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Can I fix the counterbores in the top jaws of the lathe chuck? They are hard!!!

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  • Can I fix the counterbores in the top jaws of the lathe chuck? They are hard!!!

    So I have this 8" lathe chuck with 2 pc jaws. It takes M10x25 socket head screws to clamp the top jaws to the bottom ones. The screw head goes into the counterbore of the jaw to not interfere with the lathe operation. For this chuck Chinese made the counterbores with a drill, so the bottom is not flat, but tapered. It is not even a smooth taper, but a surface with steps in it.

    I would like to put these jaws on a mill and fix the bottoms of each counterbore with a carbide end mill. The c/bores are 17 mm diameter (.669"), but 5/8" cutter would work. I know the end mill would not leave a flat bottom, but it would be way better than what I have now. The only problem with that is the jaws are hard. I measured from 55 to 60 HRC on my Rockwell press. The question is - can I do it without destroying the end mill? I am thinking about 2 flute cutter just because I have it. Most likely it will not touch the sides of the existing c/bores, but will cut the bottom.

    Any one of you experienced guys done that? Can you share the details? I know I can cut such hard steel on the lathe, but that would be OD cut with small amount of material removed. Counterbores are different and I need your help.

    How about just drilling into such hard material, let's say with a 1/4" carbide drill? Any tricks to do that right?

  • #2
    Hows about just putting a taper on the bolts, I know you said its not a nice taper on the jaws but it may be the easiest.
    Beaver County Alberta Canada

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    • #3
      It sounds like he's saying the bottom seating surface of the counterbored hole is not parallel to the underside of the jaw. And yes, you should be able to cut that with a carbide endmill. If it's very hard it will dull the endmill, but you should easily be able to skim 4 (or 8) hole bottoms flat with it before that becomes a problem.
      Last edited by eKretz; 05-03-2021, 07:42 PM.

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      • #4
        4 flute might be easier/ smoother cutting. Not an exhaustive experience, but I always find 4fl endmills less prone to chatter than 2fl when counterboring.

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        • #5
          Lock the quill into position and feed into the jaw by raising the knee of the mill. This gives you more control over the cut. If it's a lower quality import jaw, as evidenced by that counter bore, I wouldn't be too surprised if the jaw is only case hardened on the exterior. It might be a bit softer material down in the hole. If it was heat treated quickly the heat might not have had time to alter the core of the jaw as much.

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          • #6
            I would use a 4 flute carbide end mill at low rpm. A 2 flute would probably chatter a lot and chip up the end mill. The 2 flute has more of an inverted angle on the end than a 4 flute. For drilling, use a 2 flute straight flute carbide drill bit. For both, run dry with no lube of any kind. Cutting oil will just make the cutter skate on the hard steel until you build up enough pressure to shatter the carbide. Dry cutters will bite right in.
            Kansas City area

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            • #7
              Would using a boring head with a carbide insert in a boring bar work?

              Location: Saskatoon, Saskatchewan, Canada

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              • #8
                Chuck jaws will indeed be hard to machine. A couple of thoughts:

                The fact that the seats are tapered / stepped - does it really matter? I agree it's nicer for them to be flat, but are you losing too much torque when you tighten them, or are they coming loose? Maybe just live with it.

                If you still want flat bottoms, how about machining up small tapered washers and epoxying them in place? You could use hard bronze for this, or just hard steel. Either way, it'll seat better. You might need to skim the heads of the cap screws slightly, but that's easy.

                I must admit, for most times where I use counterbored cap screws, I just use a drill. Not only does it work just fine, it also centres the head in the counterbore.

                Ian
                All of the gear, no idea...

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                • #9
                  Originally posted by Ian B View Post
                  I must admit, for most times where I use counterbored cap screws, I just use a drill. Not only does it work just fine, it also centres the head in the counterbore.

                  Ian
                  You animal! Dad has a set of counterbores, they are satisfying to run and a good excuse to fire up the drill presses which sit a lot. I don't like using endmills for counterbores as they are actually a negative taper towards the middle, so you get a small high spot. Not to say I never do it...
                  21" Royersford Excelsior CamelBack Drillpress Restoration
                  1943 Sidney 16x54 Lathe Restoration

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                  • #10
                    THIS TAKES LONGER TO DESCRIBE ,THAN TO DO IT. Dykem up the hole real good.Use 3/8 four flute carbide end mill. Keep slight tension on your way locks. Drop down into the middle of the 10 mm hole .Dont go full depth at once. Feed sideways on x plus until you barely touch side of counter bored hole.Now do x minus and y the same way.Now work your way around to the areas that are still not cleaned up. Repeat with deeper cuts until done. I have actually done this on a vise jaw that was poorly made. Edwin \Dirnbeck

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                    • #11
                      I had no trouble drilling jaw tensioning holes in case hardened jaws with a solid carbide drill. If you get a four fluted solid carbide endmill intended for hard steel and go gently as already advised, it should go well. I would prefer a cutter with a small radius at the corners, and put a small chamfer to match under the heads of the screws.

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                      • #12
                        Yes those carbide end mills with just .015" corner radius are way more resistant to corner chipping.
                        EMs with dead sharp 90° corners chip if you breathe on them hard. Especially the hard and brittle
                        grades of carbide. Garr comes to mind as especially brittle, but will last in years of use if run with
                        care in a cnc mill. If you have a Bridgeport, Garr endmills are a challenge not to break, and I am
                        very careful.

                        -Doozer
                        DZER

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                        • #13
                          Originally posted by Ian B View Post
                          ...The fact that the seats are tapered / stepped - does it really matter?
                          My thoughts exactly. When it come to cheap import chucks I've had much more trouble with the fit of the
                          slots where the removable jaws mate to the master jaws. If the removable jaws seat tightly on the master
                          jaws and the capscrews lock them down solid who cares if the bottoms of the holes are not flat? Much ado
                          about nothing...

                          Keith
                          __________________________
                          Just one project too many--that's what finally got him...

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                          • #14
                            It does matter in the direction I'm pretty sure he is talking about. If I have it right he doesn't mean a taper in the diameter of the counterbore, and he doesn't mean a taper in the seat from outside to inside. I believe he is talking about a counterbore that is put in out of square to the base of the jaw. So when the cap screw is tightened, it hits on one side of the head but not the other, tweaking the head of the screw over when it's tightened and putting a shear load on the threaded diameter. Depending on the extremity, that's a good recipe for self-loosening or even breaking screws in my experience.

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                            • #15
                              I didn't read it like that. The way I read it is that the counterbores have been done with a drill, and a fairly rough one at that, rather than a proper flat bottomed counterbore. If I'm right, then yes, it would be nice to have flat bottoms, but it isn't the end of the world if they are not. I wouldn't lose sleep over it.
                              'It may not always be the best policy to do what is best technically, but those responsible for policy can never form a right judgement without knowledge of what is right technically' - 'Dutch' Kindelberger

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