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Basic CNC

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

  1. Bill Gruby

    Bill Gruby United States Global Moderator Staff Member H-M Supporter-Premium

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    I tried. this thread never even reached first base. I'm throwin in the towel. :whiteflag:

    "Billy G" :dunno:
     
  2. 7HC

    7HC Active User Active Member

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    Maybe you're just having a bad day Bill, there's been useful information and discussion from the start.


    M
     
  3. Bill Gruby

    Bill Gruby United States Global Moderator Staff Member H-M Supporter-Premium

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    From the beginning this thread has not been mine, it wasn't meant to be. I started it for all of the people here that knew little or nothing about CNC and how it works. I went as far as I could with my little knowledge of CNC and opened it up for everyone else. Nothing happened.

    I am not complaining about it, and yes there is a lot of info here but I'm just a little surprised it has not gone any further than it has. You are the first post other than mine in a couple of days. I cannot go any further with it until someone asks more basic questions. Yes I have a few more but this is not the right time to ask them. We would start jumping all over the place with this thread and that would not be a good thing.

    I have seen questions asked in other threads that were answered here before they were posted elsewhere. This is a heads up for me. I am going to leave it at that and say no more. I will watch for this thread to start again.

    "Billy G"
     
  4. JohnAspinall

    JohnAspinall Active User Active Member

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    I, for one, greatly appreciate the effort you put into this, Bill. I don't think the effort is wasted; I do have a suggestion for Version 2.

    Consider a spectrum from "student-driven" to "teacher-driven".
    Student driven:
    • I (student) know what I want to learn, and (just as important) I know what I don't want to spend time on.
    • Let's just ask questions. Sooner or later, every question will get asked, and we'll all know everything we want to learn.
    • As we make progress, we (students) will ask better questions, and we'll help each other over the hard parts.
    Teacher driven:
    • There are many ways to learn a subject, some paths are far more efficient than others.
    • Someone who already knows the subject, is better suited to picking the path.

    This is a spectrum, between those two extremes. You don't have to pick one or the other; you have to pick somewhere in the middle that is comfortable for the greatest number of participants.
    At first, especially in an online forum like this, I think students rush to ask questions and you think the process is going great. But the questions get more and more specific, and when that particular discussion is exhausted, so are the students! Everyone looks around, wondering what to do next because there's no obvious next question.

    This is what happened here (i.m.h.o.). There's no one with the 50,000 foot view. A lot of time was spent covering every single side street in one neighborhood, but now we've got to head for another town, and all its neighborhoods. Here's where this discussion needs a high level road map.

    Something else too: People have limited time, they have only a certain amount of attention to give. You give them lots of detail, how can that be bad? It can be bad by exhausting their attention that would be better spent on other things they want to learn.

    In short, this effort needs to be a little more teacher driven. That doesn't mean we have to find a single expert and convince them to write posts for hours and hours. But here's where an expert can help. An expert can:
    • lay out the landscape (the curriculum),
    • throttle the discussion when it dives too deep
    • move it on to the next big topic

    (When I say "throttle the discussion" I don't mean saying "Shut up now.". I mean saying "that's a detailed topic that might be interesting for some, but is not necessary for understanding what comes next. So go ahead and discuss it on a secondary thread, but it's ok for some people to ignore that secondary thread.")

    Concrete example: steppers vs. servos.
    50,000 foot view: They're electric motor systems that go to a commanded position. You don't care about the differences between them when it comes to topics like writing G-code, using limit switches,.... You will need to know something about the differences when we cover topics like cost and performance, estimating your torque requirements, symptoms of things going wrong.
    5,000 foot view: We're talking about choosing your motors. Both steppers and servos are systems, with driver electronics, and sometimes other components in addition to an electric motor. Because steppers are the most economical choice for many hobbyists, we will descend into more detail on steppers to begin with. If you care about servos, see...
    500 foot view: We're talking about how much position resolution you can get with steppers. Blah blah, microstepping, blah blah, gearing down.....
    500 foot view (different neighborhood): We're talking about holding torque in steppers. Blah blah cogging, ....
    5000 foot view (different town): We're talking about what goes wrong when the machine is overloaded. This is an area where servos and stepper have different behavior. ....

