CNC design & discuss

Discussion in 'The Projects Forum' started by retched, Jun 26, 2010.

  1. retched

    Thread Starter AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    Hello folks,

    In a bid to find information regarding CNC machines on this site, a few threads were speckled with various questions and replies, cramping the inital issue of stepper motor drivers.

    I have created this CNC project and discussion thread to try to take some of the strain off the other threads.

    We can start discussions and design ideas, and see if we can come up with a design that we all agree on to build. I mean, who doesn't want a CNC?

    Except BMorse, who has two... a standard and a plasma.. Hate you BMorse! ;) Kidding, thats my jealousy striking forth. :D

    OK. The stepper motor thread is located here:
    http://forum.allaboutcircuits.com/showthread.php?t=37587

    Read up. Any questions you have NOT regarding the thread topic, but still regarding the CNC project can happen here.
     
  2. BMorse

    Senior Member

    Sep 26, 2009
    2,675
    234
    I only built the standard CNC Mill/Drill, the Plasma Cutting CNC machine is one we had purchased (it is a PlasmaCam CNC plasma Cuting machine)...

    Although I already have one, I still want to build another one when I get a chance and use a closed loop DC Motor Servo drive system, so I can increase the speed of the machine, my standard mill only cuts up to 12" per minute, the plasmacam machine that uses a closed loop dc servo system cuts up to 1000" per minute!! and you can attach pretty much any kind of cutting system to it for cutting a vast array of materials from metals to plastics and even foam board and wood, i have used it to mill some PCB's before using a rotozip tool and it was quite an overkill seeing a 3"x5" PCB sitting in a 4' x 4' cutting table :) ... I usually work with a some big fabrication companies on the west side of the state, most of them have industrial laser cutters, and most of these also use servo motors instead of steppers, so my next build will most likely be a DC Servo motor based machine instead of stepper motors...

    B. Morse
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    What's really fascinating is to watch a pick-and-place machine do it's thing.

    At a former employer, they had a couple of them. You'd mount perhaps 20 cartridges that contained reels of components, and feed in a stack of boards that had the solder paste applied already. The pick-and-place machine would take the output from the PCB layout tool, and go to town putting them on the boards, three SMD's at a time, perhaps 6 components per second. The components were picked up using vacuum, then rotated to the proper orientation and plopped right on the pads.

    Maximum overkill for a hobbyist, but it was entertaining.
     
  4. coldpenguin

    Active Member

    Apr 18, 2010
    165
    9
    They have some great videos of pick and place on you-tube. There is one which moves around ping-pong balls into a pyramid, almost too quick to see.

    So what would make a table unsuitable for cutting metals? I have seen a table kit (no mill or electronics), but it states that it cannot do hard steel or aluminium.

    One of my other hobbies (which I never have time to do), is model airplanes and helicopters. CNC machining of balsa I reckon should be easy enough for any CNC, but what about machining small ALU?
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    If you want a really stable table, cast iron is the best IMHO.

    Take a look at any of the lathes, vertical mills, etc. (basically, large machine tools) that were made over the last 100 years; you'll see very large iron castings. It's very stable, and just about impossible to bend it at all without breaking it, unlike steel or aluminum, which have elastic properties.

    You want a material that has zero thermal coefficient of expansion and no elasticity. Such a material doesn't exist, of course - but cast iron comes pretty close.

    Ceramics could be another possibility. There are ceramic materials that have a very low elasticity, and very low thermal coefficient of expansion.

    However, I'm not shooting for any kind of ideal properties in my first go-round. I'm just going to use some very thick, very rigidly laminated plywood with a couple of 2" angle irons on the bottom.

    I'm really not planning on doing any metal cutting with it, and I'm not terribly concerned with tolerances as I'm going to be using drawer slides for my x-y rails. I fully expect them to be at least several mils off in repeatability, and to be about as straight as a barrel stave. :rolleyes: However, as I said before, this is much more of an experiment and "getting my feet wet" rather than an attempt to build anything that resembles a truly precision device. It should be just fine for circuit boards, and perhaps making some signs - that's all I really want it for at the moment.

    Then again, I might see how it'll do for checkering on M1911 .45 grips... ;)
     
  6. sceadwian

    New Member

    Jun 1, 2009
    499
    37
    You got any pictures of the work in progress Sgt?
    On an off note, what do you prefer, Sgt or Wookie? It feels weird calling somone wookie =>
     
  7. retched

    Thread Starter AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    Try calling someone 'sceadwian'! now thats tough. ;)
    ----

    Im a carpenter by trade, and I maintain a pretty elaborate campus, so I have access to the tool to handle quite a bit..

    Not a plasma CNC, but I have a plasma cutter..Even so, I will be going with plywood for the initial build. Upgrading to a metal later wont be a problem.

    As for the x-y, I would like to go with the bearings on angle iron approach, but that could also be upgraded later.

