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Round boring bars

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  • Round boring bars

    Hi, iv allways wondered why boring bar shanks are round... And today maybe it dawned on me when my boring bar snaged in the tight clearance hole and started rotating around in my toolholder (BXA 200 style), the cheap chinese carbide is still alive! im amazed how strong the carbide is on these cheap boring bars, wayyy better then the carbide I got on the 40 set of brazed tools from enco.. but maybe thats because I need to sharpen those first..

    Anyway, is that one of the reasons boring bars are round?
    Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

  • #2
    Since a boring bar fits into a round hole, the round shaft gives you the most cross-section that will fit in the hole.

    That's my guess.

    At the end of the project, there is a profound difference between spare parts and extra parts.


    • #3
      No I don't mean the neck (that should actualy be semi circular) but the shank itself that goes into your toolholder, like these:
      Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.


      • #4
        Rotating the bar in the holder allows you to change the rake of the tool.


        • #5
          Made more to go in a boring head then a lathe toolholder. I use them in both. Its also easier to bore a round hole than make a square hole for holding, much more economical.


          • #6
            Looks like the set I have. But, as long as I've had them, I've never had one turn...even in a heavy hogging cut.

            When I first got them, I thought they were bad...but then, a little closer observation revealed that I'd set the rake wrong. Once I got the carbide cutter raked properly, they cut nice and smooth. The finish is real nice too.
            No good deed goes unpunished.


            • #7
              Not to steal the thread, but here goes: What is the optimal way to dial in the rake of a round boring bar? I've fought just to try and get the carbide horizontal let alone at some other rake. How do you all set yours?


              • #8
                Just set the rake by eye. My chinese ones are ground flat top, but about 10 degrees of front clearance. So I usually set them slightly positive on the top rake and this reduces the front clearance slightly, but as long as it isnt dragging the tool below the cutting edge into the work all is good.

                Just south of Sudspumpwater UK


                • #9
                  If your using the bars in the photo in a boring head in a mill as they are intended then setting them by eye and how they are cutting is the best way. You can go positive a lot but if you try to go negative to far the bar can hog in and cause problems.

                  If your using them in the lathe, which is not really what they are intended to do but will work, then it is important to keep the tip of the cutter at or a few thousandths above the center line no matter whether you turn the bar to a positive or negative angle. If the tip is below center line it will hog in and do some damage.
                  It's only ink and paper


                  • #10
                    I've used mine in both a boring head and on my lathe and they seem to work well with little or no positive rake. In fact, I usually end up with a slight negative rake on the lathe, especially when boring smaller holes.
                    No good deed goes unpunished.


                    • #11
                      Primary "rake" is on the end. It cuts as it moves down the bore, not from the side. Rotation of the bar doesn't really affect that, though it can cause excessive deflection and chatter. Rotation, in this case, is more about clearance and chip control (depending on material). If you've got end rake and edge appropriate to the material, and rotation so that there is no rubbing, and as long as you don't take rotation to the extreme, it's pretty much going to work.
                      Master Floor Sweeper


                      • #12
                        BadDog, I can't agree with some of what you say.

                        It's true the leading edge is doing the cutting but with the tip on center line at all times if you tilt the cutter down your getting negative rake and if you tilt the cutter up with the tip on center line the cutter is negative rake.

                        The rotation of the cutter is not only about clearance, it's about rake as long as the tip of the cutter is on center line. If you ignore keeping the tip of the cutter on center line then rotating the boring bar gives rise to all kinds of issues, some of which will cause disaster.

                        End rake clearance on the cutter is important depending on the feed your using. If your using a low feed rate you don't need as much clearance angle as if your using a heavy feed rate. At that point you need a higher angle to give clearance on the leading edge to eliminate rubbing.

                        EDIT: Hmm, upon reflection while there is some negative/positive effect on the tilting of the bar there is also the angle of the flat part of the cutter leading edge that is neg./pos. in relation to the cut. In a neg tilt of the bar the cutter is cutting with the force pushing the bar away from the work. With the bar tilted up, pos., it would be pulled into the cut. Either of these could cause or diminish chatter or other issues.

                        I still feel the angling of the tip up or down affects the cut and the finish of the surface.

                        It's much the same with a drill bit. If you don't have the right angle of relief your drill won't cut. Also, the clearance angle will limit the feed rate of a drill bit just as it will the feed rate of a boring bar.
                        Last edited by Carld; 12-20-2009, 02:26 PM.
                        It's only ink and paper


                        • #13
                          I don't know that this is the reason, but I would bet they are round because it is easier to make a accurate round hole for them to slide into than a accurate square hole.


                          • #14
                            Let me rephrase a bit.

                            Needing to having the tip "on center line" is a bit of a misnomer. It's kind of like the "need to have the lathe level", it's really just a convenient reference.