    I hope that gives you some idea of what might be a different way to try this again.

    Disclaimer: I'm not a CNC expert. I've never built a CNC machine; I've never even operated a CNC machine. I'm simply throwing my 2 cents out there on the basis of what I read. Hope it helps in some way.
     
  5. DMS

    DMS United States Active User Active Member

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    This thread could be done. We covered:

    * Motors (mostly steppers, but some stuff on servos)
    * Screws
    * Couplers
    * Motor Controllers/Drivers
    * Software Controllers

    Yep, that's the basics. I think we even crossed over into some intermediate and advanced stuff.

    YEAH US!

    Extending what JohnAspinall said, The order listed above is probably a good order for a curriculum, and is more or less the order we covered things in.

    The things we haven't really covered are the CAM/GCode portion, maybe that is better off on a different thread.
     
  6. jumps4

    jumps4 Global Moderator Staff Member Active Member

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    we didnt cover breakout boards very much and the different types, how they work with your operating system and better options that of course cost more.
    i have been busy converting my lathe to cnc or would have posted more. I have a breakout board problem right now that is because it is too slow. even though my new controler is a usb connection it has to connect to the system i built through a breakout board. the one that came with my controlers is opticaly isolated and they are really slow ones so i ordered a hard wired db25 breakout board. this isolation thing to protect the pc can add up to be a problem. if everything you have between your pc and the drivers is optoisolated each add a little lag to the speed and the cheap ones are slow. my pc can put out 35000hz at the printer port my usb uc100 puts out 100000hz thats pretty fast but this breakout board will only process at about 20000hz so a fast pc and fast controler hits a choke point at the breakout board and slow the entire machine. the uc100 is optoisolated the drivers are optoisolated i dont need it at the breakout board also. i'm over protecting a pc that cost less than all of the other parts lol
    steve
     
  7. DMS

    DMS United States Active User Active Member

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    I realized I made a mistake on the "half step/full-step/microstep" post, and for some reason I can't edit that post, so I am posting a correction in a new post. Please refer to new visual aid:

    Everything in the original post was correct except the diagram, and the description of full step mode. Instead of one phase being on, and the other off, both phases are always on, and each phase reverses direction alternately. That's what I get for relying on my memory :shrugs:.

    stepper_diagram.jpg
     
  8. HSS

    HSS United States Active User Active Member

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    I think this thread has good information and has been well recieved and I appreciate it. I have a full-time day job, as I'm sure many of you do, and I don't get on the computer everyday. It might even be a week before I turn the computer on, so just because people aren't responding to the thread, doesn't mean it isn't working. Y'all keep up the good work and it'll happen.
    Patrick
     
  9. rogerrabbit

    rogerrabbit United States Active User Active Member

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    Billy G,

    Ok, I'll give it a shot where I would like to see this go (on a 5k ft level)

    1. Types of implementations:
    A. Lathe type CNC
    B. Mill type CNC (i would include the home built CNC router type solutions).


    For each above:
    A. how do they work? (basics right? :)))
    B. what are the differences?
    C. If I am new to the CNC world, which type should a start with assuming I have either both a mill and lathe or neither.
    D. for each, buy a machine, convert existing, or build from scratch?
    Something like
    new: high $$$, but software & machine integrate well.
    convert: med $$, but kit may not be available, anyone have experience with a kit they would like to share/
    build: low $, but you need to plan ahead to make sure software works with machine.

    d might be a stretch for a basic thread though.. your call?

    Roger
     
  10. Bill Gruby

    Bill Gruby United States Global Moderator Staff Member H-M Supporter-Premium

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    Been waitin for this one Roger. Thank you. There it is gents, your turn.

    "Billy G":thinking:
     
  11. 7HC

    7HC Active User Active Member

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    To that list you could also add, plasma cutters, engravers, hot knife machines for cutting polystyrene, probably 3D printers, and possibly even more esoteric machines like robotic arms (which might be fun to have on one of the other machines as a 4th axis for picking things up and moving them around).


