Drilling in the lathe – where did I go right?

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Drilling in the lathe – where did I go right?

Home Forums Beginners questions Drilling in the lathe – where did I go right?

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  • #451963
    Robin Graham
    Participant
      @robingraham42208

      I've been doing a bit of boring on the lathe recently, and I need to drill at least 16mm through the work to allow my 'normal' boring tool to start.

      There are lots of questions about drilling holes of this sort of size in steel on the internet, and the advice is invariably that one should start small (say 8mm) and open up maybe 2mm at a time. I've been doing that and it hasn't worked well – invariably, somewhere above 12mm, the bit snags and something gives way – the work in the lathe chuck, the bit in the drill chuck or the taper in the tailstock. It's been a pain and I couldn't understand what I was doing wrong.

      Fed up with all this, tonight I pre-clenched my buttocks and attacked directly with a 17mm bit, the biggest I have. It just went straight through 40mm deep without any complaint. I had to take both hands to the tailstock wheel to keep the pressure up and make the tool cut, but that isn't a problem.

      This was EN1A turning at 180 rpm.

      Did I just luck out somehow? Obviously I'll be doing it this way henceforth because it seems to work, but I'd like to understand what's going on.

      Robin.

       

      Edited By Robin Graham on 13/02/2020 01:00:13

      Edited By Robin Graham on 13/02/2020 01:19:43

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      #10073
      Robin Graham
      Participant
        @robingraham42208
        #451965
        Steviegtr
        Participant
          @steviegtr

          I think a lot of it is, a good quality drill. The flute sides are not worn, plenty of lubrication, The bit is sharpened correctly. One problem I was warned about & have noticed to be true is that once you start increasing the size of the hole. Which by the way I have been doing on some 316 today. The work starts to heat up. If that happens I do & would stop until the work cools down. The work does some weird things with temperature. I watched Blondiehacks (youtube) drilling deep holes, & do as she said which was to go steady & back out often to clear the flutes.

          Steve.

          P.S I'm only a beginner so I am sure someone will be along to advise you better.

          #451967
          Martin Connelly
          Participant
            @martinconnelly55370

            It could be a feed rate issue. A large diameter drill needs a faster feed per rev than a small drill.

            Going up in 2mm steps is not a good idea, the pilot hole should be just a bit larger than the web of the large drill. A small cut can chip off the corners of the drill and can also pull the drill into the work. You will end up with lots of drills which need re-sharpening. Presto recommend a pilot drill with a diameter one and a half times the chisel edge of the next size.

            If you do not have a spotting drill with an angle at the point similar to a standard drill then get one or more to start the drill on centre, keep centre drills (Slocome drills?) for when you are going to use a centre to support the work as their angle is wrong for starting a drill on centre. I think this was discussed in a recent thread on drilling holes accurately.

            Martin C

            #451969
            Hopper
            Participant
              @hopper
              Posted by Robin Graham on 13/02/2020 00:47:38:

              There are lots of questions about drilling holes of this sort of size in steel on the internet, and the advice is invariably that one should start small (say 8mm) and open up maybe 2mm at a time.

              This is the problem with the internet. It's full of self-appointed, self-taught "experts" full of dubious information — be it on climate change, coronavirus or drilling holes.

              Anyone who has actually taken a training course (back when such things were available in engineering craftwork) would have been instructed to drill about an 8mm pilot hole then drill your 16mm hole in one go. No faffing about with 2mm steps encouraging the corners of the cutting edges to dig in etc.

              "invariably, somewhere above 12mm, the bit snags and something gives way – the work in the lathe chuck, the bit in the chuck or the taper in the tailstock. "

              You don't say what lathe you are using but it sounds like possibly it is a mini lathe or other small lathe that is being overloaded by the larger drill bits? Job should never spin in the chuck unless something is wrong with the chuck, or it is hopelessly overloaded by too large a diameter for jaws to grip firmly etc. Morse taper spinning may just need a good clean out and tap the taper home with a brass hammer/bar to seat the taper firmly. And holding bits larger than half inch in a chuck is always a gamble. It's beyond what the chuck is designed for. (But I have used a 1" drill with 1/2" reduced shank in a chuck mounted on an MT1 taper in the old Drummond with success so it can be done.)

