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Why are my holes to big?

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  • Why are my holes to big?

    I've drilled 10s of thousands of holes and the last few, for some reason, are alot bigger than the drill.
    Here is the details: Drilled some Q size holes for some helicoils. The holes were drilled with a brand new bit. The holes were so large that the helicoil would fall into them... This was in aluminum.
    I drilled another hole in a one inch diameter Steel rod. It was a .375 drill bit and the hole was .407 in diameter when done. The drill bit seemed to drill fine. I didnot see any wandering and the chips comming out were nice and were the SAME from each flute. I did not predrill other than a center drill just to locate them accuratly. I indicated the runout on the chuck holding the drill bits and it was only .001 These were NOT split point drills but the end resulting hole seems to be excessivly over sized. Any thought as to why? I am having a hard time beliving its because I didnt predrill....

  • #2
    You did check the bits with a mic. didn't you? With that "lack" of runout and
    more or less equal chip from the two flutes "is a puzzlement" as a once and
    good movie actor said in the "King an I" .
    ...Lew...

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    • #3
      Twist drill bits drill holes. If you want "accurate" holes drill undersize hole and then use a reamer.

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      • #4
        What kind of machine? As Lew asked, did you measure the drill(not"bit", it's a drill)?

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        • #5
          Did you indicate the runout on the chuck body or on something held in the chuck?

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          • #6
            An off center point (unequal lips) is the usual cause of oversize drilled holes
            Joe B

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            • #7
              Originally posted by JoeCB View Post
              An off center point (unequal lips) is the usual cause of oversize drilled holes
              Joe B
              +1. That is likely the problem, a poor grind is 98% of reason for oversized holes. Whats the make of the twist drill? Just because its new doesn't mean its perfect

              Unless extreme, an eccentric action won't much matter; the drill will flex and overcome it...but if the lips aren't identical, the long one determines the hole size and Shorty is just along for the ride.

              btw, properly done, drills will drill within .002". I agree use a reamer when it matters, but it's always impressed me just how good drills can be when you sharpen them properly
              .

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              • #8
                "It was a .375 drill bit and the hole was .407 in diameter when done"

                32 thou? A full 1/32 over? that's HUGE.... without trying I get maybe 5 thou oversize with machine screw length, and sometimes bit more with jobber. What brand are you using? Hand held drill or drill press/mill?

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                • #9
                  Originally posted by Mike Burdick View Post
                  Twist drill bits drill holes. If you want "accurate" holes drill undersize hole and then use a reamer.
                  That's it. They seem always to dtill a few thousandths oversize. Part of it might be that the drill chuck has some runout - even good ones have a thousandth or two.

                  Another issue might be that used, resharpened drills are not ground on center. Thus, they may wobble when started.

                  As Mike says here, I will always drill undersized and then finish with a reamer or a boring tool if possible.

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                  • #10
                    Originally posted by JoeCB View Post
                    An off center point (unequal lips) is the usual cause of oversize drilled holes
                    Joe B
                    You'd think.........

                    BUT

                    That does not seem to correspond with the OP's statements.....

                    Originally posted by ahidley View Post
                    I didnot see any wandering and the chips comming out were nice and were the SAME from each flute.
                    Normally, if the drill has unequal flutes, it will also have rather unequal chips. With a hole size problem of 32 thou, you'd suspect that the off-center point would be clearly visible to the unaided eye, and that the chips would be very obviously different.

                    I'm puzzled unless something the OP said is in fact not the case, even though he thinks it is..... But WHAT it is, I haven't a clue.... he seems to have covered the obvious areas of problems, and as I recall from other posts, he is no dummy about tools.
                    1601

                    Keep eye on ball.
                    Hashim Khan

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                    • #11
                      You should use a "spotting drill" to start the hole. A "center drill" has a 60 deg point and the drill is usually 118 deg. With a sloppy spindle the drill can walk before it gets a good start.

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                      • #12
                        Regarding your drill, something you are sure of is wrong. Drills have few characteristics so it should be fairly light work to isolate the one that is creating the problem. I'd include the chuck and arbor in the search, and also think about what was going on when this change occurred.

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                        • #13
                          And even if the drill is correctly ground, on my less-than-perfectly-rigid drill press the combination of table and cross-slide vice allows for a bit of wobble, so my holes are nearly always over size.

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                          • #14
                            I bet you could try a different brand of drill bit and the hole would come out on size.
                            I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                            • #15
                              Totally agree with Joe and Mcgyver. A good bit will drill within a couple of thou. It's the flutes that matter. Not just the width of the flutes but also the front and back relief angles since if that differs the flutes will take off different amounts.
                              Free software for calculating bolt circles and similar: Click Here

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