    M
     
    Last edited: Oct 24, 2012
  12. rogerrabbit

    rogerrabbit United States Active User Active Member

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    good point, my thinking was to categorize it based on how the cnc moves (and feel free to correct if I have this wrong):

    how about this categorization?

    2D - CNC lathe (i don't know how to classify those that can change the rotational speed, help?)

    3D - CNC mills, plasma cutters, hot knife,engravers and laser cutters, 3d printers (not sure if its 3d, as there is a 4th stepper to control the material feed rate)

    4D - advanced CNC mills, robotic arms

    5D - ?? I've seen it mentioned..

    thanks,
    Roger
     
    Last edited: Oct 24, 2012
  13. 7HC

    7HC Active User Active Member

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  14. DMS

    DMS United States Active User Active Member

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    I take a stab at Rogerrabbit's outline

    Ok, I'll give it a shot where I would like to see this go (on a 5k ft level)



    A. how do they work? (basics right? )
    CNC is Computer Numerical Control. According to wikipedia

    So, we are using a computer (and software running on that computer) to control a machine tool. That could be any of the items listed above. LinuxCNC will control up to 9 axes. According to their website, Mach3 will do up to 6 axes. Axes are points where the machine can moved. Think X, Y, and Z axes on a milling machine. Given that, all types of computer controlled machines operate the same more or less.

    B. what are the differences?

    Number of axes, work envelope, material, type of operations capable.

    For example, a CNC lathe, just like a manual lathe, is going to produce cylindrical work, and a mill is going to produce prismatic work. We can blur the line here, but in general you are going to use a CNC lathe for the same thing you would use a manual lathe for, its just that a computer is turning the knobs based on instructions you gave it, rather than you actually turning the knobs by hand.

    Some other examples

    - CNC Router/gantry router. These are basically milling machines. They tend to have really large work areas in the X and Y, and relatively small Z work envelopes. Usually they have small, high speed spindles (routers are common), and are used on wood or plastic, but sometimes aluminum. If you are into woodworking, want to make signs, or quick prototypes out of plywood, or mdf, this is probably the machine for you.

    - Hot wire cutter. Are you into model planes? I'm not actually sure what else you would use a hot wire cutter for... anybody else?

    - Plasma cutter. Are you a welder? Want to make custom metal work? This may be for you. They tend to have similar stats to a gantry router, except that instead of a high speed spindle, they use a plasma cutting nozzle. LASER cutters would be more or less the same, except using a laser instead of a plasma nozzle. Waterjets too.


    C. If I am new to the CNC world, which type should a start with assuming I have either both a mill and lathe or neither.

    I would say, figure out what you want to make, and then get the machine that will let you do that. Mills are probably the most common and versatile, but if you want to make large things, a router would probably suit you better. If you want to cut lots of sheet steel, you are going to be disappointed with a milling machine, and probably would be happier with a plasma cutter.

    If you just want to explore the technology, and see what is possible, I would say a small CNC, desktop CNC router would be the cheapest route to this. 3D printers would be a close second. Both can be had for a couple hundred dollars US, and will fit on your dining room table, stow-able under the bed or in a closet when not in use.


    D. for each, buy a machine, convert existing, or build from scratch?

    Honestly, software comparability with home built machines is not a concern. LinuxCNC and Mach3 (though I have no direct experience with Mach) are really configurable, and can do a great deal. The main things you have to consider are

    1) How much work
    2) How much money
    3) Who do I call if there is a problem?

    Everything is gonna depend on what type and size of machine you chose. I picked up a used knee mill and am converting it myself. I am probably into the project to about $3k, and I don't even have ballscrews yet. To put that in perspective though, if I had purchased a new Tormach, I would have been up to $8k already, probably more with accessories. That being said, I would have a ready made, solid machine, with lots of accessories made for it, and somebody I could call when things go wrong ;).

    Now, if I had purchased a new knee mill, the price difference at the end of the day would have been basically zero. If you are going with a small machine like an X2, then you would have to do the math.

    The main thing to consider is that you want to stick with the basic formula. Lathes have 2 axes. Mills have 3 (or more, lets ignore that for now). The trick here is that there is a lot of software out there (CAM software) that takes CAD drawings and produces GCode to control your machine. If you have some oddly configured machine, off the shelf software is not gonna work. So stick with the basic formula.

