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Boring bars 101

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  • Boring bars 101

    I've been working on boring out a steel ring, a little at a time. This should be soft steel, but it sure seems tough. Tried different angles, sharpened the tool, etc. The piece gets pretty hot, so I'm taking a few cuts then leaving it to cool awhile, etc. But the project got me to thinking about my boring bars.

    The better results are had with my slide-mounted bar holder, ie I remove the compound and mount the holder direct to the cross slide. It's a custom mount to allow this of course. On this particular job I discovered that light finger pressure on the bar makes a lot of difference as to how well it cuts. The bar is not singing, at least not at a frequency I can hear. It's probably got every dog in the neighborhood howling though- my finger pressure isn't enough to affect anything other than a resonance. I know all about damping, etc, and this isn't the focus of my topic at this point.

    My cross slide is a flat table with two T-slots. What I'm thinking to do is make up a mount for a 1 inch diameter bar, which would be turned down to a smaller diameter for the portion holding the cutter. Probably that would be slightly tapered as well, but the focus would be on the bar mounting being on the right side of the cross slide. The mount would be about half the width of the table, and it would be secured using the T-slots. A portion of the mount would extend towards the left T-slot to give added stability to the mount. The part where the bar mounts would only go about half way across. Some of the mount might extent towards the right, giving a longer seating area for the bar. For the moment I'm not concerned with how to mount the bar.

    Some of the bars would be turned down for the portion that holds the cutter, as is typical from what I've seen. I would probably end up with several versions of it- some narrower for smaller bores, some larger, and at least one that doesn't get turned down at all, or maybe just tapered from 1 inch down to about 7/8 inch at the cutter end. Might make some shorter, some longer.

    In large part the advantage would be more steel, more rigidity, and more often the cutter would be above the table rather than overhanging off to the left. This would put the pressure from cutting over the saddle, which should help a fair bit.

    It might be best to bore the holder to take the bar, then slit the side of the bore and arrange to pinch it closed over the bar. This would be better than pinching down on the bar with a set screw or anything similar. I'm thinking also that when the bore I'm turning is longer, so will the boring bar be longer, and thus there should be little problem with the crosslide interfering with the chuck. Only when the bore is going to be deeper than 2 inches will the cutter begin to overhang the slide.

    I'm just feeling for comments, intuitions, etc. A good question might be what material would be best to make the bars from. I can rule out carbide since I'd have to sell my arms to get a 1 inch diameter bar of any length, and then I wouldn't be able to machine anyway. I could, and would set a smaller carbide bar into the 1 inch bar so it fits the holder- I can see some version of that making sense. I've thought about making the right end of the bar a morse taper- using a drawbar to suck it tightly into the holder. I'm not sure which would be the most rigid holding- my first method or the taper method. The first method would allow me to make one bar hollow and put a spindle within it- the drive would be to the right of the mount and the spindle made to hold 1/4 inch shank tooling. I would find that to be a worthwhile addition to my tool post grinding arsenal of machines.

    My ring must have cooled by now- time to put a few more minutes into it-
    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

  • #2
    If you use carbide, you can get 10x diameter per length - i.e 5 inches for a 1/2 inch bar. You can buy cheap old carbide bars with messed up insert mounts. No machining.. take off the old end (heat) and just braze on any tip etc you want.

    On my small lathe I tossed my compound for most boring operations a long time ago. Made a cross slide mounted block and bored (with the headstock) a few smaller blocks that mounted on top with a single post. Made a shims to account for a few different cutter offsets.

    Steel for bars... really doesn't matter. it all flexes about the same.


    • #3
      i have just ordered a carbon bar to try and make a boring bar. has anybody done that? another idea i have, is to wrap (epoxy) a hollow steel bar with carbon and fill it with lead shot or oily sand. another idea i havent tryied yet is to wrap the boring bar with a rubber bad with some lead glued to it. seems to work for brake discs.


      • #4
        A split cotter is a great way to hang on to round stuff w/o damaging the surface. Lots of clamping action without much force on the screws; split rings take some serious torquing sometimes. Whenever I have the room and the forethought, I'll use a split cotter.
        I'm here hoping to advancify my smartitude.


        • #5
          Just use a piece of steel for the boring bar, doesn't make much of a difference in your situation. If going with HSS, I would just tack weld the HSS piece in the end of the bar or if the bar dimensions allows, drill a hole in 45 degree angle or make a rectangular slot and use screws.

