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center drills vs spotting drills

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  • #16
    Originally posted by mochinist
    I was really hoping tiffie would have posted some sketches, wiki links and long thought out post by now on why the spot drill is better for starting holes, but I guess he is busy.

    First off I dont say they are for lathes only, although I do say to just use them when you need a center hole for a center, which more often than not tends to be on a lathe. Using some sort of indexer and a center on a mill or grinder would be the other alternatives that come to mind.

    Like said a center drill will work fine a lot of the times, be sure to use a lot of cutting oil and choose your RPM's wisely and you should be ok. If not have fun removing the broken center drill tit from your part. I would rather just use a spot drill and have zero worry about that happening.

    Lastly I do this for a living and time is money, I have full drill indexes with screw machine length drills and I use just those when the hole isn't that deep to be drilled, no spot drill.

    As stated earlier, sooner or later the tit will crunch off and you will have to deal with it.

    Personally i don't care if someone wants to use a screw length, a centerdrill, a spotter, or something they ground up bosses never gave a crap neither, they just want the job done in a timely manner and to print.

    The folks that say it should ONLY be used to drill centers are welcome to do that. I have found them quite useful to countersink and drill thru on some of the thinner details, and not so thin too. They are available in 82 and 90 deg. incl. angles, and probably others too. I have also used them with great success to start holes, and go just deep enough to leave a chamfer when the hole is brought to I said it is a futile argument.... some love em, some hate em, and some will use them cuz they work. They are also cheaper then spotters...

    I will seriously chuckle if we do get any drawings and tiffipedia links


    • #17
      I use a procedure that has been referred to as pecking and have had no trouble with breaking the tips off center drills.



      • #18
        Good discussion. For a project a few days ago I mentioned using a drill bit in the chuck to spot a hole location, and bringing the workpiece up to it- presto, accurately spotted location.

        Now I realize I should have said spotting drill- of course I don't have one but I usually grind something from an old drill bit to do that job. I should have clarified that at the time. To just use an ordinary drill bit in that application- the bit would have walked all over the place most likely. Spotting drill is the right tool for that job- center drill is the tool to create the hole properly contoured to use on a center- makes total sense.
        I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


        • #19

          There was an excellent video (U-tube) posted here not all that long ago about a bloke who was using a lathe at a pretty good speed to drill very small holes in a brass jet for an IC engine of some sort or another.

          All that he used was a free-hand-held drill chuck/vise - with no use of a centre or spot drill and no use of the drill chuck or the tail-stock quill either.

          He just simply lined the end of the drill to the centre of the job by "eye" (ie - he just "boned it in") and used a "pecking" technique.

          The hole seemed to be dead-centre - all the way through.

          I'd appreciate it if some one who has the link to that video were to (re?)post it here.

          It is a phenomenon of drilling into the end of a spinning disk or shaft that once started, a drill seems to be pretty well self-centre-ing.

          It is for this reason that I have my tail-stock quill extended and not clamped with the centre or spotting drill also extended in the drill chuck.

          A "good" "Jacob's" or "key-less" chuck may have up 0.004" (or more) "run-out - even when new.

          Who here can really say that the end of a drill in a drill chuck in a tail-stock quill is aligned dead-centre with the job to be drilled in the lathe?

          You can?

          Then I suggest that you put that centre/spotting drill in that chuck in your mill quill (if you have a MT taper) or put it in the head-stock MT in a lathe and "spin" it or put a good indicator on it and see just how accurate it really is.

          A "bit out"?

          Does it seem to matter on a lathe?


          Why so?

          Perhaps it self-centres on a lathe?

          If that isn't the case or reason - then what is?

          Many here seem to worry (too much?) about tail-stock alignment in the horizontal plane (parallel to the lathe bed or cross-slide) but few if any get too concerned about the alignment in the vertical plane?

          If the end of a centre-drill is going to snap off because of "tail-stock misalignment" then why doesn't the misalignment in the vertical plane matter?

          It sure does - misalignment in any plane normal to the lathe spindle axis will have the same effect.

