No announcement yet.

problem with locating drilled holes precisely

  • Filter
  • Time
  • Show
Clear All
new posts

  • problem with locating drilled holes precisely

    hi guys!

    i'm a newcomer to this list & was wondering if any of you could please help me with a problem i've been having for the past year or so with hole drilling on my drill press. i putz around with occasional projects involving drilling holes, generally from 3/16" to 7/8" dia., in aluminum and steel, and have been having a lot of trouble with my holes wandering off the center-punched mark when beginning the hole. i don't think i had this much trouble with them in the past, but can't think of anything that has changed in my setup or technique. it's like i'm getting worse at this, for some reaon i can't figure out.

    here's how i typically do it:

    1. lay out the hole center point with a thin scriber

    2. use a carbide scriber to carefully create a small indentation at that center point to catch the tip of my spring-loaded center punch (sort of a "pre-center punch" to avoid the impact of the actual center punch blow driving the point off-mark)

    3. use the spring-loaded punch to punch the center

    4. use a small drill bit (i've tried anywhere from 1/16" to 3/16" for this) to drill a pilot hole

    5. check the "dimple" formed by the drill bit as it's starting, in order to verify that the hole's ending up centered on my scribed lines. if i'm using a bigger pilot drill, i usually check this a couple times up to the point where the "dimple" has reached the full o.d. of the drill bit.

    6. assuming the pilot hole's located ok, drill the hole to finished size.

    the problem i'm having is with what occurs in step #5 above. more often than not, the dimple wanders off the mark (i'd guess about 1/128" to 3/128" off the proper location), and i have to try and force the bit to sort of cut the dimple sideways to move the dimple's center back onto the mark. this is a real bear sometimes, and involves a lot of eyeballing & tilting the workpiece around so as to get the bit to chew its way back where i want it -- not very precise workmanship.

    if i catch it early, while the dimple's still small in proportion to the bit o.d., i can usually bring it back reasonably well. the bigger the dimple gets, though, the harder it is to "pull" it over to the correct spot. if the dimple gets to be the full o.d. of the bit, it's usually a lot tougher to fix.

    i've had pilot holes end up slightly off location because of this, and have found that, once the pilot hole's drilled thru, it's REALLY tough to bring the finish-size hole back to location. i've tried using a countersink to sort of cut away heavier on one side of the pilot hole so that the countersink is more in the correct location (and thus eccentric to the incorrectly placed pilot hole). the idea with this is that i'm hoping that the offset countersink will guide the finish bit to stay cutting in the correct location rather than following the erroneous pilot hole. if i do this several times before drilling the finished-size hole, gradually working my way up in drill size, i can sometimes move the hole sideways to the correct location. if i try to do it in 1 step, if doesn't ever move the hole enough to being it to the corect location.

    the above technique is based on this thing my high-school shop teacher taught us where you use a sharp punch & hammer to chop away at one side of a partially drilled "dimple" that's incorrectly placed to bring it back to location.

    anyway -- to make a long story short, this all seems really crazy. a person shouldn't have to go thru all these contortions to drill a correctly-placed hole.

    can any of you offer any advice?

    here's a list of items i've already checked, that are pretty much ruled out as causes:

    1. the drill press table is as square to the centerline axis of the chuck as i can get it (there's about .003" difference with a feeler gage on either side of the table top (about 10" across), using the "bent-wire-in-the-chuck-checked-at-opposite-sides-of-the-table" squaring method.

    2. the drill press is a ching-chang taiwanese model, but it's the same one i used to use before becoming mr. innaccurate driller all of a sudden.

    3. i can feel no unusual lateral slop in the chuck quill

    4. bit sharpness doesn't seem to matter much. i've had holes end up well-placed using bits i haven't sharpened in a long while, and holes end up off with both freshly-sharpened (sometimes i eyeball-sharpen, but also often use a "general tool"-type attachment for my bench grinder) bits and new bits.

