Center cutting flat bottom end mill
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  1. #1
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    Default Center cutting flat bottom end mill

    Like the title says, I am milling a track on a rotary axis and need a flat bottom (standard end mills are not flat on the bottom and leave a crown in the middle of the slot equal to the relief on the cutter). Looking for an off the shelf 3/4 diameter 2 or 4 flute center cutting carbide end mill that has a true flat bottom.

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    afaik, no such thing off the shelf. Just have one ground flat.

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    Not sure if anyone actually stocks those as an off the shelf item. I usually just made my own. This one was made from an old drill. If you don't have grinding equipment you might have a tough time finding one. They might make some indexables just for rotary milling, but I dunno. The cutting speed near center is a problem for a flat bottom cutter and milling. Can you get to the groove with something cutting on the periphery instead, like a staggered tooth type cutter?

    20190208_154550.jpg20190208_154509.jpg

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    You say you are milling a track on a rotary axis. So this is a slot in the shape of a circular segment or even for a full circle. This would seem to involve moving the end mill or the work sideways along an arc. Aren't you aware that once you move that end mill sideways by it's diameter then the entire bottom face of the slot it is milling will have been cut by the outer corners of the end mill and therefore it will all be at the same depth. So, unless your "track" is shorter than two diameters of the end mill, it will come out with a flat bottom even if the end mill itself has the slightly angled cutting edges on it's end face that are standard.

    In short, it is only in rare cases that the geometry of a standard, end cutting end mill is a problem. And even in those cases, it may be more advantageous to cut the slot with the full diameter end mill first, and then go over it with a smaller one that can reach all of the small hill that was left. That second end mill would need to be one half the diameter of the full sized one or less and it would take three or more careful passes to fully remove the hill. CNC would make this easy. Even if it is a one-off or just a few pieces being machined manually, it could still be advantageous.

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    Nope. Rotary axis but using the rotation of the 4th to machine a radiused surface while the cutter is still. When you do this and are cutting with a milling cutter you don't get a flat cut unless you have a flat bottom milling cutter - which is why it's better to change the setup and use something that can cut on the periphery instead. I've had this exact experience milling out water jackets from the solid on bearing boxes. Except it was a water jacket, so I didn't give a rat's hairy arse if it was flat.

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    That is a simple job for most cutter grinding shops.
    Here is one.
    D & D Precision Tool & Cutter Grinding - Greensboro, North Carolina - Hardware Store | Facebook

    For high accuracy one might stand desired end mill in a V block to surface grind grind off perhaps .003 / .005 .. The with a TC grinder grind to sharp. Often one flute is to center and the others are not.
    I have made end mills with a small flat of perhaps 1/16 and then a dish to center for a smoother cutting end mill.

    EPAIII is right if not stopping in the work the end mill acts like a flat bottom.

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    The OP is correct! Milling as he described will produce a radiused bottom no matter how long the cut is. Cutting on a radiused surface like when using a rotary table or some means to rotate the part while being milled will produce a non flat bottom like a fly cutter can make a ball similar to a ball hitch since some way is needed to hold it while being cut. I've made a few flat bottomed end mills to be used as counterbores but when used as an end mill will produce a cut looking like one made with an end mill getting dull. Thought I haven't used a flat bottom mill as described by the OP.

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    Quote Originally Posted by michiganbuck View Post
    EPAIII is right if not stopping in the work the end mill acts like a flat bottom.
    No, as I understand it this is incorrect and irrelevant for what the O.P. is doing. He is using a rotary or 4th axis to rotate the work past the cutter, not moving the cutter in a straight line past the work. When this is done and the work is rotated past the end of a milling cutter, a straight line is not produced unless the cutter has a flat bottom. This is the operation we're talking about - milling cutter on center and work being rotated:

    241.jpg

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    Quote Originally Posted by eKretz View Post
    No, as I understand it this is incorrect and irrelevant for what the O.P. is doing. He is using a rotary or 4th axis to rotate the work past the cutter, not moving the cutter in a straight line past the work. When this is done and the work is rotated past the end of a milling cutter, a straight line is not produced unless the cutter has a flat bottom. This is the operation we're talking about - milling cutter on center and work being rotated:

    241.jpg
    Granted like the photo it would not cut flat...

