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Need some ideas for a different type of boring bar.

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  • Need some ideas for a different type of boring bar.

    I am gathering the parts to make a line boring machine for backhoe and loader buckets.

    All the boring bars the I have seen have a series of cross holes drilled to hold the tool bit.
    Some times as close as 4" on centers.

    Seems to me that makes for a lot of drilling and tapping,also a much weaker bar.

    How about a tool holder that slides to any position on the bar and locks in place?
    I am picturing a square or splined bar driving a positionable tool holder.

    Something like a shaft collar with a radially drilled hole to mount a tool bit,with a way to accurately advance the bit.

    A couple different diameter holders for small or large bores.
    On the larger one it should be easy to have the bit rotate on an axis that is on center line of the bar.but on the smaller one I am stumped on how to have it rotate without it being off center.

    I am talking about bores of 1 1/2" to 6".

  • #2
    Its gonna sound goofy but
    How about a tool holder that slides to any position on the bar and locks in place?
    I am picturing a square or splined bar driving a positionable tool holder.
    your description sounds a lot like the general description of an arbor for a horizontal mill...the bore range you mention is somewhat the range of horizontal mill cutters, so perhaps a 1" or 1.25" bar would be enough, spacer sleeves that are quite available could then be could, I think, make the usual key more continuous as a single "spline"...


    • #3
      Why not track down the arbor of a horizontal mill? It's keyed, set up to hold cutters and spacers and has aome sort of driver.


      • #4
        The horizontal mill arbor wont allow the cutter to advance into the section being bored .
        I would suggest a long splined shaft and have the cutter mounted on a sleeve with a leadscrew and a guide to advance the cutter..


        • #5
          If it needs to get inside a 1 1/2" hole have a spline, leadscrew and holder it's going to get a bit wimpy and complex.

          OK on bigger stuff but stick to cross drill holes on the smaller bars.

          It's not that much drilling and tapping compared to the work needed for moving tool holders and you only have to do it once.

          There was a good post on here a few years ago which resulted in Motorworks [ Eddie ] building one.


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


          • #6
            What John said.

            Of necessity, the clearances between the bar and hole are usually quite small as most of these set ups have all the rigidity of a wet noodle and using as large a bar as is possible helps correct that.

            The comparisons to a horizontal mill set up are not the reality here. A horizontal mill arbor of 1 1/2" diameter might be 3' long overall, with 6"-12" between supports. I have made and used 1 1/2" diameter line boring bars that are 5' long with 3'-4' between supports.

            And you won't be taking any great hogging cuts, so the need for splines or keys to drive any tool block really isn't necessary.
            A simple split hub with cross/clamp bolts on a round shaft is usually sufficient. If for some reason this type of clamp set up isn't enough to power whatever you are doing, it would be a fairly simple procedure to put a cross hole in the tool block that would correspond with one of the holes that are already in the bar. This would allow a pin to be inserted to drive the tool block.

            Last edited by becksmachine; 11-14-2013, 08:37 PM.


            • #7
              An old trick is to drill the cutter hole at eg 45° to the axis of the bar, so you can fit a screw adjuster and still measure easily with a mic.


              • #8
                Thanks for the link.
                I thought I may have been reinventing the wheel.


                • #9
                  From an engineering point of view and for a given diameter bored hole, you are balancing the diameter of the boring bar, the diameter of the tool, and the amount of tool protrusion to achieve maximum rigidity in the set up.

                  In the analysis below, I am assuming that the hole size is the same for all cases.

                  Consider: a pipe or hollow box shape has almost the same rigidity or resistance to sideways deflection as a solid round or square shape. What this means is as you approach the center of a round or square rod, the material at the various depths is increasingly ill relevant in terms of it's contribution to that rigidity.

                  A Traditional Bar Approach: So, if we choose a boring bar that is a large percentage of the final hole diameter, say perhaps 80%, that bar would be a lot more rigid than a bar that is perhaps 35% of that diameter. With that 80% bar, the tool will only need to stick out about 10% of the bore diameter. If we choose a tool diameter of about 20% of the bore diameter, it will be BOTH a large percentage (200%) of the amount that it will protrude from the bar and a small percentage (20/80 = 25%) of the boring bar's diameter. This 25% hole through the boring bar will have a minimal effect on it's stiffness or rigidity. Even less so if the diameter of the tool and hole are closely matched. And, the full strength of the boring bar is in effect for all of the rest of it's length.

                  A Movable Tool Holder Approach: This approach will have an additional factor depending on the size of the bored hole. For small holes, the size of a movable tool holder will be prohibitive and the idea simply will not work. So this will only work for larger bored holes and I am not sure as to exactly the dividing line will be. At some large size hole, it will work just fine. I am going to look at an intermediate sized hole case, without exactly specifying that hole size.

                  To make room for a movable tool holder, the diameter of the boring bar will have to be a smaller percentage of the bored hole size. Let us say 50% for an intermediate bored hole size. 50% is significantly smaller than the 80% that I used in the Traditional Bar Approach. So this will result in a significant reduction in the rigidity. In general, the diameter term appears to the third power in deflection equations. That means that a round bar that is twice the size will have only 1/8 the deflection under a given load (2^3 = 8). Our example has a smaller reduction in diameter, but 80/50 = 1.6 and 1.6^3 = 4.096 so the smaller bar, the 50% one, will deflect four times as much under the same forces. That is your starting point. Now you need to design a tool holder that occupies only 25% of the bore diameter and that holds a tool that protrudes perhaps 5% to 10% of that diameter so you are left with only 15% to 20% of the bore diameter to hold that mechanism. And you want to include a fine adjustment mechanism.

                  I am not saying it can not be done. I can imagine one approach. A hole to hold the tool. A hole at the base of the tool hole that is drilled at 90 degrees to it. It has a threaded section on one end and it holds a tapered pin with threads for the adjustment. And, of course, a set screw hole so the tool can be locked in place. That's two, stacked cross holes in that 15% to 20% of the bore diameter.

                  I don't know how much a given tool hole will weaken the Traditional Bar, but I strongly suspect it is not a factor of 2, much less the 4 of my example.

                  I suspect that only a large bore diameter would make this movable tool holder boring bar a workable idea. For instance, a 10" bore with a 6" boring bar would allow 2" for the tool holding mechanism. That sounds workable.
                  Last edited by Paul Alciatore; 11-15-2013, 03:05 PM.
                  Paul A.

                  Make it fit.
                  You can't win and there is a penalty for trying!


                  • #10
                    Paul; you put a new perspective on it that I didn't think about.
                    Maybe the K.I.S.S. principle is the best.

                    Now I start cross drilling the bar.


                    • #11
                      I have used a Magnetic Based drill, and a bar with the end turned down for the chuck to grab, or a morse taper. The bar would be lined up in the holes with a couple of tapered plastic bushings, and then holders for either flange or pillow block bearings to guide the bar. I achieved good success with a mag base dial, small punch, and hammer to set the tool depth, and hit target sizes. If you are doing a lot of line boring, there are systems out there that are all hydraulic, with variable speeds & feeds - a dream to operate, but priced accordingly. Some even have an attachment to set up a wire feed to weld up the bores, so you can bring it all back to standard.