Alright, I'm trying to wrap my head around this concept for basic machine design.
High TPI results in better mechanical force.
Low stepper speed results in better electro magnetic force.
If you wanted to build a high speed machine which trade off will result in the better net results.
For the sake of this I am assuming accuracy and resolution will be within desired parameters with either build.
Desired results are good strength for roughing out parts, good speed for fast pocketing, the ability to overcome any minor drag or bind points in the machine, and the ability to move a heavy carriage.
Obviously screw and nut types and quality will have a major affect as will the quality of the machine build itself. I'm looking for a general major difference here in the one aspect of the build.
For such work..one would use ballscrews rather than screw and nut types.
"A conservative who doesn't believe? in God simply doesn't pray; a godless liberal wants no one to pray. A conservative who doesn't like guns doesn't buy one; a liberal gun-hater wants to disarm us all. A gay conservative has sex his own way; a gay liberal requires us all to watch and accept his perversion and have it taught to children. A conservative who is offended by a radio show changes the station; an offended liberal wants it banned, prosecuted and persecuted." Bobby XD9
Trade off what vs. what? It _appears_ that you've settled on stepper motors, and you're looking at trading off step rate vs. thread pitch (or gearing).
In my experience, from a pair of similarly sized motors you can get a lot more useful power out of a DC motor than you can out of a stepper. Granted, there are clever circuits out there to increase the power out of a stepper -- but by the time you've reached the end of that road you're still not generating as much reliable shaft power as you can with a DC motor, you've got a circuit that's every bit as complicated as a DC motor with feedback, and one that's possibly even more obnoxiously obscure in its workings.
So _I_ think that, given the available motors and drivers out there today, that your _first_ good tradeoff is to use motors with good encoders or tachometers on them in closed loop with servo control. If you just have to follow the stepper model, use controllers that make a DC motor pretend to be a stepper.
While they are a type of screw and nut...blink blink..they impliment the differences rather dramatically.
Ballscrews are tough, strong, able to take loads that would bust a nut and screw, far more accurate and last far longer.
With ballscrews..one simply needs to use servos/steppers that are fast and strong
Or are we talking on different pages here?
Gunner
"A conservative who doesn't believe? in God simply doesn't pray; a godless liberal wants no one to pray. A conservative who doesn't like guns doesn't buy one; a liberal gun-hater wants to disarm us all. A gay conservative has sex his own way; a gay liberal requires us all to watch and accept his perversion and have it taught to children. A conservative who is offended by a radio show changes the station; an offended liberal wants it banned, prosecuted and persecuted." Bobby XD9
You're right of course, but Bob already has his steppers.
There are engineering calculators out there to tell you how much torque you need. And your stepper spec sheet will give torque vs. RPM. A bit of study will get you in the right ballpark for gear ratio to give the torque you need at low speed. We just don't have the information to tell you what ratio to go with. I can say err on the side of extra torque and sacrifice top end speed. To little torque and you screw up parts with missed steps and it only takes one little section of your program. Top end speed is just not that important for a hobbyist.
My fist CNC was a stepper on a knee mill. After I screwed up a couple parts, I went from 1:1 to the ballscrew to 4:1. And that was with 2000 oz in steppers.
Maybe -- too high a TPI (too fine a pitch) means that the shear strength of the threads is lowered.
Toss the steppers and use servos with encoders instead. They can put out lots of torque at high speeds.
Toss the (presumably Acme) threads and replace with ball screws and ball nuts -- very low torque needed to move against cutting forces compared to Acme screws. And -- the ball nuts and screws are typically not that fine, either.
Essentially, this is what serious CNC machine tools use.
If you are already considering steppers -- servos and the rest of what is in a good CNC machine are the better choice.
Ballscrews and nuts are typically quite coarse. But the major limit on your speed is the steppers, which you get around by using servo motors instead of steppers.
Bob, Your argument is wrong. The premise you have assumed is the use of stepper motors in a high load solution. Stepper motors, by their nature fight themselves. Output torque is the differential between slightly out of phase drive fields and is quite low in respect to the power applied. Under load they slip. A far better solution is the use of permanent magnet DC motors driven with PWM along with velocity and positional feedback. This solution is powerful, efficient, fast and accurate. It has been the solution of choice for computer disk drives for 30+ years. Furthermore, the parts required are available and inexpensive. Just as Gunner stated, you are locked into using ball screws, because of backlash. Ballscrews are low TPI. The more gears you use, the more backlash you build in. So, a high torque, direct drive solution is the only way to go. Steve
I have been looking at servos. inexpensive is in the eye of the beholder. Controller's, power supply and motors are about twice the price of comparable in/oz steppers. Given that in many cases the power supply is interchangeable. Most of that difference is in the servos. I'm not saying they are not better. I always figured a positive feedback encoder made perfect sense anyway.
I already have a fair number of steppers accumulate from various sources and three stepper controllers.
Basically with a stepper, the stepper motor is what's keeping track of where it is -- the motor is physically counting steps. If you try to drive it too fast, the average torque out of the motor falls to zero, period. That doesn't just mean that the motor won't push any more -- it also means that motor goes limp, and will allow itself to be backdriven. So unless you've got feedback external to the motors your machine position will be unknown until it's homed again. Even if you _do_ have feedback external to the motors, and a control loop that does something with it, what that control loop will see is a HUGE disturbance, which it will want to respond to by asking the steppers to go REALLY FAST -- which will just keep them unlocked.
