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Basic CNC

Discussion in 'CNC IN THE HOME SHOP' started by Bill Gruby, Oct 7, 2012.

  1. Ed of all trades

    Ed of all trades Active Member Active Member

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    Thanks guys, I now have more of an idea of what I Don't know. Before I just knew I didn't know anything about it, now I have an idea of the way the system works. The one thing I did not get is this. Is a ball screw a feed screw that uses balls instead of a threads on the "nut"?
     
  2. JimDawson

    JimDawson Global Moderator Staff Member Director

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  3. Tony Wells

    Tony Wells United States Vice President Staff Member Administrator

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    Built on the same concept as the recirculating balls in a Saginaw style steering box.
     
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  4. Ed of all trades

    Ed of all trades Active Member Active Member

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    Thanks Jim and Tony I figured It had to be something like that but I had no idea how it could be done.
     
  5. JPigg55

    JPigg55 United States Active User H-M Supporter-Premium

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    Okay, couple more questions on stepper motors.
    First is to do with physical size and weight.
    What is the typical physical size and weight of NEMA 23, 34, and 42 stepper motors ?
    For example, I have a Clausing 8520 mill that I’d like to install stepper motors on for direct drive of the axis, but really don’t want to have a 50 lb stepper motor that’s 12” long sticking out in 3 different directions that I'll have to engineer some sort of mounting system.
    Second, how would you decide if going direct drive or geared drive via gearbox or belt & pulley as far as required motor size ?
    If math is correct and works here, I could use a NEMA 23 CNC kit with 382 oz-in motors ($369) with a NEMA 23 5:1 planetary gearbox ($200 ea) for a total of $969 assuming 382 oz-in at 5:1 gear ratio would be the same as 1910 oz-in of torque, but with smaller, lighter, and cheaper motors.
    I know this would not be a direct correlation since the motors would have to operate at a higher speed, hence lower torque, for the same feed rate, but I think you get my drift.
    Third, what factors would you consider for sizing a 4th axis for say connecting to a rotary table or dividing head ? Since these type accessories have some degree or gearing integral to them, I’d think a small oz-in stepper could be used, but how would one decide on oz-in sizing ?
    Forth, is there an easy way to direct control stepper motors without using a CNC program and/or computer ?
    I want to add power feeds to at least 2 of my axis (getting tired of cranking, LOL).
    Accordingly, my research has shown that I’d need something like a Servo Type 140 power feed for my X & Z axis. Looking on eBay and other suppliers, a Servo Type 140 power feed runs anywhere from $650 to $800 each. That totals somewhere between $1300 and $1600 for adding power feeds to 2 axis.
    Now, looking at Automation Technologies, I can buy a 3 or 4 axis NEMA 34 CNC conversion kit with 1805 oz-in stepper motors and drivers for $1022 (3 axis kit) or $1350 (4 axis kit).
    If I needed larger steppers, they have a NEMA 42 kit with 2830 oz-in motors for $1193 (3 axis kit) or $1712 (4 axis kit) or, as stated in the second item, using smaller motors with gearboxes.
    From a purely financial aspect, it seems to make more sense to install manually controlled stepper motors than buying power feeds, plus being most of the way towards a full CNC conversion, if I so desired later.
    Considering this since I know nothing about CAD/CAM other than what I’ve read here, but not wanting to rule out the possibility for later. If I knew I wanted to go CNC for sure, I’d more likely sell what I have and just buy a Tormach or a Precision Mathews mill that’s already a CNC machine.
     
  6. JimDawson

    JimDawson Global Moderator Staff Member Director

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    I don't have time right now to compose a full answer, but 1200 oz/in NEMA 34 motors in direct drive would be more than enough. They weigh about 5 lbs, and are about 5 inches long. I'm using these same motors to run a 48x96 wood router at 150 IPM, and 300 IPM rapids. The router table probably weighs as much as your entire machine. Nothing wrong with Automation Technologies, but look at Automation Direct for steppers and drives also.
     
  7. JPigg55

    JPigg55 United States Active User H-M Supporter-Premium

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    Any particular reason for suggesting Automation Direct over Automation Technologies ? Just curious.
     