    5. cutting fluid maybe seems to help a slight bit, but i'm not sure.

    6. i recently read an online suggestion about using a center drill (like for lathe tailstock work) for pilot holes, because of their stiffness. got one, sharpened it & tried it, and am still having the problem, although a bit less. i did notice that this center drill seems to have a pretty thick web between the flutes. could web thickness cause this wandering, especially when the center punch dimple isn't all that big to begin with?

    7. i tried using a really small bit (the 1/16" mentioned above) for the pilot, figuring that, since pretty much the entire o.d. of that bit would be contained within the center punch mark, how far off could it possibly wander? this hasn't worked out too badly, but small o.d. bits like this are a lot more susceptible to deflecting from small irregularities in the center punch mark, especially if i had to punch it several times to "nudge" the punch mark to the exact spot.

    8. i've also recently tried making my center-punch marks bigger (after using the spring-loaded punch, i go back in with a hammer & punch for another hit to widen & deepen the center-punch mark), hoping to more effectively hold the drill bit's tip on location. this hasn't helped a lot.

    that's about it. can anyone PLEASE tell me how to go about drilling holes right on the money, location-wise? there must be something i'm missing. it shouldn't be this much of a headache!

    thanks very much!

    drew j.

  • #2

    If you want to place holes precisely, I would use a milling machine with a sinkable endmill and locking X/Y axis, with a very good holding vise while drilling (milling).

    Center point drill bits are good for making holes in wood or PlayDough



    • #3
      Hi,welcome aboard!

      Well I would recomend ditching the springloaded punch.Get your self a set of good quality impact punches.They will produce a deeper punch mark than the springloaded will.

      One big advantage of a impact punch is you can move a center over with one by just catching the edge of the miss-punched mark in the direction you need to go and re-punching it.

      Drill your pilot hole as you are doing now and then if possible make the finished hole in one step.
      Hope this helps.

      I just need one more tool,just one!


      • #4
        One silly question. You said you checked the quill and didn't have any runout, but did you check it under power as well. I bought a benchtop chinese wonder one time that when the machine was turning at close to 700rpm, the quill would start to wobble. When you would check it by hand, there wasn't any perceptible runout. The problem ended up being the chuck wasn't driven up on the shaft far enough. It might be possible yours is doing the same thing.


        • #5
          When I need to drill an accurately located hole, I do the layout,center punch, and then use a wiggler to center the work before clamping it down.



          • #6
            I like to sort of let the workpiece float under the drill as it's getting started. I watch for the drill bending sideways as it tries to center on the punch mark, and I peck at it with the downfeed until the bit looks like it's not going off axis at all as it starts the hole. That's no guarantee that the hole will follow the punch mark, but at least you get to notice if it's not starting in the right spot before you go too deep with the bit.
            Sometimes a more flexible bit will be better at getting the hole started in the right spot, as will one that's been reground to a sharper angle. If you peck with it and make constant corrections to the positioning of the workpiece, you can usually start the hole centered on the punch mark.

            I still have that problem anyway, just less so, and when the location of a hole is critical, I'll use a homemade spade bit to initially follow and enlarge the punch mark. This bit is made from music wire or drill rod, 1/8 inch diameter,and about six inches long. It's first ground to a point on one end, at about a 90 degree angle, then two sides are ground away carefully until the very point is all that's left at the business end. Then grind relief on the two lips, same as sharpening a drill bit. This will take some care, and maybe a magnifying glass, to get it right without overgrinding. This isn't much different than a drill bit, except that the web is thinner at the tip, and there's no helical flutes. It will look like a philips screwdriver blade but without two of the four blades. Basically it's a drill bit that's 1/8 diameter, but with a web thickness like a 1/32 or smaller drill bit.
            Anyway, I just spin it by hand in a punch mark until the mark is cleaned and enlarged somewhat. It won't usually wander from center, but if it does, you can press it sideways, making one lip cut towards the direction of correction. A couple of spins around by hand and the punch mark is ready for the pilot drill.
            This is all about enlarging the punch mark without throwing it off center, to provide a stronger initial guide for the pilot bit to follow.
            A feature I have added is a rotating dimple on the top of the handle. Kind of a live center type thing. There's a little thrust bearing of sorts there, so one finger can apply pressure to the tool while the other hand turns it. This makes it easy to use and to control. If this tool starts going off the punch mark, I know it before a half turn is done, so it's easy to correct before the mark is too well established.
            I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