          That is why I let my quill and drill "stick out" and "float" on my lathe.

          It is not as much an issue on the mill as it is on the lathe.

          I have set of ER-16 metric collets in an MT2 collet chuck that suits my tail-stock quill - but I never use it.

          Have you noticed that this phenomenon does not apply when it is the drill that is spinning in that drill chuck in a mill spindle or in a pedestal drill? Collets do help as does minimising the protrusion ("stick-out") of the centre/spotting drill. Clamping the quill a little helps to reduce "wandering off" as well.

          I rarely snap the end of a centre drill as I keep them sharp. If I do snap the end off, I will re-grind the rest of the (usually very good HSS) centre drill into a spotting drill. A trip to the pedestal grinder or belt or disk sanders and then a steady hand and eye and a good "Dremel" and some "rake" and web-thinning soon sorts that out.

          As an aside, of all the posts on the importance of sharpening drills correctly and keeping them sharp thaty I've seen here, I rarely (ever?) see much on maintaining centre and spotting drills. They just/must be put away as soon as they come off the machine and get re-used in that progressively deteriorating condition.

          What reason or justification is there for that sort of neglect? Perhaps that is also a big reason why the ends of centre-drills snap off.

          Just another case of the cause being more due to the operator than it is to/for the tool/job and/or machine?

          "It simply can't be (me??)" you say?.

          Well, it sure can be.

          If I want a "centre" to be accurate for location purposes on a lathe - say for use on or between centres where concentricity of/to "centre" and a diameter/bearing/journal etc. is important - I will run the reference part in a fixed steady rest and then "rough" it with a centre/spot drill and then use the compound slide to bore and finish the 60 degree "centre angle".

          I rarely drill on the mill as a total millimng operation as I prefer to "mark out" and use a centre punch as I am pretty consistently accurate with that - within several (not many!!) "thous". After centre punching a centre or spotting drill is not really needed. I do all or most of that drilling on a pedestal drill. I start off with a 1/8" drill to get "spotted" (it follows the centre-punched mark very accurately) and then drill as far as I can with a 1/4" drill so as to nor have to "web thin" any larger drills any more than is necessary.

          If I must drill on the mill, I will "spot" or "centre" drill all holes with-out changing the drill/s. I will then preferably take it off the mill and finish drilling on the pedestal drill.

          The perceived or real inaccuracy of the pedestal drill is of no consequence as all drills following the "spot" will follow the "spot" automatically. They will "flex" a surprising amount as the cutting edge/end of the drill bit pivots aroound the pointg in the spot and still works well as regards drilling accuracy.

          I often mark-up and centre-punch a job and if on the mill I will just use the "pointer" ("true it up" at ~1,000 RPM with my thumb nail and my "eye") and then align it to the mark-outs. I can keep to within 2 >10 "thou" (job-dependent) for accuracy as required.

          My edge-finders (for the mill) rarely get used as I rely more on "mark-out" by my digital height guage and surface guage on my very good "float glass" "surface plate" (which sits on my mill table mostly).

          If I must use the mill and say rotary table etc. for a rectangular or circular drilling, I just "spot" the job and remove it to the mill table or the pedestal drill as I can position it pretty well and can rely on the "follow-up" drills to follow the "spotted" hole. I can't see the need for unnecessary winding of mill table and rotary table hand-wheels unless necessary.

          Its easier to move or "jog" a job on the pedestal or mill table. I usually leave the jobs un-clamped as I can hold them pretty well by hand or I just put a sheet of newspaper on the table under the jobs - helps a lot.

          If I am drilling holes along a line parallel to an edge I will use the "Wiggler" "pointer" to get the drill centre over that line then bring any sort of a relatively straight edge (hot rolled bar etc. is fine too - no need for my mill/grinder parallel strips here) and then use my super-good USA-made "Kant-Twist" clamps to hold it on. I now have the off-set fixed and all I have to do it slide the job along it and use the "Wiggler pointer" and/or my eye to "spot" the marked-out lines and centre-punched marks. If it is a circular pattern in say a flange, I will use two straight edge to form a "vee" and away I go again.