  8. JimDawson

    JimDawson Global Moderator Staff Member Director

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    Just for price comparison and my preferred vendor. Both vendors are good.
     
  9. JimDawson

    JimDawson Global Moderator Staff Member Director

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    One way to look at the torque requirement is to make a disk to fit the leadscrew and wrap a string around it and pull with a spring scale, a little math will give you the torque required.

    I am going to use a NEMA 23in the 300 oz/in range on my rotary table when I get around to converting it. A RT is a 90:1 gear, so doesn't require much torque to turn the crank.

    Without ballscrews more torque will be required to move the table which is the reason I suggested a 1200 oz/in NEMA 34. You can always turn the torque down if it's too much. On my mill the X and Y DC servos are in the 600 oz.in range, and I'm am using a 1280 oz/in stepper on the Z. I cut the torque back by 1/2 and it will drill a 1/2 inch hole in steel. My mill table is 10x54

    One problem with coupling the steppers directly to the axis as a power feed is that they cog. It makes hand feeding kind of lumpy. A method of decoupling the motor would be suggested. Maybe something like a lever operated dog clutch.

    A NEMA 42 is a huge motor, 20 lbs or so, about the same torque as a 3HP motor. Here is a picture of a NEMA 42 with a NEMA 23 sitting on top of it. Way overkill for your application.

    IMG_0479.jpg

    Stepper speed controls are cheap, I bought a couple of these, one to run a NEMA 42 stepper for a project, and the other just to play with. http://www.ebay.com/itm/9-24V-Input...59466c&pid=100338&rk=3&rkt=30&sd=281570110066
     
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  10. JPigg55

    JPigg55 United States Active User H-M Supporter-Premium

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    Thanks Jim,
    Any suggestions/links for a control method other than a PC for speed, direction, and possibly distance of travel including being able to switch between full, half, or micro stepping ?
    For the purpose of using more like a power feed, considering using something like Rasberry Pi or Arduino through a keyboard interface for this purpose.
    Was also considering some sort of adjustible limit stops to prevent powering into lead screw limits of travel.
    Still trying to learn a lot of this stuff, but figure it's possible.
    I was thinking of using some sort of coupling device to better allow manual operation, but I can't figure a way to do this for my Y and Z axes unless I use something like a belt & pulley cofiguration since there's only one handle on these axes vs two on the X axis. Thinking I'd have to use dual shaft steppers and connect the handles there.
     
  11. JimDawson

    JimDawson Global Moderator Staff Member Director

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    This is a stepper speed controller that does not require a computer for control. It has limit switch inputs but will not move a specific number of pulses.
    http://www.ebay.com/itm/9-24V-Input...59466c&pid=100338&rk=3&rkt=30&sd=281570110066

    Once you add distance traveled then you are into CNC control. Most if not stepper drives have several settings for steps per revolution and power output.

    The down side of putting handles on the steppers is that steppers cog when turned and cause the hand feed to be ''lumpy''. The other thing is that the steppers become a generator when turned by hand and if connected to the drive, require extra effort to turn. For hand feeding you would want to provide a switch to disconnect the motor wires from the drive if the motor is mechanically connected to the lead screw.

    A belt non-cog system would work on the Y and Z, also on the X for power feed, but for accurate positioning it would require a timing belt..
    .
    .
     
  12. JPigg55

    JPigg55 United States Active User H-M Supporter-Premium

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    Thanks Jim,
    Would using a Variable-reluctance stepper motor get rid of the "Lump" problem of hand operation ?
    Just turn off the power and manually feed ?
     
  13. JimDawson

    JimDawson Global Moderator Staff Member Director

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    Good question, and one I am not able to answer.

    A brushed DC servo motor would not cog. There are DC servo drives that will take a step & direction input just like a stepper drive. Higher cost than stepper systems but would make a great basis when you are ready to convert to CNC. Something in the 600 oz/in (~30 in/lb) range world work in your application.
     
