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accuracy drilling small holes

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  • #16
    There is a lot of wisdom here. Virtually all drill bits will make a TRUNCATED cone, not a complete cone. The bulk of the starting dimple will be a conic section. But at the very center there will be a flat; not a dimple, but a flat. This flat is made by the wedge at the tip of the drill bit. Now, if your "small" drill bit has a larger wedge, then it will make contact with the cutting edges and not that larger wedge and it will follow the cone made by the spotting bit. BUT, if the "small" drill bit has a smaller wedge then that wedge will make the first contact with the larger flat made by the spotting bit and it will be free to wander across it. Yes, when the cutting edges make contact with the original cone, they will tend to center the small bit in the dimple but this may or may not completely correct any drift that came about when the wedge went off center.

    The above assumes that the the angles of the cones that the two drills produce are the same. This is not always the case. Drill bits come with factory grinds that often produce cone angles of 60, 90, 118, and 135 degrees. Other angles are also possible. And I have never seen any published specification that includes a tolerance on these angles. So a drill bit that is advertised as 118 degrees could, with a 1/2 degree error, actually be 117 or 119 degrees. A 1 degree error would produce angles between 116 and 120 degrees. How much of an effect these small errors may have is open to question. I have never tried to measure it and I know of no one who has studied it.

    There is even disagreement as to which way this tip angle problem should be resolved. Is it better for the wedge to make first contact or for some point along the cutting edges to do so. There are good arguments on both sides. My preference is for the wedge tip. So I like the following sequence:

    Spotting drill bit with smaller wedge AND LARGER point angle

    followed by

    Larger drill bit with larger wedge AND A SMALLER point angle

    And in order to insure that order with factory sharpened drill bits, you would need to start your hole with a 135 degree spotting bit and follow it with a 118 degree one. I do not know of anyone who does it that way. Most spotting bits have 90 or 118 degree cone angle.

    Here is how I usually work to accurately locate small holes:

    1. Mark up the part with cross hairs using the surface plate and height gauge. This is easy.

    2. Using a punch with a narrow angle at the tip AND a good magnifier I prick the intersection of the cross hairs as accurately as I can. I check it and make any adjustments needed.

    3. Using a small punch with a broader tip angle (30 degrees?) and the good magnifier I enlarge that original prick mark. Again I check it and make adjustments as needed with sideways strikes to move it.

    4. Finally I use a more standard punch with a broad tip angle I enlarge the punch mark a second time. At this point the magnifier is not really needed for making the punch, but is used afterwards to check it.

    If you use at least a 10X magnifier, with care this can consistently get you within a thousandth or two.

    5. A spotting drill that is significantly SMALLER than the final hole size is used to start the hole. I make a MINIMUM sized dimple with it. I would never go to it's full diameter so there will not be any corner edge to the dimple. Avoiding this corner is an important part of this procedure because once you have that corner edge, it will completely take over the job of guiding the next drill bit and all the above is lost. The smaller size of this spotting drill is to ensure a smaller wedge at ti's tip.

    6. Finally I would proceed to the final size drill bit. It should be guided by it's wedge to the center of the cone made above.

    If all is done carefully, then good positional accuracy can be achieved.

    I have purchased several spotting bits, but they do not make them in really small sizes. For that, I have carefully shortened and resharpened some small diameter, standard drill bits. Also center drills have that small, reduced diameter, center section that can be very useful if you don't drill past it to produce a sharp edge or the cone that was intended.



    Originally posted by J Tiers View Post
    maybe.....

    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.
    Paul A.
    SE Texas

    Make it fit.
    You can't win and there IS a penalty for trying!

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    • #17
      Would like to add, that watchmakers and clock makers pivot drills are considerable shorter and stiffer than the average twist drill. They are basically spade drills ground to a point, with the shank same diameter as the hole. Using them you have to make absolutely sure that there is no runout in your chuck etc. Jewelry suppliers carry them, relatively cheap.

