Engraving PCB with CNC router

Discussion in 'General Electronics Chat' started by CVMichael, Jun 7, 2010.

  1. CVMichael

    Thread Starter Senior Member

    Aug 3, 2007
    416
    17
    I want to make a tiny PCB to convert from MSOP8 to PDIP8

    Since I don't have a laser printer, and the necessary tools (solutions) for etching a PCB, I decided to use my homemade CNC to engrave the PCB board.

    I use L298 (with L297) for the steppers drivers, I have to cut at really slow speed otherwise the drill bit breaks (I broke 2 bits already). The problem is that the L298 chips get very hot at low speeds.

    What can I do to lower to power at lower speeds ?

    Right now I use a compressed air can (upside down) to spray the cold liquid on the aluminum heat sinks for each L298 driver to cool them down while the CNC is running.

    I am looking for a solution that I can incorporate in the microcontroller so that I can select the power from the programming, instead of manually having to "flip a switch" for low/high power.
     
    Last edited: Jun 7, 2010
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Post your schematic for your L297/L298 circuit.

    There is a Vref input to the L297, which it uses to compare the Rs1 and Rs2 from the L298 to keep the current through the steppers limited to a safe level.

    You may not have the correct values for Vref or for Rs1/Rs2.

    [eta]
    Did you know that the liquid from those "canned air" cans is flammable? Don't burn it though; the fumes are not good for you.
     
  3. Markd77

    Senior Member

    Sep 7, 2009
    2,803
    594
    Just a thought, would it save drill bits to bring it up, move, and down again repeatedly, although possibly that would be hard to do in the CNC routing.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Drills really aren't the right thing to use for routing. You really need a router bit, or a rotary file.
     
  5. CVMichael

    Thread Starter Senior Member

    Aug 3, 2007
    416
    17
    Can you give me a link so I get a better idea what kind of bits ?

    These are the bits that I have:
    http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=160409635345

    PS. SgtWookie, I am at work now, I will make a schematic for the stepper drivers when I get home.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
  7. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,694
    904
    This is the type of bit I have seen used for CNC PCB cutting: http://cgi.ebay.com/5x-Carbide-PCB-...tem&pt=LH_DefaultDomain_0&hash=item20b15d7019

    They come in various angles. One member of AAC uses the 20° bit, I believe. Here is some of his work:

    http://forum.allaboutcircuits.com/showthread.php?t=11470&highlight=cnc+pcb

    Separation is obtained by taking multiple passes. It is CNC, after all. He uses a Tormach and the linear cutting speed seemed quite rapid to me (his shop is South of Cleveland).

    John
     
    Last edited: Jun 7, 2010
  8. BMorse

    Senior Member

    Sep 26, 2009
    2,675
    234
    I usually use these kinds with some PCB's, I have even used them for some really small SMD components, I get them at a local Machine Tool Shop here in Lansing, MI.

    [​IMG]

    [​IMG]

    [​IMG]

    I like using these because it also removes the burrs that are normally present from using a straight milling bit, plus I can run them at a faster feed rate than the straight ones shown below....

    but I also use the same kind as you had shown....
    [​IMG]

    with these, slow is always better when milling with them.... I usually just use them as drill bits :) ....


    B. Morse
     
  9. CVMichael

    Thread Starter Senior Member

    Aug 3, 2007
    416
    17
    SgtWookie,

    I attached the L298 driver that I'm using. In the zip file you will find the ".dch" file that you can open in DipTrace.
     
    Last edited: Jun 7, 2010
  10. CVMichael

    Thread Starter Senior Member

    Aug 3, 2007
    416
    17
    Thanks BMorse,

    I will order some of these:
    [​IMG]
     
  11. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Brent,
    Mill Supplies Corp on Michigan Ave & Museum Drive? I have been in that store a number of times, but not very often since the early 70's.

    I remember it just like this:
    [​IMG]

    Michael,
    Unfortunately, you did not use reference designators in your schematic. This makes references to it more "wordy".