  14. JPigg55

    JPigg55 United States Active User H-M Supporter-Premium

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    Since I was thinking of being able to drive a rotary table and/or indexer, figured I'd better stick with steppers.
    I'd think that would be a better option for controlling rotation for operations like gear cutting.
    Not sure if servo motors work well for this application or not and didn't think mixing and matching both stepper and servos would be a good idea.
    I read about Variable-reluctance stepper motors here: http://www.freescale.com/files/microcontrollers/doc/app_note/AN2974.pdf
    Says they don't exhibit magnetic resistance when rotating unpowered. Did a quick Google search for them and didn't find a whole lot so not sure how available they are.
     
  15. JimDawson

    JimDawson Global Moderator Staff Member Director

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    A servo motor or a stepper will do exactly the same job and just as accurately if set up correctly. It's really a matter of the drives, the feedback (encoder), and the controller. Adding feedback to a stepper system technically turns the stepper into a servo. I have a mix of brushed DC servos(X&Y) and a stepper on my Z axis and it works well. I consistently hold 0.0001 on my Z. See that build here http://www.hobby-machinist.com/threads/z-axis-cnc-conversion.21060/

    I am going to build a RT with a stepper on it for my 4th axis. Going with a stepper just based on cost, not function.
     
  16. JPigg55

    JPigg55 United States Active User H-M Supporter-Premium

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    Lot of good info.
    One question, you stated that a 1200 oz-in stepper would work for my application. You also posted that a 600 oz-in servo would work as well.
    I thought I understood the differences between steppers and servos from reading earlier posts in this thread, but I either missed something or don't quite understand.
    Why would a servo half the power rating work for the same application as a stepper with twice the power (meaning oz-in rating) or did I miunderstand your replies ?
     
  17. JimDawson

    JimDawson Global Moderator Staff Member Director

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    Nope, you have it correct. BUT.... You normally size steppers twice the torque that you size DC and AC servo motors because the steppers can decouple under load and lose steps the other motors will just stall, but won't lose position. A stepper with feedback (servo) may decouple but won't lose position.
     
  18. JPigg55

    JPigg55 United States Active User H-M Supporter-Premium

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    So, if I understand correctly and for example, I could get by with using a 600 oz-in stepper for my intended use since I would not (at least for now) be using a CNC program to run them realize that they would be undersized for a full CNC conversion or is there something I'm missing as far as the stepper controller is concerned ? Or does it have more to do with the way the different motors work ?
     
  19. JimDawson

    JimDawson Global Moderator Staff Member Director

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    On a machine of your size, the 600 oz-in would probably work OK for CNC with ball screws. Acme thread leadscrews require quite a bit more torque for the same load.

    A lot of it has to do with the way the motors are constructed and how they are controlled. By decouple in the case of a stepper I mean that the torque overcomes the magnetic force inside the motor, and the magnetic field continues to spin but it can't grab hold of the rotor.

    A brushed DC motor has no way to decouple, it just keeps producing torque, even if stalled.

    A BLDC (AC servo) is a 3 phase motor and normally won't decouple, but I think it can, but I have never seen it happen. I think the controller would shut it down before it reached that point. Somebody correct me if I'm wrong on this point.
     
  20. LoboCNC

    LoboCNC United States Iron Registered Member

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    The torque ratings issue is quite a bit more complicated that steppers being able to "de-couple" or lose steps, all though that's part of the reason you need a higher torque rating with a stepper.

    The rated torque for a stepper motor is the "holding torque" which is the torque the motor can resist when the shaft is not moving. The minute you start stepping, the torque drops by 20% or so. And as you move faster, the available torque drops even further, essentially down to nothing at high speeds. Large steppers may have only 25-30% of their rated torque at 1000 RPM. Also any vibration in the motor drive train is effectively added to the torque seen by the motor, further reducing the available torque to drive the load.

    Servo motors have 2 different torque ratings - the continuous torque (this is the rating most often cited) and the peak torque which may be 2x - 3x times the the continuous torque for a large motor. With a servo motor, though, the torque does not drop off with the speed - most servo motors will run at the rated torque at speeds up to 1000 - 3000 RPM. The other great thing about servo motors is that the controller can run them a more than the continuous rated torque (up to the peak torque) for short periods to overcome static friction.