            • #7
              It sounds like you already know what you are doing, but here are a few things to consider:

              1) Check the runout of your chuck just to be sure it is running true. Extend the quill and put some good side pressure on it to be sure that there is no excessive play. Make sure that everything is rigid, from the cutter all the way to the vise.
              2) Use an automatic center punch to start the mark, and then deepen it with a regular punch.
              3) Find your center with a wiggler.
              4) Clamp the work. Make sure the table, vise, and head are all firmly tightened.
              5) Use a short/stiff bit like a spotting drill or a center drill to start the hole.
              6) Don’t use too small a drill for the pilot due to deflection. I avoid using anything smaller than 3/16â€‌, and use screw machine length drills whenever possible. A sharp and properly ground split point drill may help.
              7) Pre-drill with a diameter that roughly matches the width of the web of the next drill.
              8) Drill one size smaller than the final hole size to increase the chances of a round, correctly sized hole.

              If the punch mark is symmetrical (I sometimes rotate the punch while hammering on it just to be sure), your pilot drill is sufficiently stiff, the cutting edges on your drill bit are sharp and symmetrical, and your set up is rigid, you can hardly go wrong.

              You can let the pilot drill center itself in the punch mark if you like, but as soon as the tip of the drill has fully started the cut, you should probably to clamp the work down (either to the table, or in a vise) if you want to be assured that successive cutters will follow the same path. Of course the need to do all of this depends on the accuracy required.
              Location: North Central Texas


              • #8
                Heres some things to check.
                1.Check your chuck runout.It may be dirty or just sloppy.
                2.Run your quill down to check for lateral play.If it was o/k before but is getting worse with use it maybe wearing in.try to keep the work as high as possibleso you dont have to run the quill way down.
                3.Try possibly a larger dia. spotting bit not a center point.
                4.Try a different or better brand of bit.


                • #9
                  Sheesh. Just grind the tip of the punch as a triangular pyramid and it will stop the bits from walking.

                  Free software for calculating bolt circles and similar: Click Here


                  • #10
                    Evans idea may work.

                    He can drill bearing fits with a 1 3/8" drill bit. He is a very talented drill press operator.

                    Here is a little tool that may help also.
                    It is a scribewrite tool.

                    It is made to transfer dimensions from a scale, but the lense can be used as a pick up tool as well.

                    This tool comes with two lenses. Make a lense/drill bushing holder that can be clamped on the job.

                    With this tool the lines are magnified and look like a plowed field.

                    Once lined up, clamp your tool on the job, replace the lense with the drill bushing and drill the hole.

                    With a little imagination, and reducing bushings, larger than the 1/2" lense sized holes can be drilled.

                    I would still use pilot drills for drilling. Redrilling results in better size control and finish.

                    Brass, and cast iron need special treatment of the drills for double drilling. The edge must be stoned to eliminate the drill rake.

                    The operator should not be stoned in any case.

                    A little hone on the corner of the finish drill, will make an accurate, finely finished hole usually within a couple of thou. dia.

                    Using this tool will eliminate the "walkabout" you experience.

                    Search Scribewrite and yea shall find.



                    • #11
                      The operator should not be stoned in any case.

                      Should be ok to file your finger nails though.


                      • #12
                        Make a fence for the DP table. Nothing more fancy than a straight piece of hardwood with a C-clamp or QuickGrip on each end, clamped to the table. This gives you a fixed Y-axis, so the only possible error is horizontal.