    Your reference input voltage is established by the 10k and 2.2k resistors on the left, and stabilized by the 0.1uF cap on the far left.
    5v*2.2k/12.2k gives a nominal reference of 902mV.
    Your Rs1 and Rs2 are 0.47 Ohms, so since I=E/R, 0.902v/.47 Ohms = 1.92 Amperes. This is just below the maximum DC operating current of 2 Amperes.

    At that point, according to ST Microelectronics datasheet and some rough extrapolation, you will have somewhere around 4.5V to 4.8v dropped across the L298 itself (total for both sink and source), for a power dissipation of 8.6 Watts to 9.22 Watts, per H-bridge that is operating.

    You can reduce the current by adding a resistor in parallel to the 2.2k resistor, and using a uC pin to either be high impedance (input mode, for the high torque high speed mode) or as an output low, to pull the reference voltage lower.

    Another 2.2k in parallel with the existing 2.2k resistor would give you a Vref of about 5v*1.1k/11.1k gives a nominal reference of 495mV, and just over 1A current through the steppers. Your L298 will then have about 3.2v dropped across it, which will cause it to dissipate less heat. 1A*3.2v = 3.2 Watts per H-bridge.
     
  12. BMorse

    Senior Member

    Sep 26, 2009
    2,675
    234

    Actually it was at Production Tool Supply by the Airport on Grand River....
    I don't believe Mills is around anymore....

    B. Morse
     
  13. BMorse

    Senior Member

    Sep 26, 2009
    2,675
    234

    Just make sure you get the right shank size to fit your tool, I used the 1/8" shank to fit the standard dremmel collets...

    B. Morse
     
  14. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Brent,
    I think that rumors of their demise may be exaggerated.
    http://www.millsupplieslansing.com/
    336 E. Michigan Ave
    They used to have a great inventory. Like I said before, I haven't been there in quite awhile. Things may have changed. If you get a chance, have a look.

    OT: Have you been to the Olds Museum? It's right around there. Some very interesting items in the Museum. They had an Olds Limited last time I was there. They were made with a 707 cubic inch (11.58 liters) straight six! :eek:
     
  15. BMorse

    Senior Member

    Sep 26, 2009
    2,675
    234

    Me and my youngest frequent that side of town a bit (don't know why I never noticed Mill's before....), we usually take walks down the boardwalk that goes along the grand driver and end up right by the museum, there is also the Impression 5 Museum down in there, it is mostly for kids, but have some cool electronic interactive displays....

    B. Morse
     
  16. CVMichael

    Thread Starter Senior Member

    Aug 3, 2007
    416
    17
    This was not easy to do... see the pics attached.

    I did not order the bits you guys told me in the previous posts, I don't have the patience to wait a few weeks until I get them. To get the drill bits I have now, it took 3 weeks until I received them from the time I ordered them. But I will order those eventually...

    I used a thicker drill bit to make sure it does not break ( #72 - 0.025" ), and i've set it to barely touch the board, it took me a few tries to get it at the right depth.
    It took me like 20 minutes to solder the chip on the PCB board, it's tiny !!!
    I tested the traces with an ohmmeter and everything seems to be OK.
     
  17. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,694
    904
    That certainly looks usable.

    Do you have the ability to grind carbide? If so, you can take the tapered shank from a small broken drill and grind off half of it to the centerline. That should work as an emergency milling bit.

    John
     
  18. CVMichael

    Thread Starter Senior Member

    Aug 3, 2007
    416
    17
    No... How is that done ?
     
  19. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Not bad, Michael. :)

    If you were using a copper pour, your milling process would have gone a bunch faster.

    I don't know what it's called in your PCB software. In Cadsoft's Eagle, you draw a polygon on the board. The CNC won't mill away the polygon; only the edges.
     
  20. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,694
    904
    Carbide can be ground like almost any other material. All you need is the right grinding wheel. Diamond of course works, but it is a little expensive and works best if you have good support for the object and water cooling, as with a tool and cutter grinder.

    A more typical grinding wheel for carbide can also be used off hand. I usually just refer to it as a green wheel for carbide. My supplier (PTS in Cleveland) knows exactly what I need.

    You probably won't find them at building centers. If you have any sort of tool supply place -- there must be some in Toronto -- they will know what you need.

    John
     
Loading...