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Saddle lift up when internally threading or boring

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  • #31
    Call it "ramping up"?

    A bit of a real world example might be a wide drawer in a poorly made dresser. Pull or push it closed from the center and the drawer moves just fine. Try to pull or push it from one end and it cocks and jams. In Challenger's case some sort of binding makes the carriage hinge on the spot that binds and the front of the carriage ramps or cams up the slope of the ways until the limit stop that runs on the lower side of the rail stops the rise. In Dennis' case I suspect it's a combination of the high pressure point of the cut combined with the boring bar being well to the left of the foot print of the carriage generating a twisting torque on the whole thing that combines to reduce the pressure on the prismatic way and "ramp" or "cam" it's way up out of proper contact. A bigger heavier carriage and apron would counter this.

    Come to think of it the way we push and pull the carriage from the front edge in the apron isn't helping either. There's going to be a slight torque on the whole setup that tries to twist the carriage which is resisted by the prismatic way.

    Ya know.... thinking about this as I type it and considering how a lot of smaller lathes tend to use a box bed starts to make sense and make me think that the folks that designed lathes like the Myford knew what they were doing. A box bed resists these issues by using the flat edges of the bed to absorb the torque effect. So even a lighter carriage assembly can potentially do cuts that involve extra torque without twisting and, in this case, lifting.

    I mentioned earlier about the idea of using a weight or spring bar to the ceiling. And I also just realized that all you might need to do is lean on the front of the carriage a bit during a pass to hold the carriage down and in close contact with the bed. That may be all it needs. Bigger lathes with heavier carriage and aprons don't have this issue. So a little extra weight from us might do the trick.
    Chilliwack BC, Canada

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    • #32
      You may not have any ID threading to do but how about sticking the boring bar you used back in and for giggles just with the lathe off engage the thread feed and then on the end of the bar push away and to the right like the forces would be during a cut. See how much it takes to cause the front of the carriage to lift and quite possibly twist as it rises. Vary the angle of your pushing from straight cross to more inline. The way the carriage responds might indicate the cutter's resistance to some degree.
      Chilliwack BC, Canada

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      • #33
        Well, remember that the idea of pushing "up the slope" of the V requires some looseness somewhere.

        Dennis (the OP) has stated that the back way has tight gibs, but the front way does not. I do not know if we have been told what SORT of way (V or flat) is on his machine, whether the rear way is V or flat. It is a Southbend "copy" to some degree, apparently, and Southbend used front and rear V ways. Some "copies" do not.

        And, don't forget that the rear gib may prevent the carriage from lifting straight up, but still may allow twisting, or even lifting at one end or the other (rocking). If the middle is still snug, it cannot lift straight up, but if the ends are worn, it can still rock.

        The down force which is always present on the front way with normal lathe usage will add resistance to the carriage "sliding up the V", as will gibs. That "usually" make the problem non-existent.

        Two people have reported similar effects. In both cases, there is a twisting force in action, as well as a straight "push" sideways. In one case (the OP), it is when using a boring bar and threading. Despite minimizing the stickout, it is inevitable that the force will be applied farther to the left of the crosslide centerline than is normally the case. So, since there will be some radial force on the bar, there will be a larger than normal twisting effect on the carriage.

        The taper attachment, cited in the other case (post 29), will also create a twisting effect, since the friction of the taper attachment slider is out past the rear of the bed, and the driving force for the carriage is applied at the very front of the carriage.

        Even normal usage produces some twisting effect, due to the offset of the location of the tool vs where the driving force for the carriage is applied. That produces extra wear on the front and rear of the carriage wings (I mentioned this in post 11). The resulting wear pattern often allows the carriage to twist vs the ways, or rock somewhat in the direction along the ways. Often this is referred to as "rocking horse wear", due to the similarity of the action to the tipping of a child's rocking horse on the curved "rails".

        That wear can occur even with the double-V design, but is obviously easier when there is only one V guiding the carriage. It is noticeable even in normal use when reversing carriage direction produces a change in the depth of cut on a workpiece. If that happens, you have "rocking horse wear". Often the center part of the carriage is slightly relieved when the lathe is made, in order that the wear shall not allow the shifting to start quite as soon, but eventually there will be the situation whre the center of the carriage is touching, and the ends are slightly more worn.

        OK, so what?

        Well, the wear tends to effectively "deepen" the V of the carriage in the worn area. The sides of the V are farther apart, and the carriage in that area must drop down in order to come into full contact with the V of the bedway. An off-center down force is ideal to cause that.

        In one case, the carriage is allowed to twist slightly, since the V is not in full contact, producing a small gap between one side of the V and the matching surface of the carriage while the other side is brought into contact. That makes the carriage less stable. The shifting produces different cut depth depending on which direction the carriage is moved, and can allow chatter etc. And it looks to the user as if the carriage is lifting slightly, because of the gap. Or the carriage may in fact be lifting as well (see next).

        Also, the wear can be such that the carriage is allowed to rock slightly, with the end of the "wings" lifting at one end and dipping down at the other, tipping on the "high spot" in the middle. The OP is mentioning that type effect. It will probably be the most obvious when there is an offset force with a good deal of leverage, such as a boring bar, which is necessarily offset from the crosslide centerline.

        A combination of these effects can cause the carriage to ride up the V of the bedway, as well. Loose gibs obviously make that more likely.

