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accuracy drilling small holes

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  • accuracy drilling small holes

    I have brought this subject up before with out any final conclusions but I have dug the hole deeper.

    When I drill small holes, under 1/16" or so, the holes NEVER end up where I want them IE: in the center between to edges etc., so I did some additional testing. If I need a 1/32" hole right in the middle of a square or round bar they inevitably come out 5 to 10 thou off center now if I use an end mill to drill the hole they will be within 1 to 2 thou of the center. I consider that acceptable with my cheap Chinese mill, cheap Chinese collets and cheap "unknown" end mills all of which could be considered to be of questionable accuracy (maybe better then I thought).

    So it would seen that the problem is using drill bits. I always use a spot drill or a center drill (for small holes) thinking that it would place the holes where I want them to be. Apparently not so. Some times the center drill/spot drill holes have a pip in the middle but not always. The question is how do you guys get holes to come out on or near the place you want or in other words how do I get the damn drill bits to go straight.

    P.S. I always peck drill, which is suppose to help, the holes to stop the bits from bending instead of driving them down with all my strength.
    The shortest distance between two points is a circle of infinite diameter.

    Bluewater Model Engineering Society at

    Southwestern Ontario. Canada

  • #2
    Chuck run out maybe? Stu


    • #3
      Holes from a drill can be accurately placed, accurately sized and accurately round. Just seldom all 3 at the same time.

      Drilling removes material quickly. Boring locates a hole well. Reaming tends to give you a hole to the desired size and roundness. If you need holes on position, straight and on size you really have to drill to remove most of the material, bore to locate and straighten the hole and ream to take it to size.

      If you only need to get close enough you can spot drill to start the drill. This improves the starting position quite a bit. If you don't need to be that close you can start with a split point drill, they tend to not wander as much as non-split points. If you're starting on material at an angle to the drill point (or round stock even if you're sure you're on center) it helps a lot to put a flat with a center-cutting endmill before spotting.


      • #4
        If there is a pip in the middle of your center/spot drill marks then you have some significant run out and/or looseness in the spindle/quill somewhere. This assumes your center/spot drills are properly sharpened.

        1973 SB 10K .
        BenchMaster mill.


        • #5
          Spindle run out.
          Chuck run out.
          Small drill was never straight to begin with.


          • #6
            Originally posted by 10KPete View Post
            If there is a pip in the middle of your center/spot drill marks then you have some significant run out and/or looseness in the spindle/quill somewhere. This assumes your center/spot drills are properly sharpened.


            Most larger drills make a significant flat spot in the center, where the drill fails to cut, and only rubs off material, due to the way the drill "point" is made. Even spade drills can be like that. You need a spot drill smaller than the small drill. A center punch mark "usually" works pretty well for small drills

            A small drill will skate around on that flat spot and get several thou off at least. Spot drilling works best when the spot drill is actually smaller than the follow drill.

            Keep eye on ball.
            Hashim Khan


            • #7
              Center punch!


              • #8
                Are you using jobber drills? if so then try precision drills that are much shorter and don't wonder as much, the quality of the drill is very important too...


                • #9
                  First, tram the head to the table as exact as you can make it. Use a collet for the spot drill instead of a chuck. Use a quality small chuck with low runout to hold the drill. Use a good quality sharp bit and good cutting fluid. Use a light touch and let the drill do the work. If you push too hard, it will go off center.

                  Peck drilling won't make the drill go any straighter,but you have to do it often to clear the chips. Otherwise, the chips will lock up the drill bit and break it off in the hole.

                  I drill #57 (.043) bits 3/4" through steel by the hundreds and have them come out the other side on location doing it this way.


                  • #10
                    I think you're not getting a proper cone for the drill to start in. It could be various reasons as noted above. We have to assume a proper drill grind, and, once a drill is mounted, run out of the chuck doesn't matter (it may however matter when using the spot drill).

                    Of course if you are boring afterward, its not an issue...but that's a little challenging if you need a 1/32 hole.

                    Two practices in watch making worth noting. They might be useful techniques but they underscore the importance of a cone in the right spot for the drill to start. With lathe work, it is common to make the starting cone by hand with a graver (but its not easy to consistently avoiding getting a pip!). Drilling is done by hand with a pin vise. Point being with a single point tool (graver) you're guaranteed to get the cone in the right spot, and with the start in the right spot, exactly where the drill is held is minor....and they don't rely on the tailstock to get the starting position. Secondly, on a watch makers jig borer, a very accurate centre punch mark is made and that is used to start the drill. We're talking machines who's dials are graduated in tenths so its done to a very high level of accuracy. all the punch marks would be made with a special quill and then each would be align via a microscope for drilling/boring.

