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boring a long hole - workpiece or boring bar turns?

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  • boring a long hole - workpiece or boring bar turns?

    I have a long through-hole in a workpiece, 11/16" diameter by 10" deep.

    I now want to bore the hole round and straight before reaming to final size. The amount bored out isn't important; the reamer needs a straight hole to follow since the finished hole must be straight within .001". I could put up with .002". At .003", I'd have to buy another chunk of 4140 and try again.

    The largest practical size for a boring bar is 5/8". I can make bushed caps or drill supports as necessary, so the boring bar is supported close to the ends of the workpiece. [it wouldn't be "lost work"; the hole has to be that straight, and I'll be making more of those workpieces eventually]

    So the question is, do I put the workpiece in the chuck and feed the boring bar through, or do I put the boring bar in the chuck and put the workpiece on the carriage?

    Long ago I read an article that claimed one method was vastly superior to the other, for reasons I didn't quite understand at the time. Google is being cranky today and not returning anything useful.

  • #2
    I think the phrase you want to Google is "line boring".

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    • #3
      The old method was to mount the workpiece to the lathe saddle on the centre line and have a long boring bar in the chuck and supported by the tailstock. The adjustable cutting tip projects from a hole drilled halfway along the bar.

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      • #4
        When drilling, you are better off when the work is spinning and the drill is stationary. This is because the drill will function sort of like a boring bar if slightly off center.

        I don't think it makes a significant difference which way you do it when boring, although a 3/4 inch hole that is 13 diameters deep is a lot. Line boring might be a better idea for such a long, narrow hole simply because the bar is supported on both ends. It would require a lathe with more than 20 inches of carriage travel to do that 10 inch hole.

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

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        • #5
          You have choices.

          1) regular boring bar, very long... work spinning.. I'd worry about it drifting, and I would EXPECT it to bore tapered.

          2) Piloted boring bar, twice as long.. work spinning. Cutter gets adjusted out to set the hole size. Bushing in the spindle to guide the bar. I think that would work well, so long as you get the bushing and bar lined up nearly perfectly. still deflects a bit and could be tapered hole, you'd want spring cuts. Less deflection than #1

          3) boring bar like #2, but bar spins, and work is on the carriage. Also would work well.

          4) Like #2, or #3, but add a bushing in the work that is slid in place after the cutter enters the work. OK if bushing is concentric and to size, would cut down deflections. Basically line boring.

          There are other ideas also. I don't know that they are better.
          1601

          Keep eye on ball.
          Hashim Khan

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          • #6
            You want definetely line boring instead of boring bar with that kind of diameter/lenght ratio.


            Both line boring(tool spinning) and turning have their own set of potential problems especially on old and worn machines or when you have to push limits of accuracy.

            Turning: straight hole but could be tapered.
            Line boring: less likely tapered but hole can be crooked

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            • #7
              > line bore

              Thanks! That's the search term I needed.

              I don't really care of the bore is tapered; the reamer shouldn't care. I don't care if the bore is crooked; there should be plenty of stock to turn the OD concentric to the ID. It doesn't have to have a nice surface finish. It mostly just has to be straight.

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              • #8
                It would probably be best to do a line-boring type set up if you have the clearance on both ends of the part. Bushings and drill supports are all well and good but they are not rigid and could introduce potential problems with tool chatter. If you are going to eventually set up for multi part production then this job might be a good candidate for a gun drill.

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                • #9
                  I'd think about boring the hole through an 1" bar, centering the bore and then turning the bar to diameter or dia plus grinding allowance and get it ground concentric to the bore if that's what is important, just a thought
                  Mark

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                  • #10
                    Originally posted by J Tiers View Post

                    1) regular boring bar, very long... work spinning.. I'd worry about it drifting, and I would EXPECT it to bore tapered.
                    Why expect it to be tapered? The boring bar doesn't flex any more at the end of the cut than it does at the beginning; it's unsupported the same amount the whole cut. The only other reasons to end up with a tapered hole are technique, setup, or inferior tooling that dulls during the cut.

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                    • #11
                      Originally posted by Yondering View Post
                      Why expect it to be tapered? The boring bar doesn't flex any more at the end of the cut than it does at the beginning; it's unsupported the same amount the whole cut. The only other reasons to end up with a tapered hole are technique, setup, or inferior tooling that dulls during the cut.
                      Because the cut often DOES taper.

                      The cause seems (at least it is as good a theory as any) to be that there is usually a radius on the cutter corner. During the cut, that radius has both an end-wise and a radial component of force from the cut. The tool is forced inwards during the cut by that radial force, until it is balanced by the resistance to bending of the bar.

                      As the cut progresses with more passes, the tapering tendency is continued.

                      You end up taking a lot of spring cuts, and either the tool dulls or the spring just is not enough, but unless I use a sharp cornered cutter, I see a tapered hole. Not always MUCH taper, but there it is even if it is small. I have checked for dulling, and that does not seem to be the issue. The bars I use most of the time are available with a radius or a flat front, and the flat front do better, even if not perfect (Criterion and Bokum).
                      Last edited by J Tiers; 07-12-2017, 10:39 PM.
                      1601

                      Keep eye on ball.
                      Hashim Khan

                      Comment


                      • #12
                        I would look into 4140 tube. Comes in different wall thicknesses.

                        Bob

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                        • #13
                          Line bore.
                          Big as you can go boring bar between centres.
                          Work piece accurately bolted to the (preferably flat with t slots) cross slide.
                          Power feed preferable.
                          Turns the lathe into a sorta horizontal line borer.

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                          • #14
                            I'm thinking a line boring bar as well. But since you're after a final reamed size of 11/16" you're looking at drilling with a 5/8" drill and then using a line boring bar that is slightly longer than 20" long and slightly under 5/8" diameter. That boring bar is going to be as stiff as over cooked pasta. But with a few light passes and a couple (or ten) spring passes it should true up the bore to a good degree axially and get it ready for the final reaming.

                            I don't see how to do this without removing your compound and fabbing up a special work piece holder. It might be simply some sort of tool maker's vise attached to a base that fits into the compound's mounting table. But either way you need some method to hold the bar stock. You're going to want to have the line boring bar made up and ready to go and then mount up the bar stock and align it and then lock the gib on the cross slide down tight so it can't shift for the entire process. First off you'll drill with something like 5/8 or maybe 21/32" so you can use a 5/8 boring bar. Then you'll take a few light passes and a couple or six spring passes to get the hole bored out to true and the right size to ream. But 21/32" sure does not leave much room for any wandering of the drill bit. I'd be looking more at something like 9/16" for the boring bar and drill to 5/8" so you have more wiggle room to deal with any runout.

                            All in all a real brain teaser in how to do a job to that degree of accuracy.

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                            • #15
                              make a long boring bar with the cutter in the middle so it can be held between centers. make a fixture the hold the part in the carriage. feed the part over it. it is called boring between centers. I have done it before.

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