          Centre or spotting drills are optional.

          I see a lot of comment here about super/very heavy drilling in a lathe using the tail-stock quill and really hammering the tail-stock quill and its bore and the relatively light "key" which is all there is resisting the torque applied to the drill and quill by the drill in the job in the lathe. The lathe tail-stock has to be the worst designed and weakest part of the lathe and yet people hammer the $hit out of it and expect super accuracy when turning between centres on it.

          Odd that when the same people would not even think of abusing the usually better-built/designed mill quill in the same way.

          My lathe is fairly (very?) "light" and it only has an MT2 taper and no facility for engaging the "tang" on either a drill chuck or a MT-ed drill - so it is more likely to "slip" in the tail-stock taper which will not help the taper at all.

          I prefer to start the drilling in the lathe in those cases and then mount the job in a lathe 3-jawed chuck on the mill table (usually) or the pedestal drill (some times) as both have slots intheir quill spindles to engage and drive the drill/chuck "tang" where there is little if any chance of a "slip" in the MT in the quill/s.

          Its quite easy to clamp a 3-jaw chuck to the mill table, put a job in the 3-jaw chuck and them use a good indicator (a "Co-axial" indicator - "Chinese" and "Clones" are fine too - is ideal here, and align the centre of the job to the mill quill axis. After that it just a matter of putting a drill chuck in the mill quill and from there on use the lower speeds and power of the mill spindle as a drill (which any vertical mill is).

          Realistically, the best if not the only way to be sure that a hole is aligned to the axis of the lathe head-stock spindle or the mill quill spindle is to drill (which may "wander off") and then bore the hole if positional accuracy is that important.

          Boring can be used for either or both positional accuracy as well a "roundness" and /or "straightness" on the one hand and accuracy of the hole size on the other.

          A reamer can be used for hole diametrical accuracy but it will "follow" the pre-drilled or pre-bored hole for positional accuracy.

          The very best "spotting/centre-ing" drills I ever saw - and used - were set up in the capstan/turret of the capstan/turret lathe. They were great and worked just as well in the tail-stock of a normal lathe or in the quill of a vertical mill or pedestal or radial drill. Here is a pic of the general construction and principals involved - they are not hard to make:


          • #20
            That centering drill is pretty much what I grind when I need a spot. I have one which is manual, with a 'cup and bearing' on the hand end. Some pressure on that, spin the shaft with my fingers, and I get a dimple which will definitely start a drill properly. It's a lot easier to locate this thing in a center punch mark because you get to feel what's going on. That particular one is made from music wire.

            My biggest problem is getting the center punch mark right on the button. I need more light, a hands-free magnifying glass, and a steadier hand- not to mention better vision.

            I'll take a broken drill bit and cut it off leaving about 1/4 inch of flutes, then put a similar grind on it. That's what I'll be using in the lathe when I go to spot the holes in my latest jig. The flutes don't do much except help you to see to grind the flats centered.
            I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


            • #21


              It may help of you have two punches - first a "prick" punch that has a fine(r) point - say an included angle of say 60 or lesser degrees - and a "centre" punch with a larger angle - say 90+ degrees.

              They should be HSS as it keeps a good edge. An old end-milling cutter is ideal. I have some that are 1/4" square or 5/16" round HSS tool bits (not expensive) and they have lasted me for years.

              Keep them sharp on your pedestal grinder.

              Use the prick punch first and then use the marks from that to locate the centre punch.

              Its much easier if you use "Dykem" or the like to mark the job out with a good scriber to get a good sharp visible line. The "scriber" can also be hand-held along a rule or the "sharp end" of a scribing extension on a digital height guage - the method is not important but the quality of the line/s is/are.

              Have a good light illuminating where you want to punch so that it is not in shadow and that you can see it clearly.

              Lay the punch of choice away from you with the point as near as you can get onto the place/point you want to centre punch. Put the point on the mark and keep it there as you lift/raise the punch to the vertical.

