Basic CNC

setting the best step rate for the normal use of the machine depends on what it is doing. you can set the machine to have really fast rapids and it will have bad vibration at slower cutting speeds. you can set higher microsteps to smooth out slow feeds but the top speed of the motor is much lower. that is why if you can you want big motors to be able to have smooth low speeds and enough torque left to still move at high speed during rapids. i included 2 videos of my machine as i was still trying to work out the proper microstep i needed for the way i will use my machine. the first on is at a half step (400 pulses per revolution)note the vibration but at this setting the mill will move at 200 inches per minute rapids. the second video is set at 5000 pulses per revolution and note how smooth it sounds cutting at feed rate of 9. the mill will now only rapid at 100 ipm any faster and the motor does not have the power it needs and falters missing steps. you can actualy hear your motor missing steps anything that is not a steady humm or buzz is a missed step and recovery.so if it dont sound right it isnt right.

first run
[video=youtube;3PXo4sqKEyY]http://www.youtube.com/watch?v=3PXo4sqKEyY&feature=plcp[/video]

200 ipm listen to the motors
[video=youtube;1jWF1JW1JnE]http://www.youtube.com/watch?v=1jWF1JW1JnE&feature=plcp[/video]

my final setting for smooth motion at feed rates
[video=youtube;DcsYpc4I7mY]http://www.youtube.com/watch?v=DcsYpc4I7mY&feature=plcp[/video]

steve
 
sorry the last post was so big but i wanted you to be able to hear the sounds
steve
 
Bill Gruby said:
Not me. The motor moves the tool. The tool is doing the cutting. How do I know how much torque it will take to make that cut? I understand that from this I will know what size motor to choose. I there a formula or something else?

"Billy G"
Click to expand...
that is a very good question bill i'd like to know the math or what ever is really needed to decide myself
to tell you honestly my choices were made by watching videos of machines like mine, how they sounded and how fast they would move for the motors they had. i didnt like what i heard or seen so i went bigger. i already had a cnc with too small of motors and wanted to avoid that at all cost even if i had to wait for more money. the small motors work on my sherline but it is always just working on the border of failing and messing up the part.
steve
 
If you have never held a stepper motor in your hands and fiddled with it, you may be unfamiliar with detent torque. If you have access to a motor (those of you that don't will have to close your eyes and imagine), pick it up. Hold the motor in one hand, and turn the shaft with the other (no power connected). You will feel a sort of "clicking". The motor won't turn smoothly, it jumps from step to step. The force required to get the motor to move from one step to the other is the detent torque. It starts out high, and as you gets lower and lower, until it hits zero (halfway between two steps). Go a little further, and it starts rising, but it's reversed now, pulling you towards the next step, instead of back to the last. That's what John is talking about. If you power the motor, and turn the shaft (if you can), you will feel this in a _much_ more pronounced way. Basically, the torque is greatest when the rotor is aligned with a pole, this is why you get less torque out of a half or micro step setup.

As far as measuring the torque, 7HC, you could probably do that, it's probably a pretty tricky setup to do it right. Once you get that number, make sure you use a safety factor, because you never _ever_ want to reach the limit on your torque with a stepper, cause if you do, you're gonna miss step (and that is bad, mmm'kay).

To BillGruby, I don't know of an equation for motor sizing. This stuff gets pretty complicated, mainly because there are so many things to account for (friction, mass of the moving members, rotor momentum, cutting force on the tool). I've never actually seen an equation or table for the force required to move a tool through a workpiece (I have looked, though not for a while), so I think that is the trickiest part.

That is why my advice is, copy your neighbor. This is your best bet, unless you are converting a really uncommon machine, especially if nobody has done a conversion on one (or published their result).

If you really want to try calculating your torque needs, I would do this.

1) Turn up a disk about 3 inches in diameter, with a center hole that will mount in place of your handwheels.
2) Mount it on one axis, and wrap a piece of cord around it. To the other end of the cord, attach a small container (small plastic pail, etc)
3) add sand or shot to the bucket (slowly!) until the axis starts moving (if it sticks somewhere, add more weight till you get past that).
4) Measure the weight (including the bucket). This weight times 1/2 the diameter of your disk is the torque required to move that axis.
5) Wind the cord around the disk the other way, and repeat (depending on how worn the screws/nuts are, they may be different).
6) Repeat on all axes
7) Take the maximum value you get here, and double it. Then look for a motor that has enough torque to meet or exceed this value, at an RPM that will give you about 100IPM (check the torque curve).

