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Problem drilling holes on the centre line of items

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  • Problem drilling holes on the centre line of items

    1 - put block in vice
    2 - pick up edges on sides and use divide by 2 function of DRO
    3 - OK it should be right on the center line of the block

    But some times it is off.

    I pick up the edge with an edge finder and even a 3D Taster. I use a center drill or if it is a bigger drill a spotting drill to start the hole so the bit should be in the correct location but it seems to deflect or jump to the wrong spot. Frequently I will double check the location, with a caliper after putting a tiny mark with the spotting drill and it will appear to be in the correct location. I know twist drills are not the best for accurate work but getting a .005 or even .010" error in position seems to much. I know others with the same problem.

    Any ideas or solutions.
    Last edited by loose nut; 04-06-2018, 11:09 AM.
    The shortest distance between two points is a circle of infinite diameter.

    Bluewater Model Engineering Society at

    Southwestern Ontario. Canada

  • #2
    Center drills are for lathe work, use a spotting drill. If a round object, mill a flat.
    I think the error is in your use of the edge finder and DRO. Are the dimensions of your block accurate? Divide 2 by what exactly? You can measure the workpiece with the DRO, then divide by two. Edge finders are simple, but can be off by .002 to .003. If the round types deflect out, that’s beyond center, need to bring them back a tiny bit. Unless you are NYCCNC, and spend money to make up for lack of manual machining skills, spend extra time on metrology skills and use a liberal amount of DTI’s and Noga holders and actually verify things are what you think. Also, if your DRO isn’t glass scales or something better, get glass scales or something better.


    • #3
      The only thing that comes to mind is the tramming problem combined with parallax that one of the threads talked about a few weeks ago.

      In his case, the head was not in proper tram, so when he used used a short center finder and a longer drill the hole moved over. Are you using two tools with the same extension?

      Is the error always in one direction? That would point to the parallax problem as mentioned above.

      At the end of the project, there is a profound difference between spare parts and left over parts.

      Location: SF East Bay.


      • #4
        If you are using spot drill or end of centre drill to start, my first suspicion is that you're not really where you think you are vs the drill somehow going off 10 thou from there. The short stiff nature of a spot drill shouldn't let move about. I would usually pick up one edge and move in 1/2 the part dimension, but I doubt that matters. Not sure what it could be....if you pick up the two edges of something of a known length, does the DRO tell you to go to the predicted middle? i.e. 1/2 part, the DRO tells you go in .500"?. Are the machine tools ways tight - no overt movement of an indicator needle if you try and manual move them about? how about the quill? These may or may not be likely suspects but you have to start with elimination.

        edit - yes of course, tram as Dan suggests, curious how that checks out
        in Toronto Ontario - where are you?


        • #5
          One step further, if you really need the middle.

          Sticky or worn edge finders , old eyesight and, maybe DROs that do not tell quite the truth can all add up to put you off by a few thous. I usually find both edges with the centre finder, divide by 2 then zero the readout and carry on.( I build steam engines not rocket parts) However, if I REALLY want the middle I go again to the edges. Quite often I find a thou or so difference, so I rezero as appropriate and go again until both readings are equal. IF possible use the centre finders against the part and not the vice jaws , tilted vice jaws may throw you off a bit. Hope this helps. Regards David Powell.


          • #6
            Check your edge finder technique by putting a test indicator in the spindle and seeing if you get the same reading picking up the high point on each side of the part or the faces of the opposing vise jaws (if the part is sunk below the top of the jaws). If that turns out <<0.010, then that rules out edge finder and DRO issues, and points more to drill wandering, and possibly tram depending on whether Z changes a lot between edge finder, indicator, and drill. Your tram would have to be very far out of whack to accumulate 0.010 lateral movement in a few inches Z move. Also try drilling the hole and then sweeping it with an indicator and see whether the hole is on the spindle axis.


            • #7
              For those without a DRO with the "1/2" function, it's almost foolproof. Assume a 4 x4 inch block in the properly aligned vise. Assume you just want the center of the X axis.

              You pick up one edge using the handwheel for the Y axis. Zero the X axis there. You don't even care what the diameter of the tool is. Then you move the table to find the far edge. The DRO should then be reading 4.000 plus the diameter of your edge finder. Press the 1/2 button and it shows you the coordinates for the center of the block.

