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? 4 "tween centers" turners

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  • ? 4 "tween centers" turners

    What is the rule of thumb for choosing the size of a center drill?

  • #2
    One British publication advises that you use the smallest sized hole that will "hold" the centers. He says that over-sized center holes on finished work looks tacky, or words of similar import.


    • #3
      Seems to me the typical center drill is 1/8, followed by a 1/4 in countersink. Purpose made, hss, some are double ended. A lot must depend on the shaft size, obviously a small diameter shaft won't have the meat to allow this, and a larger shaft will need better support. Look at the end of some motor shafts to get a better idea.
      I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


      • #4
        Current textbk I am using (Krar, 5th ED)
        gives following guidelines in table 42.1:
        CD 1 work upto 5/16"
        CD 2 " 0.5"
        CD 3 " 0.75"
        CD 4 " 1-1.5"
        CD 5 " 2-3"
        CD 6 " 3-4"
        CD 7 " 4-5"
        CD 8 " > 6"
        What CD 0 and 00 are for, is not mentioned.
        Center and spot drilling only I presume.


        • #5
          I find myself using a BS4 centredrill most of the time (Shank 5/16", drill 1/8") for the simple reason it is robust enough to take fairly aggressive drilling (in mild steel) and not break. Once you go smaller than this, you better take care. This assumes your centredrill is sharp too, tired old centredrills are a menace.

          If I was turning something smaller than say 1/2", then I would go smaller, especially if the "looks" mattered.

          Edit note, I should have taken more notice of your question, I am mainly going on about when using No.4 centre drill prior to drilling, you are talking about centre drilling prior to turning between centres, so disregard the above...

          [This message has been edited by Peter S (edited 12-02-2003).]


          • #6
            Thanks, sch. That's the kind of info I was looking for. [Ok, "for which I was looking" ;-)]



            • #7
              Not exactly on topic but close: I recently made an extension chuck for a Dremel tool. It needed a hole with a tapered entrance for the Dremel collet and I was going to grind a drill bit to an approximation of the angle when I spied the center drill set sitting there. The # 4 had a central drill that was smaller than the hole I needed and the OD was sufficient for the taper. The angle looked perfect (and I didn't risk destroying my delight by measuring). Center drill, then drill for the body of the collet and presto, a "perfect" fit. Well, I didn't check too closely but it worked A OK. Made the estension from a piece of hardware store CRS in the 3/8" collet and it ran dead true at top speed in the Dremel. Used it to grind the jaws on an old 3 jaw.

              What size center drill should you use? First, the central hole should be small enough to allow any further drilling that may be needed. This may be a consideration for the size of the taper also. Remember, you don't have to go all the way in with the center drill. On very small stock I often only drill to 50% of it's OD. Second, the 60* taper in the work should be at least 1/4" at it's OD if the work allows this (normally not more than 90% of the OD of the workpiece). Third, I would go for about 20% to 25% of the OD of the stock or mandrel you are working on if other factors allow this. But these are just rules of thumb, many workpieces will require differences. Heavy cuts or tough material may require a larger supporting taper. Oh, and how big is your center? I would almost never exceed the OD of the center itself - but if you really had to....

              In high school I was in a drill team and one of our leader's favorite commands was, "Make it Fit!" I think that applies to many situations in life. Make it fit. Make it work.

              Paul A.
              Paul A.
              SE Texas

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