    The other possibility is that you write the GCode yourself (it's very doable), but for complicated things, it can get... tedious.
     
    Last edited: Oct 25, 2012
  15. jumps4

    jumps4 Global Moderator Staff Member Active Member

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    my suggestion for anyone who is interested in cnc to start small and with a mill, a cnc lathe is nice but will never be used as often. if you can master a small cnc mill like a sherline you will be a wizz with a big mill. the small mill uses small pieces of material and keeps cost of learning to almost nothing for materials. the process is exactly the same only your making smaller parts so you will be making more parts more often and mistakes cost nothing in materials. a friend of mine laughed when he seen my sherline so i drilled 5 holes through the head of a pin and gave it to him the next day. he still has the pin on his desk. my point is to see if you really want to learn cnc and if not you will always be able to get your money back on a small machine. if you do enjoy it then big parts will be easy and you use your little machine to build the parts for your big machine. the parts made in the pic were done on a sherline cnc mill and cnc lathe.
    steve

    DSCF0433.JPG
     
  16. lseguine

    lseguine United States Active Member Active Member

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    Hummmmm great idea.

    This consept of a place for people that are intrested in the basic's of a subject s fantastic. Following what I have seenso far i see a common problem with open discussions on a spacific topic. Some one asks a simple question then diferent people ,with diferent levels of knowledge or experince respond at what they preserve as "BASIC".. then the topice gets runoff in many diferent directins. never fails...:))

    But that said, I still found some informitive info here and would like to see this thread turned into something useful to thoes that are intrested in basic info on the subject of CNC..

    I think we need three modirator volunteers to make this work. or mabey 4. one with advanced knowledge of a subject, one with a good workers level of understanding, and a student level person to tell when the real basic answer has been reached. and finally a major overseer to collect answers to a spacific topic and post them in a seperate file that others seeking the answer could go to and see the cleaned up version.

    When a question comes in the modirator could eithed open a new discussion or point to an already answered file.

    i have several basic questions, or at least what i think are basic, the guy next door might think as advanced and the guy over at palmer machine think as below basic.:thinking:

    any whooo keep up the good work

    larry
     
  17. 7HC

    7HC Active User Active Member

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    I'd say go ahead and ask your questions Larry, even if it's as basic as "What's CNC?"

    There are no dumb questions on this forum. ;)


    M
     
  18. OakRidgeGuy

    OakRidgeGuy Active User Active Member

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    I have started reading this thread and a question came to mine. If a brushless motor that powers the spindle on my HiTorque mill is basicly the same as a servo motor, it brought me to thinking.

    In CNC motion of a machine, why could you not use a stepper or servo to control the spindle on the mill? I mean, in my thinking, it would be prudent to have a spindle that can go both directions instead of just counter clock wise. To effect, if the spindle can go either way with the same amount of power, then in essence, you could have the cutter always in a climb cut no matter which side of the part it may be on.

    Make sense?
     
  19. Tony Wells

    Tony Wells United States Vice President Staff Member Administrator

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    On "real" CNC milling/machining centers, the spindle is under the controller's command. It set's the cutter surface speed and direction. Additionally, if the feedback encoder and control allow it, and most do support this, it will do what is called "rigid tapping". To do that, the spindle RPM, Z axis feedrate and position are all programmed to follow the tap's lead, stop at the (hopefully) bottom of the hole in blind tapping, and reverse and back out. And even then, go back in if you need to clear the chips and go deeper.

    It is common practice, and generally preferable to up-mill, or climb cut. This takes advantage of the very low backlash of the ballscrews normally seen on CNC machines.
     