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      • #18
        Use collets and tungsten carbide PCB drills. PCB drills are reasonably cheap and sharpened way better than any HSS drill you ever find.
        Flat surface, no scribing, no centerpunch marks (even tho carbide PCB drills usually just chew the centerpunch mark away if its 20% off)

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        • #19
          One thing that has not been mentioned is that a center punch may cause work hardening on some materials, which may cause the drill to chatter and cut unevenly. Stoning the raised portion flat to the surface may help, and using a cobalt or carbide bit may work better. An end mill might drill a more accurate size hole, and perhaps a ball end or chamfering type might center itself on the center punch mark better than a flat end mill.
          http://pauleschoen.com/pix/PM08_P76_P54.png
          Paul , P S Technology, Inc. and MrTibbs
          USA Maryland 21030

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          • #20
            Here are two threads that have lots of information on this topic

            http://bbs.homeshopmachinist.net/thr...ighlight=small

            http://bbs.homeshopmachinist.net/thr...ighlight=small


            bob

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            • #21
              I think BC rider and Paul Al have shown what my problem is but the solution is a bit problematic. In the case of center punching many of these parts a small and round so it would be difficult if not impossible to do this, that is why I have used the coordinate method. The info on why a spot/center drill dosen't work some times is helpful. Maybe I will just have to start the holes with an end mill (they seem to locate properly) and finish with the drill, at least for those that are to deep for an end mill. Thanks.
              The shortest distance between two points is a circle of infinite diameter.

              Bluewater Model Engineering Society at https://sites.google.com/site/bluewatermes/

              Southwestern Ontario. Canada

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              • #22
                I find a ball nose end mill works very well as a spot drill. I have ball nose end mills down to 1/32". They are available in sizes smaller than that.
                Last edited by Toolguy; 02-10-2019, 10:08 AM.

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                • #23
                  As several of us mentioned, center punch....

                  Center punch makes a cone of a good size for the follow drill, if the drill is small. Stoning off the raised area is actually a decent idea for small drills, so they do not "catch in" the raised stuff and either get pulled off center, or break (if quite small).

                  BUT, there are center punches and then there are center punches. Some of them still are not pointy, and make a mark that is not a good cone. Plus, punches wear like other tools. I like to have a standard punch, and also one sharpened to a steeper and sharper point, often called in the UK, a "prick punch", because it makes a smaller mark, not like a big and somewhat clumsy punch can. The thinner sharper punch can find the intersection of scribed lines... that is how sharp I mean.

                  Now, I do not know why you think a punch is not possible to use. Maybe you do not want to ht the part with a big punch, or the like. Maybe NO punch is good.

                  So, the jeweler's flat drill is one possible. Or just make your own.

                  Get a small spade drill and stone the end thinner than the tip of the drill you want to use, which would be less work than making a spade drill.

                  Now use that to do your spot drilling. If you made it small enough you will get a cone that your small drill will land in nicely.

                  The good part about a spot drill that works, iis that you are going to be on-center with the follow drill if you change out the spot drill and put in the follow drill without moving the work.
                  Last edited by J Tiers; 02-09-2019, 08:07 PM.
                  1601

                  Keep eye on ball.
                  Hashim Khan

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                  • #24
                    If the drill press spindle upper end can be accessed, put a 60 degree sharpened center punch in and locate the hole. Then LIGHTLY tap the upper end of the spindle, making just enough of a mark to help in locating a larger center punch to deepen the mark before drilling. The second center punch could be used with a guide to keep the punch vertical.

                    If the center punch is not perpindicular to the workpiece, it will move from location as it is punched. Using a guide can help maintain location.
                    North Central Arkansas

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                    • #25
                      Their are drill bushes for this thing down to 0.2 mm,I assume their are drill bits that small.I don't think my eyes would see them.

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                      • #26
                        Using drill bushings would be awesome. But for one or two holes and not knowing the sizes needed that could end up being a LOT of bushings. And they still need to be placed in some manner of holder which is then located over the work piece. That seems like more of a production situation.

