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Basic Machine Questions

Discussion in 'CNC Mills/Routers' started by CNCMD, Oct 9, 2017.

  1. CNCMD

    CNCMD Well-Known
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    Hi All - Just wanted to introduce myself as I've just completed my build. Here are the details:

    Sphinx 1000x500 build w/ Limit Switches
    800w Water Cooled Spindle
    CNC xPro Board - Hint, $4=1 - That had me messed up for about an hour
    Nema23's
    Fusion 360, and GRBL Panel

    I had a long weekend of struggles in learning how to get coordinates setup, stepper motor steps adjusted, machine homing correctly, etc. All in all I would consider it a pretty good success. Got some test routing done and didn't break any bits yet.

    One of the projects that I plan to do is routing recessed holes into some aluminum plate. The aluminum plate will already be final sized. With that said, I am trying to find out how to do my G54 offset properly, as I want to ensure the holes will be exactly where I expect them to be. So, in determining 0,0, where would the bit sit. The model has it in the lower left corner.

    My question is a bit more precise, meaning, would I sit the bit inside the corner, outside on the corner, half on/half off the corner, etc. - Edit: In further reading, it would appear the the 0,0 point is the exact center of your bit, in both directions. Thus, it would seem the center of the bit would be placed right on the corner? Right?
     
    #1 CNCMD, Oct 9, 2017
    Last edited: Oct 9, 2017
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  2. Jonathon Duerig

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    Once you have made your machine as precise as it is going to be, consider using the machine itself to help you set up the proper stops.

    So, for example, set a stop of a relatively soft material approximately where you need it. Then create a profile which will cut through that material at a precisely known coordinate location. Then you place any stock against that stop when clamping it in place. And you know that the edge of the stock is precisely located at that known coordinate. Then you drill holes or cut shapes accordingly in your software.

    If you have homing switches, those stops can be permanently installed on your machine and can be used for precise placement of any other jig or template you put on your machine bed.
     
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  3. CNCMD

    CNCMD Well-Known
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    Thank you for the input. I hope to get back to making some chips soon enough.
     
  4. Joe Santarsiero

    Joe Santarsiero OB addict
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    Yes. Center of the spindle on 0,0.
     
  5. CNCMD

    CNCMD Well-Known
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    Thanks Joe for the reply. That is so far what I have determined based on quite a bit of reading. Appreciate the help.
     
  6. Joe Santarsiero

    Joe Santarsiero OB addict
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    No problem. Did you get to cutter compensation yet?
     
  7. CNCMD

    CNCMD Well-Known
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    I honestly can't say that I know what that even is. I'll do some searching.
     
  8. Jonathon Duerig

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    Cutter compensation is part of calibrating your machine. When you set up your software to cut, you need to tell it how big the bit is. Then when it is cutting the outside of a shape, it can offset the path by half the cutting diameter to give a precise outline that you want. But you don't want to just use the manufacturer's specs. There are various factors that can modify the effective cutter compensation to be larger or smaller than the factory specifications.

    For example, I do most of my cutting with a bit that is 2mm wide. Some factors (like the runout of the spindle) make it cut in a diameter that is slightly larger than 2mm. Other factors (like the dullness of the bit or the nature of the material cut) make it cut in a diameter slightly smaller than 2mm. The only way to know is to cut a slot in material and then measure the width of that cut. The width you measure is what you specify as cutter compensation in your software.

    In my case, even though I have a 2mm bit, the actual cut width is closer to 1.9mm because the material I use is slightly springy and so deforms just a bit when it is being cut. For other materials (like metals), the runout of your spindle will probably dominate. But the only way to be sure is to measure.

    -D
     
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  9. CNCMD

    CNCMD Well-Known
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    Thank you for the information, that is a good explanation. Would that, or wouldn't that be achieved in the software? Meaning, when I tell Fusion 360 my bit information, is that already written into the gcode?

    I understand the part about the actual size the cutter makes versus the advertised size.
     
  10. Jonathon Duerig

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    Accounting for cutter size is entirely done in the software. So when you generate your gcode, the paths there already account for the assumed cutter size. And if you change cutter size, you need to re-generate your gcode.
     
  11. CNCMD

    CNCMD Well-Known
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    Ok, in looking at Fusion360, I've been using the "In Computer" Compensation type. I'm assuming this is what I want to do, and obviously the accuracy is contingent on the actual size that the bit cuts in the material versus the advertised size.
     
  12. CNCMD

    CNCMD Well-Known
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    Wanted to provide an update - Was able to get the 8x10 2D contour pass perfect.

    However, there are 12 holes that are made using 2d adaptive clearing, that continually come out too small. I've checked my F360 options,and am NOT leaving any stock. I'm at a loss on this. The holes are slightly undersized.
     
  13. Joe Santarsiero

    Joe Santarsiero OB addict
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    Bump your offsets at the controller if it supports it.
     
