Hello All, I am planning on building a cnc router for my art students use at my university. I own and use a workbee at home but I am looking for something more sturdy and a bit faster for school. Classes are 3 hours long and I want all my cuts to be less than that time period, whenever possible. We won’t be cutting anything harder than hardwoods (nothing harder than janka 3000). I’ll be using linear bearings and 12mm lead screws. The travel size will be approx. 3’ x 4’. I was planning on using the blackbox controller and the 345oz steppers that are sold in the parts store here. Then I remembered that my workbee was limited to 100ipm cut speeds. Is this a limitation in the parameters set by the profile for the workbee? And if so what is it based on? The stiffness of the machine? The stepper motors? The drivers? The horsepower of the router/spindle being used? Could I increase the cutting speeds in the profile? What would I need to do to increase those speeds? I know I can just change the numbers in the parameters but I want it to be reliable. Any observations or experience that could help me make a decision would be greatly appreciated. I know I can just buy a professional machine and not worry about these issues but my schools budget is in shambles and it becomes harder and harder to justify spending 6 to 10 grand when I can spend 3 grand. Thank you, Michael Yeomans
Most major limitation is stepper drivers, the BlackBox has some beefy 3.2A RMS (4A peak) drivers, that can help push things along quite a bit faster than say an xPro (1.5A RMS max drivers) etc The next major limitation is flex in the frame, see How to calculate V-Slot® deflection - the smaller and stouter the machine, the faster and harder you can push it. Cutting loads can flex the extrusions leading to chattering or breaking bits. The Grbl profiles are set conservatively (well on the safe side) so you can absolutely play with those. Bumping up max feedrate, acceleration etc will all contribute. The CAM profiles of course set the actual feedrate, so spend a little time dialing those in too And lastly, of course, leadscrews are slower but more precise and have better torque. Depending on what you are cutting (if its just foam for example) Belts can be faster too (though less precise and with lower torque which can lead to skipping in harder materials)
Grbl-side, the limiting factor is pulse rate of the controller (most often Arduino Uno), which is around 30k per second while still stable. On a 8mm leadscrew this translates to a maximum of 72 m/min (8mm per rev / 200steps per rev * 30 000 pulses per second * 60s), and even more for a belt driven of course. This of course would neccessiate a very high acceleration rate, no microstepping and like mentioned in the previous post, some rather high power driving on the steppers (for the 345 oz-in ones I recall the optimum voltage being over 70) and massive rigidity to be able to actually cut in the theoretically usable feedrates. But, with a "normal" setup you should be able to rapid at about 5m/min (200 ipm) without problems. Cutting hardwoods, however, you're not going to be cutting at that speed with a non-reinforced Workbee.