          I would like to hear more of your boring operation that you mentioned, like pictures of the tool, setup and more in detail. Has your tool enough clearance under it? Is it a little bit over the center height so that if it tries to dig in it will ease off of the material and thus reduces tendency to ring and snap the tool. Mystery metal? Speed? Feed? DOC? Machine model?
          Amount of experience is in direct proportion to the value of broken equipment.


          • #6
            Interesting thread darryl,

            I have a project going that requires some boring also. I am trying to bore a piece of 2.5" X 6" A36. The first inch neeIds to be bored out to 1.004", the next 1/2" to 1.25" and the rest to 1.5077".

            The bar I am using is home made from mystery metal (it's pretty soft), about .8150 diameter and 14" long. I drilled and tapped a 1/4"-20 hole in the end of it to capture the tool bit. Then crossed drilled a 1/4" hole about 3/4" from the end of the bar I then drove a 1/4" HSS tool bit through it (poor mans broach). I have about 7.5" of bar hanging out of the tool holder.

            I have tried HSS and cheapy carbide bits and still the surface finish is terrible. I started with a 7/8 hole and so far I have got it out to .975". I have got figure out how to get a better finish on the bore before I get it out to 1". From 1" I will hone it out to the final 1.004". I think my bar has too much flex in it for the stick-out that I need. It starts to chatter about midway through the bore.

            Lakeside, you refer to a "10 X the diameter of the bar" as a stick-out limit for the carbide bars. What is the limit for a steel bar? I was looking at a 3/4"X10 oal hardened steel bar from enco model #377-1016 that uses a triangular carbide tool bit. would this work any better than what I have?

            What is meant by the term "lead angle". The Enco bars I was looking at list one bar at a -5deg lead and the other at a -1deg lead.

            Sorry darryl, not meaning to hijack your thread. Hopefully these questions will yield some helpful information about boring bars in general.

            Last edited by tmc_31; 02-15-2013, 10:20 AM.


            • #7
              Steel is about 3X.. but at all depends on the depth of cut etc. I've hung them out way more on lighter cuts, but with carbide inserts you quickly run up against minimum DOC.

              Terrible surface finish on A36? lol.. yep.. although if you turn it really fast, take a decent DOC and use finishing grade positive rake carbide, it can often look pretty good. A36 can really suck, and is variable from piece to piece. Keep it for projects where it doesn't matter, or for welding, and buy decent material.

              Emco insert boring bars? Will they work better then your homemade bar with HSS - probably not. They will work fine as a bar, but toss their carbide in the garbage and buy some quaity brands. The difference is night and day. I have dozens of inherited bars.. many from the 70's when the old guys made their own from 3/4 to 1 1/4 round mild steel; HSS attached to the ends often with a slot and set screws. They still work fine for bigger work. I also have some carbide 5/32 diameter that takes inserts!. My "go to" bars are a couple of 1/2 inch carbide.

              Lead angle is the angle the insert makes wrt the work. 5 is typical as it allows the swarf to clear without rubbing. On the other hand you won't be able to make a flat bottom cut unless you clean up the end with a cross feed cut.

              You won't cure a floppy machine with a stiffer bar. Ditching the compound on a small machine goes along way to "cure".

              Boring block... on compound (takes two bars).

              AXA on riser block. Boring block also fits on top. And.. all made from A36

              Last edited by lakeside53; 02-15-2013, 01:05 PM.


              • #8
                When I worked in Auto Repair....we would wrap brake drums and rotors in a big glorified rubber band to absorb harmonics when machining them. I applied this same theory to my boring bar a few times and it worked. I wrapped the bar with rubber many as I could fit around it, and also wrap the workpiece with rubber bands. All but one time, it has made an improvement. Now that I've got a fairly rigid machine, I rarely find the need any more.


                • #9
                  Been working my way through the "Turret Operators Manual" and for me there maybe a few interesting (odd?) items coming from that...there is a fair bit on, shall we say, "efficiency" of doing the work, stuff like multiple cutters, large DOC etc. so one of the other topics is of course rigidity.
                  The solution in the turret situation regarding boring appears to be [a] use of a secondary support arm (I always think "overhead". I guess it does not have to be but is most of the time), [b] use of a bushed boring bar. More or less what it amounts to is getting the boring bar as rigid as is possible, often more to increase the depth of cut than improvement of finish (though both certainly apply).
                  In the case of the overhead support, the turret becomes more connected to the lathe headstock and in the case of a bushed boring bar both ends of the bar have support.
                  I suppose the other thing to mention is the option of "pull boring" where, rather than pushing the cutter in, you put it to the far end of the hole already existing, dial in the cut and pull the bar out. My understanding of the latter is the different dynamic of the forces involved since you are pulling.

