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    Default Machine not cutting accurately

    The cut is always out between .2 and .4 mm undersize

    The steps have been set, there is no steps being dropped and no noticeable backlash, and the z axis is rock solid.
    I set the steps with a 900mm rule, and checked the settings by moving the x axis about 10 times while holding it back with all my strength. still spot on.

    I have checked the 6mm bit and I use aspire for the code.
    I have made 25mm, 50mm and 100mm square pockets, 25mm 50mm 75mm slots, they are all out 0.2 to 0.4mm

    I have tried Mach3 and Uccnc, both the same.

    UC300-lpt5 motion controller and HG08 Breakout card
    Stepper motors are direct drive, onto 2005 ballscrews (2 on x axis linked not slave), with dial gauges at the x and y homes (just to check)

    I have even tried cutting slots, like for a guitar fret board

    35.64 equated to 35.98
    33.64 equated to 33.86
    31.75 equated to 32.00

    not much cop for guitar fretboards

    I have looked at other threads and site but no real fixes

    Any Ideas?

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    Member ger21's Avatar
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    Default Re: Machine not cutting accurately

    First, it sounds like your pockets are too small, then it sounds like your slots are in the wrong place? These would be two different issues..
    For your pockets, cut one climb cutting, and one conventional cutting, and compare the sizes. The conventional will likely be larger.
    I have even tried cutting slots, like for a guitar fret board

    35.64 equated to 35.98
    33.64 equated to 33.86
    31.75 equated to 32.00
    I don't understand what you mean here?

    Are these the slot locations? How are you cutting the slots, in a single pass, or back and forth?

    Gerry

    UCCNC 2017 Screenset
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    Mach3 2010 Screenset
    [URL]http://www.thecncwoodworker.com/2010.html[/URL]

    JointCAM - CNC Dovetails & Box Joints
    [URL]http://www.g-forcecnc.com/jointcam.html[/URL]

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


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    Default Re: Machine not cutting accurately

    In mach3, you can calibrate the steps. There is a wizard that leads you through it, i think under motor tuning. Basically it will move the axis being tuned to a distance it thinks is right, then you measure the actual distance. Mach 3 then calculates reality and you should be good to go. A longer test movement will result in a more accurate calibration.

    Sent from my SM-G920T using Tapatalk



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    Default Re: Machine not cutting accurately

    I set the steps with a 900mm rule, and calibrated it in mach3 and Uccnc, they are spot on

    I have cut slots, like for a guitar fret board

    Slots like this I I I I spaced apart 35.64, 33.64 and 31.75 @ 3mm depth with a 1.7mm end mill at 0.5mm per cut 1.7mm wide

    The actual spacings were 35.98, 33.86 and 32.00



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    Default Re: Machine not cutting accurately

    Quote Originally Posted by piccyman View Post
    I set the steps with a 900mm rule, and calibrated it in mach3 and Uccnc, they are spot on

    I have cut slots, like for a guitar fret board

    Slots like this I I I I spaced apart 35.64, 33.64 and 31.75 @ 3mm depth with a 1.7mm end mill at 0.5mm per cut 1.7mm wide

    The actual spacings were 35.98, 33.86 and 32.00
    Not to be flip but if it was calibrated it wouldn't be giving you this problem. I don't know what the 900mm rule is but i know measuring the distance and setting the steps per inch or whatever measurements you use will always calibrate the axis for it's drive screw/mechanics. looking at your measured distance versus instructed distances they appear to all be getting larger, which also seems to indicate in incorrect calibration. I could certainly be wrong but that's how it seems to me. If i am wrong i will be really interested in the solution. Learning new stuff is awesome. Good luck!

    Sent from my SM-G920T using Tapatalk



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    Default Re: Machine not cutting accurately

    When I check the axis I looked at the distance at every 100mm , it was spot on
    Also I am not a newbie, I have been using the machine for at least 2 years, just never checked if the parts were slightly out, i didn't really matter until now.

    0.34,0.22, and 0.25 are the errors that order

    So how can the calibration be out?



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    Default Re: Machine not cutting accurately

    Quote Originally Posted by piccyman View Post
    When I check the axis I looked at the distance at every 100mm , it was spot on
    Also I am not a newbie, I have been using the machine for at least 2 years, just never checked if the parts were slightly out, i didn't really matter until now.

    So how can the calibration be out?
    Well, the only other thing i can think of, and I'm no newbie either, is the machine is loose somewhere. Mechanical slop. Those are the two places to look other than software which we'll assume is not the problem. Bottom line is it was working, now its not. Something changed. Have you checked for backlash?

    Sent from my SM-G920T using Tapatalk



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    Default Re: Machine not cutting accurately

    Quote Originally Posted by piccyman View Post
    When I check the axis I looked at the distance at every 100mm , it was spot on
    Also I am not a newbie, I have been using the machine for at least 2 years, just never checked if the parts were slightly out, i didn't really matter until now.

