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Determining milling depth of cut

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  • Determining milling depth of cut

    I was milling slots in .25" mild steel plate today with a .25" end mill.
    Drilled a through hole first then started the slot with .050" DOC.
    It was going so well I just increased the DOC to full thickness and went ahead with a conservative but purposeful
    feed speed.
    No problems at all.
    I guess my question is, how much of home shop machining like that is "whatever you can get away with and not ruin anything" as opposed to "strictly by the book".
    I know...dumb-ass question since all of life is pretty much whatever you can get away with.
    Just wondering if I should be more conservative?
    Len

  • #2
    The problem is, you find out what you can ('t) get away with by ruining things. One of my many areas of expertise.
    "A machinist's (WHAP!) best friend (WHAP! WHAP!) is his hammer. (WHAP!)" - Fred Tanner, foreman, Lunenburg Foundry and Engineering machine shop, circa 1979

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    • #3
      Dunno about the home shop, that's just whatever your budget can do. At work, it's whatever you can get away with.

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      • #4
        I sometimes contracted to a fellow who had a lot of experience in managing machine shops for others and who, when he went on his own, built an excellent world wide reputation for his speciality work. He always encouraged folk to " Push the envelope" saying that the cost of an occasional bust up or failure was far less than the waste of not reaching the productivity and quality that was possible If you were prepared to experiment. In a much simpler way one mechanic I knew, on asking him " how tight should a nut be he replied, tighten till the thread strips then back off half a turn !!!!"
        Regards to all David Powell.

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        • #5
          Originally posted by QSIMDO View Post
          I guess my question is, how much of home shop machining like that is "whatever you can get away with and not ruin anything" as opposed to "strictly by the book".
          I know...dumb-ass question since all of life is pretty much whatever you can get away with.
          and you are getting more life out of the cutter by using more of its cutting edge. A 1/2" pass is going to give 10x the life 10 50 thou passes, more or less, given you're using .500" of cutter rather than than putting 10x the mileage on the bottom .050"

          imo a good approach, with goal being max removal and max cutting live, is this. Go as deep as as you can without breakage. A few x the cutter diameter is not impossble. Your limitation on cutting and machine and set up is removal rate.....so by going with a big DOC you may not be able to feed as fast (if say the width is the width of the cutter). Then....you reduce your feed so as to not break a cutter, and reduce your speed to so you making a decent chip not dust. Carbide is different. HSS can be slowed down and wear is linear whereas carbine needs a certain surface speed to work well. You can still be aggressive, I just did slot in steel, 1/2" thick in one pass with a 3/8 cutter at speed with an aggressive feed.

          In practice you adjust the approach as required and makes sense, machines and set ups vary, but that's how I often come at setting the levers of DOC, Feed and Speed
          Last edited by Mcgyver; 02-28-2020, 07:58 AM.
          .

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          • #6
            It depends how square / straight you want your edge.
            End mills deflect more as the side load increases.

            -D
            DZER

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            • #7
              Slotting at 2x cutter diameter is pretty aggressive, but as you have found it was manageable at the right feeds, and using more of the flutes is good as mentioned above. On a manual machine there is plenty of opportunity for the cut to give the operator feedback, and 1/4" endmills are cheap...

              Maritool has this note on the speed/feed charts for end mills:

              Feed Per Tooth is based on:
              - Slotting - depth of .5 x Cutting Diameter, for deeper slotting reduce feed and speed
              - Profiling (normal edge cut) - Axial depth up to 2x Cutting Diameter and Radial Depth of .25x Cutting Diameter,
              when profiling less than .25 cutting diameter increase feed and speed

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              • #8
                I was doing similar on 3/16" flat stock with a 3/16" end mill. After about the third slot, I went full depth. Only broke one end mill so adjusting the feed worked for me. Probably didn't save any time but it sure broke the monotony--Had to make about 30 slots in three pieces of flat stock. These were to become my lathe tool holder rack.

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                • #9
                  If you can get away with a depth of cut equal to the cutter diameter, then you don't have to use the end to do the cutting. That is assuming that the part of the cutter projecting through the work does not foul anything. Using the flutes close to the shank of the cutter has the advantage of increased stiffness and using the less used and sharper edges. A cutter can last a lot longer if every part gets a go at cutting rather than just the tip. Also if the work can be raised up in this fashion, the chips will not build up in a slot. Be careful, as a 1/4" cutter may well cut a wider slot in these circumstances.

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                  • #10
                    Originally posted by QSIMDO View Post
                    I guess my question is, how much of home shop machining like that is "whatever you can get away with and not ruin anything" as opposed to "strictly by the book".
                    ?
                    Keep in mind that the recommended speeds and feeds are to get maximum production with an acceptable tool life. Read a bit closer to see the tool life of just minutes is common. As in less than an hour of making chips.

                    I use a chart of suggested feeds/speeds to start and adjust as needed for a gold finish.

                    Dan
                    At the end of the project, there is a profound difference between spare parts and extra parts.

                    Location: SF East Bay.

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                    • #11
                      Keep pushing it until something breaks, then back off a bit. You paid for the whole tool, so if your machine and setup allow for it, you should use the whole tool. So long as deflection and HP use are withing tolerance for your application, push it

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