               

               

               

              Edited By Hopper on 13/02/2020 01:31:13

              #451983
              Chris Evans 6
              Participant
                @chrisevans6

                Plus One for spotting drills, I have used them for the last 30 + years. If I do use a centre drill for when the work requires a centre I always shorten the length of the parallel portion of the centre drill to prevent breakage. This tip was shown to me in the first month or so of my apprenticeship 56 years ago. I also agree with the say 8mm drill followed by the bigger one. Running the lathe to fast is a necessity on some small lathes that lack torque but can wear the flutes of the drill.

                #451990
                not done it yet
                Participant
                  @notdoneityet

                  Think about the metal cutting process being undertaken. The centre of any drill is cutting at zero speed – while at the circumference, the cutting speed is maximum. It makes sense to drill a pilot hole (small diameter, high speed) before using a very large bit at lower rotational speed.

                  Larger drill bits may not cut too well at the centre unless sharpened precisely.

                  Only go up in steps that the machine can tolerate. That means that a 6mm drill as a pilot could be followed by a 16mm as the cutting surface will be the difference between the two areas. Trying to go to 26mm as the next size up is likely impossible on a small machine.

                  28mm^2 for a 6mm pilot – easy for the machine.

                  175mm^2 for the jump from 6mm to 16mm

                  330mm^2 for the jump from 16mm to 26mm

                  Some machines may not cope well with the jump from 6mm to 16mm in one go, as that requires over six times the power used to cut the pilot. Clearly, to increase the hole from 16 to 26 mm (in one jump) would require 18 times the power used for the pilot! That is one reason why larger holes are bored.

                  For a mini-lathe, 6mm, 12mm and 16mm might be a sensible progression. I’m sure the advice is not ‘invariably’ 2mm steps, even on the net. Any linear diameter increase is mathematically flawed (area of a circle is proportional to the diameter squared) and that would indicate the first hole should be 2mm (an increase from 0mm to 2mm as the first step) – patently just absolute rubbish.

                  #451992
                  Nigel McBurney 1
                  Participant
                    @nigelmcburney1

                    The advice is ok to shorten the front portion of a centre drill,if dealing with tough materials,a lot depends on the type of work each company does and their methods.When shown how to centre drill now over 60 years ago I never saw anyone deliberately shorten a centre drill , though they do get worn down to resharpening, be careful with these as when drilling centre holes to turn between centres you may find the point of the centre bottoms on the hole and there is no support from the tapered part of the hole.Also in those days the centre drill was the accepted method to start a drilled hole in the lathe,mill and drilling machine.The spotting drill seems to have got popular since introduction onc/cnc machines..The one time I use a shortenened centre drill is when making cast iron pistons for stationary engines 3 to 6 inch diameter,I usually hold the piston skirt in the chuck and centre drill the crown,too long a centre drill point can sometimes get close to going through the crown ,so its a case of short drill and a older rotating centre with point ground off..As for drilling larger holes,lots of methods,drill manufacturers recomend for ordinary jobber twist use pilot drill and then open up with the correct size not steps,though this is not possible on many small lathes and on larger lathes can be hard work, so do what suits your machine ,one way to reduce the wear on the edges of the drill is to reduce speed to about half recomended speed, on larger lathes try making an adaptor so that drills are held in the tool post or a cross slide fixture. My dickson tool post on the Colchester has a holder having a 2 morse taper socket, and I have crosslide mounted 4 m/t socket allowing the use of a 2 inch drill. the Dickson holder comes in usefull on one job which requires 9/16 dia holes 2 inch deep in en 9 and up to 40 components ,thats where the power feed does its job,plus it saves wear and tear on the tailstock and on me.

