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  1. #1
    Neale's Avatar
    Lives in Plymouth, United Kingdom. Last Activity: 9 Hours Ago Has been a member for 4-5 years. Has a total post count of 1,005. Received thanks 170 times, giving thanks to others 5 times.
    I was doing some cutting earlier in a really difficult material - 3mm MDF. I was cutting some small components which needed to slot into each other, and after the first attempt none of them fitted. Measuring, I found that I was getting dimensions consistently about 0.7mm out. That was averaged over a dozen or so different dimensions, internal and external. I regenerated the gcode using a cutter of nominal dimension 2.3mm instead of its theoretical 3mm, and all the pieces cut and fit perfectly. Question is, is it my crap machine bending with the strain of a 1.7mm DOC in MDF, or is it likely to really be cutter size? It's a single-flute carbide cutter from CNCRouterParts, but I can't check its diameter directly as it is single-flute. I don't have much experience with these kinds of cutters - do you need to do trial cuts to find out their effective cutting diameter before use? I'm happy to blame the machine, but curious about cutter accuracy as well.

  2. #2
    Hi Neale,

    Doesn't seem like a particularly heavy cut so it's a bit odd that one. I cut a fair bit of 3mm ply, liteply, and the occasional bit of MDF and use a single flute 3mm TCT router bit without problems. Here is my home design Spitfire which all locks together nicely . . .

    Click image for larger version. 

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    Can you do a plunge into a semi-hard material (e.g. hardwood) to drill a hole and then check the diameter? This gets around the single flute problem for diameter.

    Some of my bits are quite worn, especially the lowest part of the cutter, and they are less than the original diameter. Should throw them really but they are still OK for roughing or non-critical jobs. How old are your cutters?
    Building a CNC machine to make a better one since 2010 . . .
    MK1 (1st photo), MK2, MK3, MK4

  3. #3
    That Spit looks like it going to be lovely - which Mk are you going for?

    Sorry for the hijack....I have a thing for Spitfires (except maybe the Mk5 with that intake)

  4. #4
    Haven't you by chance turned on/forgotten to turn off tool radius compensation in the controller program? 0.7 mm is too big difference in tool diameter. You can measure the diameter at the stem, where the flute ends. Accurate enough to give you an idea. This if the shank is of diameter greater than 3 mm.
    Drilling a hole or milling a groove is also a good approach.

  5. #5
    If the machine was flexing then in one direction the error would be positive and in the other direction it would be negative.

    Also +1 re. Washout's comments.
    Spelling mistakes are not intentional, I only seem to see them some time after I've posted

  6. #6
    Neale's Avatar
    Lives in Plymouth, United Kingdom. Last Activity: 9 Hours Ago Has been a member for 4-5 years. Has a total post count of 1,005. Received thanks 170 times, giving thanks to others 5 times.
    No, I don't think that it is a tool compensation problem as the gcode preamble includes G40 which should turn it off. That amount of error is strange - too big for cutter diameter error, I would have thought, but too small for any significant axis calibration error (which I have checked in the past). The error is also pretty much the same, within the limits to which you can measure MDF anyway, along both axes, which I would have thought meant that it's probably not machine bend as I would expect that to vary with axis. However, this is my MDF router, so anything's possible. At least the error is consistent enough that I can tune it out by tweaking cutter diameter in vCarve. Sometimes I have to pretend to be an engineer and not a scientist (it works, dammit, so forget the theory!).
    Thanks for the thoughts, guys, and I shall try putting a mike across the plain part of the cutter (3mm single flute on 6mm shank) as a check.
    This is what I'm playing with.
    Click image for larger version. 

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    It's a cable chain idea based on something I found a while back; it needs reasonably accurate cutting to make it all stay together (no glue or loose fixings) while still being able to articulate.

  7. #7
    There's a few of those designs on YouTube if you search for 'diy cable chain'
    Spelling mistakes are not intentional, I only seem to see them some time after I've posted

  8. #8
    Neale's Avatar
    Lives in Plymouth, United Kingdom. Last Activity: 9 Hours Ago Has been a member for 4-5 years. Has a total post count of 1,005. Received thanks 170 times, giving thanks to others 5 times.
    Indeed - that's where I found the original. However, I've redrawn it to be a little bit bigger as the original was tight to take CY cables, etc. I was just taken by the idea of hinged links that didn't use any kind of conventional hinge, and it's an interesting CAD->CAM->machining exercise.

  9. #9
    Wow, that spit looks great. G.

  10. #10
    Quote Originally Posted by Neale View Post
    It's a cable chain idea based on something I found a while back; it needs reasonably accurate cutting to make it all stay together (no glue or loose fixings) while still being able to articulate.
    Don't see the point in cutting these other than the exercise.!! . . . They will cost more in Elecy than they cost. . Lol

    The cutting differance will be MDF frame swelling in the morning dew try cutting in afternoon when it's warmed up. .

    Now seriously I would check for backlash or end float on the screws and because it's consistant the differance could be the amount of backlash. Try cutting something similair with a larger cutter and see if error is same amount I'd wager it will be.!
    After this then check steps per again and make sure it's moving the exact amount it's told. That amount of differance can really only come from either cutter comp or wrong amount of movement, either thru backlash or steps.
    Last edited by JAZZCNC; 03-03-2014 at 11:20 PM.

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