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how to drill correctly with a center drill?

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  • #31
    Dont forget that as well as being misalighed horizontally, the tailstock can be misaligned vertically as well, usually due toi wear. This can break centredrills just as easily. if the pilot is only 1mm or less, what speed are you usiing? A drill that small needs to be pretty fast. Running too slowly is another common cause of small drill breakage.
    'It may not always be the best policy to do what is best technically, but those responsible for policy can never form a right judgement without knowledge of what is right technically' - 'Dutch' Kindelberger

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    • #32
      A quick test would be to put centres in the headstock (or chuck a short piece of round bar and machine a 60 deg point on it) and tailstock, bring them together with a piece of shim steel between them. Any misalignment will be very obvious.

      If it's horizontal misalignment, it can be taken out if the tailstock is offsettable. I'd try to get that right first. Then the harder one to fix is vertical misalignment. Various ways to fix this:

      - Shims between the 2 parts of the tailstock (assuming the tailstock is low)
      - Line boring the tailstock, new oversized barrel
      - Offcentre boring the taper in the tailstock barrel (never seen this done, but it should work)

      Alternatively, just learn to live with it and work within the accuracy of the lathe as it is - this is what I did for years.

      Ian
      All of the gear, no idea...

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      • #33
        [QUOTE=Richard P Wilson;n1941964]if the pilot is only 1mm or less, what speed are you usiing?QUOTE]

        This is exactly what, yes, the pilot is 1mm or less. but I learned not to break them by relaxing the quill in the tailstock. I was wondering if this method of drilling with self-aligning the drill in the center affects the accuracy of the center hole.

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        • #34
          Originally posted by Ian B View Post
          A quick test would be to put centres in the headstock (or chuck a short piece of round bar and machine a 60 deg point on it) and tailstock, bring them together with a piece of shim steel between them. Any misalignment will be very obvious.

          If it's horizontal misalignment, it can be taken out if the tailstock is offsettable. I'd try to get that right first. Then the harder one to fix is vertical misalignment. Various ways to fix this:

          - Shims between the 2 parts of the tailstock (assuming the tailstock is low)
          - Line boring the tailstock, new oversized barrel
          - Offcentre boring the taper in the tailstock barrel (never seen this done, but it should work)

          Alternatively, just learn to live with it and work within the accuracy of the lathe as it is - this is what I did for years.

          Ian
          I understood everything, thanks

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          • #35
            I am still learning, so I just tried Ian's suggested "quick test" with two centers, on my little PM 10x22 lathe. It is almost a new lathe with only a couple hours on it so far. The two points come exactly point to point and with a little light, my magnifier, and looking from different angles, it would be very apparent had there been any misalignment. Now, I don't know if my chuck, when remounted, would bring any error to the result. But then, I have not yet broken a center drill using the light pecking method to start.

            I am very grateful to all who post all kinds of ideas like this. It helps me understand more machining concepts, and also confirms my understanding of some. Thanks guys!
            S E Michigan

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            • #36
              Originally posted by pensioner View Post

              I understood everything, thanks
              Looks like you're getting good advice on things to look for and ways to correct issues with the broken drills.

              I'm wondering what spindle speeds have you been using for these drilling operations. I've found that running a drill too slow is the largest contributor to drill breakage.

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              • #37
                Originally posted by tom_d View Post

                Looks like you're getting good advice on things to look for and ways to correct issues with the broken drills.

                I'm wondering what spindle speeds have you been using for these drilling operations. I've found that running a drill too slow is the largest contributor to drill breakage.
                I usually have a spindle speed of about 1000 rpm when centering, sometimes less or more, it depends on the material being processed. The spindle of the machine is on bronze bushings, and the center drills are from HSS.

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                • #38
                  Originally posted by pensioner View Post

                  I usually have a spindle speed of about 1000 rpm when centering, sometimes less or more, it depends on the material being processed. The spindle of the machine is on bronze bushings, and the center drills are from HSS.
                  I would be running faster than that. While it would be better to up the rpm, I don't think lack of sufficient cutting speed is the primary culprit. If you are not feeding the drill too heavily then 1000 rpm should work. If all other conditions mentioned in previous posts have been addressed then it could just be a poor quality center drill causing the excess breakage.