              In theory, it even cancels the small offset that edge finders tend to have, where they 'kick' a few thousandths after contact. As long as it kicks the same on both edges the DRO takes care of it.

              After using it for years I finally realized it works for symetric holes too. Even if you start by being offset to one side or the other, you end up with the center of that axis. Then you repeat on the other axis and you end up with 0,0 at the center of the hole. It's quicker than using a wiggler, although a wiggler does work too.

              At the end of the project, there is a profound difference between spare parts and left over parts.

              Location: SF East Bay.


              • #8
                When you zero the back of your vise you should always clamp something in it to put force against the back (fixed) jaw before entering zero into your DRO.
                If that's how your zeroing your DRO.

                Loose spindle bearing or drill walk could also be coming into play. There are a whole lot of things that could be causing your error.
                An off center chuck arbor or chuck jaw could also cause this.

                Other possible contributing factor..... is your work square??? Is your vise tipping the work when you tighten it??

                If I'm going for real precision I always square up my work before milling or drilling. Flat bar, even ground flat bar is not perfectly square.

                Last edited by JoeLee; 04-06-2018, 12:00 PM.


                • #9
                  Some of your answers may need tweaking.... he has checked location with caliper, and verified it is where it should be.

                  It seems that the problem is not with locating the spot, but with having the drill follow the spot and accurately locate where the spot was (which was verified)

                  SO either the calipers are always "off" just enough to make the spot seem to be on center even though it is somewhere else (quite a co-incidence), OR the issue is with getting the drill to go down where the spot was.
                  CNC machines only go through the motions


                  • #10
                    Originally posted by RB211 View Post
                    Center drills are for lathe work, use a spotting drill..
                    I respectfully diagree. I ran a Moore jig bore for years drilling and borring thousands of holes ON LOCATION. for interchangable stamping dies.I found the best location came from using a new 5/16 usa made center drill. Many times you could skip the borring operation . Just center drill, drill and ream..And of course siting at the Moore it was easy to go back over your finished work with the special Moore jigbore indicater holder and verify locations Especially in later years when they had digital readouts installed If the original poster puts a 5/16 good centerdrill in a 5/16 collet ,the hole WILL BE ON LOCATION THE TIP DIAMETER ON A GOOD CENTERDRILL ACTUALLY HAS RADIAL RELIEF AND CAN BE USED as a short stuby endmill I say this to ilustrate how small and ridgid it is and how well it resists the inevital forces trying to push it of center.Another illustration is trying to spot a hole on and angled surface ,say 45 degrees. A spotting drill will walk all over the place.Where a spotting drill exells is locating the hole prety good AND SIMULTANEOUSLY CHAMFERING THE HOLE ..Edwin Dirnbeck
                    Last edited by Edwin Dirnbeck; 04-06-2018, 01:08 PM.


                    • #11
                      Originally posted by Edwin Dirnbeck View Post
                      it was easy to go back over your finished work with the special Moore jigbore indicater holder and verify locations Especially in later years when they had digital readouts installed.Edwin Dirnbeck
                      This is what Loose needs to do, but without a special Moore indicator holder.


                      • #12

                        Locking the table before drilling? It is not unknown for the drill to "kick" the table, especially with smaller mills. Or kick the work, if not securely clamped {"but all I am doing is drilling holes, not pushing the work into a cutter").

                        Not spotting in a way that actually is bigger than the center chisel edge part of the drill point. The chisel edge on a drill tends to make it "walk around", and it is best to make the "spot" larger than the chisel edge of the following drill, so that the drill will "locate" and not "walk around".
                        CNC machines only go through the motions


                        • #13
                          Originally posted by RB211 View Post
                          Center drills are for lathe work, use a spotting drill.
                          yes, but if you're just using the end point of a centre drill, it is essentially a very rigid spot drill. Otherwise, agreed.
                          in Toronto Ontario - where are you?


                          • #14
                            I'm thinking most likely slop in the quill. When I want a real accurate hole, I scribe, prick punch, center punch, then run a flat file over the surface to knock down the metal displaced around the punched divot.


                            • #15
                              Spotting drills didnt become widely known until everyboy bought vertical machineing centers. People started saying "why not chamfer the hole at the same time that we center drill them". Edwin Dirnbeck