Centre drill sizes

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Centre drill sizes

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  • #599180
    Anthony Knights
    Participant
      @anthonyknights16741

      I am about to do some turning between centres and was curious as to the relationship between the centre hole size on the work piece and it's diameter. I have tried various searches on the internet to see if a table of dimensions exists, but without success. Of course, trawling the internet is an art, which means devising the correct search terms to enter to get a sensible result. Some people are better at this than others. Can anyone point me in the right direction?

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      #28715
      Anthony Knights
      Participant
        @anthonyknights16741
        #599181
        Emgee
        Participant
          @emgee

          **LINK**

          Emgee

          #599182
          Hopper
          Participant
            @hopper

            I don't know that there is an exact formula for what size centre drill to use for what size job. Although, I suppose some busy bee somewhere has sat down and worked it all out. Mostly it's a matter of common sense, which of course is usually far more common than sense. These days with revolving centres that don't need a large bearing suface, it's probably even less critical than ever. Whatever works will work. One thing you want to look out for is if you are building models or tools or stuff for display, then honking great centre holes left in the ends of small pins or shafts etc look a bit off. So keep them small for that kind of thing. Or turn the end off afterwards. Otherwise use a good size centre drill so there is less chance of snapping the "tit" off the end, but don't drill it all the way in on the tapered section. Halfway in or a shade more will do the job. I tend to use the centre drill with about a 5/16" or 1/4" shank for most jobs. Smaller for tiny stuff. Bigger on the industrial stuff.

            Edited By Hopper on 25/05/2022 11:07:46

            Edited By Hopper on 25/05/2022 11:08:45

            #599192
            Chris Evans 6
            Participant
              @chrisevans6

              I always shorten the "Tit" on a centre drill to lessen the chance of breakage. My understanding of sizes is Number 1 is 1/8" 2 is 3/16" 3 is 1/4" 4 is 5/16" Rarely have the need for anything bigger.

              #599193
              Nigel Graham 2
              Participant
                @nigelgraham2

                Recommended in one of my reference books, is a minimum 1/32" (0.8mm) depth of the coned part of the drill; irrespective of drill diameter.

                (I happened to find that while in this session, when replying to another question!)

                I tend to go further than that. It does no harm, except perhaps to appearance if in a visible component on a scale model.

                #599246
                not done it yet
                Participant
                  @notdoneityet

                  I would just use the largest that I consider practical. We (mostly) don’t often make really large items on our machines – if we did, we might use a fixed steady to help matters?

                  The small pilot is there to both prevent the centre bottoming in the hole and also to contain some lubricant, for when running at the tailstock end with a solid centre (which is likely regarded as better practice) unless a good quality revolving centre is available.

                  It is likely that cutter interference with the centre may be a problem, so small tool-holders/cutters may need to be selected, extra length left for turning off later (as per Hopper, above), or half-centres used. Don't need any centre holes left unless the part is expected to be reconditioned in the future.

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