                    #451994
                    old mart
                    Participant
                      @oldmart

                      The best way to avoid the risk of a less than perfect drill is to get a smaller boring bar to start with. They don't have to be expensive.

                      **LINK**

                      #451996
                      Kiwi Bloke
                      Participant
                        @kiwibloke62605

                        Oh, thank goodness! Martin Connelly and Hopper are spreading the light!

                        The tradition of sneaking up to size by a succession of drill diameters, increasing by small increments, has had its day and needs to be laid to rest. It may have been appropriate in the days of treadle-driven lathes, but is wrong-headed for the reasons stated. If your machine really can't take the cutting loads, you'll have to use multiple steps, but each one comes with the risks stated, so try to use as few increments as possible. And start with a 120 degree spotting drill, never a centre drill!

                        #452000
                        Clive Foster
                        Participant
                          @clivefoster55965

                          Provided you have the power to drive the drill at something close to the recommended speed and rigidity to keep it straight best technique is indeed to start with a pilot hole a shade bigger than the centre point then switch straight to the big drill.

                          Getting a good start can be an issue with big drills on small, inevitably less stiff machines. Until the drill has a decent cone a significant fraction of the diameter to work in it may well vibrate, wobble and generally complain about the job. Especially if its not as sharp as you'd ideally like. Cure is to use a smaller drill or two in succession to create a wider starting cone of the correct angle. Just create a cone, not a hole.

                          I suspect that the 2 mm step thing comes from someone trying to cope with poorly hand sharpened drills with off centre points. If the cut isn't too wide the effect of concentricity errors is minimised and the drill rapidly becomes stabilised by the side of the hole. Clearly if the cut step is too big then the off centre cutting cone will generate enough side force to wobble the drill causing enlarging the hole. Its a fearful bodge but most of us don't have facilities to sharpen larger drills well and accurately.

                          As has been said a sharp drill cutting the main hole in one pass at the right speed and feed actually runs cooler than nibbling the hole out. When things are running correctly heat should go more into the chip than the work or tool. Book speeds and feeds will surprise most folks. Scary fast really. For the likes of us half should be fine but not too much lower or the chips misbehave.

                          Concerning sharpening. I have just got my Clarkson drill sharpener attachment up and running after far too long on the "get round tuit somewhen" list. Its impressive. As usual proper professional tools are leaps and bounds above the usual offerings at a amateur friendly prices. I can, with some concentration, get reasonable results on a swing jig or Plasplugs mulit-sharp but the Clarkson puts a super sharp edge on with impressively casual effortlessness. Big slow down is getting hand sharpened drills back properly concentric. Surprised me how many of my good looking larger ones were hand sharpened and not quite right.

                          That thing could put a point on a (small) rolling pin. Max size it will take is only 3/4" tho' so now I have to come up with something equivalent for the bigger ones. My collection goes out to 1.5".

                          Clive

                          Edited By Clive Foster on 13/02/2020 10:18:19

                          #452007
                          SillyOldDuffer
                          Moderator
                            @sillyoldduffer

                            I agree whole-heartedly with advice not to drill big holes with a series of small steps. Apart from anything else it wears all the drills used disproportionally on their outer edges.

                            But, one reason it might be done is when a small lathe doesn't have the grunt needed to push a big drill through in one go, perhaps because the material is difficult or the drill is blunt, and/or the operator is inexperienced.

                            My mini-lathe struggled with big twist-drills due to the type only having low torque ( 'turning power' ) at the low speeds preferred by big drills. (No backgear.) I fixed the problem by buying a small carbide boring bar! No need to remain loyal to a big one, my small boring bar fits comfortably into a 9.5mm diameter hole, which my mini-lathe could usually make in one go with a twist drill. Then I could put the mini-lathe back into it's comfort zone and finish by boring the hole out at carbide speed. (Drill speeds slow down with increasing diameter, carbide works better at high speed, which is easier to get as the hole grows in diameter.)

