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  1. #1
    Join Date
    Feb 2019
    Posts
    9

    DIY CNC not cutting correct size

    Recently attempted to cut a box with lid. Both bottom and top of box were same file/cut. Gcode was generated in fusion360 and run on Mach3. In fusion all components fit together correctly. Once cut top and bottom did not fit. Y axis cuts were pretty close, close enough to make work. X axis cuts were off by over 0.075. I dont believe its a step issue because i believe parts would still fit together if steps were off due to both parts sitting side by side on the x-axis. Any ideas would be greatly appreciated.

  2. #2
    Join Date
    Dec 2003
    Posts
    1206

    Re: DIY CNC not cutting correct size

    Is the machine flexing?If the Y axis runs along the gantry and is pretty close and the travel of the gantry is along the X axis,thats where I would start looking.A photograph of the machine might help,together with a description of the depth of cut and feed speed.

  3. #3
    Join Date
    Feb 2019
    Posts
    9

    Re: DIY CNC not cutting correct size

    I dont believe it is flexing but cant be sure. With my machine the X-axis runs along the gantry and the gantry runs along the Y-axis. I will get a couple photos with details for the cut and post them.

  4. #4
    Join Date
    Feb 2019
    Posts
    9

    Re: DIY CNC not cutting correct size

    I have included a couple files to this message to display what is going on. One is a picture of the actual piece and what the actual measurements are, I have also included a picture of my machine and pdf of what the actual dimensions are supposed to be.

    I am using a 2 flute 1/4" flat endmill:
    30in/min
    10000rpm

    Total depth of cut is 1.1"
    Maximum stepdown is 0.15"
    Stepover is 0.2375
    Climb milling

    Again any help will be helpful

  5. #5
    Join Date
    Sep 2005
    Posts
    1740

    Re: DIY CNC not cutting correct size

    I would start with some basics. Draw up a square a certain size and see if your machine will cut it the proper size. Same with a circle, both large enough to do some accurate measurements. Adjust your steps if needed to make the creations the proper size. Do this before you waste anymore good wood on a project. If you get your steps right and you still can't make a decent square consistently then its your machine.
    Retired Master Electrician, HVAC/R Commercial. FLA Saturn 2 4x4 CNC Router Mach4 Kimber 1911 45ACP

  6. #6
    Join Date
    Dec 2003
    Posts
    1206

    Re: DIY CNC not cutting correct size

    As suggested above try cutting a circle and a square and measure them as accurately as possible.For further confirmation you might do an additional pair and compare climb cutting with conventional cutting.It also looks like a lot of cutter projection was used and clearly any slack in the spindle bearings could be an additional factor.In the photograph of the project if you look really closely at the outer wall of the right hand recess there appear to be small steps in the surface and this is the classic sign of a spindle slightly out of square to the table.Do you have a way to verify the tramming of the spindle?

    One other small point,what should the dimensions have been and what clearance were you allowing for the fit of the lid?

  7. #7
    Join Date
    Feb 2019
    Posts
    9

    Re: DIY CNC not cutting correct size

    Thank you wmgeorge and routalot,
    I did try cutting a 2" square and circle with conventional milling and both were off by a few hundredths. Thank you routalot for picking up on the spindle being out of square, I do have a gauge that I will try tonight and see what that tells me. I also found that my x-axis cradle that travels on the gantry has a little play. Could that be throwing the x-axis cuts off? I think that my best bet would be to cut another cradle piece and see if that takes the play out of the v-bearings on the x-axis. Hopefully that will be a bit improvement. I am still completely new to this so all information is helpful and thank you to all that have replied.

  8. #8
    Join Date
    Sep 2005
    Posts
    1740

    Re: DIY CNC not cutting correct size

    I meant a 10 or 12 inch or larger square you can't tell much on a small one. Same way cut a larger circle, you do not have to cut very deep just enough to measure, in fact you could do it with a marking pen.
    Retired Master Electrician, HVAC/R Commercial. FLA Saturn 2 4x4 CNC Router Mach4 Kimber 1911 45ACP

  9. #9
    Join Date
    May 2005
    Posts
    3920

    Re: DIY CNC not cutting correct size

    Any play in your bearings or leadscrew will impact accuracy and cut quality. It looks like your machine is using a DIY solution for linear bearings. If so realize that to be effective they have to be built to a high standard of precision. Sadly you can end up spending a lot of time and effort trying to get these DIY bearings to work well. To be honest if I went this route I would want a way to preload one set of bearings against another set that are rigidly held in place.

