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IndustryArena Forum > WoodWorking Machines > DIY CNC Router Table Machines > Incorrect internal feature dimensions
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  1. #1

    Join Date
    May 2020
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    10

    Incorrect internal feature dimensions

    Hi All,

    I have built a custom CNC router.

    In external features (e.g. bosses) I manage to hit tolerances of +/- 0.05mm

    In internal features (e.g. holes / pockets) I get a poor tolerance of negative ~0.2mm (they always come out smaller than designed).

    Anyone has any suggestions about why that might be?

    Some details of the machine:
    - Aluminium extrusion frame built
    - Ball screws + Linear rails (cheap aliexpress stuff)
    - 2.2kW spindle
    - Closed loop NEMA 24 steppers
    - GRBL controller + UGS + CAM in Fusion 360
    - Tested material: Delrin
    - Cutter: 6mm single flute

  2. #2
    Community Moderator Jim Dawson's Avatar
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    Dec 2013
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    5171

    Re: Incorrect internal feature dimensions

    Try a second pass on the finish cut. Or you can always set the axial Stock to Leave at a negative number to increase the size.

    Is your 6mm cutter actually 6mm cutting diameter? Sometimes they are a bit smaller.
    Jim Dawson
    Sandy, Oregon, USA

  3. #3

    Join Date
    May 2020
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    10

    Re: Incorrect internal feature dimensions

    I do repeat passes on very feature of importance to make sure that tool/machine deflection is kept under control.

    I have not measured the endmill size precisely yet. If it is undersized does that explain the issue?

  4. #4
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    Jan 2018
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    Re: Incorrect internal feature dimensions

    If you kknow it's always out and is always out by this amount.
    Easy way is to set 'stock to leave' in your cam as negative -0.1mm.
    Now it will remove an extra 0.1mm of material all around the iinner diameter bringing it to tolerance without effecting anything else.

  5. #5

    Join Date
    May 2020
    Posts
    10

    Re: Incorrect internal feature dimensions

    Yeah it's pretty consistent so far (same cutter - same material).
    I have been doing the negative stock to leave trick with success.
    However I do want to find out what the root cause of the issue is
    Thanks!

  6. #6

    Join Date
    May 2020
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    Re: Incorrect internal feature dimensions

    So I have some interesting results.
    I measured the diameter of my tool using 3 methods.

    1. Taking a pass on the right hand side of stock, then left hand side. Then measure the part across the cut faces -> calculate tool dia.
    Result: tool is 5.85mm

    2. Indicate the tip of the flute with reference to the tool's shank diameter (which was measured exactly 6mm)
    Result: tool is 6.05mm

    3. Cut a slot with the tool and measure the slot width.
    Result slot is 5.98

    I am not sure which method is more trustworthy ?
    Any interpretations of those results?

  7. #7
    Community Moderator Jim Dawson's Avatar
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    Dec 2013
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    5171

    Re: Incorrect internal feature dimensions

    I think #1 is the most accurate. This takes into account any tool runout and machine/tool flex.
    Jim Dawson
    Sandy, Oregon, USA

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