  20. lseguine

    lseguine United States Active Member Active Member

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    Hi Doc,
    Ok heres one for you.

    i have my sheen set up and running, i'm using moch3 updated with the mochmill pro software on a piter with windows XP, through a CNC4U C11 breakout board controling 5056D drivers to the largest steppers in the nema 23 size.

    on the drivers are a set of switches that 1. controle current 2 controle microstepping. in reading the manual it appeared to me that setting the microstep switches to all on set things up such that mock3 was setting the steps per rev.

    so now having read here and in the CNC home threads what is the story with the switches vs the computer for microcontrole??
    Larry better known as Docwishbone:))
     
  21. OakRidgeGuy

    OakRidgeGuy Active User Active Member

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    Tony,

    That was my thinking, but as I was mulling this, I was having the smaller machines in mind. With that, it seems to me that a really candidate for a hobby CNC machine would be the Super X3 due to the fact that the spindle does go both ways. I have been talking to Steve about possibly doing a CNC conversion to the machine that I have now. But reading this has me thinking that I should wait a bit, get another machine, a SX3 to take advantage of the spindle motor being able to go in both directions. But would that be a fourth axis? So in essence, if you really wanted a 4 axis machine, to mimic more of the actions of the bigger machines, you would really need a 5 axis system to complete a set up on a SX3 machine. Not only did being about to do continuious climb cutting, you would be able to do power tapping as well.

    Doc
     
  22. DMS

    DMS United States Active User Active Member

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    As Tony said, on "real" machines this is the case, and on most small commercial machines, and even home built machines too. My spindle is currently not connected to the controller, but it is in the plans.

    Mostly for spindles on CNC machines people use brushless DC or 3 phase induction motors with variable frequency drives. Newer VFDs give you very good control over position and low end torque. All you need is the ability to feed a control signal from your PC to the motor controller. This is usually stream of pulses sent by your controller, the frequency corresponding to the desired spindle speed.

    In order to do rigid tapping, you need a spindle encoder. The signals from the encoder feed into your PC, so it knows where the spindle really is, and can adjust the frequency of the pulses it is sending to the motor controller. This is a lot like the way a servomotor works.

    For the X2, I don't think the stock controller allows external inputs, but I may be wrong. You may need an add-on motor controller to be able to control the spindle speed through software.

    Oh, and steppers are not used for general cutting because they have very low torque at high RPM, but they can be used for slow speed stuff like rigid tapping.

    [video=youtube_share;vGTfVlsA9Ww]http://youtu.be/vGTfVlsA9Ww[/video]
     
  23. Tony Wells

    Tony Wells United States Vice President Staff Member Administrator

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    The spindle itself is not considered an axis. The fourth axis (sometimes called the B axis) on a mill is a rotary table/superspacer type of rotating workholding device.

    There is no reason you can't do this on a smaller machines. I was only citing typical operations on a full scale, factory built CNC machine.


    Doc, I don't know anything about Mach3 software, or setting it up with stepper motors. There are some here that do, however, so they will probably chime in.
     
  24. DMS

    DMS United States Active User Active Member

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    The spindle isn't considered an axis in general. The method used for rigid tapping or for single point threading on a CNC lathe is called "spindle synchronization". Basically it is a mode you enter that tells the cutter to track the spindle position. You don't need a 4th axis for this.
     
  25. OakRidgeGuy

    OakRidgeGuy Active User Active Member

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    That is what I am saying, the SX3 already has a controller so to speak to reverse the motor that drives the spindle, so all it would need then is to be connected to the breakout board so that it can be controlled by software.. am I right in my thinking?
     
  26. OakRidgeGuy

    OakRidgeGuy Active User Active Member

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    ok, so I miss placed my thinking in terms.. but what I am getting to is being able to control the spindle and have it go both ways. I will be wanting to set the machine up so that it will have a true fouth axis when I do this.
     
  27. OakRidgeGuy

    OakRidgeGuy Active User Active Member

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  28. lseguine

    lseguine United States Active Member Active Member

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    Doc

    I have mach3 running machstdmill from calypso ventures pro. and it definatly has reverse spindal control.. you can also controle the spindal speed ect.
     
  29. OakRidgeGuy

    OakRidgeGuy Active User Active Member

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    Just a lil note, sometimes I think way outside the box. So far out, it would seem that I don't have a grip on what is going on. And sometimes that is true too.
     
  30. David Kirtley

    David Kirtley United States Active User Active Member

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    A few points as to the sizing of motors and microstepping. This is a bit rambling but here is a description of the factors involved in the calculations.