              When you are satisfied, just lightly tap the top of the punch with a good light engineers ball-pein hammer or equivalent (your "call").

              Lay the punch back or lift it clear so that you can see whether you got it right or not. It is very easy to see either in a single scribed line or a line intersection.

              There are several ways to "draw the mark back" if it is out.

              You can get in line with the direction of error and either put the sharp point of the punch inside or outside the raised edges of the initial mark and lay it over slightly and then tap the punch with a hammer.

              It will "come back".

              It needs a fair bit of patience and effort to get it right but its as much an art as it is a skill - but very easy to keep up with a bit of regular practice.

              You will find that your setting up/out will be much easier if you get it right - especially on a mill - as those lines are easy to pick up and just as easy to "split" when you need a line to machine to.

              Its nearly always better to use the centre-ed part of a circle for locating one leg of a good pair/set of engineers dividers and marking where the circle intersects any lines - mark it out just a bit larger than the hole/s you want to drill. Prick or centre punch where the circle intersects any line/s as it will soon show you if your drill etc. is "running out" or is "off-centre".


              • #22
                My understanding is that it is very important that the angle of the "spot" be the same as the angle of the drill to follow. If I use a center drill, I only use the "pip"; I find that the countersink diameter causes a lot of chatter, and rather randomly drives the drill off the desired center. (I don't use center drills for spotting since I started using carbide spotting drills; they make a huge difference.) For larger size drills, I drill a pilot hole about the size of the web on the "full size" drill to follow.

                In an old edition of Moore's book on drilling holes, I believe that they comment on the fact that a non-symmetrical center punch burr will drive the drill bit off center; it makes sense, but I never would have thought of it on my own.

                I bought a Dankroy center punch years ago ( I believe that they may currently be a it pricey, but they are absolutely head and shoulders better than those which are more commonly featured in the tool catalogs (USA made or not).



                • #23
                  The use of either the tip or both parts of teh center drill makes the following drill start on an 'edge"... the first contact is somewhere up on the cone of the following drill tip.

                  maybe on one side only, which means it will be "pulled" off center, and has to work its way to center again... or maybe it just chips the edge. or maybe it works. I suppose teh one advantage is that it automatically clears a hole for the web of the following drill, so in that way a center drill can have some potential goodness, but the hole isn't very deep, and doesn't give much to aline the following drill if it starts 'off".

                  The spot drill makes a cone that contacts the tip of the follow drill, either all along the cone or at the web/tip. So the follow drill actually follows a 'started hole" that is what it would have made if it had started on-center.

                  Also, the sharp edge of the spotter tends to 'cut" a centering spot even if the surface is not perfectly flat, etc.

                  IMO spotters should ideally be a WIDER angle than the follow drill so the first contact is the tip/web of the follow drill. But I think that is maybe a pretty fine distinction......

                  I don't care what you use.... I just notice the spotter usually works better for me.
                  Last edited by J Tiers; 12-31-2009, 09:13 AM.

                  Keep eye on ball.
                  Hashim Khan


                  • #24
                    In the past I always used center drills to start a hole and then I started using machine stub drills but they tend to wander as much or more than the center drills. Lately I have used the spotting drill and I find it doesn't wander as much as the other two. I kind of like the spotting drills now.
                    It's only ink and paper


                    • #25
                      Originally posted by mmambro

                      I bought a Dankroy center punch years ago ( I believe that they may currently be a it pricey, but they are absolutely head and shoulders better than those which are more commonly featured in the tool catalogs (USA made or not).

                      Wow are they still going ?
                      I spoke to Mr Dankroy ? about 5 or 6 years ago and he'd retired and had no spares left, I wanted some of the lenses to fit into machines for lining up timing marks on the drive.
                      Got his number off the box of the punch set I have, they are nice but I believe mine was £49 when £49 meant something.

                      he did tell me they were not easy to make and the perspex lens was ground to a finish, nice chat with the old boy.
                      Looks like someone else has taken it on, I like the patent applied for

                      No one heard of prior art ?


                      Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.


