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Plunge Cutting with an End Mill to Make a Slot

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  • Plunge Cutting with an End Mill to Make a Slot

    I was reminded of plunge cutting with an end mill today...no pics...sorry

    I had 2 pieces to make for a customer. 1" x 2" x 10" solid bar with a 3/4" x 3" slot (located on the 2" side, extending 1" deep and out the other side...did I explain that right???)

    Anyway, I chain drilled 4 x 5/8" holes leaving 1/16" - 1/8" between the holes in both pieces to take out the worst of the metal.

    I then proceeded to cut the slot by plunging with a 3/4" end mill about 0.200" and then cutting over the 3" required. Plunge another 0.200", cut back...repeat... The problem was I went through 3 end mills! Ran about 1000rpm (too fast???). I can't remember the feed, but I don't think it was excessive. (The mill has a 5hp motor.)

    On the second one, I scratched my head a bit and decided to start with a 5/8" end mill and plunge cut the web remaining between the drilled holes. I then ran the full 1" deep cut to clean out the remaining metal. Switched to 11/16", (yeah, I was chicken...I only had a single 3/4" 2-flute end mill left!), and ran the slot full depth again. Then 3/4" and run the slot again.

    Cut the time in half (including tool changes) and burned up 0 end mills using the second method vs 3 for the first run (2 cheap china 4-flutes & 1 USA-made roughing 4-flute cobalt).

    What I learned:
    1) I gotta remember this for next time...
    2) I'm gonna hafta order some more 3/4" end mills too!

    Andrew

  • #2
    Hi Andrew,

    If you are cutting mild steel with a high speed steel end mill, try slowing your spindle down to around 450 to 500 rpm's.

    Also, a little coolant will extend the life of your cutters. A bottle with a tiny hole in it will work. I have seen people use coke bottles for this. Just make sure you don't drink it by accident.

    Brian
    OPEN EYES, OPEN EARS, OPEN MIND

    THINK HARDER

    BETTER TO HAVE TOOLS YOU DON'T NEED THAN TO NEED TOOLS YOU DON'T HAVE

    MY NAME IS BRIAN AND I AM A TOOLOHOLIC

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    • #3
      Solid bar of mild steel??? I'm guessing so, HSS cuttter? I'm guessing so.

      So yes, too fast. How did I know, I took 3 seconds to make a calculation. About 200sfm. Quite a bit way too fast, about double.

      Don't think in terms of RPM's, think 'surface speed'. The same 3 second calculation I made could have saved you 3 endmills. Even if you didn't have a calculator, had to find a pencil and a piece of paper and look up the surface speed, it might have taken you a minute.

      Some will tell you that they'll "know it when they see it". I've "seen it", flying past my head and into the wall, and also a 4 minute job taking 2 hours.

      I hope you were plunging off center, not directly on top of the web?

      You can also ramp pretty darn easily on a manual mill. Its easier than an etch-a-sketch, much easier than rubbing your head and patting your belly.

      Sorry about the demise of the endmills, it happens, it'll happen again, learn from it, I still wreck stuff and learn from it, just a bit less frequently.

      Comment


      • #4
        Next time don't drill any holes, use a 3/4" roughing endmill at 400-500 rpm and cutting oil. It will speed up the job quite a bit.
        It's only ink and paper

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        • #5
          For an approximation of the correct rpm of HSS tools in mild steel with cutting fluid I use 100sfm max.

          100 sfm equals 1000 rpm with a 3/8" diameter cutter. That's easy to remember. Adjust rpm to correspond to cuter diameter.

          When doubling the tool diameter, 3/4" for the current discussion, half the rpm to 500. Likewise, halving the cutter diameter, double the rpm.

          This works for drilling, milling and turning.

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          • #6
            Just a thought from someone with little experience but similar.

            I think the two flute end mills are better for plunging. They seem to provide more clearance for chips.
            VitŮŽria, Brazil

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            • #7
              You might also take a look at your end mills to check that they are actually capable of centre cutting. If the teeth stop short of the centre leaving a cylindrical hole, then plunge cutting is not going to work well or at all. In UK we used to distinguish between slot drills (can plunge) and end mills (not intended for plunge cutting). However many end mills are now ground to be centre cutting so the distinction is not as useful as it was.
              Bill

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              • #8
                Yes, all of the end mills were center cutting. The 2 china ones were HSS, the USA one was a roughing end mill made of cobalt/HSS blend of some sort.

