[LeeWay]

One thing I can say about the 495's that I have on the mill, they have 3/8" shafts. This plus the fact they were Nema 23's and nearly as large as I'd seen was my deciding factor in buying those. The ones he offers now are different motors with only 1/4" shafts.

My choice now for his larger 23's would be the 382 oz model. It does have 3/8" shafts and would work best at 3 amps and 75 vdc. Thats with the inductance formula and it would be 72 with the 20 times rated voltage.
Still only going to get 2/3 top speed with them but likely plenty of torque.

I think we have all been throwing around directly coupled motors too. A belt drive might change things up some.

[ger21]

As I understand it, you always want to use the largest power supply possible to get the most you can out of your motors.

Only if you're going to be using every bit that you can get out of the motors. Say for instance, that 50V gives you 2000 usable rpm. But you'll never run your motors faster than 800rpm. In that case, a 24V power supply would give you the performance you need. The extra voltage would cause your motors to run hotter, and nothing more in this case. And a 24V power supply would most likely be a bit cheaper than a 50V.

While those motors can be run on 50V they will not put out the full torgue that they are capable of. I don't know how to calculate the decrease in torgue caused by using a power supply less than the maximum possible, but if it is directly proportional then with those motors and a 50V power supply it would put out 59.52% of the torque as opposed to using the 84V power supply. If this is correct then you would only get 253 oz-in out of those motors with a 50V power supply.

Again, not entirely true. At very low speeds, a stepper motors torque is proportional to current, not voltage. If your turning at 10rpm, you'll have the same torque using a 24V supply as you will with a 50V supply. But as a steppers rpm increases, the torque starts to decrease. The point at which the usable torque falls off, however, is proportional to voltage. So if you can run your machine at 500rpm with 24V, you should be able to run it at 1000rpm at 50V.

There's really more to it than just finding the motor with the lowest inductance, unless you've designed your machine to utilize the maximum rpm those steppers are capable of. If the 270oz motor will give you 2000 useable rpm and the 425 will only give you half that, but you only need 1000 rpm, then the 425's will give similar speed performance (But with more force). Or, you could use the 270's, gear them 2:1, and have even more torque than the 425's for less money, although the gearing will cost more.

Keep in mind that I'm making up these rpm numbers to try to get my point across.

[wolfdagon]

Say for instance, that 50V gives you 2000 usable rpm. But you'll never run your motors faster than 800rpm. In that case, a 24V power supply would give you the performance you need. The extra voltage would cause your motors to run hotter, and nothing more in this case.

I really don't know how fast I will want to run my motor. Until I actually se the machine running and start cutting with it 2000 rpm (200 inch/min) vs. 800rpm (80 inch/min) are just numbers that don't really mean that much to me. So for me, it seems better to go for the bigger power supply just in case I decide I need the extra speed. As for the motors running hotter, this should not be a problem with the drives we are talking about,if I remember correctly from the test results that Mariss posted.

Again, not entirely true. At very low speeds, a stepper motors torque is proportional to current, not voltage. If your turning at 10rpm, you'll have the same torque using a 24V supply as you will with a 50V supply. But as a steppers rpm increases, the torque starts to decrease. The point at which the usable torque falls off, however, is proportional to voltage. So if you can run your machine at 500rpm with 24V, you should be able to run it at 1000rpm at 50V.

There's really more to it than just finding the motor with the lowest inductance, unless you've designed your machine to utilize the maximum rpm those steppers are capable of.

Thanks for the explanation about torgue fall off. I'll have to remember that. Like I said though, until I get the machine up and running I won't really know whether I need the extra speed and torque. So even though it might not be the right choice I will still try to use the larger power supply.

Kenith

[Crevice Reamer]

With 5 TPI Ball screws, you can get away with less torque and less RPM. With 10 TPI Acme screws though, you need all the torque AND speed that you can get. If you gear down with pulleys, Then you need as much torque at the highest speed you can get.

CR.

[ger21]

With 5 TPI Ball screws, you can get away with less torque and less RPM. With 10 TPI Acme screws though, you need all the torque AND speed that you can get. If you gear down with pulleys, Then you need as much torque at the highest speed you can get.

CR.

This starts to get complicated. In a perfect world, 1/2-10 acme can deliver the same force as 5tpi ballscrews and only requires half the torque to do it. Twice as many turns per inch doubles the mechanical advantage. However, once you factor in efficiency, ballscrews are twice as efficient as 1/2-10 acme, so it's basically a wash. However, the problem is that as rpm's increase, the 1/2-10 needs to spin twice as fast, and since torque falls off as rpm's increase, the acme will run out of juice sooner. But at lower speeds, they should perform similarly.