In my formative years as a control system engineer I had a very bad experience with steppers. They had let a mechanical guy with no responsibility for driving the motors select them, with an electrical guy who didn't understand motors designing the circuits, and a software guy who barely had the mechanical ability to zip up his pants* writing the code. I got tossed into a project as a troubleshooter** after we had product failing in the field, and all I could do was offer management the choice between being reliable without meeting spec, meeting spec 70% of the time with spectacular failures the other 30%, or completely redesigning the mechanism.
So I tend to be down on steppers. But if you have time, space and power to waste, they're a great way to turn a shaft in a predictable way.
Not to denigrate the guy on the whole -- he was an absolute wizard when it came to processing video, and absolutely made up for any lack of mechanical smarts with his attitude and aptitude in other areas -- he just couldn't screw a bolt into a threaded hole to save his life, and should _not_ have been asked to drive a motor!
** The real trouble was that they demoted a vice president of _programs_ for crying out loud into a project management position -- so he's pissed off, disconnected, and not an engineer. This in a company that doesn't understand the meaning of systems engineering, which means that the systems engineering is either a consensus amongst the project members, or entirely up to the program manager. His response to systems engineering questions was "you've been hired to build a system -- go build a system".
OK, the basic problem is that stepper torque falls off rapidly with increasing speed. At some point, the available torque is less than the drag (friction plus machining forces) and the stepper starts to lose steps. To REALLY know what is going on, you have to get manufacturer's data charts on the torque vs. speed for the specific motor, drive and power supply voltage. This is the reason why so many setups use a stepper with a 1:1 drive ratio to the leadscrew. If it weren't for this loss of torque, you could use lots of reduction as is often used with servo motors, where the torque is pretty much flat across the speed range.
Once you have a chart of torque vs. speed, you can then figure linear force that can be delivered at a specific IPM feedrate. You can then compare what happens at 1:1 or at 2:1 reduction. Probably, in most cases, you will find that the belt reduction doesn't buy you anything except at standstill. When you near that motor speed where the supply voltage limits the drive, then the reduction will actually hurt you, you will get more torque to the screw without reduction.
OK, here's where you run into trouble. Since you are already looking at the difference between steppers and servos, I'll get into that shortly.
I'm not sure what "bind points" are, but mechanical binding will almost certainly cause steppers to skip steps with no warning to the computer. This not only can cause ruined parts, it can cause the machine to start cutting into itself!
Stepper motors are good up to about 200 W of mechanical power, ie. 1/3 Hp. Above that, you really need servos. Larger steppers just don't deliver much more power, but lots of torque at lower speeds.
All steppers suffer from a loss of torque at higher speeds. For instance, some particular stepper may generate 200 Oz-In of holding torque, and that will be the given rating of the motor. But, if you can get the curves on the motor, you will find that it only gives 100 Oz-In at even low speeds, like 600 RPM. At 2400 RPM, the delivered torque will be almost zero, just touching the shaft with your finger will cause it to stall. (These numbers are just typical for some size 23 motors, some are better, some worse, and apply ONLY to a specific combination of motor and drive, at some specific supply voltage.)
Another problem with steppers is resonance. Since the motor moves in STEPS, it produces a vibration every step. At some speed, this vibration matches the mechanical resonance of the motor, leadscrew and other parts. These vibrations grow until the motor loses synch, usually accompanied by loud rattling sounds. I would not use non-microstepping drives in ANYTHING today except toys. Comparing the difference between a full-step drive and a good microstepping drive like a Gecko is an astonishing night and day difference!
Another problem with steppers is heating. Modern drives reduce motor current at idle to reduce motor heating, but there is also SELF-heating. The moving magnets in the motor cause iron losses in the motor's laminations. If you spin a typical stepper motor at 2000 RPM, with no electrical connection, it will get quite hot. In fact, it will burn out in less than 30 minutes! The stepper motor makers will NEVER tell you about this nightmare! So, if you plan a machine that will run motors at high speed for a long time, it will self-destruct.
Servo motors generally don't suffer from these problems. Torque is roughly constant up to the speed where the self-generated voltage of the motor nearly equals the power supply voltage. Self-heating is usually quite small, and is proportional mostly to motor LOAD, not speed. Resonance is non-existent in motors designed for servo use, as the windings are set up to minimize velocity and torque ripple. A modern brush motor is just a little more complicated than a stepper, and brushless motor is no more complicated. Due to the efficient heat removal of the brushless motor, a smaller, and therefore cheaper motor can often be used and still provide far better performance.
Keling supplies some awesome size 23 brushless motors for $52 in single quantity. You still have to add an encoder, but the CUI encoder from Digi-Key is now down to $28. I make a drive for these motors for $150, and a 4-axis controller for $250. So, a 3-axis system can be built for $940, assuming you already have the power supply. Now, this is NOT picking parts out of your junkbox, but it will perform a LOT better.
You haven't defined "high speed" or "heavy carriage", so I have no idea what scale of things you are talking about. High speed machining means
75 Hp spindles running at 80,000 RPM and removing 640 Cu In of aluminum, if you ask Boeing. it might mean 1 Hp at 10,000 RPM and 1 Cu In per minute to a guy in his garage. Giddings and Lewis might think a 25,000 Lb gantry is pretty heavy, you might think 100 Lbs is heavy.
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