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Thread: Newbie questions: How do you use a counterbore bit?

  1. #1
    Join Date
    Jun 2010
    New Hampshire, USA

    Default Newbie questions: How do you use a counterbore bit?

    I would like to drill holes for socket-head cap screws to permit the head of the screw to be recessed so that it is flush with the surface of the material that I am drilling.

    I believe that one tool designed for making such holes is a counterbore bit.

    For example, I'm considering getting the set of HSS counterbore bits shown at

    Although I have a general idea of their proper function, I'm not completely certain how to use them.

    Do I drill a through clearance hole in one operation and then use the counterbore bit in a second drilling operation, using the small "pilot" end of the bit to center the bit before plunging in with the 3-flute drilling/reaming portion of the bit?

    I believe that I have seen counterbore bits that have cutting edges for the pilot section as well as for the socket-head-cutting section. The photo in the eBay listing link above is a little unclear - are the edges of the pilot section used in those bits smoothly-ground or do they have cutting edges?

    Is there an alternative to using these bits to do counterboring for socket-head cap screws? For example, do they make reamers with pilots, or are these bits essentially just reamers with pilots?

    These will work well in 6061-T6 aluminum, right? Guidance on what rpm to use with these bits in aluminum would be appreciated. Do I need to use a cutting fluid or can I get away with using them dry? Are these things foolproof, or are there tricks of the trade that will prevent me from messing things up?

    I haven't ever used a reamer before (other than one of those hand-held pointy things). I suppose that the answers to my questions are buried somewhere in my Machinery's Handbook, but I'm lazy, sorry...

    The smallest bit in this set is for a #6 screw. I'd like to find a counterbore bit for a #4 screw and also for a #2 if possible - any pointers would be appreciated (my search capabilities seem to be failing me).

    That's probably enough questions for now, thanks...

  2. #2
    Join Date
    Mar 2005
    Oroville, WA


    This operation, more than most any other, brings out the bungie nature of my small mill. I've tried using end mills to face drill, spot mill, and counterbore and each time I seem to end up with a bent or broken mill. And I use collets.

    What I do now is drill the central hole then drill part of the counterbore, then use a boring head to face the seat of the counterbore. And the reason I do this rather than use counterbore drills is I'm cheap!

    The boring head is adjustable and does a good job regardless of the size needed (admittedly I've never needed a counterbore smaller than 1/4"). And that means I don't have to spend any money on counterbors of various sizes.

    However - if I had to drill a lot of holes with counterbore, to save time I would buy the needed counterbore drill for the job. Until I get a more rigid mill and do some tests of spot drilling with mills, I'm done with that.

  3. #3
    Join Date
    Mar 2005


    Maybe this'll get me 'posted' at the boys metal working club, but more often than not I just drill the counter bore. I know its not technically the correct way, that there's reasons why a flat bottom hole works better, but its not often its critical. when it does matters use a counter bore after dilling the proper hole for the pilot. Use at your lowest speed - its a big wide cut. Squirt of coolant wouldn't hurt either. Make them from drill rod if you don't want to buy them but it would only take a couple to cover most common sizes/ needs

  4. #4
    Join Date
    Mar 2008
    Barrie, Ontario, Canada


    I use them all the time. First drill a clearance hole completely through the peice you are working on. (the clearance hole should match the diameter of the small end of the counterbore tool.) Without changing anything on the set-up position, remove the drill and put the counterbore tool in the chuck. Without turning the mill on, use the Z axis feed to lower the c'bore tool into the clearance hole untill the larger diameter stops against the surface of the part being c'bored. Set your Z axis feed readout to zero. Back the c'bore off a bit, turn the machine on, then advance it, untill the readout informs you that you have reached the required depth of c'bore. Remove c'bore tool, reinstall clearance drill, and then advance to next hole to be drilled and c'bored. this is kind of a slow, pain in the a$$ way to do it, but if you want you can drill all of the clearance holes first, then try and get the c'bore tool lined up with all the drilled holes "after the fact" which is even worse.---Brian
    Brian Rupnow

  5. #5
    Join Date
    Apr 2007
    South Texas

    Default Counterbore use -

    First you drill the body drill sized hole for the screw you are using - that should match the size of the pilot on the end of the counterbore. Then you feed the counterbore the depth you want and you have a counterbored hole. If you have multiple holes you can set a depth stop on the spindle to have them all come out the same depth. Should work in most materials using the same feeds and speeds as a twist drill.

  6. #6
    Join Date
    Dec 2007
    Montezuma, IA


    Most counterbores I've had to cut have been small sizes...#6, #8 and #10. I have "official" counterbores for the two smallest, bigger than that I drill with a regular drill bit, then use a drill that I've hand ground to a flat bottom profile, essentially an un-piloted counterbore that starts in the previously drilled hole. Since I'm only cleaning out the drill point angle, it works for me.

    David Kaiser
    “You can have peace. Or you can have freedom. Don't ever count on having both at once.”
    ― Robert A. Heinlein

  7. #7
    Join Date
    Feb 2010


    I don't know what the cost difference would be, but the interchangeable pilot version is more versatile as compared to the solid pilot version shown in your link.


  8. #8
    Rosco-P Guest


    Quote Originally Posted by brian Rupnow
    but if you want you can drill all of the clearance holes first, then try and get the c'bore tool lined up with all the drilled holes "after the fact" which is even worse.---Brian
    That's why counterbores have pilots. If you have the part loosely clamped in the vise, the part will self-align on the pilot, at which point, you can tighten the vise and counterbore to depth. Counterboring is routinely done on a drill press or gang drill, doesn't have to be done on a mill.

    OP might consider buying one of these books:
    Can't beat $5.07 for a $50 college level machine shop text. Much easier to have a reference to tools, machines and techniques at hand, instead of having to rely on getting a speedy answer from a BBS, every time a question comes to mind.

  9. #9
    Join Date
    Nov 2006
    Newtown, CT


    For occasional use I make counter bores from drill rod. They work well and are easy to re-sharpen if you make the guide pin an easy press fit.

    Directions are here:

    Go to "Articles" in his menu, then get the pdf on "Pilot Cutter".

    Use as you describe, where the initial hole should clear the pilot pin size. Use WD40 or similar for aluminum.


  10. #10
    Join Date
    Apr 2001


    The few times I've needed a counterbore, I've used an appropriate-diameter end mill, or made the counterbore from drill rod. I think I do have a couple of "real" counterbores, somewhere. If you use an end mill, it may have to be an odd size (something-32nds or something-64ths), but I figure an end mill, even an uncommon size, is more generally useful than a single-purpose counterbore.

    If your counterbore has a pilot, allow a few thou at least for clearance so there is no danger of the pilot seizing in the hole.
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