                  When determining the cutting speed there are many variables from job to job that will influence things, but it is possible to come up with some approximations that can be used as a starting point.
                  Cutting speed rate is commonly expressed in terms of Feet per Minute, of FPM. When High Speed Steel, or HSS, is used as the cutting tool, the following approximations can be used:

                  for hard to cut tool steel alloys - 50FPM
                  for higher carbon and stainless steel alloys - 75FPM
                  for low carbon and free machining steels - 100FPM
                  for brass and softer bronze materials 150 FPM
                  for aluminum alloys, cast of or wrought - 200FPM

                  Note that these are approximations only, and are used as a starting point. To convert these numbers into a spindle RPM use the formula 4 x CS / Dia. where CS is the Cutting Speed in Feet per Minute and Dia is either the diameter of the work if on a lathe, or the diameter of the cutter if on a mill. As an example: half inch end mill cutting a low carbon steel would be 4 x 100 / 0.5 = 800RPM. Turning a 3 inch diameter piece of aluminum on the lathe would be 4 x 200 / 3.0 = 266RPM. When using a drill divide the speed by 2, and divide by 4 when power reaming. These numbers are for HSS cutters. If using carbide multiply the speeds by 3. Again, these are meant to be used as a starting point.
                  Last edited by tom_d; 05-09-2021, 12:42 AM.

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                  • #39
                    Originally posted by tom_d View Post

                    I would be running faster than that. While it would be better to up the rpm, I don't think lack of sufficient cutting speed is the primary culprit. If you are not feeding the drill too heavily then 1000 rpm should work. If all other conditions mentioned in previous posts have been addressed then it could just be a poor quality center drill causing the excess breakage.

                    When determining the cutting speed there are many variables from job to job that will influence things, but it is possible to come up with some approximations that can be used as a starting point.
                    Cutting speed rate is commonly expressed in terms of Feet per Minute, of FPM. When High Speed Steel, or HSS, is used as the cutting tool, the following approximations can be used:

                    for hard to cut tool steel alloys - 50FPM
                    for higher carbon and stainless steel alloys - 75FPM
                    for low carbon and free machining steels - 100FPM
                    for brass and softer bronze materials 150 FPM
                    for aluminum alloys, cast of or wrought - 200FPM

                    Note that these are approximations only, and are used as a starting point. To convert these numbers into a spindle RPM use the formula 4 x CS / Dia. where CS is the Cutting Speed in Feet per Minute and Dia is either the diameter of the work if on a lathe, or the diameter of the cutter if on a mill. As an example: half inch end mill cutting a low carbon steel would be 4 x 100 / 0.5 = 800RPM. Turning a 3 inch diameter piece of aluminum on the lathe would be 4 x 200 / 3.0 = 266RPM. When using a drill divide the speed by 2, and divide by 4 when power reaming. These numbers are for HSS cutters. If using carbide multiply the speeds by 3. Again, these are meant to be used as a starting point.
                    Thank you very much, this information is useful to me and I will remember this in my work.
                    I'm not a professional, this is the reason for my mistakes😃

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                    • #40
                      Great thread !
                      I use a loose quill lock for center drilling, since my quill nods up a bit when tightening.
                      12x industrial, work in tool steels, work involves high precision in low volume aka prototypes only.

                      Mostly use medium/large size center drills, these never break.
                      The smallest/r ones have broken, maybe 4 in 2000 hours of use and 200 center drilled holes, as a guesstimate.

                      Holes tend to be 5 mm - 12 mm, upto 12 mm drilled and 50 mm bored.

                      Drills are not initially on center because the quill is not perfect, and neither is the (good industrial Rohm) drill chuck.
                      But they follow the center drilled hole and make excellent fairly on-size holes, in tool steel.
                      To about 0.02 - 0.04 mm in D, and pretty good in cylindricity.
                      Very good in straightness.

                      A 6 mm hole drilled 120 mm deep, from 2 ends ..
                      304 SS, about 16 mm D,
                      the 2 ends when drilled 6 mm D had perfectly concentric bores, and the resulting bore in the tube looked perfect.
                      As gun-stuff guys know, any error in a small-long bore will be very visible against a light.

                      For holes smaller than 2mm, or 3mm, that need to be true and on center I would use mounted tools on the cross-slide, gang tools in my case, or toolpost mounted tools in a collet.

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                      • #41
                        A thing not yet mentioned, but possibly relevant is that centre drills are a combination pilot and 60 degree drill, effectively replacing 2 operations - pilot and centre with a single one. They were invented to improve production efficiency.
                        Unless you are going to use the centre for tailstock support it’s usually better to use a spotting drill for the start of a drilled hole.
                        The pilot on a centre drill can also be shortened a fair amount and not effect it’s usefulness. The pilot hole is to make sure the centre is running on the cone not the tip, and to provide a small lubricant reservoir.
                        If you are using a live centre in the tailstock then there is no need for the reservoir, only to clear the point, and if a dead centre then most drills produce a pilot which is larger than required. Shortening the pilot reduces the chance of breaking it.

