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Thread: accuracy drilling small holes

  1. #11
    Join Date
    Jan 2007



    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.

  2. #12
    Join Date
    Jan 2014
    Edmonton Alberta


    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.

  3. #13
    Join Date
    Jan 2010
    Kansas City area


    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.

  4. #14
    Join Date
    Mar 2005


    Quote 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.

  5. #15
    Join Date
    Dec 2015
    Chilliwack, BC, Canada


    Quote 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.

  6. #16
    Join Date
    May 2002
    SE Texas


    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.

    Quote Originally Posted by J Tiers View Post

    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.

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

  7. #17
    Join Date
    Jul 2017
    Buffalo NY USA


    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.

  8. #18
    Join Date
    Dec 2016
    Helsinki, Finland, Europe


    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)

  9. #19
    Join Date
    Oct 2013
    USA MD 21030


    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.

  10. #20


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


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