    I have a bearing supplier, but they have a 100 order minimum, so If anyone else wants to go that route, Ill buy the lot, and peel them off to you folks for
    (total cost)/100 * (# of bearings you want) + s/h.

    If thats a way anyone else wants to go.
     
  8. sceadwian

    New Member

    Jun 1, 2009
    499
    37
    I know my names a pain, especially considering even I don't know how to pronounce it. It's based off an old english word, which means 'to shade, to protect' which is a play on words from a handle I used when I was a kid "Shadow Knight". The etymology I've found is a little vague too. I just found the play on words so perfect I couldn't think of a better 'short' name to be called. The pronunciation .. requires knowledge of this...
    http://en.wikipedia.org/wiki/Old_English_phonology
    ˈʃæɑdwiɑn

    I think it sounds like Shead'wian although it flows better than that crude approximation. Not she as in her but she as in shea butter. It's not quiet like shade, the C intonation is there.

    I tend to tell people in the real world that it's Ski Aye dwee-an but that's to avoid the dirty looks when I say Shead and spell it S C E A...

    ElectroNerdy and a few others I've chatted with tend to juts call me SC, otherwise known as glorious pain in the rear.

    Your name is a little more clear cut retched ;)
     
  9. John P

    AAC Fanatic!

    Oct 14, 2008
    1,634
    224
  10. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    What, you want to see the messy pile of parts on my desk? :confused: :D
    Wookie is a nickname that's stuck with me since my commanding officer declared me that back in 1977. ;) I was a tall skinny Corporal with my hair and moustach at the bleeding edge of the regulations, and our Marine fighter squadron had just come back from the premier of "Star Wars" at the base theater (we got to see it a couple weeks before the public did) - and my last name is very similar.
     
    PackratKing likes this.
  11. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Interesting.

    I do have a few pneumatic cylinders around here, one with an 18" displacement. That might be used, linked to an accurate positional encoder, and would be fast as the dickens. Trouble is, I'd have to run my air compressor continually, and it is a noisy sonofabi... er, gun.
     
  12. retched

    Thread Starter AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    That would be rather interesting, but I think I will settle for ACME.

    It would be awful interesting to set up a linear encoder or even a laser ranger. I wonder how accurate a DIY laser encoder would be?
     
  13. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    I don't see pneumatic cylinders ever being accurate enough, they just don't have that kind of control or precision.
     
  14. retched

    Thread Starter AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    PPPBBTTTTT... You have never seen me on a backhoe then. ;)

    I can scratch my ba...back with it with no 911 call needed. ;)
     
  15. Syniva

    New Member

    Jun 21, 2010
    25
    0
    I also have to suggest using DC motors instead of stepper motors.

    I once built a really crappy CNC mill, but it got old real quick when after listening to the Dremel run for two hours it still hadn't drilled even half the holes on the board. It also wasn't accurate enough, partly because stepper motors are weak and it doesn't take a whole lot of opposition to make them lose their position. Another part of the accuracy problem was that there was that it was possible to wiggle things around a little since the nuts that moved along the screw drive weren't precision, just junk from the hardware store. I figured I could fix that by attaching a weight to always pull the table to one side, but I couldn't do that with the stepper motors since they could barely move things around as they were.

    So, I did my best to see what kind of DC motor I would need, and as best I could calculate, even the cheapest DC motor I could find would perform better than the best stepper motors I had. Then I kind of lost interest in the project when I couldn't find an inexpensive source of gears.

    Sometime last year I found some cheap gears, and so built a little test rig with the motor and screw drive, and sure enough, DC motors easily beat the performance of stepper motors.

    I have yet to build another CNC mill, but I've thought of a clever way of controlling the thing that I think would work nicely.

    I'd want acceleration and the ability to cut curves and other fun stuff, which would require complex floating point math, which wouldn't be easy to do on a microcontroller, so what I was thinking of doing is making the hardware end nothing but a FT245RL connected to an AT89S52 microcontroller connected to a motor driver connected to a DC motor, and a feedback system that consists of rotary encoders connected to the microcontroller. On the PC I'd have a Perl script which simply calculates where the machine be each milisecond or so, taking into account acceleration and all that, then it simply outputs that data to the microcontroller as quickly as possible, rather than try to do it in real-time, since real-time on a PC isn't very reliable. Then the microcontroller simply reads that position data at a fixed rate, compares it to the actual position, and controls the polarity of the motor voltage based on whether it is ahead or behind of where the script says it should be. As long as the script takes into account what the hardware is capable of, the actual position of the hardware should always be just about where it should be, and so the Perl script wouldn't actually need any feedback, other than some sort of "major problem detected" feedback.

    ...and one last recommendation: Build a small CNC mill, something you can easily toss in the closet when you get bored with it. The first one was two feet cubed and got thrown away when I realized I just didn't have the room for it. My second one (never completed) was three feet square and five feet tall, and only lasted about a month before it was in the trash. My third sits happily on a shelf, still incomplete, but in no danger of becomming garbage since it is only one foot square and so it isn't in the way.