And if you gear down, you increase the force, so it again balances out until you get to higher speeds.

But if you compare all screws with the same motors, fewer turns per inch will win out. Many have proven this by building Joe's new machine using 1/2-10 5 start acme (2 turns per inch), which has shown to deliver the fastest speeds with smaller motors.

Not sure if I clarified this at all, but I have to run and am trying to rush through this.

[Mariss Freimanis]

ger21 is right.

It's power (Watts) that gets things done, not torque and not RPM. Power is torque times RPM. 1 oz-in of torque at 100,000 RPM is exactly the same as 500 in-oz at 200 RPM. It is 74 Watts (1/10 HP) of power.

1) Step motor power is proportional to the power supply voltage. Double the voltage and you get double the Watts from the business end of your motor.

2) Step motor power is inversely proportional to the square-root of its inductance. Sounds fancy and complicated but it's not. A 1mH motor will have twice the power of a 4mH motor at the same power supply voltage (and motor size).

3) A step motor's low speed and holding torque is proportional to phase current. You cannot monkey with this value because it's set by the motor's manufacturer. If they say it's a 3A motor then set the drive to 3A.

4) A step motor is a constant power output motor. This is important to understand. From zero speed to the corner RPM, torque is constant (flat) and power output (Watts) increases linearly with RPM. Past the corner RPM, torque drops off inversely with speed. Double your speed in this region and you get half the torque. Same power either way.

Double your supply voltage and you double the corner RPM. The holding and low speed torque stays the same. You get twice the power because the corner RPM doubled; same torque, twice the RPM gives twice the power.

Change out a 4mH motor for a 1mH motor and you get twice the power. Why? The corner RPM of the 1mH motor is double the 4mH motor's corner RPM. The holding and low speed torque remains the same.

5) The Secret to Everything is gearing. You probably wouldn't be happy with the results if you attached a 1 oz-in at 100,000 RPM motor to a 5TPI screw. You would be happier with a 500 in-oz at 200 RPM motor on the screw. The power is the same; 74 Watts either way.

So, how do you gear the motor correctly? First, you want to run the motor at its corner RPM. You bought the motor and you expect it to give you the power you paid for. You get that power at the corner RPM. Knowing that speed is important enough that the G203V has a yellow LED that lights up when the corner RPM is reached. It marks the end of the flat torque region and the beginning of the inverse torque region.

It is not easy to determine the corner RPM without instruments. The G203V senses when the motor current refuses to reach the current set value and lights the indicator LED. This change can also be read with a multimeter set to "AC Amps" in series with a motor phase wire.

Once this speed is determined, gear the motor to the mechanism so that it's maximum design speed coincides with the motor turning at its corner RPM speed. There will be plenty of torque left over for rapids if needed.

I will run some experiments in the coming weeks to correlate multimeter AC Amp readings against a dynamometer and try to come up with an easy formula everyone can use.

Mariss

[Santa Fe Al]

Hi Guys,

WOW !!! You guys sure know your stuff.

In anticipation, I attempted to turn my X-axis 1/2 x 10 turn screw - 54 inches overall length, with 47 1/2 inches between bearings and using two bearings separated 1/2 inch with a spacer at each end.

I managed 827 rpm, by digital rpm counter, and had very minor whipping action. I had stretched the screw tight by using nuts on the outside of the bearings to get the stretch.

I can't wait to get this whole show up and running. Just waiting for one more piece of material to finish my Z-axis - oversight by me.

Then I will be incorporating all of your data for the stepper choice. Hope I do that right - I should if I learned anything at all.

Mariss, you are so far over my head, I get nose bleed looking up that high. As for the rest of you guys, my body only has so much blood to lose. I do appreciate all of your knowledge and information that you are providing. I can even understand some of what everyone is saying.

What I now know, thanks to ALL of you, compared to what I used to not know, I feel as if I have been sent to school for a crash course with megatons of information.

Without ALL of you, I probably would be making major mistakes which translates into unnecessary expenditures. At least now, I can do what I set out to do and make the necessary compromises at a reasonable cost.

Al

[LeeWay]

I don't think you need to sweat the small stuff at this point, Al. Just about any of the motors we have been discussing will give you good service with the little Gecko's. Some just a little better than others, but not enough to make or break a project. The money involved in the difference in motors is also minor.
In other words, get what you would like to get and I don't think you will be displeased.