                Yes, it was mild steel.

                I was using cutting oil, but I guess the speed was too high. I'd better print out a chart, there's lots of them floating around the ol' web, or jot down some numbers at least.

                I should have been clearer...I drilled 5/8" holes in both pieces to eliminate some of the metal.

                Carld - Why not drill holes first? A drill bit is cheaper and easier to sharpen. I will admit the extra tool change will take longer, but if time isn't an issue, why not?

                Andrew

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                • #9
                  Drilling first may or may not be a good idea. It does remove excess metal fairly quickly, but it also creates a lot of entrance/exit cuts for the end mill to deal with, which can cause the end mill to grab and cut to one side, or chip.
                  ----------
                  Try to make a living, not a killing. -- Utah Phillips
                  Don't believe everything you know. -- Bumper sticker
                  Everybody is ignorant, only on different subjects. -- Will Rogers
                  There are lots of people who mistake their imagination for their memory. - Josh Billings
                  Law of Logical Argument - Anything is possible if you don't know what you are talking about.
                  Don't own anything you have to feed or paint. - Hood River Blackie

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                  • #10
                    What kind of machine is this? 5hp spindle motor and it'll only take .200" depth of cut?

                    How did the end mils fail? Chipped teeth or break?

                    Comment


                    • #11
                      When taking a cut with an endmill passing a hole the endmill tends to grab and jerk as it passes the hole. Sometimes it can even break the endmill or shift the metal or jerk the table travel.

                      When milling past a hole it's best to snug the table lock so it won't get jerked into the cut.

                      If you drill a hole at the starting point that is ok but the job will go better if you let the roughing mill do it's job and not spend time drilling holes.
                      It's only ink and paper

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                      • #12
                        Originally posted by DR
                        What kind of machine is this? 5hp spindle motor and it'll only take .200" depth of cut?

                        How did the end mils fail? Chipped teeth or break?
                        Milling machine is a Gambin 2n...French made (1950's?? I think), refurb'd in early 90's in Scotland. Don't know how it came to Canada??? It would likely take a deeper cut. It's still fairly new to me, so I haven't been pushing it...yet! It was originally a 7.5 hp 3-phase, but I had a spare 5hp single phase laying around, so that's what went on it.

                        The cobalt end mill broke off at the top of the flutes. When I pulled the fluted section out of the slot, the end was very deformed. I'm guessing too much heat???

                        The HSS ones chipped badly.

                        Andrew

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                        • #13
                          Originally posted by Carld
                          When taking a cut with an endmill passing a hole the endmill tends to grab and jerk as it passes the hole.

                          If you drill a hole at the starting point that is ok but the job will go better if you let the roughing mill do it's job and not spend time drilling holes.
                          That's something I hadn't considered before...never looked at it that way.

                          That's why I like these forums....lots to learn!

                          Andrew

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                          • #14
                            RPM = cutting speed x 4 / cutting diameter s x 4 / d

                            Mild steel 80 to 100 feet per min, Alloy steel inc stainless. 50 fpm

                            Cast iron, 70 fpm Brass and Aluminium 2-300 fpm

                            These are for HSS tools If the tool is rotatiing that is d. If the work is rotating (turning) the cutting dia that you are working on is d. Hope this quick head math works for you. If you are using a 1/4" drill then d is 1/4 so you are dividing by that so you must invert and multiply.

                            eg; 1/4' drill in 1020. 80 x 4 / 1/4 = 320 x 4/1 Some get confused with fractions so I thought I'd add this, not that you would make that error, Peter
                            Last edited by Oldbrock; 11-23-2011, 06:04 PM.
                            The difficult done right away. the impossible takes a little time.

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                            • #15
                              Originally posted by davidwdyer
                              Just a thought from someone with little experience but similar.

                              I think the two flute end mills are better for plunging. They seem to provide more clearance for chips.
                              That would be why we British call them slot drills!
                              Paul Compton
                              www.morini-mania.co.uk
                              http://www.youtube.com/user/EVguru

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