  14. CNCMD

    CNCMD Well-Known
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    I'm not sure what that means. I'm using a CNC xPro.
     
  15. Joe Santarsiero

    Joe Santarsiero OB addict
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    Some controllers will allow you to adjust the tool offset for the diameter/rad of the tool and do the math to adjust the tool path.
    I looked around and didn't see it for any of the GRBL software controllers. I use mach3. If a pocket or slot comes out small(ideal case) then I make an edit right at the controller and rerun the code til the size of the feature is where I want it. No need for me to have to regenerate a new toolpath in CAM.
    I'm not up to date on GRBL, its use of the D term for G41/G42, or if there are any work arounds/scripts for this. Last I knew it doesn't support 41and 42 and the toolpath isn't adjustable on the fly. Offsets are generated in the output and are final. Maybe one of the GRBL gurus can chime in to check me on this.
    Basically you don't want to have to go back to your CAM software to edit then regenerate all new code to dial in a tool, but if you have to then you have to.
    BTW, all this is assuming the machine has been calibrated.
     
  16. CNCMD

    CNCMD Well-Known
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    Thank you for the input, I'm trying to learn as much as I can.

    Here is some additional fun facts that might play into things.

    1) My spindle rotates ccw. It has the ability to go in the other direction, but I thought it was worth mentioning. I have been using it in ccw rotation. (My manual mill rotates cw.) I'm starting to think that is wrong, and it should be going cw rotation.

    2) Ran the part again, everything is perfect, however in order to get the holes right I had to add(remove) negative stock. What is strange to me, is that the outside cut out, the 8x10 is precisely that. As noted there are 12 holes, all are exactly spaced as per the CAD.

    Machine has been calibrated to my best ability. Both axis' were measured to their fullest, and movement is exact to the mm over the length of the access. I used a ruler, and a .1mm v-bit.
     
  17. Joe Santarsiero

    Joe Santarsiero OB addict
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    So you have been cutting with the bit in reverse.....?

    How are you measuring this? How much more did you have to cut?
    Aside from other things, the structure of the machine, the tool, runout, etc all have different dynamics in linear cuts verses circular cuts. I can get into more details as to why, but I'd rather refrain from tailoring an hours long written article on this. Just know that if you're seeing a hairs difference then this is likely the case. There could be a software and controls side to this as well in regards to how circles are handled as lines or arcs.
    You should borrow or purchase a dial indicator, a dial test indicator, or should already have a set of calipers laying around. A rule doesn't have the resolution to calibrate these machines for the tolerances you're looking for and every eye is subjective(even when looking at dials. Although dials bring it in a lot closer than a rule).
    DIY Machine calibration is a pretty highly covered topic at this point. I recommend you check out as many vids on this by people not using rules, but something more accurate and precise. It is also good to know what resolution your machine and settings are at before chasing that Dragon.

    Joe
     
  18. CNCMD

    CNCMD Well-Known
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    1) Yes apparently so. I don't know how much that is contributing to any error rates that I'm getting. But again, the only error I can see is on the circles. Edit: Just went to rewire the spindle, the directions from the place I got it from were not correct.

    2) The outside 8"x10" is measured by rule. Edit - I now have used calipers and confirm 8"x10" exactly. The circles I did negative stock of -0.35mm.

    3) I have all of the mentioned tools, I am going to get a longer set of calipers.
     
    #18 CNCMD, Oct 14, 2017
    Last edited: Oct 14, 2017
  19. Joe Santarsiero

    Joe Santarsiero OB addict
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    I'm lmao. Okay CNCMD. Get that spindle wired correctly and let me know how it cuts in the other direction. Still lmao.
    No offence. I'm really here for ya. Update when possible.
     
  20. CNCMD

    CNCMD Well-Known
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    No offense taken, I'm laughing at myself.

    I can tell you that now the 3.175mm bit cuts exactly 3.175mm. So that takes the variance in the bit size out of the equation.

    The holes are still slightly out, .20mm.

    I'm going to revisit my stepper steps using a caliber this time, I can only do a 6" span, but its worth while to verify. I'm going to get a 12" set soon.

    Thanks for being here for me. I think I've been working too many late nights...lol.
     
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  21. CNCMD

    CNCMD Well-Known
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    Further updates:

    Ok so, I finally caught onto the fact regarding the use of a dial indicator over a rule. I would say there is as much misinformation out there as there is information. I've honestly googled and found countless calibration exercises using a rule.

    Since I honestly own quite a few measuring tools, I broke out the digital dial indicator and used that to calibrate steps, and as expected by Joe, there were variances found. Ironically the first noted variance was about 0.35mm, which was the amount that I was adding as negative stock removal. Coincidence? Maybe. But probably not.

    All in all I was able to go though all 3 axis' and each one of them were off by some amount. So I can report in that I feel my steps are now correct. I need to re-run the part, and see what the overall effect is.
     
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