[Mariss Freimanis]

Al,

My comment would be leadscrew pitch, 10-TPI in your case, is gearing as well. Gearing changes the mix of force and velocity. Linear power is force times velocity. If you want the units of power expressed in Watts, force in Lbs and velocity in IPM, use:

Watts = Lbs * IPM / 531

Your screw converts rotary power into linear power. RPM and in-oz get converted into IPM and Lbs of thrust. Use:

Lbs = pi * TPI * in-oz * eff / 8

eff is your screw efficiency, 0.95 for ballscrews, as little as 0.15 for the crappiest Acme screw. The more TPI, the less efficient the Acme leadscrew. Another way of looking at it is a really crappy (inefficient) Acme screw needs a motor over 6-times as powerful as what's need on a ballscrew (0.95 / 0.15). The ballscrew may cost more but the motor and drive costs less. There must be a happy medium in there somewhere.:-)

The other downside is what happens to worst case lost 85% power being put into a really inefficient screw? It goes up as heat in the nut. I once talked to a guy who was using a 500W servomotor to drive a really bad screw. He wanted to know why oily smoke was coming from the nut.:-)

827 RPM means 82.7 IPM (IPM = RPM / TPI). Thrust per in-oz of torque becomes:

Lbs = 3.14 * 10 * 1 * eff / 8 or about 4 Lbs per in-oz on a ballscrew, 2 Lbs on a really good Acme thread (eff = 0.5). This means a 250 in-oz motor gets you 1,000 Lbs and 500 Lbs force respectively.

It's not true "The more torque the better". Assume the single most important Law of Nature is "There is no free lunch" and ask yourself "What is the extra torque at the expense of?" The answer will be "Speed". Without more power, it always comes at the expense of speed. The 500W servo guy found out there are limits to power as well on a poorly balanced design (cheap mechanicals / powerful motor).:-)

I design electronic circuits but I think the following applies to machine design as well:

Outline the specifications for what you propose to do. Calculate the power necessary to meet the goals. Identify all the variables affecting the design. Many of them will be conflicting, especially the $$$ one. Enter them into a spreadsheet. Get a feel for how changing one affects the others. Massage the numbers to tune the design, usually to minimize the $$$ variable. Then put pencil to paper, design it and order parts.

Sorry for the incoherent rambling.

Mariss

[Santa Fe Al]

I don't think you need to sweat the small stuff at this point, Al. Just about any of the motors we have been discussing will give you good service with the little Gecko's. Some just a little better than others, but not enough to make or break a project. The money involved in the difference in motors is also minor.
In other words, get what you would like to get and I don't think you will be displeased.

Thanks Lee.

When this machine is finally up and running, the gnawing results will tell whether I built correctly or not. A few of the things I am anxious about are:

tuning of motors - ??????
precision - reasonable
appearance - looking good so far
functionality - ??????
repeatability - ??????

Then, of course, I will have to make sure my CAD drawings are correct and I convert them to proper G-code.

Not too long ago - about one year, I would never have even considered any of these things. However, a friend of mine had a Harbor Freight vertical mill which he converted to CNC and the bug bit me. It's been a year now and I am almost there.

Six plus months of figuring out what a Router table is and how it works and what to do with it. What G-code is and how it works, etc.

Six months of designing and finally building and learning as I do my build.

Now I am getting to the point where I succeeded or wish I had never started.

I blame my friend for turning me on to CNC and all of you for me being at this stage in my life - referring to CNC.

I wouldn't trade it for anything.

Al

[Santa Fe Al]

Hi Guys,

Here are a few photos of my machine. All is finished except for the Z-axis, table top, Steppers, Drivers, wiring, etc.

It really is near completion. These photos are part of my build record. All aluminum was cut with my Radial Arm saw, Drill press and hand file. A rectangle isn't always a true rectangle.

Despite the colors shown, it is White and Warm Yellow

Al


#1 - Base unit
#2 - Incomplete table and Z-axis
#3 - Router table and Gantry
#4 - Incomplete Z - waiting for one more part before final assembly. The outside corners really are square.

[Mariss Freimanis]

You sir, are a craftsman.

Mariss

[ger21]

A few of the things I am anxious about are:

tuning of motors - ??????

"Tuning" steppers is a bit misleading, imo, because you don't really need to "tune" anything. All you really have to do, is set your steps per inch (20,000 in your case) and tell your software how fast you want to rapid and accelerate. Al you need to do is a bit of trial and error testing.

