A few questions before I start building one

This topic contains 87 replies, has 6 voices, and was last updated by  gera229 5 years ago.

Viewing 15 posts - 1 through 15 (of 88 total)
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  • #2447

    gera229
    Participant

    I’m still a high school senior graduating this year and love creating things like these.

    Never wanted to buy a CNC machine for a lot of money to create little parts that I need for some other projects so when I came across this it inspired me.

    Looks like not only will the end product create parts for me, but building this thing is just another project for me over the summer.

    I wanted to make this CNC first, and then use it to create parts to make a RepRap. Funding for me is a problem so I do not know how far I will get.

    A few questions I wanted to ask were:

    What is the difference between unipolar and bipolar stepper motors? Why do you guys use unipolar?

    Which are more accurate and/ or better?

    Another question is:

    Do you guys have any problems with these machines?

    For example, the positioning? Does accuracy fade over time?

    When you set the home position, can you save that position so you won’t have to mess with it again?

    Do I need any programming knowledge or do I just draw a file and then save it as a certain file time and just import it to the software and have it cut it for me? (This is the process I used for a plasma cutter at school engineering class)

    #3985

    gera229
    Participant

    Anything?

    #3987

    T4b
    Participant

    Stepper Motors: http://en.wikipedia.org/wiki/Stepper_motor#Two-phase_stepper_motors

    Unipolar ones are larger and heavier (for the same output) but easier to drive (cheaper), as of my understanding. I think there is no accuracy difference, which one is better depends on the project. I don’t know why exactly unipolar ones where chosen here, but I can imagine that the weight isn’t too big of a problem here and that maybe the driver board only supports unipolar ones.

    I’m quite sure the home position is lost when the machine is turned off, as there is now way for the machine to know where it is at the moment and it assumes something might have changed while it was turned off.

    You shouldn’t need programming knowledge, but it probably won’t be as easy as just drawing it, sending it to the machine, waiting and taking the finished part out. I’ve recently found http://lcamtuf.coredump.cx/guerrilla_cnc1.shtml. You can skip the part about casting and mold making if you want, but there are lots things about cnc milling which are interesting to know.

    I personally have no cnc mill although I want to build one (v2 of this one) and have only used the cnc mill at my school a few times, so probably you shouldn’t trust anything I wrote. ;-)

    (it seems the forum system doesn’t let me post. I’ve already tried to post this lots of times)

    #3988

    ReidJD
    Participant

    I’ll try to answer a few of your questions, but I can’t help with all…

    A little about stepper motors.

    A unipolar motor typically has six wires. Unfortunately, this is not a hard and steady rule. These six wires are divided into two sets of three. Each set of three wires has a center tap, and two “coil” wires. These motors are usually a little easier to build controllers for, but offer less holding torque (power) than their bipolar counterparts. These motors work by activating each set of windings in the correct order. Note that by not using the center tap and using a bipolar controller, a unipolar motor can be used in a bipolar configuration.

    A bipolar motor still has a single winding per phase, but there is no center tap. In order to make these motors work correctly the controller must alternate the current (reverse the current) on each phase in the correct order. This makes it significantly harder to build a controller for. The trade-off is more torque in a smaller package.

    Most electronics for repraps are setup for BIPOLAR operation in a NEMA 17 motor size (to get more torque in a smaller package). The DIYLILCNC is designed for a larger UNIPOLAR motor (to make it easier to control).

    Neither is really more accurate or better, personally, I’d start with a unipolar (DIYLILCNC) and work my way to a bipolar (Reprap) to get a better understanding of how they work.

    Hope my info helped. I haven’t build my DIYLILCNC yet, so I can’t give you any more info on it, but I have built a reprap Mendel.

    #3989

    gera229
    Participant

    I wonder why a RepRap would require a bipolar and use more torque than a CNC? Doesn’t it take less torque to print than it does to cut?

    Also what is the different between 6 and 8 wire unipolar meotors? (For bipolar the way I understood it is you connect only 4 of the wires while for unipolar you connect all wires be it 6 or 8, but not less than that. Correct me if I am wrong)

    Finally, was it hard to get the programs and coordinate functions set up correctly so that it does things accurately? Can you convert a drawing directly with the dimensions already on it without any programming knowledge?

    #3990

    ReidJD
    Participant

    You are correct that milling probably requires more torque, but the reprap uses that extra torque to get more speed (you can run the motors faster without it stalling out) which is huge if you have print that is hundreds of layers thick.