                            The tool edge touches is always on some radial from the center of rotation. Other than loading into the tool stack of a lathe (compound, cross, saddle, bed, screws), as far as the tool cutting is concerned, it really doesn't matter where the boring bar touches the bore. And with the tool rotating in a boring head, this is even more obvious, and becomes clearly irrelevant.

                            And end rake/clearance is the primary concern because the cut direction is (typically) axially along the bore.

                            So the real point is tool edge orientation with regard to the bore, the typical goal is to have it more-or-less aligned with the radial of the bore connecting the tool contact point to the center point, and suitable side clearance provided to work in this orientation, accounting for any flex. And assuming even rake and/or chip breaker, chip direction is mostly down the axis of tool and variable with ductility, rake, etc.

                            What difference would be had by changing cutting edge angle? If you tilt it so that that angle between cutting edge and bore contact tangent is acute, then it gives the edge properties similar to lead angle on an OD cut, wants to direct the chip into the freshly cut bore, and push the bar more to the center potentially causing more chatter. Angled so that the edge forms an obtuse angle throws a chip more to the center (again, depending on material) and encourages the bore to want to "hog", again somewhat like an obtuse lead angle on an OD cut. In this obtuse orientation, clearance can be increasingly difficult to achieve, and edge end/point support is reduced.

                            If you are stuck with a cutter that can't rotate, then all this has to be done with moving it up/down combined with the grind. But it boils down to the same thing. For instance, if you have too much flex and not enough clearance, and can't/won't change the related edge angles, you can accomplish the same thing by moving up to get the clearance, and also push away to center to help eliminate the problematic hogging/rubbing otherwise associated with flex "down". Something you can't really do with a boring head or some lathe bar holders, which explains why the commercial bars are generally round.

                            And lots of things change depending on grind and material. But as long as you orient the cutting edge to be roughly on the radial (or slightly acute) so as to minimize side loads, and have clearance to deal with bar deflection without rubbing, it's going to work.

                            Again, given proper rake and clearance on the end, along with sufficient clearance on the side to handle bore shape and deflection, simply placing the cutting edge roughly on a radial of the bore is all that's required. If it rubs and you can't/won't change the side clearance, you can rotate/raise/lower to deal with that (which is what led to my earlier summarized statement), but I would generally say that's a deficiency in the bar design/grind. It may also force you to deal with introducing other less desirable traits due to the effective changes in "lead angle". The only way I can see rotation (other than slightly to the acute) helping anything is if the bar cutting edge design is otherwise wrong, and the rotation helps fix that problem.

                            At least that's how it works in my head. Hopefully that's a better explanation of what I'm trying to convey about how I think when dealing with boring bars in both lathe and boring head.
                            Last edited by BadDog; 12-20-2009, 04:10 PM.
                            Master Floor Sweeper


                            • #15
                              Having the tip on center line is not a misnomer at all. A boring bar in a lathe is entirely different from a boring head in a mill. You can adjust the height of a boring bar in a lathe but you can't in a boring head.

                              I agree that in a lathe it is best to have or start with the tool horizontal and on the center line and that the leading edge is doing the cutting and in that configuration your cutting a negative rake on the leading edge. If you don't believe it draw it out on paper.

                              The difference obtained by rotating the boring bar to an acute angle would produce a more negative rake than the horizontal setting as long as the tip of the cutter is on center line. If you rotate the bar up to give an obtuse angle and keep the tip on center line then the tool will hog in as you state and be a positive rake.

                              If your stuck with a bar that can't rotate then if you go below center line your going to have all kinds of problems and believe me I have seen them. It will not work well with the cutter horizontal and the tip below center line. You can raise the cutter tip above center line and have good results with a non rotating bar.

                              I have several square bars and several round bars and I prefer the round bars for difficult jobs for the reasons I have mentioned. A round boring bar in a lathe has many options for angles of cut and I prefer the round bars.

                              Basically it is important to keep the tip of the cutter in the boring bar on the center line in a lathe. You can go high with the cutter horizontal but your asking for trouble if you go below center line with the cutter horizontal. The horizontal center line is your friend in a lathe.

                              Now as to a boring head in a mill, all you can do is rotate the bar in the boring head. What happens when you rotate the bar is if you rotate it toward the direction of rotation your going positive rake on the cutter and also above the center line of the bore. If you turn the bar away from the rotation of the bar your going negative rake. You have to be very careful how much you turn the bar. Slight movements can make or break your good cut. You don't have a lot of options with a boring head.

                              EDIT: we are saying much the same with the exception of keeping the tip of the cutter on the center line. It is very important to keep the tip of the cutter on the horizontal center line in a lathe for good results.
                              Last edited by Carld; 12-20-2009, 09:39 PM.
                              It's only ink and paper