    0.34,0.22, and 0.25 are the errors that order

    So how can the calibration be out?
    Sorry, just saw you already checked the backlash.

    Sent from my SM-G920T using Tapatalk



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    Default Re: Machine not cutting accurately

    Nothing is loose, i have dial gauges at home on the x and y axis, and they read the same every time each axis is homed.
    Also this morning i grabbed the x axis as it was doing all 10 moves from 0 to 900mm and it was spot on at the end, nothing is moving

    Its really odd, not sure whats left to check, the climb and conventional cuts surely couldn't make a difference to a profile slot cut, could it?



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    Default Re: Machine not cutting accurately

    Another thing to check is the size of the bit itself. Is it truly the size you think it is? They do wear down, and sometimes they do get manufactured funny. I would take a mic to the edges and see where it is. But something is definitely off, and if you recently replace the bits or got a batch of bits that could be at as well.

    Sent from my SM-G920T using Tapatalk



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    Default Re: Machine not cutting accurately

    Quote Originally Posted by piccyman View Post
    the climb and conventional cuts surely couldn't make a difference to a profile slot cut, could it?
    Yes, if you 're making one pass in one direction, deflection will usually show up.

    Gerry

    UCCNC 2017 Screenset
    [URL]http://www.thecncwoodworker.com/2017.html[/URL]

    Mach3 2010 Screenset
    [URL]http://www.thecncwoodworker.com/2010.html[/URL]

    JointCAM - CNC Dovetails & Box Joints
    [URL]http://www.g-forcecnc.com/jointcam.html[/URL]

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


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    Default Re: Machine not cutting accurately

    Its not the end mills, I measured the 1.7mm @ 1.7mm and the 6mm @ 5.99mm, and set them up correctly in aspire.



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    Member Biggs427's Avatar
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    Default Re: Machine not cutting accurately

    What kind of linear motion? Have you measured the movement with a dial indicator. Not at home but each 150-200mm. You place the indicator, and measure if a 10mm movement is really 10mm in the indicator.

    I could not imagine that 0.2 - 0.4 mm difference is the linear motion but it could be the sum of multiple causes.



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    Default Re: Machine not cutting accurately

    There are lots of interrelated aspects that can affect accuracy and precision of cuts. Please do not take these questions as criticisms, I am providing them as a checklist of things for everyone to consider, you may have already considered many of them.

    How did you measure the spacing of your fret slots? Did you use the same 900mm rule, at the same room temp? At a meeting of a CNC group I belong to, we had several 3-5 foot rules (that different people owning same brand of machine had used to calibrate their steps per inch) lined up against each other. The variation between them was much larger than anyone expected. The differences between a steel rule and an aluminum one changed visually between cool of morning and warmth of afternoon. Of course wood also moves with changes in temp and humidity.

    Not measuring the spaces between frets, but what were the measurements from one end (your zero point on the part) to each one? Does your g-code match what you expect from your cad model?

    What is the resolution of your machine (without micro stepping)? What micro stepping settings are you using?

    How does your measured/calibrated steps per mm compare to the calculated one based on steps per revolution and pitch of your ballscrews? Did you independently calibrate x-axis and y-axis travel? If using same pitch ballscrews, are the numbers the same?

    Are you using one of the spiral couplers between stepper and ballscrew or an Oldham type? Are the couplings and ballscrew end support bearings still tight since you say you have been using machine for some time.

    Have you recently done the largest 3-4-5 triangle you can fit on your machine to check how square your gantry is? How about tramming your spindle?

    Have you checked runout of your spindle? How much of the 1.7 mm bit was extending from collet? How often do you clean your collets and the inner bore of the spindle?

    If you cut a large circle is it round? How does size compare cutting in a climb direction versus conventional? Any noticeable steps or “jaggies”? Try to make sure your CAM is using arc (G2) commands for this, not fitting lines (G1) commands. Measure the diameter at several angles.

    Tubular latex pressure vessel configuration engineer


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    Default Re: Machine not cutting accurately

    Your rule may not be that accurate.

    Ball screws have pitch error, and this varies along the screw.

    There are many potential contributors.

    My pockets were coming out small. Eventually I realised that my 6mm bit was less than 5.9mm. Only way to measure it accurately was cutting a slot. Single flute bits are hard measure accurately directly.



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    Default Re: Machine not cutting accurately

    Keep calibrating in Mach 3 until you get the results you are looking for. This can take several times.
    A rule is not a good measuring instrument for perfection. It is a guide to help make seemingly straight lines with a pencil and a woodworking measuring device. Most woodworking measuring tools are not considered precise.
    I have been all over the trouble shooting methods that you are using now and once I started calibrating the machine with the wizard to achieve the sizes and spacing I needed, it was perfect for my needs.


    PS. Calibration is the main bane in 3D printing. It is, however the way you achieve perfect parts on a cheap machine or several thousand dollar machine.

    Lee


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Machine not cutting accurately

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