Posted by: davidhayden | March 15, 2010

Proven Tips, Tools and Tactics: Understanding Interpolation


Your son or daughter wants to borrow the car. Don’t want to know a few basic things like where they are going, how they are going to get there, when they will get there, when they are coming back?

Well, guess what, without knowing it, you understand interpolation.

Ok, What does this have to do with CNC Programming?

Everything.

When you send a tool into the workpiece you might as well be sending a teenager into a strange city. The tool needs clear instructions about where to go, how fast to get there and so on.

Imagine you are asking the machine to move 10 inches along the X axis but only 2 inches along the Y axis. Let’s say you programmed a very safe 10 inches per minute feed rate.

If both axis move at exactly 10 inches per minute, it will take the machine 1 minute to move the full distance to the X destination, but only 12 seconds to make the Y move. Instead of an 11 degree angle, you will have a dog leg.

Something like this.
   ___________________________
 /
/
The above is an example of simple positioning, much like you would expect from a G00 code.
 
 
Whether commanding linear interpolation (G01) or circular interpolation (G02/G03) you are telling the machine that you want ALL axis to arrive at the programmed point at the exact time.
 
If you are programming an angled line and one axis leads or lags behind one of the other axes, you will not have the angle you programmed.

The same is true with arcs. If one axis arrives late you will not have a good arc.

A topic for a future article will discuss accuracy of CNC machines and how backlash and lead/lag error can diminish the accuracy of your machine.

So next time you give the keys to your CNC machine and send it on and errand, know that because of its ability to coordinate the moves of the axes via interpolation, the tool will go exactly where you tell it to go in exactly the right way.

Now, if only we could teach our children to interpolate our thoughts.

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