Steps per mm is 800, on ramps with DRV8825. Cutting speed is 480mm/min, not sure about rapid, probably somewhere around 2000mm/min.
800 step/mm on an 16mhz chip has a theoretical max of 50mm/s we have found it to be much lower than that. On top of that stepper torque drops off significantly with RPM and tends to actually be the limiting factor most of the time.
40,000"steps"per second 16mz Marlin max/800steps per mm=50mm/s I think 35k-38k is a safer bet.
8mm per revolution, 200rpm (3.3Rev/s) you lose a lot of stepper power=8*3.3=24mm/s and you are still pretty safe 300RPM (40mm/s) and you are at about half peak power. any slight binding will cause skipped steps, even too aggressive accelerations.
I keep forgetting to reply to you. I think the mechanical advantage of the leadscrews counteracts the loss in torque. With the belts I couldn’t get the steppers to stop unless I put a lot of force into it. With the leadscrews I could not get them to skip steps.
So did I, we used threaded rod initially 4500steps/mm. Insane mechanical advantage. Anything above 8mm/s and it would not even move. I know it doesn’t seem like it but there is substantially more friction compared to belt as they are the most efficient (beside balls screws), screws vary dependent on lubrication, anti-backlash protection drag, and the actual torque it takes to stop and start the rotation of a giant heavy metal rod (even more so in your case those things are very long).
In engineering everything is a balance, nothing is perfect, meaning there is 100% every single time a trade off. speed, power, cost, efficiency, ease of use, etc.
Measure this in real life, hands are horrible pressure gauges. Get a consistent material (MDF is great). Make slot cut 2-5mm deep, new cutters, fastest system wins (pulley Vs screw). Keep cutting slots at the same depth faster and faster until it skips steps. Now also test fastest rapid, 50mm/s is the firmware max. Then the “engineering” part is easy if there was a price difference for the winner was it worth it.
Now remember it does not matter if you can lift 100 lbs, we use a rough number of about 3-5lbs max cutting force divided by a minimum of 2 steppers, usually some combination of 4, at an average speed of 8-15mm/s. You will find belts to be the most efficient (ball screws I think are slightly better), most accurate (very little backlash and does not drift much over time), most cost effective solution (more so with added length). BUT You have yours built, please test your build. Very few actually make one, over the years sooooo many have asked about it, you are one of very few, if not the only one, to actually do it.
Right now I’m gonna start making christmas gifts, so no formal testing, but I’m sure I’ll see if there is a problem. I do have extra leadscrews, if you want I can send you them and the files for all the parts.
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I forgot to reply directly to you again.
I can tell you, from helping people in the forums. Using a 1 start leadscrew, for example, will make your max speed lower. The lifting force at the ends is higher (at low speeds), but the torque turning the screw is lower (because the steppers are moving faster). But the friction is the same, or more. I’ve helped a lot of people with 1 start leadscrews who skip steps in Z and we adjust the max z speed down on those machines.
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These leadscrews are 4 start.
I get that, but you can apply that lesson to these leadscrews too. The motors have to spin a lot faster (4x or so) to travel at the same feedrate. Yes, you’re getting more force (at the trucks) than belts. But the friction is higher and the motor torque is smaller at the higher speeds, so friction can make you skip steps. Increasing the voltage of the power supply (a 24V, but that can’t be used directly with the RAMPS) can give you more torque at higher speeds.
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