• On the top of the box draw an L and a label the vertical axis Y and the horizontal axis X and write G17 in the center.
• On the front and back of the box draw an L and a label the vertical axis Z and the horizontal axis X and write G18 in the center
• On the right and left sides of the box draw an L and a label the vertical axis Z and the horizontal axis Y and write G19 in the center

This is the basic set for a vertical milling machine. You can tell from this, if you are milling an arc:

• on plane parallel to the top of the box, you use a G17.
• on a plane parallel to the front or back of the box, you use a G18
• on a plane parallel to the right or left side of the box, you use a G19

To learn how to use the G02 and G03 to cut arcs click here .

Like G94/G95 and G20/G21 The plane selection command is usually set at the beginning and not changed again unless a canned cycle or arc is to be cut on a different plane than the one currently active.

The default setting for most Vertical Milling Machines (VMC) is G17. For most lathes, the default setting is G18, because most lathe work is performed in the ZX Plane.

Learn from my mistake. If you are working with various planes, before you invite your boss over to show off, make sure you have told the machine the correct plane on which to work.