I am new to CNC and I am planning a CNC Router. As I like to tinker I want to do most of it myself. So far I have stepper motors, a controller and my own software which drives the stepper nicely via the controller. I intend using off the shelf software to generate G code files from a DXF file. Now, the G code defines only the geometry of the toolpath. A workable toolpath must also have a description of the kinematics (motor speeds and accelerations). There is stacks of info out there on most aspects of CNC, but this critical, and non-trivial aspect of kinematics is nowhere to be found. Is there a conspiracy amongst kinematics gurus or can someone help me?

Many Thanks
CNCMP

Hi Guys
Thanks for all the useful comments. I especially liked Hufungdung’s “First you get good, then you get fast”, actually I will happily settle for good I don’t much care about fast! I think my concerns stem from a lack of experience with CNC cutting.
Working out a sequence of speeds and accelerations is easy enough for a single segment where the tool is at zero velocity at the start and end of the segment. But when you have two or more sequential segments and you want to keep the tool moving the problem becomes tricky. Is it OK just to plan each move of a large sequence with the tool at zero velocity at the end of each move? I have visions of a CNC router constantly stopping and burning the workpiece. Is it the case that in practice as the electronics works at high speed and the “stopped” periods are momentary the system runs smooth and continuous despite all the theoretical stops?

MP

For acceleration, the software that runs your steppers needs to take care of that. Typically you set the acceleration rate that you want in the software, and then the software needs to figure out when to accelerate and decelerate based on the moves your making and the feed speeds. The feed speeds are usually in the g-code, unless you run everything at a default speed that, once again, your software should have settings for.

Once you set the acceleration, you should know how long it will take to reach the commanded speed, so it should be fairly easy ?? for you to know when to accelerate and when to decelerate.

Typically, you have g-code something like this:

G0 X0 Y0 Z1
G0 X5 Y0 (rapid move, rapid speed should be maximum speed set in controller software)
G1 X5 Y0 Z-.25 F3 (move at commanded "F" speed, 3ipm)

Kinematics...that's cute....vectors and direction.....so checkout linear and circular interpolation....gotta remember most CNC stuff is designed to be user friendly....so, the gory details are hidden in the software below the G-Code and pretty much vendor specific....checkout the EMC stuff on NIST, I believe they delve more into what's under the kimono....

CNCMP,
Check here and download the documentation for MACH 2.

http://www.artofcnc.ca/DownLoads.htm

There are whole sections dedicated to how to set up acceleration, speeds, etc.

Most driver software is similar in concept, even though the particulars are different.

Ya, if you like all the gritty details in your face all the time, you can use something like a Galil motion controller card and program in native Galil commands. It makes g-code programming seem like tranquility personified :D