    Most of the time the only difference between a six and eight wire unipolar stepper is that on an eight wire motor, each of the 4 coils has two wires coming out of the motor. On a six wire motor, the 4 coils are paired up, with a “common” wire going to each pair so that there are three wires per pair. An eight wire stepper can be made into a six wire stepper by binding the “common” wire together for each pair of coils.

    Converting drawings into gcode is really the tricky part to a CNC or 3D printer. Most people call this the CAM toolchain or workflow, and there are many different ways to do it. It’s usually not that difficult, but you have to find a way that works for you (depending on what OS, device, electronics package, etc). A good place to start with this is Inkscape and Pycam.

    #3991

    Chris
    Keymaster

    Hi guys, great q&a you’ve got going here. You can find some more information on steppers, bipolar/unipolar wiring & more in this doc.

    The main reason we use use unipolar wiring is the HobbyCNC board; this is still the lowest-cost & most robust controller we’ve found that suits our needs on the DIYLILCNC. That said, you could certainly use a different controller that supports bipolar wiring. We haven’t really been in a situation where torque has been a limiting factor; generally, the Dremel is going to crap out before the steppers do.

    For the software, it’s a little more involved depending on what you want to do, and you have lots of options for software. Check out some of our CNC tutorials on youtube to get a feel for what’s involved. There’s also a list of CAD/CAM programs here that you can download & try.

    #3992

    gera229
    Participant

    Hey @ReidDJ I wanted to build a RepRap just as a hobby, but also to possibly create things that may be used. Can you create plastic replacements parts? Or is there anything in particular that you use your RepRap for? Do you find it useful?

    What motor do you use and where did you get it? Also how much torque does your bipolar stepper motor produce? What controller do you use?

    Would you use the DIYLILCNC suggested controller and motors for your CNC build or the same things you used in your RepRap (since it’s bipolar/stronger)?

    Did you follow a tutorial to build your RepRap or just built it yourself?

    Controllers are probably harder to make for Bipolar, but we aren’t building controllers are we. We are just using controllers already made to function so I do not think bipolar controllers being more complex is a problem since we are not making them, but correct me if I am wrong.

    Looked at this: http://www.stepperworld.com/Tutorials/pgBipolarTutorial.htm

    And pretty much explains the difference between 8 and 6 wire motors. Looks like the 2 wires on each coil on 8 wire motors can be put together to create “1″ wire that will act as the 3rd wire like in 6 wire motors.

    Otherwise, one thing I don’t understand is why 8 wires when they could have just made 6 wires?

    Hey @Chris, I also wanted to ask you something.

    After looking at all the costs, it was claimed to be under $700. Is that excluding or including all the fees such as shipping?

    #3993

    gera229
    Participant

    You guys should also check this out: http://www.instructables.com/id/Easy-to-build-CNC-Mill-Stepper-Motor-and-Driver-ci/

    For building circuit boards for these stepper motors. I think it’s for bipolar, but correct me if I am wrong. Also let me know of any disadvantages of building one yourself (not creating one, but building one that is already designed)

    Also http://planet-cnc.com/ in the DIY section. Sorry for posting so much. Thought you guys might be interested and if you make your own sell it for cheaper than the other ones lol.

    #3994

    gera229
    Participant
    #3995

    gera229
    Participant

    I managed to find the cheapest printer build ever without having to create your own (Probably cheap because they build their parts from printing which tells me that it’s a good printer also. They were funded by indiegogo.com just like DIYLILCNC was funded by kickstarter).

    Their threads:

    http://forums.reprap.org/read.php?94,106939,106939#msg-106939

    http://forums.reprap.org/read.php?94,126166,126166#msg-126166

    RepRaps seem more complicated than CNC (which I find much easier to understand with only 1 controller and separate “axis” controllers, and since I’m on a very limited budget and do not yet understand much of electronics I think I will learn plenty from this machine:

    http://shop.seemecnc.com/H-1-Complete-Kit-71599.htm

    And this to make custom add ons:

    http://shop.seemecnc.com/H-1-Configurator-H-1-Configurator.htm

    You should actually check out the entire website. Learned about it from reprap.org forums, don’t have time to find and read reviews on it, but I think it’s good correct me if I am wrong.

    Also found this board:

    http://shop.seemecnc.com/3-Axis-CNC-Board-26703.htm

    Also this board seems cheap and according to the specs it supports 3A and 36V (Similar to the HobbyCNC board which supports 3A and 42V with 35V recommended) Correct me if I am wrong, otherwise maybe you can try it.