                            I use the same method on my WM280. Although considerably more powerful than a mini-lathe, it too works best at higher speeds. I've found it useful to own 3 different sized boring bars, and when I need to bore a long cylinder I won't hesitate to make or buy a long one with an adjustable cutter that can be supported by the tail-stock. Having the right tool saves a lot of bother.

                            Dave

                            Edit: removed hateful unwanted smiley.

                            Edited By SillyOldDuffer on 13/02/2020 10:56:18

                            #452008
                            ega
                            Participant
                              @ega

                              A very effective way of opening up a pilot hole is to use a core drill with three or more flutes.

                              In default, the pilot hole can be opened up to fit the larger drill with a boring bar even if this is too big to enter the target hole to any depth.

                              #452010
                              blowlamp
                              Participant
                                @blowlamp

                                The main thing with drilling larger holes is to get rid of the centre section by drilling a pilot hole of reasonable size first.

                                I find that doing this allows the large drill to cut freely without snatching. Conversely, stepping up in small increments provokes snatching.

                                Centre or spot drill, pilot drill, then drill to size unless a very large diameter hole, in which case maybe use an intermediate size followed by finish size.

                                #452034
                                Martin Connelly
                                Participant
                                  @martinconnelly55370

                                  Have you considered that 180 rpm is too low. Recommended rpm from various sources for 16mm HSS into steel is about 500 rpm. This is a maximum speed but since I don't know if you are controlling rpm with pulleys, gearbox or electronic speed control it may be important. Electronic speed control causes a drop in power as the rpm is turned down.

                                  Martin C

                                  #452437
                                  Robin Graham
                                  Participant
                                    @robingraham42208

                                    Thanks for replies, informative as ever. I should have said that that this was on a 12×36 gear head lathe with a 2HP AC motor, so quite a bit of torque at 180rpm. It may be that the advice I'd read to go up in steps is more appropriate to smaller machines. Anyhow it doesn't sound like I've done anything terribly wrong. It'll save me some time and frustration if I can whizz through 17mm in one go safely.

                                    There is always a difficulty for someone in my situation (no engineering background or hands-on tutor) – you learn by a combination of trying things and reading advice in books and on the internet. If something works works for me, then it should be OK, but I never know what's lurking round the corner to bite me in the bum… hence my questions here.

                                    I take Hopper's point about self-styled experts on the internet. I once had to give a talk on energy, and went to Wikipedia refresh my memory of the the virial theorem. Summat not right here I thought. Turned out the article was written by a sophomore who had just got back from a thermodynamics lecture and was so chuffed with himself that he couldn't help but make his misapprehensions public.

                                    I'm enjoying the journey into this (to me) unknown territory – I'd probably have given up long ago without this site.

                                    Robin.

                                    #452505
                                    IanT
                                    Participant
                                      @iant

                                      Well, not trying to court controversy but I use Centre Drills to start holes and will most likely after all this time continue to do so

                                      However, to be clear, I do NOT drill a "centre" with them when doing so…

                                      I seat the drill right back in the chuck and get the tailstock body as close to the work as possible – this then gives a very short and rigid set-up (as much as my lathes will provide at least). The centre drill is then used to make a deep 'dimple' (more than a centre punch but less than a hole!). The slender front piece is not pushed into the work and no 'cone' is produced. However this dimple is quite enough to give the next drill a good start without wandering.

                                      I do the same in my drilling machines, although this work is usually already marked out (which it generally isn't in the lathe). Usual caveats – my work is mostly small in nature and I don't [normally] 'drill' in my mills – not having drilling quills on them as yet.

                                      It may not be modern practice but this works for me..

                                      Regards,

                                      IanT

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