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Need Help to Improve Surface Finish

Discussion in 'CNC IN THE HOME SHOP' started by TomS, Aug 5, 2017.

  1. TomS

    TomS Active User H-M Supporter-Premium

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    This is wierd. I clicked Reply on your post and it brought me to Homebrewed's post. Yeah, I'll get it done just frustrating.

    Homebrewed - I like your idea about twisting the drain wire and signal wire. Being that I've got some additional wiring ahead of me tomorrow I'll give it a try.
     
  2. TomS

    TomS Active User H-M Supporter-Premium

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    I swapped the X and Y driver wires at the BoB. If it was a wiring issue I would have expected the X axis driver fault light to come on. It didn't. The Y axis driver fault light is on so it looks like a bad driver. Looks like I'll be ordering a new driver. The price isn't too bad, about $40 to $45, but it's coming from China. Probably going to take 2 to three weeks to get here.
     
  3. jbolt

    jbolt United States Active User H-M Supporter-Premium

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    If I recall you used the DQ860MA drivers for the X & Y? Both of mine eventually went bad. There are rectifier diodes on the back of the PBC that kept failing. I went to the KL-11080 from Automation Technologies. It is similar to the DQ2272 I have on the Z axis.
     
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  4. homebrewed

    homebrewed United States Steel Registered Member

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    I've never heard of it causing a problem like you're describing but it's possible for the different steppers to talk to each other as well. If your wiring configuration for the steppers has changed since your initial build, you might want to take a look at it with this in mind. You wouldn't want to bundle all the stepper cables together because that would be worst-case as far as coupling is concerned. Bundling each stepper's wires together should be OK because the currents should -- more or less -- cancel out (i.e., you've got the same current flowing IN as OUT so the mag fields should cancel). You actually end up with a dipole type of magnetic field, which drops off in intensity much more rapidly than the field from a single wire.

    I recall a story one of my electrical engineering professors told long ago. He was contacted by a mining company that was having problems with a new 3-phase electric motor they had gotten. The motor was running as though it had insufficient voltage. The company was testing it before they hauled it into the mine, and he noticed they had coiled and stacked the (very long) wire bundles on top of each other. On a hunch he told them to separate the coils: and the motor began to operate correctly. The problem was the mutual inductance between the phases, exacerbated by their close proximity.
     
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  5. spumco

    spumco United States Active Member Active Member

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    The problem was the mutual inductance between the phases, exacerbated by their close proximity.

    Interesting, and is a parallel to what I was taught in the Navy about extension cords. We were told not to use extension cords or welding cable coiled up but to spread them out to reduce heating. Of course, we didn't want to do that on some big old 00 welding leads when we only needed a couple feet for the job, but you pretty much say Yes, Chief and get on with it. I get my NDE inspectors to do the same thing now when we're doing mag particle inspections running 600 amps through giant cables. You can measure the voltage drop in a 50' cable when it's coiled vs. spread out.

    I also was advised by someone here or on CNCZone to twist my VFD conductors inside the conduit to help cancel out noise. Twist the three leads, but do not twist the motor ground with the bundle. Each conductor is looped through an individual ferrite ring right at the VFD end, and then the twist starts. Everything is run inside metallic conduit with ferrite and so far I've had zero VFD noise issues.

    Not that that's an issue with Tom's current woes, but I figured I'd mention it anyway since we're swapping electrical pixie stories.
     
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  6. TomS

    TomS Active User H-M Supporter-Premium

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    Received my new driver today and got the mill up and running. I ran a quick test with the mag base mounted to the column and the DTI on the vise jaw as in the previous test. Needle movement was the same as the first test. Tomorrow I'll run the test as jbolt suggested.

    I've been doing a lot of reading while waiting for the replacement driver. There are a few settings in CamBam (my CAM software) and the Mach3 post processor that may need to be tweaked. In CamBam there is a arc fit tolerance and auto arc fitting setting. In the Mach3 post processor I found arc center mode which can be absolute or incremental. There is one absolute setting and four different incremental settings. Default is incremental (C-P1). One CamBam user has been using absolute for years without issue so I'm going to give that a try. Then there is arc to line tolerance. The default is .01. I did find a reference that .01 is the default for working in metric. Couldn't find a reference to the default for inch mode but .01 of an inch is much greater than .01 of a millimeter. Looks like I need to make a change.

    For info my X and Y power supplies are set at 60 volts output. I've read that higher is better but how high is too high?

    I'll post more test results tomorrow.
     
  7. spumco

    spumco United States Active Member Active Member

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    Good to hear you're making progress. I had the same problem when I switched from Mach to UCCNC; the default settings are for metric units. My machine profile imported correctly, but all the variables not in my profile caused a few pucker moments until I went through every single thing and set it up for proper units.

    As far as the driver supply voltage - it's too high when the smoke comes out :) If you're still using the DQ860MA driver, it's rated to 80 volts. If I recall my internet stepper theory lessons, power is a function of voltage, and torque is a function of current (all else being equal). Increase the voltage, and the power (torque x speed) goes up. The low-speed maximum torque won't change, but the point where it starts dropping off will be at a higher RPM.

    The way to increase low speed torque is to raise the max current. The downside to this is motor heating - so just set the driver to the motor rating and be done with it.

    (Hopefully someone smarter will chime in and confirm my hillbilly stepper motor hypothesis.)

    I suggest you leave them at 60v and get your machine sorted out. Once you have the surface finish problem fixed and you've cleaned up the copper octopus in your enclosure, then you might try bumping the voltage and seeing what feed rates you can achieve in a standard cut before you start losing steps. Increasing the voltage will probably permit you to take more aggressive or faster cuts, assuming your spindle isn't the limiting factor. Again, this assumes your power supplies are capable of maintaining 80v to the drivers at max current draw.