                        Sharpen the drill on any of the sharpeners that allow grinding a split point (Drill Doctor does this). Or purchase split point drills, available at every price level.

                        I also use an automatic center punch, Starrett 18B. I rarely use a pilot hole if drilling 1" or under. It introduces an additional step that I've not found to be necessary.

                        Shorter twist drills are more rigid. Screw machine length work well if the material is no more than two diameters or three diameters thick.

                        Barry Milton
                        Barry Milton


                        • #13
                          Nothing wrong with the spring punches.... put a light mark in, and check location. If wrong, you can "draw" it over a bit. Then deepen if needed with a hammer and punch.

                          Best thing is to get an x-y table for the DP. Put a chiwan one on your chiwan drill.

                          Then you clamp down and use the cranks to get it under the drill precisely. Check teh drill as you drop the quill, if it moves you are still off, move to bring the point back straight. When it drops right onto the mark, go.

                          Spot it, and check spot location, then drill if OK. If not, you can draw it over any of several ways.

                          Don't try to fit your 1" drill into that punch mark, use a pilot drill of small size.

                          Another issue...

                          Many chiwan drills have ONE bearing, just above the chuck. The top of the splined spindle is held only by the splined drive pulley, and it is loose. That means the drill will wander all over because it has no support to keep it in line.

                          Make a bushing that holds the spline centered in the splines of the drive pulley, and you will be surprised at how much better it is.

                          Keep eye on ball.
                          Hashim Khan


                          • #14
                            Many ways to skin the cat but here’s what works for me (I think its pretty textbook) …first off, start with a prick punch then a centre punch. the prick punch is basically the same tool but with a more acute angle. with the prick punch, you should be able to pick up the intersection of the scribed lines by feel, when there, give it tap. you could, if accuracy demands or you are unsure, inspect with magnifying glass, shift the mark over by lightly tapping at an angle to where you want. when its where you want it, deepen and spread the indentation with a centre punch.

                            For a higher class of work, us an optical centre – these are neat and should get you to within a couple of thou at most. Again, follow-up with the centre punch

                            Before you use a twist drill, you need to start the hole with something stiffer, like the centre drill. Actually, the right tool is a spotting drill – stubby like the centre drill but creates a conical cut with the same angle as a twist drill so you don’t get the drill bighting in like it can on a centre drilled hole. Don’t clamp the vice to the table for this! It has to float to find the centre of the punch mark.

                            If safety permits, don’t clamp the drill press vice to the table for the balance , let it float. I keep a hefty piece of angle clamped to the table in case something gets away – the work doesn’t touch it but it’s a short distance away in case something catches (keep fingers out of the gap!)

                            Hope that helps
                            in Toronto Ontario - where are you?


                            • #15
                              I was just over at the main shop.
                              One good friend and machinist has a problem with flatness on a milled out gusset.

                              He has milled away 90% of the material to make these parts.
                              Parts are out of flat up to .020"
                              The print allows .005.

                              I told him you have to clamp it in the "Free Condition" which means run an indicator ovet the surface with the part loose, mark up the high spots, and how much.

                              Then you have to shim it while clamping to achieve the same readings clamped.

                              You pull it down, and cut it, guess what?
                              When you release it it's gonna spring up the way it was.

                              Yes, you have to test every part this way!

                              You guys have to learn the steps necessary to do the job and not cut corners.

                              This layout, punch method will not RELIABLY produce accurate work.

                              Evan is the exception here.

                              That method was used when holes were designed 1/16" to 1/32" diameter oversize for clearance.

                              You line it up perfect, put pressure on the spindle, and the spindle will push out, in, or the direction of least resistance.

                              The feed pinion is in the back of the spindle so I guess the drill is gonna push out to the front.

                              The alignment is lost.

                              From the Bull of the Woods.