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Thread: Wrong cut

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  1. #1
    Hi all!

    I have a "small" problem with my DIY CNC. For some reason, I'm not able to get the right dimensions when I cut a 40x40 mm square in wood. I get 40 mm when I cut along the wood fiber and 39.7 when perpendicular to the wood fiber.I have tried to reduce the cut deep from 2mm to 0.5 but the result is almoust the same.If I cut in mdf ,I will get 39.8 mm on one axis and 39.9 on the other(so here the differences between the axis is smaller).
    The backslash on all axis is less than 0.03
    The deflection is around max 0.04-0.05mm when 15 kg applied on each axis
    I'm running Uccnc with ETH400 and on each axis, I have a DM856.
    Also 2.2 China spindle+VDF "NowForever"(China)
    The cause of my problem can be the high noise made by the VDF. Probably I'm losing steps (when high cutting force), but I'm not sure about it.

    Any idea?

    THX
    Stefan
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  2. #2
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Last Activity: 2 Days Ago Forum Superstar, has done so much to help others, they deserve a medal. Has been a member for 9-10 years. Has a total post count of 2,908. Received thanks 360 times, giving thanks to others 8 times.
    It's more likely to be spindle deflection.
    You say you've measured deflection on each axis, but what about on the spindle nose/cutter with the spindle at cutting depth?
    Avoiding the rubbish customer service from AluminiumWarehouse since July '13.

  3. #3
    I already checked the spindle and has no runout(0.01-0.02) and also no deflection. The cutter is a 4mm x22mm (not the best quality) and for sure will deflect. Tomorrow I will check this too.
    However, the cut was done first at 2000mm/min and second at 1000mm/mm and decreasing the feed rate /deep of the cut improve the result, but not dramatically(from 39.7 to 39.8).In case of cutter deflection changing the speed/deep should improve the result.

  4. #4
    Quote Originally Posted by cncnescu View Post
    I already checked the spindle and has no runout(0.01-0.02) and also no deflection..
    Have you investigated the side-force generated by your cuts and applied that force via a bar in the chuck, with the same protrusion as your cutter, to measure deflection?
    If not then your measurements are not valid,
    Regards,
    Nick
    You think that's too expensive? You're not a Model Engineer are you? :D

  5. #5
    Quote Originally Posted by cncnescu View Post


    The cause of my problem can be the high noise made by the VDF. Probably I'm losing steps (when high cutting force), but I'm not sure about it.

    Any idea?

    THX
    Stefan

    Hi Stefan,

    If you think losing steps is the cause,

    Have the spindle return to a known position after cutting.

    Setup a dti there.

    Run the program return to that position and check for zero on the dti.


    Grtz. Bert.




    Verstuurd vanaf mijn SM-A320FL met Tapatalk

  6. #6
    Quote Originally Posted by driftspin View Post
    Hi Stefan,

    If you think losing steps is the cause,

    Have the spindle return to a known position after cutting.

    Setup a dti there.

    Run the program return to that position and check for zero on the dti.


    Grtz. Bert.




    Verstuurd vanaf mijn SM-A320FL met Tapatalk
    Yes, it will return to the position, so it's not the noise.

  7. #7
    It was from the cutter. When I changed to a new one(3.175x9mm) the situation improved to 40mmx39.95mm.Now I have to find a 22 mm cuter that will not deflect so much because the 9mm that works it's too short for what I want to cut

  8. #8
    Bad news!!... Yesterday I made a mistake. The cutter was not 3.175 but 3mm. After changing in the Estlcam the cutter diameter to 3mm (the real one) I get again wrong dimension.This cutter it's 9mm long so the deflection is minimum at 1000mm/min.
    I really don't understand what is happening.

  9. #9
    Quote Originally Posted by cncnescu View Post
    Yes, it will return to the position, so it's not the noise.
    So this also rules out reversed polarity problems... right?


    If you are convinced the total combined rigidity/flex cant be the problem....


    You can try with a 6mm cutter with a lot less flex..



    Grtz Bert.



    Verstuurd vanaf mijn SM-A320FL met Tapatalk

  10. #10
    Today I discovered that my VDF was set on CCW rotation and the cutter used for MDF was a right hand one. After changing to CW on MDF I get the right dimension.
    However on the wood plate on a 40x40 square it's all perfect, but when I switched to a bigger 80x80 square I get again wrong dimensions 80x79.8.
    Normally it should work also on a bigger square but at least half of the issue was solved.
    Last edited by cncnescu; 23-09-2018 at 12:09 PM.

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