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    OK, the sketch helps. I was not visualizing it properly.

    I see the problem and agree that a flat bottom end mill would be ideal.

    But, you could still do it with a standard, slightly concave end mill. Referring to the sketch, start with the end mill's axis directly over the rotational axis of the work and make a full rotation of the work. That would cut the corners of the slot to the full depth and leave them square. Then offset the end mill by some small increment, tangent to that rotational arrow and make another complete rotation of the work. That should cut two paths just inside of the corners to full depth. Continue moving the end mill in small increments and rotating the work for each until the edge of the end mill is directly over the rotational axis of the work. That should produce a real close approximation of a flat bottom for the slot.

    If you just want to to eliminate that center hump, then two positions of the end mill, one radius apart, should do it.

    Rough calculation:

    You specified a 3/4" end mill so it is 3/8" (0.375") from center to the corners.
    If the cutting edges have a 1 degree angle, the height of the center hump is 0.375" TAN(1) = 0.0065"
    Assuming a 0.001" hill in the surface is acceptable (+/-0.0005").
    0.0065" / 0.001" = 6.5 steps.
    So stepping the end mill over 7 times by a 0.536" will result in a surface flatness of less than +/- 0.0005".
    The trade-off here is more steps produce a flatter surface and fewer steps a rougher one.

    And, yes the machining time would be increased. Factor that time against the cost of a special end mill. It may depend of the total number of parts needed.



    Quote Originally Posted by eKretz View Post
    No, as I understand it this is incorrect and irrelevant for what the O.P. is doing. He is using a rotary or 4th axis to rotate the work past the cutter, not moving the cutter in a straight line past the work. When this is done and the work is rotated past the end of a milling cutter, a straight line is not produced unless the cutter has a flat bottom. This is the operation we're talking about - milling cutter on center and work being rotated:

    241.jpg

  16. #11
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    Like I said, better to avoid the problem entirely and preferably switch the op around a bit so it can be done with a tangential cutter if possible.

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    Tangental cutter? Am I assuming correctly that it is similar to a woodruff key cutter or a milling cutter on a stub arbor? If so that will not work unless the head or the rotary device can be tilted at the same angle as the lead of the spiral milled. If not the sides of the slot will be something other than square and straight. It will have a radius plus the width of the slot will be wider than the cutter.

  18. #13
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    Yes. And you are correct about the angle but we don't know if the O.P. is even milling a helical groove at this point. And that is why I mentioned changing the op/setup to use the tangential cutter.

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    My Bad! I assumed rotary access was helical.

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    It may be, no telling without further info... These days plenty of places do mill turning though - one particularly common use I see a lot are crankshaft manufacturers using the process to rough out main and rod journals.

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    Sandvik have true flat endmills for cylindrical floor milling.


    CoroMill Plura solid carbide end mill for turn milling

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  23. #17
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    No not a helical slot, but a barrelimg_20190205_165044933.jpg cam track configured like sketch in reply #8, so i can't use any kind of key cutter in another plane. Back to my original question, does anyone make a true flat bottom center cutting end mill that is available off the shelf?

    What most people fail to visualize is the leading edge and the trailing edge of the cutter are not cutting at the bottom of the track, only the center of the cutter at the tangent point of the rotary axis.

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    You had better ignore the advice given re small cutter, you will not get a true sized track otherwise.

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    Ah yep, not doing that track with a tangential cutter. Looks like Gregor found a good source. Make sure you zing that RPM as fast as you can. And yeah I agree with camscan, get the flat bottomed cutter, don't dick around.

    For some of you not grasping the way the bottom of the groove would be formed, just have a look at that print from RJT - he's got a sketch on there of the profile that would be formed in the bottom of the groove if he were to use a regular endmill with relief in the center - right at the top of the print.

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    CoroMill Plura solid carbide end mill for turn milling

    Only available in metric. Anyone else have a source?


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