I’ve gotten quite enamored with the fancy cycles for lathes, such as G71 for rough turning. They really amount to a poor man’s CAM system. For example, with G71, you need only provide a profile such as this one:

Simple G71 Profile

A simple profile consisting of 3 G01 segments with a G00 to the starting point…

The profile is created in g-code out of lines and arcs, and then the G71 tells your CNC controller how to turn that profile into a finished part with a series of cutting passes. Here is the g-code with the G71:

And here is the code that was simulated:

% Top to bottom, right to left
( One line G71 )
G0 X4 Z1.0 (Start Position before commanding the cycle)
G71 P1 Q4 D1.0 F2 U0.2 W0.1
N1 G0 X1.6
N2 G1 Z-4.0
N3 X2.5 Z-5.0
N4 G1 X4

As you can see, there’s not much more needed than the profile to get the job done. Here’s a simulation (using the G-Wizard G-Code Simulator, of course) of what the toolpath will be from that G-71 and profile:

G71 Simulation

The simulated G71 makes 2 roughing passes and then cuts the profile…

G71 can save you a whole bunch of time and make it pretty easy to crank out some parts without needing to access a CAM program. For simple profiles, it’s really easy to crank out the g-code. But, what about a complex profile, one where you may think you need a CAM program?

It turns out you can use your Mill CAM software to create that profile g-code. In another installment for the CNCCookbook G-Code Tutorial, I go through all the steps necessary to use your Mill CAM software to produce profiles suitable for lathe programming. You can find the Mill Cam for Lathe tutorial by clicking this link.

The steps are surprisingly simple and easy to automate to go from this (a OneCNC engraving program for the mill):

Mill CAM for a Lathe Profile

OneCNC engraving toolpath traces the desired lathe profile…

to this (the same profile all set up and simulated as a G71 rough turning operation):

Swap X to Z and Z to X

Our familiar profile made the journey from Mill CAM to suitability for a lathe cycle!


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