I think you could do pretty well with this method, but it's not what I used. I basically got the largest servos I could find and went with those ;)
 
missed steps can be real bad, it is not just accuracy problems. say your into the part at some depth the pc thinks the motor made its move on x and it didnt it then moves y for an example at full depth into a location not intended you will remember the sound of your motor grinding to a complete halt or the cutter snapping off, parts flying dislodged clamps screaming belts. this i know how to do well and avoid at all costs lol
steve
 
7HC said:
John, I've read and re-read your post but I'm afraid that it it just went to prove that I'm not nearly as smart as I thought.

I'm ashamed to admit that I have no idea what 'detent torque' is, and knowing that detent torque is a sinusoidal function that completes one full cycle for every natural step position didn't help at all.
Same goes for the entire content of paragraph four, it left me :headscratch:
I won't go on because I'm just demonstrating my ignorance, and I'm obviously thinking that 'basic cnc' is much less basic than I'm able to under stand. :confused:

However, if you or someone else could provide a 'Dummies 101' of when and why half stepping or micro stepping should be used, and what the pros and cons are, I'd be grateful.


M
Click to expand...

OK, sorry about that. I'm so close to it it's hard to step back far enough for it to make sense to someone who isn't a motor guy also. I have had technical discussions with many regarding paragraph four. It's a hard to comprehend topic. What's the worst of all is people who argue that it can't be a problem for their application only because they don't understand it.

OK, so here's the low down... Ever turn a stepper motor by hand? Notice it doesn't rotate smoothly. It wants to stay in certain discrete positions even though nothing is connected to it. That's detent torque, also known as cogging torque. Some motors are designed to have low or zero detent torque (like servo motors). Some motors are designed to have high detent torque for appliocations where it is desireable to have high holding torque in an unpowered condition.

Detent torque and microstepping are generally not good friends because the detent torque adds/subtracts (depending on where you are) to the motor's powered torque resulting in less than smooth motion.

I would say that full stepping or half stepping (as opposed to microstepping) are best for a CNC application as they will yield, by far, the most reliable motion and the most consistent torque output. After all, what good is CNC when you can't count on the tool moving at a desired rate or being in the proper spot?

John
 
DMS said:
If you have never held a stepper motor in your hands and fiddled with it, you may be unfamiliar with detent torque. If you have access to a motor (those of you that don't will have to close your eyes and imagine), pick it up. Hold the motor in one hand, and turn the shaft with the other (no power connected). You will feel a sort of "clicking". The motor won't turn smoothly, it jumps from step to step. The force required to get the motor to move from one step to the other is the detent torque. It starts out high, and as you gets lower and lower, until it hits zero (halfway between two steps). Go a little further, and it starts rising, but it's reversed now, pulling you towards the next step, instead of back to the last. That's what John is talking about. If you power the motor, and turn the shaft (if you can), you will feel this in a _much_ more pronounced way. Basically, the torque is greatest when the rotor is aligned with a pole, this is why you get less torque out of a half or micro step setup.
Click to expand...

Thanks DMS. I couldn't say it better! My only comment is that detent torque hits a maximum 1/4 of a step away from each detent. It starts low, peaks at 1/4 of a step, hits zero at 1/2 step, then starts to ramp up in the opposite direction until 3/4 of a step, then falls again to zero when you reach the detent. In essence, detent torque is a sine wave between detents. In other words, the rotor will be able to stop wherever there is a position that has zero torque (like a detent). If there's torque, the motor will move until there's no more torque.

John
 
Gonna try that method of measuring the torque tomorrow. I will post the results with pictures.

"Billy G" :))
 
the method described for measuring torque required does work but it is leaving a lot of factors out. for it to really be useful it would have to be done while cutting something hard and deep at a desired feed rate that weighs a lot. it is just figuring what it takes to move the axis nothing else. i'm not trying to correct the information just adding to it.
steve
 
I'm going to do it cutting and non cutting Steve. Cutting will be with aluminum and a .040 depth.

"Billy G" :thinking:
 
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