Originally Posted by mitip  Thanks guys, I am trying to understand the differences between what we used to do and what these benchtop machines can do. We used to post as Fanuc, so that is OK. Also I read that steppers are nearly as accurate as servos, but you are suggesting a servo set up would be better?
You make a point about the accuracy of the machine needed, I am not clear how rigid these smaller machines are or whether it is worth considering an older machine and doing a refit. I came across a machine that is half way between a Bridgeport knee mill and a benchtop, very solid cast construction, I could scrape the slides and get it working well but would there be any benefit? All the parts are heavy, especially in z, would the torque needed be too much for steppers?
Last point, I gather with Mach3 the data drip fed to the controller via parallel, is the point you are making that this is not a limiting factor? I recall early Interact controllers that would simply stop and dwell when they ran out of data. You mention 1000 lines look ahead, which I recall was a figure from the past, is this OK when doing high feed rates. I have a brand new NSK 65,000rpm air head worth about $4000 I was planning to use but it does not have much torque but should be OK with 1-1.5mm cutters although someone mentioned electronic heads that are OK for 36,000rpm. One of the guys who did our programming never used more than 20,000rpm but for plastics I found the faster the better as long as you can get the heat and swarf away and stop burning. What is the emc2?
Mike |
mach doesnt drip feed in the normal sense. mach IS the controller, and simply outputs step and direction pulses. 2 pulses = one command to move one step.
the limiting factor for mach speedwise is the number of pulses you can send out per second. in the current incarnation of mach3, using my 2.8ghz pentium 4 pc i can safely output the maximum of 100,000 pulses per seconds. safely, you could average 40,000 pulses per axis per second in typical 3 axis 3d contouring. thats 2.4 million pulses per minute.
in microstep mode, you have 2000 pulses per revolution on a stepper motor. using my sieg kx1 (desktop mill) which has 4mm pitch screws i need 12700 pulses to move 1 inch. this means i have a practical speed limit of 188 inches per minute for 3d contouring and an absolute maximum of 472 ipm one axis at a time. reliable positioning accuracy barring mechanical considerations is .0004" which is good. the acceleration on my machine is tuned to about .05g if i want to get to 188ipm. this is decent enough for my use, but not great. this is a limitation of the stepper motor power (190 oz-in).
so, properly configured, my machine is accurate and more than fast enough for the 5000rpm spindle, but nowhere near high speed and under stepper control wont get there without going to a larger lead screw and sacrificing accuracy and even more acceleration.
using mach with a servo system allows alot more fun. you can use the servo drives internal "digital gearing" to make better use of the steps from mach - instead of microstepping each step could then corespond to a line of the encoder. you could easily get up to 10 times the speed yet maintain the same accuracy.
from EMC2 you could use the servos via torque control as well, which as my understanding goes can allow another 2x speed up with the same amount of pulses. you would also get the most torque in this mode making the best of acceleration... my math says my 300oz-in (peak 6 seconds) nema 23 motors could push my particular mill at .5g happily.
BUT. and this is the big one with these little machines... the mecanical limitations of my kx1 (which is a nice inexpensive but high quality machine) wont allow this. at best, 250 ipm would be great on this little machine given the bare iron dovetail ways, tiny ball screws, play and flex in they system. top speed is also inhibited by how much it can ramp up over the mere 5" of travel on the y axis. even at .5g, it take a few inches to get to the theoretical 1880 ipm, so in most contouring with small sharp details, its barely going to get past 2-300ipm in the best case scenario (this is true also for many super expensive machines).
so basically what im getting at is "high speed" is alot more complicated than slapping on a 60k spindle and hoping for the best
to get the best of it, you want a linear way machine with peppy servos and that starts costing 5 figures real quick.