straight bit actual cutting diameter vs specified cutting diameter


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    Question straight bit actual cutting diameter vs specified cutting diameter

    Noob here to CNC. I have noticed with my DIY CNC router that parts that I cut are not exactly as I have them drawn. Ex. I cut a 3 inch square part and it is to large for the 3 inch square hole that I also cut. The block is a bit oversize but maybe .005 and the hole is a bit undersize by about the same. Not a lot but I cannot force fit it.
    When you get a .25 inch straight bit do you assume it has .25 inch cutting width or do I have to measure the cutting width some how and plug that into the tool library so the software generates a tool path based on the actual cutting width?
    Thanks for any feedback.

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    Member mactec54's Avatar
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    Default Re: cutter specified width vs actual cutting width

    Quote Originally Posted by tkms002 View Post
    Noob here to CNC. I have noticed with my DIY CNC router that parts that I cut are not exactly as I have them drawn. Ex. I cut a 3 inch square part and it is to large for the 3 inch square hole that I also cut. The block is a bit oversize by maybe .005 and the hole is a bit under size by about the same. Not a lot but I cannot force fit it.
    When you get a .25 inch straight bit do you assume it has .25 inch cutting width or do I have to measure the cutting width some how and plug that into the tool library so the software generates a tool path based on the actual cutting width?
    Thanks for any feedback.
    The cutters are almost aways under size and like you said, you now know the size of your cutter and can make the cut to suit by using you tool geometry in your cam program, or use a G41 /G42 in your code and then you can adjust the cut of your part, sometimes it is just easer to just know you tool diameter, your cutter does seem smaller than normal .0025" lower quality cutters you can expect cutter size variation.

    Mactec54


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    Default Re: straight bit actual cutting diameter vs specified cutting diameter

    You absolutely must measure the bit size and use that value in the Tool Diameter in the CAM program. It is not uncommon for the actual flute cutting diameter to be a bit less than the stated tool diameter. As an example, the cheap carbide 1/4'' router bits I sometimes buy from the local hardware store typically measure 0.245 to 0.246'' rather than the nominal 0.250'' If you use 0.250 in you CAM setup, your part will come out oversize on the outside and undersize on the inside. Computers are pretty good at doing exactly what you tell them to do, rather than what you want them to do.

    Jim Dawson
    Sandy, Oregon, USA


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    Default Re: straight bit actual cutting diameter vs specified cutting diameter

    Sometime can be tricky to accurately measure.

    You can cut a slot in something like MDF and measure the slot.



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    Default Re: straight bit actual cutting diameter vs specified cutting diameter

    Quote Originally Posted by pippin88 View Post
    Sometime can be tricky to accurately measure.

    You can cut a slot in something like MDF and measure the slot.
    Especially if it is a single flute or 3 flute most would not have anything they can measure these cutters with, other than doing a trial cut and measure the cut part.

    Mactec54


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    Default Re: straight bit actual cutting diameter vs specified cutting diameter

    The cut diameter also depends on the runout of the spindle.



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    Default Re: straight bit actual cutting diameter vs specified cutting diameter

    Quote Originally Posted by elfrench View Post
    The cut diameter also depends on the runout of the spindle.
    I would add that it can also be influenced by a dirty collet or collet nut.Beside which,the OP mentioned a DIY machine and they aren't immune from flexing.I suggest measuring a groove,as recommended above and then cutting two test squares-one with climb cutting and one with conventional cutting and comparing them.If the groove size corresponds with the stated tool size and the test squares aren't identical-look to the machine.



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straight bit actual cutting diameter vs specified cutting diameter

straight bit actual cutting diameter vs specified cutting diameter