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  1. #1
    Hi all,

    I recently got my diy cnc up and running. As i was testing it i noticed if i cut through a piece using say 3 passes there was a line left and in a couple of places a step. It runs on rack and pinion on the x ( long axis) and a ball screw on Y and Z. I initially assumed it was the rack and pinion but im starting to think maybe its from the ballscrew on Y. I'm thinking this because of the location of the bump. I attached some pictures of a few circles i v grooved to test, and also of some text. Any help with this would be greatly appreciated.

    Philip
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    Last edited by philgiggins; 08-10-2015 at 10:42 PM.

  2. #2
    Have a look in your code to see the start and finish positions of your circles. It seems that they may be different.

    As for backlash, yes ballscrews do have backlash if they are not preloaded, you will also get it from the bearings. Try cutting a square, it is usually the best way to see if you do have backlash. Also use a flat endmill not a ball. A very quick way to test is to measure the actual movements you are doing to verify them.

    If you really want to test the machine have a go at the NAS Test also known as the Circle Diamond Square:

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  4. #3
    Ill try the NAS test tomorrow evening when i get a chance and post the results. Theres definitely something amiss. I need to eliminate possibilities one at a time !

    Philip

  5. #4
    To me it just looks like the tool geometry is wrong, as Komatias said are you using a ballnose ?. If so and the software thinks it is an end mill, then you will be left with material like you have in the pictures on the end points of the circles. You can change to an endmill or you can simply tell the machine to overrun the toolpath by a few mm.

    If those steps were caused by backlash you would see signs of it on the top and bottom of the circles on the direction change, but they look fine, so it looks to me like you just have the wrong settings in.

    The reason the steps arent in a straight line is becasue i assume each vector is starting in opposite directions and that is just the bow wave effect pushing material infront of the bit, so you get that alternating step effect, if you ran the same program with all the startpoints and directions the same on the vectors you would still have the steps but they would all be in a straight line, either overrun the bit or use an endmill, and even then i would overrun a few mm.
    Last edited by howser37; 09-10-2015 at 05:45 PM.

  6. #5
    Quote Originally Posted by howser37 View Post
    To me it just looks like the tool geometry is wrong,
    +1 on this. If you had backlash issues that V carved lettering would show it straight away.!

  7. #6
    There's a kerning fault with the lettering, but the cutting looks good.
    You almost always need to manually kern in cad as very few software packages do the job well.

  8. #7
    Tool geometry was incorrect on that toolpath, quite a mistake on my part. I had issues earlier that with steps as i have said before and that evening i thought ill try some circles it should be a good test and when they didnt join i just assumed .... and then posted it ... thats what i get for rushing it!
    Thanks Philip

    To me it just looks like the tool geometry is wrong, as Komatias said are you using a ballnose ?. If so and the software thinks it is an end mill, then you will be left with material like you have in the pictures on the end points of the circles. You can change to an endmill or you can simply tell the machine to overrun the toolpath by a few mm.

    If those steps were caused by backlash you would see signs of it on the top and bottom of the circles on the direction change, but they look fine, so it looks to me like you just have the wrong settings in.

    The reason the steps arent in a straight line is becasue i assume each vector is starting in opposite directions and that is just the bow wave effect pushing material infront of the bit, so you get that alternating step effect, if you ran the same program with all the startpoints and directions the same on the vectors you would still have the steps but they would all be in a straight line, either overrun the bit or use an endmill, and even then i would overrun a few mm.[/QUOTE]

  9. #8
    Nothing to do with Mach3 so forget that.!! . . . . They are correct thou that those motors are more than good enough when run at that voltage and with those drives.
    It's a common mistake to blame motors and go for larger motors only to find out they weren't the problem and now your performance is less.!!

    If really is loosing steps(I don't think it is.!!) This will be down to setup with motors being over tuned or mechanical issue causing binding.
    My own feeling is that some coupler or pulley is slipping.? Missed steps tend to be one of two states.? Incremental showing slowly over the course of job in tiny amounts. Or Random with no ryhm or reason.?
    Incremental often comes from wrongly setup motors/drives. Random from Electrical noise or poor wiring.

    Give us some info on the tuning for each axis. Also Info on the Pitch of pinions or screws etc.
    Last edited by JAZZCNC; 11-10-2015 at 01:22 PM.

  10. #9
    The easiest way to measure the backlash is with a dial indicator and manual jogging. There are a lot of vids on youtube about it.

    To find out if you had lost steps, just program and run in the air a straight line for each axes and the same speed you did the circles (with a higher Z) and try to stop the movement by grabbing the Z axis with your hands and determine how much force is needed for the motors to start skipping. You can even measure it with a hook scale.

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  12. #10
    Quote Originally Posted by komatias View Post
    If you really want to test the machine have a go at the NAS Test also known as the Circle Diamond Square:
    Does anyone have an STL or Gcode for this.

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