    #3996

    Chris
    Keymaster

    great info guys, please keep it coming! Rolling your own controller is definitely a whole additional can of worms on top of building a DIYLILCNC, but it’s something that can be very useful for a range of projects. Keep in mind that the DIYLILCNC motors require a high amount of current (at least compared with a repRap or other less torque-hungry machines) which has implications for what driver chips & additional circuitry you’ll need. I’ve found the Adafruit motor shield for Arduino is another great (& cheap) controller solution for smaller projects–very simple to use with the AFMotor library.

    @T4b & @gera229, if you’re having issues with the forum please contact us, rather than, say, posting duplicate entries or complaints. This site is here for you all to use, please help us keep it useful.

    @gera229, I’ve increased the post editing window to 48 hours, so if you’d like to clean up anything that would be appreciated.

    as to your questions:

    Otherwise, one thing I don’t understand is why 8 wires when they could have just made 6 wires?

    Most likely just a quirk of mass production — the motors carried by HobbyCNC were probably not manufactured specifically for the board. It’s a common feature of stepper motors to be wired like this, and as long as you connect the wires properly the motors will work just fine.

    After looking at all the costs, it was claimed to be under $700. Is that excluding or including all the fees such as shipping?

    The cost would not include shipping, since you’d be getting parts from several third-party vendors. It also doesn’t include costs for a computer to run the CNC, or for laser cutting costs (if any). These things are all pretty variable, and you can definitely source many of the parts for very cheap through places other than McMaster-Carr, which tends to be more expensive than a local hardware store.

    #3997

    gera229
    Participant

    Hey for the seemecnc.com board I listed I also noted the specs and the amount of power they can handle seems to almost match the board used for the motors on the DIYLILCNC machine. So I assume that the board I linked can handle the high current and voltage rendered by those motors.

    Also do you guys know anything about servos and servos vs stepper motors?

    Well I do not think high end servos are really necessary unless your mass produce parts and cut super strong material. Although servos can detect exact position with sensors (for example, steppers cannot detect position especially when they lose steps which is probably highly unlikely with these high torque stepper motors unless we cut steel or something). But it’s still interesting to know.

    Oh my goodness such an amazing read T4B and makes me want to forget about building a 3D printer and facing all the problems involved since a CNC machine is easier to build and mold casting seems like a fun and more accurate process than 3D printing. :P

    Right now I am wanting to build a CNC instead of a 3D printer and cast things out and forget about building a 3D RepRap printer because of the advantages of casting over printing (since RepRap resolution is nothing compared to casting imo and takes forever to building multiple components).

    I actually read this entire blog: http://lcamtuf.coredump.cx/rstory/

    I want to make a CNC with a larger Z-Axis which is at least 4″ for extra space.

    #3998

    gera229
    Participant

    Aside from my previous post, I recently found out that I will have access to a pretty large CNC machine that is like 5′x5′ so I’m set on cutting out the parts. Although I will redesign it to have the Z axis going at least 4″ high unless there is a reason to not do it haha. I think it’s beneficial to do it though.

    Another brilliant idea that came to my mind is if the router can reach the base material (wood plate), then possibly carve out a grid and make ruler markings where the home position would be (0,0)? That way when you put material on there you can approximate where it is and send that into the program instead of having to taking a long time to manually adjust the machine for any reason.

    What do you think?

    #3999

    Chris
    Keymaster

    @gera229, yes definitely that is an easy mod to make your workflow quicker.

    You can also easily add limit switches like these to your CNC that can be used to ‘home’ the machine before cutting. Basically, you locate the switch near the zero position of each axis, and when it trips you can assume that the machine is where it needs to be.

    Boards vary as far as having built-in support for limit switches (I know the HobbyCNC does, some others I’ve seen don’t), and some controller software (EMC2, for example) has built-in support for a homing routine you can run when the machine starts up.

    re servos vs steppers, steppers are definitely the way to go in a build like the DIYLILCNC. While there is no feedback as to the real position of the motors vs the virtual position, the motion of stepper is reliable enough to be successful almost all the time (unless, as you mention you’re losing steps due to bad wiring, etc., or the machine is mechanically jammed). If you went with servos you would absolutely have to build in some sort of encoder to give feedback as to the servo’s position, as their movements are not nicely incremented like steppers are.

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