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  • Boring a precise hole-advice needed.

    Gents,

    I am attempting to bore a hole 0.625" ID, 0.450" deep through hole in 4140.

    The boring bar is 12mm using a carbide inset, triangular in shape, TCMT? It is being held in a Chinese BXA QCTP mounted on the cross slide of a Colchester Student lathe.

    Using a 4 jaw chuck to hold the part and it is showing less than 0.001" runout.

    The problem that I am experiencing is that I keep producing a slight taper in the hole, taper runs from the tailstock being the widest to the chuck being the smallest. The taper is about 0.01" consistently during successive cuts.

    I imagine my problem relates to the boring bar not being dead on parallel with the part, I have tried to ensure that all is parallel by using an engineers square on the toolpost to the chuck. Still getting the taper. I do not get this taper when I use a HSS or carbide turning tools when cutting the OD of a part, it is only when trying to bore this hole.

    I have tightened my cross slide gibs and am feeding in using the carriage, my RPM is 600 and feed has been varied to see if it helps, it has not.

    Looks like I have a simple alignment problem, however, I would appreciate any advise on how to set up the boring bar and tool post absolutely dead on for a parallel hole.

    Appreciate any feed back.

    Mike

  • #2
    Hi Mike
    It sounds like your boring bar may be deflecting. Try choking up on the bar if you can and take a few spring cuts.
    Location: Long Island, N.Y.

    Comment


    • #3
      Thanks Rich,

      I am a noob and am not sure what 'chocking up on the bar' is mate?

      Happy to learn though, how do I do this?

      Mike

      Comment


      • #4
        Originally posted by mikerolly View Post
        Thanks Rich,

        I am a noob and am not sure what 'chocking up on the bar' is mate?

        Happy to learn though, how do I do this?

        Mike
        Decrease the amount of stick out of the bar from the bar holder to the minimum You can get away with to make it a more rigid set up.

        Steve

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        • #5
          Thanks fellers, will give it a go.

          Appreciate the help.

          Mike

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          • #6
            Did you ever play baseball. It was common to Choke up on the bat when bunting the ball. But on second look I see you are an Aussie. Use only enough Stick-out on the bar that's necessary to get to the back of the hole.

            Are you locking your cross slide? Maybe it is drifting a bit. Using a sharp tool?

            Edit* Well I was typing too slowly.

            Comment


            • #7
              You might try and step up the rpm's also, You are only at 95 surface feet per minute on Your cutting speed and I think that is a little slow for carbide. Small boring bars tend to have a lot of flex so light depth of cut will help also.

              Steve

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              • #8
                Maybe the insert does not have enough clearance below the cutting edge and is rubbing and pushing the tool out of cut?

                Ian.

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                • #9
                  Get it close and ream it to size.

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                  • #10
                    When trying to produce an accurate bore, I'll take a number of spring cuts. Making repeated passes with the tool at the same setting to remove taper. Your insert might be the problem here, many carbide inserts have a minimum depth of cut.
                    Paul Compton
                    www.morini-mania.co.uk
                    http://www.youtube.com/user/EVguru

                    Comment


                    • #11
                      Thanks for the responses fellers, all taken on board and applied today. Now have a nice smooth and consistent bored hole.

                      It was a case of choking my boring bar, spring cuts and using a HSS boring bar for the finish cut. It all came together to give me the interference fit I was aiming for, happy days.

                      Hope you US blokes are not too cold, been watching the news and it looks nasty in some areas. My concern was not sweating on my lathe at a balmy 36 Celsius / 97 F.

                      Mike

                      Mike

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                      • #12
                        24 deg F in the Chicago suburbs this morning. About 4 inches of new snow, We're good.
                        Glad the boring bar worked out for you.

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                        • #13
                          Even with 4140, which will eat HSS for lunch, it can pay to ditch the carbide to finish the hole.

                          Carbide isn't really good at light cuts when you are final sizing a hole, and getting the taper out. Sharp HSS is, and it will last long enough with 4140 to size a hole like that. Good for "spring cuts".
                          CNC machines only go through the motions

                          Comment


                          • #14
                            I agree with everything said. Lots of spring passes are needed to take taper out of bored holes. Sharp HSS is great for this like mentioned!

                            I always try to use the biggest strongest bar I have with as minimal stick out as possible.

                            Andy

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