    Also keep in mind that the force generated by a motor directly coupled to the lead/ball screw will produce a force (in pounds) of: (rated torque in oz-in) x (number of threads/inch) x 2 x pi x (lead screw efficiency) / 16. Therefore, 600 oz-in of torque applied to a 5 TPI ball screw (~90% efficient) would produce a force of 1059 lb. This is a fairly dangerous amount of force that would likely damage something on your machine. 200 oz-in would be more appropriate, but if you are using a 600 oz-in stepper, you may only have 200 oz-in of torque available at higher speeds.
     
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  21. JPigg55

    JPigg55 United States Active User H-M Supporter-Premium

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    Okay, been doing some looking around on servos. It appears one has the choice between AC or DC servo motors.
    From a hobbyist point of view, what's the difference between the two and what would be your recommendation in as far as ease of use and simplicity in set-up ?
    Some of the things I read seemed to imply that it may be a good idea to use gear reduction with servos.
    What are your thoughts ? Would say using a 2:1 gear reduction reduce the required servo size by half ?
    Lastly, I've seen the term "Holding Power/Torque" used in reference to stepper motors. Do servos have a similar process for holding position ?
     
  22. JimDawson

    JimDawson Global Moderator Staff Member Director

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    Great explanation!
     
  23. JimDawson

    JimDawson Global Moderator Staff Member Director

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    Primary differences are cost, construction, and operating principal. The AC servos are the modern equivalent of the older DC servo, normally less maintenance, but that is really a relative view. The only real maintenance on a DC servo it periodic brush replacement. Probably not something you would ever have to do on a home use machine.

    From a setup perspective, AC and DC servos are comparable. Both require tuning, and the controls are slightly more complex that s stepper system. Older DC servo systems are a bit more complex to set up, but not bad.

    You could do a 2:1 reduction and reduce the motor size, unless you really need 400 IPM rapid moves.;) A reduction is a good idea.

    As stated by LoboCNC, the torque curve is almost flat from zero RPM to max speed. A servo system has the same or more holding torque as a stepper of similar size.
     
  24. JPigg55

    JPigg55 United States Active User H-M Supporter-Premium

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    Thanks a ton, last couple questions.
    I've read a lot about tuning, but have never heard an explaination of what it really is.
    Do you have a CNC for Dummies explaination for it and is this more to do with the CAD/CAM programs and is it something I could basically ignore for now in as far as using them for what would basically be power feeds ?
    What would you say is a good speed range for Rapids ?
    Lastly, do you have any recommendations for a controller for use in manual operation of servos ?
     
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  25. JimDawson

    JimDawson Global Moderator Staff Member Director

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    Hmmmmm....Tuning Servos.. There have been entire books written on this subject. From a user perspective it really means getting the system, in this case your milling machine, to operate smoothly and accurately. This is normally as simple as making a few controller adjustments. It has to be done with with all drive system types.

    Lets use the car analogy. You are the controller, the car is the motor, and the feedback is all of the information that you are processing (sight, sound, seat of the pants feel), and you output signals to the throttle, brakes, and steering. This is a closed loop system. For simplicity we will ignore everything except the throttle and the brakes for this explanation.

    Let's say you are sitting at a stop sign and a block down the street there is another stop sign. So the ideal motion profile is to accelerate smoothly and quickly up to the target speed (speed limit), proceed down the street at the target speed, then decelerate smoothly and quickly to reach the target position (the stop sign). A new driver might accelerate too quickly or too slowly, not be able to control the speed, and brake too hard or not enough. So in the case of the servo system, you have to teach the controller how to operate the car. This is tuning.

    Many modern servo drives can be operated manually with very simple controls. A FOR/REV switch, an ON/OFF switch, and a speed pot much like a VFD. The setup is easy by making adjustments to the on-board software via the front key pad or connecting it to a computer. The parameters are normally Acceleration, Deceleration, Max Speed, and the Proportional Integral Derivative parameters (PID). I'm not going to try to explain the PID here, but normally the only value that you have to adjust the P parameter. This sets the ''tightness'' of the system, set to high and the system becomes unstable and the motor will oscillate, set too low the system is mushy. For manual operation not so critical, but for CNC operation it has to be right. Not difficult to do, just takes a bit of playing around to see how the machine reacts to the changes you make.