                        A lot of the suggestions seem to be aimed at far larger hole sizes than what the OP asked about in the opening post. The focus here is supposed to be small drills of around 1/16 and smaller. Things like drilling with an end mill and spotting with a ball nose mill. Great ideas for larger size options but not so great for sub 1/16".
                        Chilliwack BC, Canada

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                        • #27
                          If you're using coordinates to find the 'spot', then why not mount a center and use just a bit of downfeed to place the mark. You can get a decent sized mark in most materials with a reasonably light pressure. For a small hole like that it's likely to be enough.

                          For a 1/16 hole, I would make my own center from say 1/8 inch drill rod or music wire. Grinding the point well centered is then the challenge. This isn't that difficult though. You would probably make it about the length of the drill bit you're going to use so you won't need to change any settings- just swap out the center for the drill bit and continue on to drilling.

                          Another method which has some merit is to simply drill a hole in some scrap- perhaps 1/8 thick, making sure that when it's started, you allow the drill bit to bring the scrap piece to center before you drill all the way through. Then once you have the coordinates set for the hole location, you slip this jig up onto the drill bit, bring the bit down to barely touch, or not quite touch, then allow the scrap to settle onto the piece you want to drill. The drill bit locates the scrap piece, and you then hold it in place with your fingers while the drill bit starts its location. This has the potential to prevent wander.
                          I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                          • #28
                            Just in wood but this improvised drill guide improved things a lot where it wasn’t practical to use a short carbide bit, or spot or center punch first.

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                            • #29
                              I would go for the carbide pcb drills, the shortest available, but with a larger shank. Just start the hole with them and then change to regular drills. Spotting drills tend to make a finer starting indent than centre drills which are intended for a different purpose entirely. With the pcb drills, you may not need anything else beforehand. Check runout of the chuck, even 0.003" would be too much for the sizes of drill you are using.

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                              • #30
                                To add my 2c, from my experience in drilling small holes is slightly different than the generally held approaches. The small holes I drill are typically between 1/64 (.0156) and 1/32 (.031). Usually on small gas engines. The first thing I learned was runout is the bane of small accurate holes. For example, when drilling a .020 hole and if you have a chuck with .003 runout, you have effectively a 15% runout relative to hole size. Not going to get a round, straight hole. That is the equivalent of .075 runout on a .5 hole. The .5 bit would look like it was shaking all over the place.
                                Get rid of runout and you at least have a start. I have a special chuck that can be made to run at 0.00 runout, ie none. It is special Dumore chuck. So strive to get the runnout as good as you can. Most of the time I use a good small chuck and I am fine with the larger holes above .025.

                                Next I spot the hole with a center drill that has a point smaller than the diameter of the drill I am using. For a .020 hole I would use a 5/0 center drill with a point of .015. That way the bit is not just dropping in the hole it is hitting the tapered side. Even though the taper is not the same it seems to center very well. Most of the time I am dialing in the hole position from a reference with a DRO. If the hole has no reference edge, I have just spotted the cross marks. Usually in that kind of a setup being off by several thousands is not an issue.

                                Next is what was told to me by a production engineer many years ago. When running a small drill at high RPM’s you must have a method of controlling the feed rate based on the material being drilled. The concept is easy to understand if you have ever drilled stainless with even a 3/16 drill. If you do not feed it fast enough it work hardens the stainless and you destroy the bit. The point he made to me was I could not feed a #76 (.020) bit properly at high speed so don’t bother.

                                I drill the above size holes at about 100 – 200 rpm sometime a lot less. If you watch the cut with some degree of magnification you will see the chip forming. With practice you get a feel for the feed and you end up with real nice holes. Since I do not drill small holes enough to develop a touch, I just take my time on each occasion.

                                I use the Dumore see pictures for about half the holes I drill, the rest are done on a Clausing 8530. The special chuck works like edge finder , you use your finger nail against the bit to center the magnetic holder and its base in the drill chuck.

                                Bob



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