    There are no set rules or formula that you can choose the "perfect solution." The reason being that *it depends*. When you set up a system, you have to look at how you are connecting things mechanically. You are connecting a rotational movement to a linear movement. They mesh together different ways. All of them will create some backlash because they have to move against each other relatively freely. The tighter you couple them, the less backlash will be present but also you are increasing friction as you tighten them together. You need stronger components and more power to overcome this friction. Let's deal with them individually.

    Stronger components.

    As you get into larger and larger motors, the frame gets heavier, the shaft gets thicker and the coils get larger. This is pretty straight forward. There are some drawbacks with getting bigger motors. One is that they are heavier. Part of the load you are moving (depending on configuration) is the motor itself, say for a z-axis on a mill. The heavier weight means you have to have more power to move it but more importantly, you have to overcome the inertia of the extra mass to start and stop it. This requires more power. The larger coils also use more power (and weigh more) and make the cogging stronger so you get a more pronounced step in the motion rather than smooth motion. You can smooth part of it out with microstepping but you are also pushing and pulling at the same time which means that you don't have as much power available in the direction you want to move. So you get a bigger motor to overcome the drag of microstepping. See where this is going? Same thing happens when you beef up the drive components. Get a stronger leadscrew that is bigger and heavier, you have more inertia to start it and stop it. Needs more power.

    Getting more power.

    Ok, we have different ways to get more power. One is from pure mechanical advantage. As Archimedes was famous for saying, "Give me a long enough lever and I can move the Earth." Your options are most commonly a screw or a gear train. The finer the pitch of the leadscrew or the larger the gear ratio, the more power you can deliver to move something. Of course, there is no free lunch. The greater the mechanical advantage, the more rotation you have to put in to get the same movement. This is fine if you don't care about how fast you move but generally it is an important part of the overall decision. You want to be able to move fast enough to effectively cut things. Move too slow and you are not getting enough chip load and the friction will kill your cutters. Move too fast and you can't get chips to clear fast enough and you put too high of a chip load on the cutter and it breaks. Beyond that, you have to look at the differences in the motor types. Stepper motors are wonderful at slow speed. They deliver a huge amount of torque for their size. As they move faster though, they lose that power. On the other end, other types of motors such as servos, have more power at a higher speed but lose power as you slow down. You can also increase the power by putting more electricity through the motor. You can increase the voltage or the amperage to get more power.

    Now, here are all the things that bite you in the rear.

    Lets say you have a leadscrew. You want to move fast, the leadscrew has to turn faster or the pitch of the leadscrew has to give more motion for each rotation. Move too fast, and the screw can start whipping around and at the least create vibration and heat from friction on the nuts. At the worst, it can rip your drive system right out of the machine. Increase the pitch so it doesn't have to turn as fast, and you lose mechanical advantage, it has more torque on the screw which can bend it and make whipping worse, and there is more pressure on the nuts. Go with a gear train and each component of the train has it's own backlash that adds up to the overall backlash. Each component also has it's own inertia that will decrease the effective power transmission. Finer pitch on the gear train will give smoother motion but more friction. Coarser will give rougher motion. Go with stepper motors to get more low speed torque or servos to get more high speed torque. Gear up a stepper to get more speed, you get more cogging and rougher finishes. Gear down a servo to get the low end torque and you can't move as fast overall. Increase the voltage, you need to specify electrical components that have higher voltage ratings. Increase the amperage, you need to increase wire sizes and dissipate more heat.

    What you really end up doing.

    You can't just plug the numbers into a program and get the sizing you need for a system. All this can get pretty theoretical and looks like you need a degree in Electrical Engineering and Mechanical Engineering but the answer is that a lot comes by trial and error. You look at other systems and what they used and how well it worked. You also make decisions based on what you are going to be cutting, how much mass you are moving around, the power of the cutter, and how fast you need the system to move. Most of the factors can be accounted for. Backlash can be compensated for in software and it can be minimized by adjustments to the relationship of the components. You can control microstepping and go with fine steps when you want precision and use single steps for fast motion. You can do prediction of movement to slow motion down gradually instead of slamming on the brakes. Same for acceleration.
     

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