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Thread: CNC Mill cutter speeds ?

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    No Hope For Me Coils's Avatar
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    Default CNC Mill cutter speeds ?

    First to explain why I'm asking.
    I wanted to cut out a basic rectangle with rounded corners and some holes from a piece of (about) 1/4" aluminum with an 1/8" end mill, and doing the boarder in three or four passes.
    The holes went fine but doing the edges the end mill broke before it finished the first pass, ok slowed down the feed rate and it broke the second end mill during the second pass.
    I have the mill spindle set to the highest speed, 2500 rpm by the chart.

    So my question is, is that to low of a spindle speed for that small of an end mill?
    I ask because I see some mills running at much higher RPMs and seem to cut fine with small end mills.
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    GuncoHolic twa2471's Avatar
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    Possibly your mill could have been chucked out to far and it caused a "micro wobble", or allow it to bend just a titch might have broke it,, not enough cutting oil, a constant bath works best IMO. I've found those small end mills get real hot real quick so I always did a test piece of the same material the end product was made out of to adjust the speed for optimal cutting with out any extra stress to the bit/ end mill. Keeping it cool and clean and chucked close is probably the most important things.


    Those might be a couple causes I could think of off the top of my head. I doubt you scrimp on bits, ,,it will cost ya more in the long run by fucking stuff up if you do.

    I damn sure miss not having access to all the good stuff we had at work now!! The stuff was old,, but all high end commercial quality stuff from the 40's and some older. But over all they worked pretty good. The lath was starting to get a bad main bearing behind the chuck and it would allow the chuck center to vary about .002-.003,, it was pretty hard to get to precise with that going on!!! But every thing else was OK nice press's, mills, every kinda bit and mill known to man, and a nice stock of all kinds of steel and stainless,, even some exotic stuff. The owners father had one of the main salvage yards in the state, and stuff from GE, IBM, and places like that would end up there. So had a pick of the cream of the crop of materials too,,,, boy do I miss that!!!!

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    No Hope For Me Coils's Avatar
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    I was thinking the end mill was out to far with the first one, but the second one I put it so the flutes were as close to the collet as they could get by eye balling it.

    I'll just have to mess with it more and figure out what's the smallest size end mill that works well and go from there.
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    Gunco Regular mg555's Avatar
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    Is there a reference book out there that gives you basic starting speeds for cutters? I have a hard time getting speeds right myself. There are just so many varibles, tool size, material, number of flutes, rough cut or finish etc. I have a small amount of knowledge(just enough to break a lot of tooling) and could sure use such a reference book.

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    GuncoHolic twa2471's Avatar
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    Ya,, mg555,,, been there done that too, I haven't seen one either. But for me a mute point,,, no access to good tools anymore sense "forced retirement".

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    Happy Camper hcpookie's Avatar
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    The smaller ones need to be ran slower. When in doubt go slow. "Slow and steady". Also how deeply you cut can be a huge thing too. Like anything start big and get it close, then slow down with the more precision cutters. Aluminum is usually soft enough, but it can clog the bit and cause you to need to slow down. Clogged and it can "jump" and the torsion can break it. Slower, cooler, less clogging. With aluminum you can also use an irrigation system to good effect.

    There's a machinist reference manual which you can buy at many places (forget the exact name of the book) and I think I even saw it at Harbor Freight.

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    Gunco Regular Ken3030's Avatar
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    Yeah, I'd run it slower and not take much of a cut at all. I've never had much luck with 1/8" cutters, break real easy. Make sure you're cutting in the right direction too. I forget which way is which, but cutting in the wrong direction causes the end mill to dig in.
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    No Hope For Me Coils's Avatar
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    I have a small amount of knowledge(just enough to break a lot of tooling)
    I know the feeling and after reading that part I was LOL'ing because I've been there plenty of times



    HCP, I've heard the opposite, as you go larger you slow down and speed up for the smaller ones (Need a confused looking smiley scratching it's head right now)
    But I'll give it a try next time.

    I wasn't trying to go deep, so I don't think that was an issue, the piece was 1/4" and I had it set to do like 4 passes.

    When it was a manual machine I could watch, listen and kind of feel the feed rate for whatever speed it was running at with whatever size mill/cutter/drill I was using.

    Oh well I'll just mess with it a little more, if it seems like it isn't going to work for me (the cnc part) I'll just go back to a manual setup and watch or build something for cnc use later. Hmm maybe a plasma table or router setup.





    Ken I had to look up the cutting directions because I forgot what they were called too, but "conventional" is the best way, and the other is call a "climb cut" (this is the one that can pull the cutter or material), I try to use a conventional as much as I can.
    here's some pics
    File:Conventional Milling 01.png - Wikipedia, the free encyclopedia
    File:Climb Milling 01.png - Wikipedia, the free encyclopedia



    MG555 want something that can be confusing to read?
    Milling: Cutting Speeds & RPM Calculations
    Just Google "mill cutting speeds" and a lot come up



    And this is the type of thing I was thinking of, ok can't copy & paste it, go down the page a little and look for the red text like this
    Quick RPM/Spindle Rate Calculations: Lathe, Mill, Drill (HSS Cutter)
    Cutting Speed and Feed Rate Calculations
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    Gunco Veteran Viper Dude's Avatar
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    Hello Coils,
    As a rule of thumb... use a depth of cut no more than 1/2 the diameter of your cutter. That would be a max depth of 1/16" on your tiny 1/8" diameter end mill.

    Use plenty of cutting fluid such as "Tap-Matic - Aluminum", kerosene, diesel, lamp oil, or even lighter fluid to flood the work. This type of fluid helps keep the end mill from buttering up ie clogging.

    Check the Feeds-Speeds-DOC math in the book Machinerys Handbook or similar text !!! These days there are charts and software to do the arithmetic for you. Roughly speaking use high RPM's and slow feed rate coupled with the proper depth of cut.

    I'd also use a 3/16" 2-flute end mill and take 4 passes to cut through that 1/4" aluminum plate. Aluminum oxide skin really does wear cutters though the metal itself is rather soft stuff.

    VD in AZ

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    No Hope For Me Coils's Avatar
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    As a rule of thumb... use a depth of cut no more than 1/2 the diameter of your cutter.
    Don't think I heard that one before, thanks.

    As mentioned I had it setup to do 4 passes, which by your advice would be right there at the "half the diameter" thing, if I try it again maybe I'll try 6 passes.


    And didn't know that about the aluminum oxide skin, thanks again
    "Government is not the solution to our problem, government is the problem" Ronald Reagan

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