    I don't know if I'll ever finish the one I have. I mostly wanted it to make circuit boards, but I've come to believe that simply drilling holes for components into plexiglass (much cheaper than prototype board) and then soldering it together using wrapping wire is a much simpler process than making a circuit board. With a circuit board I have to spend hours arranging components and routing tracks in some software, whereas just soldering wrapping wire to the pins, I can put the components wherever I want and not worry about routing traces or increasing the board szie to make room for more trace routing and jumper wires.

    Here's what I mean in super-high-resolution, for a limited time only:

    Z80 with ROM, RAM, crystal, and chip select generator:
    http://www.ecstaticlyrics.com/secret/Z80_component_side.jpg
    http://www.ecstaticlyrics.com/secret/Z80_solder_side.jpg
    Bus window with address decoding for each slot:
    http://www.ecstaticlyrics.com/secret/Z80_bus_window.jpg

    I've tried to design circuit boards for the same thing, but routing the traces, even on a board twice as large, requires so much thought and effort that just soldering the thing together manually seems trivial, if a little time-consuming. Between board design and the fact that I can't even make a double sided board and even if I could I'd still have to solder the components in place and probably solder a bunch of vias or jumpers together as well, I wouldn't even consider making a board unless I needed a dozen of this sort of thing. For anything less, making a proper circuit board would seem to be a waste of time, and more than likely I'd simply have to pay someone to make them and that's ridiculously expensive for anything other than ridiculously small projects.

    So I try to find DIP components whenever I can, and when I can't (which is presently only true for the FT245RL) I make a little breakout board, using the laser printer toner transfer method and cupric chloride as an etchant.

    http://www.ecstaticlyrics.com/pinnwand/messages/new_at89s52_programmer.html

    I went ahead and put the microcontroller on that board too, but only because I already had to have a board and so I figured it might as well have some traces. If the FT245RL were available in DIP, it'd be easier to just to solder the things together on prototype board, and if the routing would have been more complex, I would have simply etched away all of the copper where the microcontroller goes and soldered wrapping wires to it as if it were just a prototype board.

    Anyway, my plans for the milling machine were all born in a day when I envisioned the idea of putting together a Z80 system on prototype board to be an impossible task, or at least very expensive after buying all of the wire wrapping sockets. Once I figured out that standard sockets stick through a board far enough to solder wrapping wire to, suddenly I didn't have to make circuit boards anymore, and once I figured out a sensible bus design, the wire wrapping method became rather easy. (The first one I made was a real rats nest.)

    ...and really, between the cost of making a board and the cost of solderless breadboard, if I had to choose I'd just stock up on solderless breadboard and use it for permanent circuits. Between buying boards and drill bits and component sockets, the cost isn't much more, and solderless breadboard is definitely easier to work with. Besides, circuits usually don't turn out to be that permanent anyway, and so it can be reused eventually.
     
  16. retched

    Thread Starter AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    Friend, your whole problem was your steppers were either under-powered, or the wrong size for the job.

    As for your linear drive, if you used all-thread and a nut, it would be no good. ACME makes an all-thread-like rod with deeper threads and special female fittings that ride them at tight tolerances.

    I honestly can not see how your standard DC motors would outperform a proper rated stepper in a CNC application. They will spin faster, but you will not get the accuracy like a stepper.
     
  17. CVMichael

    Senior Member

    Aug 3, 2007
    416
    17
    I have a question about the stepper motors for my CNC getting quite hot.

    I don't have a device to measure the temperature but when I touch the motors I can't keep my hand on it.

    I am using the stepper drivers that I bought from here http://www.motiontek.ca/StepperDriver.html , driver D56 for a 5 Amp motor, and D42 for the other 4 Amp motors.

    I set the "Current selection" to 4.3 amps (for the 5 Amp motor), and it's still getting hot.

    Is it OK to get hot ? or am I worried for nothing ?
     
  18. shortbus

    AAC Fanatic!

    Sep 30, 2009
    4,019
    1,540
    Steppers do run hot. Part of it is because in most CNC situations one set of motor coils is on ,even when the motor is not moving. This is done to keep the position "locked". With ball screws and linear motion slides if the motor wasn't locked the cutter force would move the table/work uncontroled by it's self.

    Since one set of coils is constantly on it causes the motor to heat-up from the coil resistance.

    shortbus
     
  19. sceadwian

    New Member

    Jun 1, 2009
    499
    37
    Electronics can safely run at temperature which will cause second or third degree burns to a person past 5 seconds. Hot to the touch is nothing abnormal. Determining the temperature it's running at and if it's in a safe operating parameters is... pretty important =) So you might want to invest in thermal measuring devices, they're not always easy to use, I bought a non-contact IR thermometer before I realized that it was VERY important to know what you were pointing it at to read the actual temperature.
     
Loading...