    A little bit of trig will help you understand how a bit of clearance in the bearings can lead to large reflections at the cutting tool. You need to make sure your machine saddles and the gantry are stiff in all directions.

    It is also interesting that some of your dimensions are spot on. This makes me wonder if your acids are chain driven? I ask because you may be seeing errors inherent in chain drive systems. Chain drives can introduce some periodic error in motion due to their mechanical realities. Also many commercial chain drive components are not exactly high precision motion control components. You might want to see what happens if the dimensions are changed a bit.

    From what I saw in your photos you are off both in the X and the Y but again only with some dimensions. That could be an issue with mechanical slop. One thing to try is to see what happens with higher spindle loading. That is increase your depth of cut and compare the results with test pieces cut with lighter loading.

  10. #10
    Join Date
    May 2018
    Posts
    69

    Re: DIY CNC not cutting correct size

    I think wizard has it right. Another thing you can do is cut the square and the circle forward and backward (clockwise and counterclockwise). If the two cuts are both off in opposite directions (mirror images), it will show any backlash in your axes.

  11. #11
    Join Date
    Sep 2005
    Posts
    1740

    Re: DIY CNC not cutting correct size

    I don't see how you can draw conclusions from those pictures. He needs to make the large test square and circle as I suggested and take the measurements with a ruler and a square. Thats the only way your going to tell. If its out of square is adjustments to the machine, if its square and not the right size its the steps adjustment.

    Ball screws and I wonder if there is any adjustment in either the X or Y axis to square if needed.
    Retired Master Electrician, HVAC/R Commercial. FLA Saturn 2 4x4 CNC Router Mach4 Kimber 1911 45ACP

  12. #12
    Join Date
    Feb 2019
    Posts
    9

    Re: DIY CNC not cutting correct size

    There are no chains on this machine. It is driven by steppers connected directly to acme lead screws. X and Y axis seem to be pretty square. I am going to remake my z axis plate to tighten up tolerances and start from there. I will attempt to cut a larger square and circle and see what that tells me. I do believe that it is mechanical because when I slowed the cut down and took a smaller bite and it appeared to be closer to correct dimensions. Also when I switched from climb milling to conventional milling it also seem to have helped with dimensions.

  13. #13
    Join Date
    Dec 2003
    Posts
    1206

    Re: DIY CNC not cutting correct size

    I'd say you have it right.Something is flexing and since the big discrepancy is along the X axis,you could have flexing from the motor mounts adding to a bit of movement on the two sets of rails.With a direct drive to the lead screw you really shouldn't have any issues with steps per inch as these are easily determined by the thread pitch and the steps per revolution.In which case you should find the same amount of deviation from the desired dimension on a larger test piece as there was on the smaller item.The obvious question is what can you do about it?I suppose preloading the bearings is about it really.

  14. #14
    Join Date
    May 2018
    Posts
    69

    Re: DIY CNC not cutting correct size

    Quote Originally Posted by jlmort View Post
    There are no chains on this machine. It is driven by steppers connected directly to acme lead screws. X and Y axis seem to be pretty square. I am going to remake my z axis plate to tighten up tolerances and start from there. I will attempt to cut a larger square and circle and see what that tells me. I do believe that it is mechanical because when I slowed the cut down and took a smaller bite and it appeared to be closer to correct dimensions. Also when I switched from climb milling to conventional milling it also seem to have helped with dimensions.
    Chains, direct connect, it doesn't matter. All machines have backlash. It all depends on how much is present and how it is managed.

    But, I think you are on the correct path to fixing it. Keep playing with it and it will work out.

  15. #15
    Join Date
    Jan 2005
    Posts
    15362

    Re: DIY CNC not cutting correct size

    Quote Originally Posted by jlmort View Post
    There are no chains on this machine. It is driven by steppers connected directly to acme lead screws. X and Y axis seem to be pretty square. I am going to remake my z axis plate to tighten up tolerances and start from there. I will attempt to cut a larger square and circle and see what that tells me. I do believe that it is mechanical because when I slowed the cut down and took a smaller bite and it appeared to be closer to correct dimensions. Also when I switched from climb milling to conventional milling it also seem to have helped with dimensions.
    This means you have flex in your machine or backlash most likely both if you have anything that is loose or has any movement it will be off with the cut by the amount it can flex, climb milling will show you this straight away

    Your first cuts show you this by doing an inside cut and an outside cut, the amount they are out is the amount of backlash or flex
    Mactec54

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