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Bolt circle help

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  • Bolt circle help

    Im trying to drill a bolt circle, in 2 inch thick aluminum (6061) Im using a .250 drill. My question is , if everything is set up perfect on top. I do all my drilling and flip it over , Its .050 off in every direction on the bottom?
    Feel free to put me on ignore....

  • #2
    A few problems could be occuring.

    What RPM are you doing? What feed rate? (Just roughly how many seconds it takes to drill 1 inch deep)

    Are you peck drilling? (Withdrawing to remove chips from the flutes), How often if so?

    Are you center punching or spot drilling first with a short, rigid drill like a spot drill or center drill?

    I found one of my biggest problems was the drill would skate just a few mils before it would bite into the material, this 'bends' the drill and then it follows the angle of the bend, not the angle of your drill press. This can produce a VERY off hole after 2" with just the tinyest amount of skating, Using any drill thats too stubby to bend REALLY helps this, if just to start the hole and prevent the next drill from skating. (Basicly, any drill with next to no *flute* length and fits nearly all the way in your chuck. Flutes REALLY weaken a drill)

    Feed rate is also very important as if you feed too fast for the RPM, you'll push the drill off center. Too much chip buildup can do the same, as can having a poorly ground drill to a lesser extent.

    At 1/4", 1000rpm gives you 65SFM, So anywhere beween 1000 and 2000rpm should be fine in aluminum. (Maybe even a little slow, But I perfer slow, keeps from burning out drills)
    (More like 800~1200 for steel btw. Bigger drills require MUCH less rpms. Allways go by calculated SFM)
    Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.


    • #3

      Its .050 off in every direction
      Can you clarify the "Every Direction" portion of that statement please.

      .050 over two inches is alot, an awful lot. But in this case it matters "how" it's off... every direction would lead me to question both the drill and the method. Honestly you shouldn't have that much drift with a .25 drill.

      IS it properly sharpened? If not that can and will cause drift.

      Second, the method itself. How are you drilling these holes? Hand drill, Drill press, Mill...? IF your using a machine tool, check the tram.

      Deep holes in aluminum at .25 should be a cake walk. Peck after the first 1/2 inch of depth, use a lubricant. Kerosene, WD 40, Tap Magic... doesn't really matter, as long as you have something.

      Black hit on some real good stuff too.


      • #4
        How can it be .050' off in all directions?
        Is the head of the Mill or Drill trammed correctly ?


        • #5
          By "every direction" I'm going to assume you mean it is random.

          Blue Moons & Walter have said every thing I would, so follow their advice.


          • #6
            Originally posted by luthor
            How can it be .050' off in all directions?
            Is the head of the Mill or Drill trammed correctly ?
            Hell, my first question was how can it be so good?


            • #7
              Im trying for.005... It is a mill and it is trammed. I was peck drilling, but after some suggestions, I dont think the drill is as sharp as it could have been. Thank you guys for the input!
              Feel free to put me on ignore....


              • #8
                Originally posted by GKman
                Hell, my first question was how can it be so good?
                Hehe. You'd think drilling a hole would be the easyest of all tasks... But it really does require more skill and math then the average drill operator is willing to admit if you want to drill a high accuracy hole, and get good life outta your drill bits.

                I can now drill dozens of holes, inchs deep, with the same bit without dulling it. Even in steel. I used to as a kid dull a drill to nothing but rubbing in 1/4" thick steel before getting through. I actualy thought steel was too hard to drill as a kid. (And it never helped that most the drills I had from my dad where duller then nails, He did'nt know how to drill either... and mainly did woodwork, So dull as nails was pertty sharp for his work.. even if the wood did smoke often)

                Now that I have learned how to drill efficently, speed and tool life wise, I need to learn more about accuracy. Go figure, all these years and im still learning how to drill a hole

                As for accuracy... after 2" deep, 0.05" or more was what I was getting from about 0.01" or less of drill skate at the top of the hole. (Random exotic hardwoods/acrylics, 7mm drill (Near enough to 1/4") I barley even noticed when retracting and inserting the drill that the hole was not totaly in line with the relaxed position of the drill.

                Center punch, spot drill/center drill, or 'raming' the unspinning drill into the work (Then turning the drill press on with the drill still in contact) all seemed to greatly reduce the skate error. I don't recommed raming on anything harder then wood or plastic however. And its likey not a good idea, just a fast one. :P Feeding too fast also amplifyed error. (More RPM's let you feed faster, Its Feed per Revolution thats important. Too many RPM's (SFM really) overheats the drill, (and in the case of plastic for example) and material.

                With spot drilling, very slow feed, often pecking, highish rpms, 0.005" should be achivable with a decent machine at 2" in aluminum with a 1/4" drill.. IMO
                0.010" Should be pertty easy.
                Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.


                • #9
                  It may help to locate, then pilot drill the holes maybe 3/16" dia 1/2" deep, then go back and open them out to 1/4" for that 1/2" depth with a 1/4" center-cutting end mill. That will get the first 1/2" of all the holes spot-on location and provide a guide for drilling the rest of the way.

                  And using a sharp, accurately-ground top-quality drill bit is a huge help. If the two flutes aren't ground the same you're probably doomed to less than ideal results no matter what you do.
                  Last edited by SGW; 03-26-2011, 10:13 PM.
                  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
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                  Don't own anything you have to feed or paint. - Hood River Blackie


                  • #10
                    I find that using some sort of a stub drill- even just a countersink-
                    improves things by an order of magnitude. The web on a twist
                    drill loves to wander, sometimes even around a punch mark.

                    I also found that a bad bearing in your spindle will thwart most attempts
                    at accuracy!

                    rusting in Seattle


                    • #11
                      I have some 1/4 inch deep hole aircraft step drills left. Would you like a few? This is what they are intended to drill and the result will be dead on. If you want some just shoot me a PM with your address and I will mail them out on Monday.
                      Free software for calculating bolt circles and similar: Click Here


                      • #12
                        Thanks to all. After over 50 years I'm learning something about drilling. What does "peck" mean exactly.


                        • #13
                          In and out, in short increments. To clear the chips.
                          Feel free to put me on ignore....


                          • #14
                            Thanks Evan... I was going to reply to your PM but your box is full.

                            Im in no hurry. I dont want to ruin any more metal. So take your time and dont make a special trip if not needed. Thanks again! Dave
                            Feel free to put me on ignore....


                            • #15
                              You are welcome. My mailbox is cleared out too.
                              Free software for calculating bolt circles and similar: Click Here