                  "Odd" since I am seriously considering a sound method to attach some sort of over arm support.

                  Edit: post #14 shows how such an overhead bar looks/works
                  Last edited by RussZHC; 02-15-2013, 04:44 PM. Reason: to add link


                  • #10
                    Your thoughts on boring bar holders in the OP mirror what I found last fall when poking around on the net for hints on how to build one. One point that made sense to me was that the bar is most securely held by a well fit split clamp arrangement. Most of my boring bars are Everede with an accurate 3/8" shank so I reamed the holder to that size and use 3 screws to clamp bars in place. This is very solid and doesn't chatter despite the flip up capability, see:

                    I can't suggest much on making boring go faster - on my little lathe patience while boring is a virtue.

                    On surface finish, the vertical shear bit often works well. I use it on external surfaces but BryceGTX really got into it and found it works well for improving finish while boring too, more info if you look through this thread:

                    It's a finishing tool - only good for very light cuts, one or two thou, but seems to work well on many different materials, may be worth a try.
                    Location: Newtown, CT USA


                    • #11
                      Centre drill and bore a hole down the bar, fill with lead shot, use upside down at the back of the work so the chips clear better and better visibility
                      The lead shot will damp the harmonics of the bar (plug with Allen grub screw)


                      • #12
                        Originally posted by lakeside53 View Post
                        You can buy cheap old carbide bars with messed up insert mounts. No machining.. take off the old end (heat) and just braze on any tip etc you want.
                        I've acquired a fair selection like that. But in my case the damage was TIG welded and then machined back to form a good pocket. And I've also gotten spoiled, I don't like using any boring bar that is NOT solid carbide, they are so much nicer.
                        Master Floor Sweeper


                        • #13
                          My favorite boring tool is a carbide rod 1/4 inch diameter, held in a custom holder. I've shaped the end into sort of a D bit, and ground some front and side relief on it. It has to be angled for the cutting edge to work and to keep the shank away from the bored hole, but that's fine. I can only go about 3/4 deep with it because it's too short. I think I will look for some longer pieces, wider also.

                          The one I've been using for this latest job is brazed carbide, some cheap set made in India probably. The cutting edge is not on center, so I have compensated by turning it in the holder so the back rake is 0 to positive some degrees. The cutting force is not directly down, but more towards the front because of this. It's not a very ideal setup, but it is the most solid holder I have currently. I can usually get it to work fairly well, but it's not ideal.

                          I got my bore chewed out to within 40 thou of target (about 1.5 inches) and then sharpened the cutter. It did pretty good until I got to about 10 thou from target, then the going was tough again. I repositioned it so I could use a slightly different portion of the nose radius, and it cut well until I was almost there. About 3 thou short of goal it got tough again. Seems to me the steel is eating the carbide. It's not chipping it, just wearing it down . You can see a bright polished spot developing which is erasing the relief. At that point there's a burnishing action going on, which by the way is giving a great looking finish, almost mirror and beautifully smooth. The metal I'm turning is from the magnet assembly of a speaker. I'm not turning the magnet, just the center pole piece. It's probably a high silicon content alloy, and I'm thinking that the silicon and the silicon carbide cutter aren't mixing too well.

                          But all that is beyond my quest here, which is to develop the best boring setup I can manage with my 8x18 lathe. I have run into it's limitations a few times lately-
                          I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


                          • #14
                            I made up a couple different bars now out of old cummins valves. I would like to make one into a holder for inserts of some sort. Not sure how though since they are so hard and impossible to drill and/or tap.



                            • #15
                              I use the HSS boring bars that are made for boring heads as much as possible, they work well with little to no chatter and are very easy to sharpen. A two flute HSS end mill with one ground off works well also, easy to sharpen and cheap. These seem to be stiffer (if that is the right term) then straight steel boring bars, for tool inserts, so less chatter.
                              The shortest distance between two points is a circle of infinite diameter.

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