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  • Drilling error, what to do different next time?

    Pic of setup before any drilling



    Needed to drill 3 new holes to adapt this plate to a different div. head. Locked plate onto another plate, one from the destination head, for a pattern, twisting pattern plate 120 degrees to new hole location, and transfer-punched new hole locations as you see in the blued areas. Center-punched those little transfer-punch marks to enlarge the cone.

    Now here's where I probably omitted a careful machinist's step but not sure what it would be. I setup as shown using a 5/8"-1/4" center drill which would leave correctly-chamfered and sized hole and figured proper drilling depth. Then I visually aligned the center drill on each punch using good light and magnifier, and drilled. When checked with transfer punches thru both plates, only two would pass, third hole was out of place by many thousandths so I'll have to set up again and stretch that hole by side-milling or drill new hole overlapping mis-drilled one, or ream it larger.

    What procedure should I have used? I think the thick, somewhat blunt center drill point isn't suitable for an accurate visual alignment, even though I started each hole by drilling a few thou than stopping to see if it looked right, but got no clues that one hole was off.
    Last edited by Cannonmn; 11-03-2016, 10:08 AM.

  • #2
    I'd say your assumption about the drill point is correct. I use a pointed center-finder or a coaxial indicator with a point probe to align the drill.
    Southwest Utah

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    • #3
      In my opinion, if you need two or more holes aligned to a few thousandths, transfer punching etc will NEVER succeed. For your operation. a rotary table and a bit of math would be the way to insure a fit.
      ...lew...

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      • #4
        "I think the thick, somewhat blunt center drill point isn't suitable for an accurate visual alignment. "

        Yep.
        If the original lay out had no error, letting the spinning drill float the part in before clamp down would have got you there.
        Or use a pointy wiggler.

        'Transfer punches do have inherent accuracy issues being smaller than the nominal size by .002" up to about an inch, larger, more.
        Bolts aren't typically used for locators so they work pretty well.
        Last edited by Carm; 11-03-2016, 10:11 AM.

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        • #5
          Originally posted by Cannonmn View Post
          I setup as shown using a 5/8"-1/4" center drill which would leave correctly-chamfered and sized hole and figured proper drilling depth. Then I visually aligned the center drill on each punch using good light and magnifier, and drilled.
          there's your problem......don't visually align with the drill point, use a wiggler. That should have got this accurate enough. EDIT - agree with Carm, enlarge the mark with a centre punch then drill with it floating on the drill press table would have been best/fastest.

          To do a better job, because with the above approach there are sources of big errors that can accumulate (big as in fit of transfer screws, eyeball alignment etc), you could measure off the plate:

          measure bore with telescoping gauge and mic. measure ID of screw clearance hole - small hole gauge. measure with caliper distance between bore and clearance hole. There is some error built into that op, but its small. Workout PDC for three holes - Often this is some nominal number your measurements will point topward. ie, if you measure and calculate a radius of .997", you can assume it should have been 1" and there were some measurement errors, so go with 1". Clamp the plate down, centre with an indicator or edgefinder and use machinery's handbook to x/y coordinate drill the three holes. The whole done with simple tools all should have.

          btw, unless they are unusual bolts, the angle of the centre drill is 60 degrees and you need an 82 degree countersink.
          Last edited by Mcgyver; 11-03-2016, 10:25 AM.
          in Toronto Ontario - where are you?

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          • #6
            That's why they make wigglers.

            The best you can expect with scribe lines and prick punches is =/-.003.
            Throw a center drill on that and you will be lucky to hit withing .020 of where you want to be.

            If the work is done on a mill, DON"T CENTER PUNCH, and just use layout lines as a check on your dial numbers.
            If the work is done on a drill press, DON'T use a center drill. Start with the scribe lines, then punch , follow with a small flexible twist drill, then drill to size and C'snk.

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            • #7
              IF your punch marks are accurate
              I would start the holes with a small drill bit
              like 3/32" or the like
              and let the part float
              like hold it by hand.
              Then follow with your clearance drill
              and countersink.

              -D
              DZER

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              • #8
                Thanks, very useful info, but being a newbie, need def of PDC. The wiggler must be one of the things in that plastic pouch of five or so Chinese edge and center finders. I love transfer punches but haven't had to do much beyond their level of accuracy, which from reading here, is poorer than I had thought. I have the tools to do these things numerically, will just have to make that first choice to get more accurate work when needed.
                Last edited by Cannonmn; 11-03-2016, 10:56 AM.

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                • #9
                  The simplest method is to clamp the two parts together and drill through the existing holes.
                  Jim H.

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                  • #10
                    Originally posted by Cannonmn View Post
                    Thanks, very useful info, but being a newbie, need def of PDC.
                    Pitch Diameter Circle?
                    Location: Long Island, N.Y.

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                    • #11
                      Originally posted by Cannonmn View Post
                      Thanks, very useful info, but being a newbie, need def of PDC. The wiggler must be one of the things in that plastic pouch of five or so Chinese edge and center finders.
                      yes, pitch circle dia - use most often when referring to a bunch of holes in a circular pattern. my bad on mixing up the initials.

                      The pouch should say Starrett or lufkin on it btw. Its the pointy end you use. turn the spindle on, gently grab the spinning pointy end up top where it clamps into the body. Draw your fingers carefully down - this will get the point running true to the spindle axis. Now you've got a very fine point you can bring very close to the work and view its location from two directions. Every old boy's tool box has one, but a lot today don't seem aware of it......its a needed tool for mill and drill press. I also use mine in the lathe for 4 jaw work - held in the tailstock chuck, the point goes to a spot on the work i'm trying to indicate in
                      Last edited by Mcgyver; 11-03-2016, 11:23 AM.
                      in Toronto Ontario - where are you?

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                      • #12
                        Originally posted by Mcgyver View Post
                        <snip>
                        I also use mine in the lathe for 4 jaw work - held in the tailstock chuck, the point goes to a spot on the work i'm trying to indicate in
                        Using it as a pump center then, not the spinning centering pin as when it's held in the spindle? Newbies might benefit from a fuller description here.
                        .
                        "People will occasionally stumble over the truth, but most of the time they will pick themselves up and carry on" : Winston Churchill

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                        • #13
                          While posted quite awhile ago, this article was "on the money":

                          http://www.metalwebnews.com/howto/wiggler/wiggler.html

                          metalmagpie

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                          • #14
                            If the transfer punch marks are close enough, wiggler to find center and a spotting drill (short, stiff bit with a point designed for starting on point without wandering) to start the hole.

                            If not, rotary table or indexer to locate the holes.

                            If not available, then locate holes on a mill using coordinates (DRO or careful reading of the x and y indices on the dials). If locating using the dials, be sure to take up the backlash in the same direction on each axis (I run to the left and to the back by habit) for each hole, as well as when picking up your center. DRO compensates for the backlash automatically, since it measures true position of the table.

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                            • #15
                              Also your center drill will not have the correct countersink. They are 60 degrees and standard flat head screws are 82 degrees.

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