                      • #26
                        Not sure if this has been mentioned.......spotting drill points also break fairly often. The extra thin web makes them weak at the tip.

                        The difference is, a broken center drill tip is difficult to remove ( books with hints on machining all have a trick or two to remove a broken center drill tip).

                        A broken spotting drill tip usually can be removed easily by hand.


                        • #27
                          "The use of either the tip or both parts of the center drill makes the following drill start on an 'edge"... the first contact is somewhere up on the cone of the following drill tip."

                          Sorry, I should have mentioned that I use(d) the tip of the center drill only for the smallest sized drills (so that a cone is provided for the drill tip).

                          Re the Dankroy center punch: The beautiful sliding fit of the perspex lens in the punch body is one of the things that really distinguishes the Dankroy from the others; it fits as well as the punch does. I was shocked to discover that the lens from optical punches made by other manufacturers are a loose to sloppy fit!

                          It sounds like I paid more for my Dankroy than my brain wanted to remember ;-)



                          • #28
                            Sharp Carbide is best.

                            There was a package of small carbide center drills in some stuff that I bought in an auction. They turned out to be very sharp and worked great. There were some carbide spotting drills also guess I need to try them. Sharp carbide continues to amaze me. I also got a bunch of reground milling cutters in both carbide and HSS. This place went out of business. Sure wish I knew who their grind shop was.
                            Byron Boucher
                            Burnet, TX


                            • #29
                              Re: Center Drills

                              There has always been several schools of thought on precisely starting a hole. Some preferred center drills, some like spotting drills, and each has some merit that the other does not. I have found that "installing" a center drill in a chuck takes a little care. First, the drill chuck must be in reasonable condition and free of chips and grit. When you start the spindle you need to look at the body of the center drill and see if there is runout. If so, clean and try again. Same holds true for spotting drills. Second, as GhopShop said, you need to peck drill or at least be gentle or the point will likely wander. When I was first starting out I would occasionally break the point off a #1 or #0 center drill. It mostly happened when I was in a hurry or got heavy handed with the downfeed. I have had pretty good luck with breakage, having a couple #00 center drills I have used for the last 5 years. Another thing that helped was getting the ability to resharpen the points on old, well used center drills. The Drill Doctor does a really nice job of this. I also split the points, which makes them less likely to want to wander. You can resharpen about 2 to 3 times before the tip is gone. I have had great success with rehabing used tooling this way.

                              Another issue that comes up a lot is picking up a center punch mark accurately. There were two way explained to me years ago. The lead man in the assembly department at the Tand D plant I worked said to use a center drill and carefully line up the punch mark. One of the old diemakers told me, after the boss had left, that I would have better results if I used a really small drill and just touch the dimple left by the punch. You can see which way it deflects and correct with the BP table until it lines up. Then you step up by drill size and the hole comes out on the money. A lot of the layout punch marks were in big die sets and were for spring pockets so it was not such a big issue but that lesson has been handy when working on "touchier" layout jobs such as face pannels for electronic devices.

                              Just my $0.02
                              Jim (KB4IVH)

                              Only fools abuse their tools.


                              • #30
                                From my experiences locating from a center punch mark is marginal at best. The issue of getting the center punch mark in the exact place is a hit and miss affair.

                                I use three ways depending on the needed accuracy of the location. For within 1/32" accurate I use a center punch and a wiggler. For within a 1/64" I use cross hairs marked on the work and a wiggler. For within +/-.001" I use home on the corner of the work and the DRO on my mill.

                                Using a center drill, spotting drill or stub drill will only give results equal to the tolerance of the chuck and spindle bearings no matter how well you locate it on the work.

                                There are reasonable expectations and unreasonable expectations when marking or laying out work. Many machinists have higher expectations than the machine is capable of.

                                For instance, you can expect better tolerance on a mill than a lathe as far as using the tailstock is concerned. The tailstock has more slop than a worn out mill will have so be careful of your expectations.
                                Last edited by Carld; 12-31-2009, 02:28 PM.
                                It's only ink and paper