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Thread: Broke my dovetail cutter

  1. #1
    Join Date
    Apr 2014
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    Default Broke my dovetail cutter

    First time cutting a dovetail.

    I was cutting in 1018 with a 60* .75 face width 8 tooth HSS cutter at 435 rpm, 2.6 ipm feed and mist cooling, conventional cutting. Pre cut a slot, raised the cutter about .010 to make clearance so it wouldn’t rub and made a few cuts already before it broke. First pass was .010 into the sides (left to right, then right to left on the other side of the slot), second was .020 and the third was .030. The left to right pass on the far side of the slot was ok but near side right to left pass the cutter broke entering the cut.

    I read somewhere that it's good to halve whatever sfm I calculate (I was shooting for 90) so I think I had my rpms too high by a factor of 2. Do you guys think if I lower the speed the chance of failure will be much lower? Anything else I can do to improve my odds of success?

    Thanks
    Bill

  2. #2
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    Jan 2008
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    Default

    Climb cutting? How much backlash is in your system?

  3. #3
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    Default

    If it broke entering I'm wondering if you tried to climb and jumped and broke? there are lots of variables, worn cutter, quality of cutter, how rigid the machine and set up are and so on. I'd do it in one roughing pass and one one finish pass and not expect it to break. otoh, if you are breaking cutters, you can reduce the force on on the cutter which can be done by slower feed/rpm, but I wouldn't think that is the issue.
    .

  4. #4
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    Default

    You mentioned "conventional milling" in the first post. And if so then that should not be an issue.

    You were also only a total of .060 sideways into the cut. So the tooth load should not be all that high.

    Halving the SFM is rather drastic overkill. And on top of that 1018 is a mild steel. Charts I'm seeing show 300 sfm being a good lower SFM speed for 1018. So even at 90 you were WAY under. And in fact perhaps too low for the teeth to cut well. You'd have to do a check to see what the chip load per tooth was for that RPM and feed speed.

    I'm thinking though that the tooth load was too high and the impact forces were too much for the narrow neck of the cutter even at the low engagement depth.

    Here's A LINK to the SFM table I was using. The table of data is about half way down the page. Perhaps use the minimum values for each class of or specific material for the future?

    My thinking is that the low RPM and perhaps high tooth load due to the feed rate for that RPM led to chunking of the chips and too much impact load on each tooth. And if the cutter teeth were a little dull at the same time and you're using the motor feed you may not have felt the amount of torque needed to produce the chips. Another good reason why it's not a bad idea to make at least one pass by hand? I know I can tell right away if the cutter is sharp and cutting well or if it's dull and rubbing. At least on things from 1/4" and up.
    Last edited by BCRider; 07-31-2019 at 01:06 PM.

  5. #5
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    Quote Originally Posted by BCRider View Post

    Halving the SFM is rather drastic overkill. And on top of that 1018 is a mild steel. Charts I'm seeing show 300 sfm being a good lower SFM speed for 1018. .
    not for a HSS cutter in steel, should less than 100.
    .

  6. #6
    Join Date
    Apr 2014
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    BCRider and all – Thanks for the response.

    Yes, conventional milling. Was using power feed, electric motor type on a Bridgeport.

    The link you had was to Destiny Tool, looks like all they do is carbide so the charts there don’t apply to the cutter I was using.

    That cutter was new, never used and felt sharp. Got it from Enco before they ceased to be independent.

  7. #7
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    Apr 2014
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    Oh and the chip load, didn’t put that into the initial post thinking if anyone wanted to know it they could figure it. I calculated .00076. Seemed low to me so the halving of the speed made all the more sense. The chips weren’t really chip like, more of a grit.

  8. #8
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    Jan 2008
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    Climb milling works in both axis.. Could have the non-cutting axis moved and broke the cutter?

  9. #9
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    What brand of cutter? How was it held? What machine? Another thing, the current crop of 1018 can be awful to machine, not your father's 1018 (thank you Mr trump for tariff'ing all the good imported steels).

    With rare exceptions dovetail, key slot, etc type cutter have neutral tooth rake making for generally poor performance.

    I have a couple insert type 60 degree dovetail cutter$ with proprietary positive rake inserts that cut very well, with prices to match.

  10. #10
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    Nutz, I missed the carbide reference. Sorry 'bout that.

    So it seems like there should be no reason for the cutter to break. The mystery is compounded.

    You were still only into the material by .060. And for a dovetail cut that is only about .090 vertically. So there is simply no reason why it should have loaded up and snapped off. The only thing I can think of is that the low chip load may have caused the tips to rub too much and go dull? That's a bit of a long shot I know. But shy of the cutter being defective with a hidden fracture there's just no reason for that size of cut to break the cutter on you.

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