    Speed range for rapids..... I have my mill set to 100 IPM rapid. Plenty fast. Slow jog is set to 30 IPM. Some modern CNC machines run 1000 IPM rapids, that's just crazy and a lot of horsepower on the servos.

    I also want to note here that the reason I suggested 1200 oz-in steppers in my post above is that you are not using ballscrews in the current setup. An Acme leadscrew requires about 8 times the torque of a ballscrew under the same load conditions. Should you decide to put in ballscrews later, you can always turn the torque down on the motor. A simple adjustment. It's better to have it and not use it, than to need it and not have it. You can also adjust the torque on servo motors, again a simple adjustment. My Z axis stepper for instance is turned down to 50% torque output, which gives me about 350 lbs of max down force on my quill before the motor decouples. My shear screws will shear at about 375 lbs in case something goes horribly wrong.
     
  26. JPigg55

    JPigg55 United States Active User H-M Supporter-Premium

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    Think the light is starting to come on.
    I understand PID, we use a lot of controllers at work (Nuclear Power Plant) for various things. As an operator, I'm trained and tested on all the system and controls all the time.
    A different department works on them, but from a functionality perspective, I know how the work.
    Still trying to decide between stepper and servo, but at least have a little more know how.
    Decided to look at specs for a Servo Type 140 PF (recommended for my mill), here are some of the specs they list:
    • Peak Torque (half-wave series motor): to 140 in.- lb. / 15.8 NM torque
    • Intermittent Torque: 105 in.- lb. / 11.9 NM
    • Continuous Torque: 90 in.- lb. / 10.2 NM
    • Variable Feed Rate:
      .75-25 IPM / 19-635 mm/min (table & cross)
      .5-12 IPM / 13-305 mm/min (knee)
    • Rapid Traverse:
      35 IPM / 889 mm/min (table & cross)
      12 IPM / 305 mm/min (knee)
    • Gear Reduction Ratio (motor to screw shaft): 72:1.

    From the suppliers I've looked at, most sell gear boxes designed for either NEMA 23 & 34 stepper or servo motors. The two models most carry are either a 5:1 or 10:1 ratio planetary gearbox.
    Probably comparing apples to oranges, but thought I'd try using this data (from a mathematical perspective) to see how it would equate for servo motor drive with gearbox reduction for sizing.
    Planning on spending some time in my shop today to start a small project I need to make. Figured I'd start with counting threads per inch on my lead screws and measure the torque required to move the knee, table, and crossfeed although I'm guessing some sort of fudge factor has to be used since this wouldn't be torque with a load on the table.
     
  27. JimDawson

    JimDawson Global Moderator Staff Member Director

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    I think you have it under control. :encourage: Understanding a PID loop is half the battle.

    Spend some time looking at the torque curve charts for the various motors you are looking at.
    .
    .
     
  28. JPigg55

    JPigg55 United States Active User H-M Supporter-Premium

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    Thanks, not in any big hurry beyond my old arms getting tired of cranking handwheels. LOL
    Just seems to make more sense money wise. being able to have power on 3 or 4 axes for less than the price of 2 PF's
    Not to mention being a computer and program away from a total CNC machine basically.
     
  29. JPigg55

    JPigg55 United States Active User H-M Supporter-Premium

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    Got thinking about going ahead and putting a drive on at least one axis to test things out and get a feel for it.
    As such, decided to see how much more it would cost buying the parts piece-meal vs going with 3 axis kit for the 600 oz-in servo motor kit (http://www.automationtechnologiesin...ema34-850-oz-in-72v20a-psu-g320x-gecko-driver).
    At least on the Automation Technology site, I was surprised to find that buying the parts listed in the kit was actually $15 cheaper than buying the kit.
    You'd think one would get a better price buying an entire kit over the pieces individually.
    Huh, before posting, double checked and noticed the link says "850 oz-in, but goes to the 600 oz-in kit page. Wonder if their website messed up.
     
  30. JimDawson

    JimDawson Global Moderator Staff Member Director

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    That's a bit confusing:confused:
     

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