[Mariss Freimanis]

Pretty much right except for when you use our drives. The "ADJUST" trimpot allows you to tune out any remaining residual motor vibration. Miss-set, it's just another ho-hum microstep drive. Set correctly, your motor turns as smoothly as a servomotor. Sometimes the difference is like night or day.

Mariss

[ger21]

I was referring to the software setup aspect of "tuning". Thanks for all the clarifications.

[Santa Fe Al]

Thanks Guys,

From my posts, you can see how inexperienced I am. I worry about everything, and still have normal blood pressure.

Actually, I am in an area I knew nothing about and still need to learn a lot.

Mariss and Ger, I think I'm going to stay with the Nema 23's and of the bigger size. With a decent Power supply - after proofing my build - and using the G250's or G540, I hope I'm not going to regret my decision.

Mariss, Do you accept trade-ins??

If things turn out not to my liking of how it operates, I can always make any necessary changes.

I want to thank everyone for ALL of their inputs. Even so, it hasn't been easy. Just learning all of the variables is a challenging and ongoing process. I have learned that I need to learn a lot more. So many things to cram into this old brain !!

However, in the hopes of not delaying any longer, I am going to proceed with the project and continue on reading and refining the data that keeps coming forward.

Once this part is done, expect me to start asking not so bright questions about bits, cutting methods, G-codes, and a multitude of other things which you guys already know how to do.

At least I'll be doing my share about posting so I won't receive those, "You haven't Posted in a while", comments when I log on.

Thanks again,

Al

[Santa Fe Al]

Thanks for letting me drift off-subject in this thread. I originally wanted to get an idea about stepper sizes. It seemed to take off on it's own, with my help, even though we kept coming back to the steppers.

Al

[Mariss Freimanis]

I apologize if this is a little personal. I'm assuming the picture in your posts is you. If it is, I'm your age. You are not some jerk-faced 20-year old who won't tumble on which side is "up" in life for another 5 or 8 years. I saw your work and I saw you are a craftsman. You have lived long enough to have been bitten in the rear end a few times, enough to be careful about what you do now. It's called experience and only many years on the planet earns you that.

You do things carefully and consider the consequences of a misstep. You may be doing something new with this and experience tells you to be careful. My view is if you miss, it won't be by too much. You are on track.

Learning about anything is not a big deal. Most knowledge is very simple and straightforward. Avoid a**holes that turn what is simple into something mere mortals cannot understand. Most everything I know could probably be summarized in 50 pages of text requiring only a simple calculator to unravel it. It's not a big deal and you should never consider people like me big deals either.

Mariss

[Santa Fe Al]

I apologize if this is a little personal. I'm assuming the picture in your posts is you. If it is, I'm your age. You are not some jerk-faced 20-year old who won't tumble on which side is "up" in life for another 5 or 8 years. I saw your work and I saw you are a craftsman. You have lived long enough to have been bitten in the rear end a few times, enough to be careful about what you do now. It's called experience and only many years on the planet earns you that.

You do things carefully and consider the consequences of a misstep. You may be doing something new with this and experience tells you to be careful. My view is if you miss, it won't be by too much. You are on track.

Learning about anything is not a big deal. Most knowledge is very simple and straightforward. Avoid a**holes that turn what is simple into something mere mortals cannot understand. Most everything I know could probably be summarized in 50 pages of text requiring only a simple calculator to unravel it. It's not a big deal and you should never consider people like me big deals either.

Mariss

Hi Mariss,

You caught me. That is me in the funny Golfing hat and I'm 63 - acting like 45.

As far as holding you up high, I believe you deserve it. Your knowledge is, for me, overwhelming.

If it helps any, you are not alone in the esteem I hold for people. Too many to name on this thread, and others, who know so much. I'm just learning these things after all my years.

As far as being a craftsman, I make what I need, or want, and it usually works as designed. If not, I make the fix.
Sometimes it even looks pretty.

On the Z-axis thing, this is my first and unprepared attempt at working with aluminum. I've cleaned it up since the picture was taken. Square actually looks square, now.

Basically, I see, I like, I try to build (sometimes with modification to suit my need) and wonder why I didn't think of it. You see - I do have an ego.

Al

[Mariss Freimanis]

No big deal. I just want to tip my hat, nod, and pay my heartfelt respects to another greybeard like me. I shave, you don't. If I didn't I'd be just as distinguished looking as you are.:-)

Mariss