                    That's a couple of ways small holes (sometimes a few thou) get located properly by those who must get them accurately placed. The takeaway is what matters is having a cone of the right shape in the right spot. I'd start by looking at the cone you're getting with a loupe or microscope and go from there.
                    Last edited by Mcgyver; 02-09-2019, 12:23 PM.


                    • #11

                      First, the old "drill, bore, and ream, mantra does not work for tiny little holes. The biggest issue when drilling tiny holes is lack of enough spindle speed I have found. That and wanting to drill 3" deep.

                      Few drill presses and mills can provide the proper speeds needed to consistently drill a straight hole under 1/8". You need 5000+rpms to really have a good shot at it.

                      So what can we try since few of us here don't have a 10,000rpm spindle handy.

                      If you need to drill a perfect #60 hole 2" deep in stainless, you need to rethink just what it is you are doing. Ain't going to happen in your average home shop.

                      Consider using a stiffer tool. When the holes get below 1/4" and the roundness, location, and straightness matter, I toss the HHS and use solid carbide drills. Expensive, but when it matters, good results costs money. I've had very good luck with solid carbide spade drills if the holes aren't stupidly deep. Though fluted carbide drills are definitely better than HSS.

                      If the hole isn't seriously deep, using an endmill can also work if you can get a pilot hole in the piece. Again, endmills tend to be stiffer than drills.

                      If you are really up the creek, then your best option might to plunge edm.
                      If you think you understand what is going on, you haven't been paying attention.


                      • #12
                        I assume the pieces your drilling are small,this drill jig is pretty accurate.The smallest I've drilled is 1/16" and seems to keep drill in place.The smallest collet I have 1/32" but have not used it yet,my drill press only goes to 2800 rpm.


                        • #13
                          The small drills will still cut at any rpm, you just have to feed accordingly. I have drilled small holes just fine at 500 rpm. The recommended rpm is for a production setting where you need to make the best time.


                          • #14
                            Originally posted by Toolguy View Post
                            The small drills will still cut at any rpm, you just have to feed accordingly. I have drilled small holes just fine at 500 rpm. The recommended rpm is for a production setting where you need to make the best time.

                            A drill speeder or Servo type drill press is very nice to use, but not a necessity and has little to do with accuracy.


                            • #15
                              Originally posted by Forestgnome View Post
                              Center punch!
                              To my thinking for doing small holes this is the winner.

                              Small drills flex. So the tip will try to wander. As mentioned any sort of spot or center drill must have a web in the middle. And that web is going to leave a flat or pipped up center. So spot drilling isn't helping.

                              The flexibility of the drills removes chuck runout as a factor in actually centering the holes. It may cause a slight bell mouthing if going deep but it's not a factor in centering provided the drill starts in a true feature such as a clean conical center punch mark. If trying to start without a good conical center mark to hold the drill then any chuck runout will likely contribute to the end walking off the intended drill point.

                              Tram of the head won't do anything for us either. Other than if trying to start without a conical center mark that it would contribute to the drill point walking.

                              So why is a conical center punch mark the way to go in this case? Consider that on larger sizes the key to a pilot hole or spot drill starting hole is that it takes the center web edge out of the picture for the first part of the hole. So the drill doesn't try to walk around. Instead we get an instant bite out on the cutting lips which self guide towards center assuming a symmetrical tip.

                              But drills of about 1/16 and under are so small that the flat center of a spot drill mark is going to be a lot larger than the center web of the small drill. So it's like we start the drill on the flat surface anyway.

                              So we get back to the idea that for small drills of about 1/16" and under that a good well shaped pure conical center punch mark is one way of producing a good starting spot for such sizes.

                              For accuracy perhaps the trick would be an optical center punch setup. The ones where you set a carrier and use the optical insert to put the cross hairs right on the intended point then switch the optical viewer for the matching center punch and give'er a whack.

                              Alternately a fine tapered prick punch first to set the initial mark dead on the scribe lines intersection then follow up with a broader angle well shaped center punch. Then drill.

                              There may be something to be said for stoning away the surrounding eruption of displaced metal too. If the metal does not rise up evenly the mark might be close enough but it's the unevenly short wall on the one side that is chewed away first then lets the drill slide over and get started. So a fine file or medium stone to bring the center punch mark down flat to the surface would likely be a big help. Especially when the drill bit is at all larger than the diameter of the punch mark.
                              Chilliwack BC, Canada