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GCode is Complicated
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Combined Rotary and Linear Feedrate for Milling

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I am brand new to CNC and am trying my best to understand how feed rates work (in a G01) when using combined linear and rotary axes.  I understand that in pure linear motion, the feed rate is specified in inches (or mm) per minute, and in rotary motion, the feed rate is specified in degrees per minute.  But what happens when I want to move, say, 20 degrees in C, and 2 inches in X?

If I want to, say, mill an involute profile using the generation principle, then I would want my rotary axis to rotate at a certain constant velocity, and my linear axis, which must be tangent to the base circle, to travel at a certain constant linear velocity.  That linear speed must be equal to the speed of a point on the base circle as it travels around the rotary axis.

So, in the above situation, if I had a base circle with a radius of 5 inches, and I wanted to do a 45 degree rotation, the linear motion must travel the length of the swept arc, which would be 3.927 inches.  How do I specify a feedrate for such a command?  Does F specify a tool velocity relative to the workpiece, and calculate the individual rotary and linear rates?  Or does it assume in/min and just spread the rotary motion over the duration of the move so that it comes out even, or vice versa?

 

(Initially at X0Y0C0)

G01 X-3.927 C45 F?

 

Would I need to use inverse time feed specification (G93) for this?  I guess it could be broken up into lots of small segments, but it would be really convenient to be able to do something odd like this in a single command.

Just something I’ve been thinking about as I try to learn this stuff, and there doesn’t seem to be a single, concise, clear answer that I’ve been able to find so far.

 

  • Jesse Groover asked 1 year ago
  • last edited 1 year ago
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