    Cant wait to hear how The Jbolt Method (TM) works.

    -S
     
  8. TomS

    TomS Active User H-M Supporter-Premium

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    I'm taking it a step at a time. I want to specifically identify the root cause so I can pass it along to others. The "copper octopus" is gone. I ran the test today to see if the wiring changes had an effect. They didn't so now it's on to the next test.

    Thanks
     
  9. jbolt

    jbolt United States Active User H-M Supporter-Premium

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    Here is what kept failing on my DQ860MA drivers. Exploding diodes! The diodes are only a couple of bucks and easy enough to fix but after the third one I gave up. Wantai to their credit offered to fix them under warranty but failed to understand that the shipping charges back and forth to China would exceed the replacement cost. They did give me the US spec for a replacement diode.

    20150615_191024.png 20150615_191040.png
     
  10. homebrewed

    homebrewed United States Steel Registered Member

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    If the diodes are blowing themselves off the board they are severely overstressed! The important parameters are the diode type (speed rating), breakdown voltage and current handling capability. I found a teardown log here: http://blog.bouni.de/blog/2015/02/03/taking-a-dq860ma-stepper-driver-apart/ that provides a link to the diode data sheet. The diodes are "ultra-fast" type with 2 amp/50 amp average/peak, 280/400V RMS/peak ratings. The key to finding a US replacement is the speed -- they MUST be fast-recovery for a PWM application. Of course, you want the current and voltage ratings to be equal to or better than the OEM.

    BTW from the same teardown log I note there are some indications that the driver manufacturer may be using counterfeit chips -- the sanded-off surface of the controller IC either says it's counterfeit or the manufacturer did not want their board to be reverse-engineered. If it IS counterfeit, they also may be using diodes that were pulled from other boards. If so, they may or may not be fast-recovery. That could explain the early failure of the diodes.
     
  11. TomS

    TomS Active User H-M Supporter-Premium

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    Thanks for the info. Not inclined to try a repair as the cost for a replacement driver is only about $40.
     
  12. TomS

    TomS Active User H-M Supporter-Premium

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    I ran a multitude of tests this morning. The first test was mounting the DTI as jbolt suggested and running gcode. The DTI needle bounced around quite a bit. I read on another forum that you can run a Fanuc post processor with Mach3. So I tried it. No surface finish change using the standard PP settings. I then set the Fanuc PP Arc Center mode to Absolte and made the same change in Mach3 General Configuration. Again, no change. Made the same change to the Mach3 post processor with no change in surface finish quality. Out of desperation I changed Arc Output from Normal to Convert to Lines. Nada! All these tests were run with Arc Fit Tolerance set at .0004 and Spline to Polyline Tolerance set at .0001. The last test were test cuts made at three different Z heights to see if the faceting lined up. They didn't. See the pictures below.

    Still have to run the drain wire disconnected test as jbolt suggested. If that doesn't show anything then I'm leaning towards a motor tuning issue.

    20170817_114442_resized.jpg

    20170817_114505_resized.jpg
     
  13. homebrewed

    homebrewed United States Steel Registered Member

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    Sometimes it's the down-time that is more important than waiting for a replacement driver. If the driver is that unreliable maybe you should get one or two backups? Depends on whether or not the down-time delays delivery to a customer vs. delaying a personal project.

    I suppose your decision to replace or repair also depends on your ability to solder/de-solder surface mount parts without damaging the board or other components. I've done that at work but now, lacking a good stereo-zoom microscope, SMT would be a bit more challenging for these old eyes. I thought I might be able to use a cheap USB microscope but the frame rate is so low it's really difficult to place small components. This is a case of you really DO get what you pay for.
     
  14. TomS

    TomS Active User H-M Supporter-Premium

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    I hope I didn't come across as flippant. My mill has been running for about three years now so if I have to replace a driver again in three years I'm OK with that. This driver failure was a first for me. Everything else has been running fine.

    I'm with you on soldering small electronic components. My eyes aren't what they used to be and I don't have the proper equipment. I'd rather spend my money on tooling for the mill and lathe.
     
  15. jbolt

    jbolt United States Active User H-M Supporter-Premium

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    My drivers are the ones that kept blowing up diodes, not the OP's. I believe this is the first driver failure for Tom and I have no idea what the failure is. I was just sharing my experience and why I moved away from them. As surface mount components go they are fairly large and easy to replace with a fine tipped variable heat soldering iron. I fixed mine a few time since it only takes a little time to do vs waiting. I'm not suggesting he abandon them but if it is a diode that failed to be aware.

    Tom - if you exhaust all options with you current setup you might try reinstalling the old BOB and motion controller. Not fun but it might tell you if it is something in the 126, 107, ESS combo.

    Grasping at straws here but I have seen issues (non CNC) with some cheap Ethernet cables. I make sure to use high quality shielded cables, keep them as short as possible and away from power cords.
     
  16. TomS

    TomS Active User H-M Supporter-Premium

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    I understand your logic but I really don't want to reinstall the old breakout board and the UC100. I've changed so much that it would be nearly a complete rewire job. I've got a couple of new things to try first. I'm going to lower the power supply output voltage and see what happens. Read that on Practical Machinist where the lower voltage altered motor resonance and improved the surface finish. I read somewhere else that raising the voltage did the same thing. Who knows? It's an easy thing to do.

    I did run the drains disconnected test you suggested. Surface finish didn't change so my assumption is the grounding system is sound.

    You bring up a good point about the ethernet cable. It's Cat6 but that's all I know. I'll check into it and make sure it's shielded.
     

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