                        Dave
                        Just south of Sudspumpwater UK

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                        • #42
                          Reminds me of an interesting article I saved on center drills. Not truly germane to the OP, but related:
                          Centerdrills Don't Center Drills - Neill-LaVielle

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                          • #43
                            I am sorry, but I have to disagree with misalignment being the primary cause of center drill breakage. I am sure it can be if the misalignment is severe, but how can that happen? I see two ways: the spindle and the chuck. If the spindle is loose, has a lot of side play, then the drill bit can center it as the drilling starts and the bit will not break. It would be very rare to find a spindle, either BB or journal bearing type, that would have a pronounced amount of run-out without there being any side play so run-out can be discounted. Yes, I know there will be a ton of examples of that but I will stick to my statement that it is rare. And if you do have that, then for Pete's sake, FIX IT!

                            The other way is for the chuck to have a lot of run-out: that will cause the tip of the bit to wobble and perhaps break. But that can be easily fixed with another, decent quality chuck. I have two chucks for my larger drill press, two for my mill, at least three for my SB, and two more for my Unimat. None of them are so bad that they will break even the smallest of center drills. When a drill chuck is or gets that bad, I stop using it on any machines and keep it in a drawer for manual uses, like deburring.

                            So, I said it before and I will say it again, the primary reason for center drill breakage is that those really small flutes on the reduced diameter tip are just that, REALLY SMALL. So they quickly fill up with chips and that causes friction with the walls of the hole and heat. Too much of that and the tip, which is the weakest part, breaks.

                            Use a pecking technique to allow the chips to fall or be brushed out of the flutes. Clean both flutes with a small brush on each retraction. Use a good cutting OIL with good lubrication qualities to lessen that friction. Do those two things and you will have very long life in your center drill bits. Oh, and of course, keep your center bits DEAD SHARP. For the smaller sizes, one or two strokes at the correct angle, on a fine stone will ensure that.

                            Tail stock misalignment? Well, if you have that, then ALIGN IT. Align it to the point of a center that is held in the chuck you will be using for your drilling. There is always a way to do this. And YES, do get some spotting bits for starting ordinary holes. Or make them from used bits: I have. Use the center bits only for creating holes for centers.



                            Originally posted by Ian B View Post
                            As someone coming from a series of old, worn out hobbyist lathes (Zyto, Box-Ford 3 1/4", Myford ML10 etc):

                            It's a combination of what size centre drill you're using, and wear on the tailstock & ways. Some of the model making centre drills have a body diameter of 3/16", meaning that the tip is less than 1/16". The reason that you don't break a normal 1/16" drill but do break the tip on a 3/16" centre drill is that the normal drill is more flexible, and absorbs the misalignment. It's also why normal drils make lousy centre drills - they run out on contact with the work.

                            Does the centre drill find the centre? Measure it and find out. Make a hole with a centre drill. With the drill in the hole it made, put a dial gauge on the body of the centre drill and apply a little feed, check for runout. Highly unlikely there'll be any.

                            Do what you can to improve concentricity of the headstock spindle and tailstock quill, and use the largest centre drill that you can for the application.

                            If you're using a center drill prior to driling larger holes, stop doing this, buy a spotting drill and use that. Problem solved.

                            Ian
                            Paul A.
                            SE Texas

                            And if you look REAL close at an analog signal,
                            You will find that it has discrete steps.

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                            • #44
                              Originally posted by greystone View Post
                              Great thread !
                              I use a loose quill lock for center drilling, since my quill nods up a bit when tightening.
                              12x industrial, work in tool steels, work involves high precision in low volume aka prototypes only.

                              Mostly use medium/large size center drills, these never break.
                              The smallest/r ones have broken, maybe 4 in 2000 hours of use and 200 center drilled holes, as a guesstimate.

                              Holes tend to be 5 mm - 12 mm, upto 12 mm drilled and 50 mm bored.

                              Drills are not initially on center because the quill is not perfect, and neither is the (good industrial Rohm) drill chuck.
                              But they follow the center drilled hole and make excellent fairly on-size holes, in tool steel.
                              To about 0.02 - 0.04 mm in D, and pretty good in cylindricity.
                              Very good in straightness.

                              A 6 mm hole drilled 120 mm deep, from 2 ends ..
                              304 SS, about 16 mm D,
                              the 2 ends when drilled 6 mm D had perfectly concentric bores, and the resulting bore in the tube looked perfect.
                              As gun-stuff guys know, any error in a small-long bore will be very visible against a light.

                              For holes smaller than 2mm, or 3mm, that need to be true and on center I would use mounted tools on the cross-slide, gang tools in my case, or toolpost mounted tools in a collet.
                              yes, my "good industrial röhm" (spiro series) has a runout of 0.08 mm. (and thats after i returned the one with 0.10 mm.) it lives on the shelf.

                              and if using a center drill for spotting, its a good idea to only make a dimple with the 118° tip.

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