        The solution is to re-scrape at least the carriage, so that to ends of the carriage V ways come back into full contact with the bedway. Alternately, simply relieving the middle area of the carriage V ways may help the situation, moving the contact out further toward the ends of the "wings", and tending to eliminate the "high spot" in the middle.

        It would be very good to have pictures of the lathe, and also if possible pictures of what the OP is seeing.
        J Tiers
        Senior Member
        Last edited by J Tiers; 04-22-2021, 02:17 PM.
        3313 5160 4357 4344 3174 9120

        Keep eye on ball.
        Hashim Khan

        Everything not impossible is compulsory

        Birds are NOT real, they are spying on you

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        • #34
          Originally posted by DennisCA View Post
          .....

          I will also try and get some pictures of the areas in question, until then the best place to see what lathe it is is to go here:
          http://www.lathes.co.uk/rexvalter/

          It's the 118.
          I found a few pictures of the 118 on the general web and this lathe does in fact use front and rear V ways for the carriage.

          Chilliwack BC, Canada

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          • #35
            Originally posted by BCRider View Post

            I found a few pictures of the 118 on the general web and this lathe does in fact use front and rear V ways for the carriage.
            In which case it is likely that both front and rear have the "rocking horse wear" to some extent. How much? Is it enough to be an issue? We do not know, it sounds like it is a factor, but there could be some totally other thing going on.

            There can be enough wear on the rear way that it does not "seem" loose, but still can move. One cannot always judge the "slop" from lifting, usually other movements are easier to find slop in. And of course the gib may be worn in the same way, depending on how it is tightened, so that it will not lift,, but allows twisting.

            It looks like a nice machine.
            3313 5160 4357 4344 3174 9120

            Keep eye on ball.
            Hashim Khan

            Everything not impossible is compulsory

            Birds are NOT real, they are spying on you

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            • #36
              According to the link, it has both V and flat ways:
              The 6-inch wide and unusually deep V and flat-way bed had cast-in feet, was hardened to 230 Brinell and braced between its walls by substantial U-shaped cross members.
              http://pauleschoen.com/pix/PM08_P76_P54.png
              Paul , P S Technology, Inc. and MrTibbs
              USA Maryland 21030

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              • #37
                Originally posted by PStechPaul View Post
                According to the link, it has both V and flat ways:
                There are pictures of the 118, but nothing shows the bed, there is just the text, which has no confirmation, and might be wrong. The more square looking VF 140 clearly has double V showing in the picture of the bed, no idea if they shared that or not.

                A picture of the OP's machine would make all clear.
                3313 5160 4357 4344 3174 9120

                Keep eye on ball.
                Hashim Khan

                Everything not impossible is compulsory

                Birds are NOT real, they are spying on you

                Comment


                • #38
                  Originally posted by BCRider View Post

                  I found a few pictures of the 118 on the general web and this lathe does in fact use front and rear V ways for the carriage.
                  Yes that is true

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                  • #39
                    Should show pretty well what travels in what


                    I've planned to take the apron off some time so I can show the tab, will see if I get the time to try some boring or test threading over the weekend.
                    DennisCA
                    Senior Member
                    Last edited by DennisCA; 04-23-2021, 12:36 AM.

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                    • #40
                      Yes, very good thank you.

                      it serves also to make some reminders......

                      1) the rather heavy looking apron is present, and tends to hold down the front of the carriage, although that is not an absolute prevention of lifting by any means.

                      2) There is also a leadscrew. It is a discouragement of lifting as well. The pictures of the innards indicate that there is a part which the leadscrew goes through, which tends to hold down the carriage, as does the pinion for the rack. Of course, the fit is a bit loose, no doubt, and that also is not an absolute prevention of lifting.

                      3) the carriage wings extend toward the headstock significantly. If the boring bar is at minimum extension for the job, it may not extend past the carriage wings, so it has less ability to lift any part of the carriage. Again, not an absolute prevention, though.

                      4) The carriage seems longer than it is wide, which should give good alignment, and tend to reduce twisting action, etc. But even that is not a "prevention".

                      5) If there is much wear on the bed, the carriage will be lower than originally intended, and the leadscrew may be tending to lift up the carriage a bit, especially when the halfnuts are engaged. That might be periodic, and lift farther if the leadscrew has a bend in it.
                      3313 5160 4357 4344 3174 9120

                      Keep eye on ball.
                      Hashim Khan

                      Everything not impossible is compulsory

                      Birds are NOT real, they are spying on you

                      Comment


                      • #41
                        Based on what you posted about the wear and how when set back towards the tail it binds that wear on the bed is always going to be matched by wear on the saddle. So JT's "saddle wear" issue is certainly present to some degree.

                        I'm still thinking that this is mostly about a relatively light carriage and apron combined with an insert that needs more pressure to do the cut than it might if you used one intended for aluminium (the keener edge and better relief angles for a more aggressive bite) or used a keenly fresh off the grinder HSS cutter. I know that even fighting with a cheap insert cutter how once the nose crumbles even a touch how much more pressure I needed to use at the handwheel. And how much I needed to use some fairly high magnification to see the damage.

                        That or I'm still wondering if you tried to use an OD threading insert on the boring bar?
                        Chilliwack BC, Canada

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                        • #42
                          Try running the operation in reverse. This will push the carriage out instead of in.

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                          • #43
                            Originally posted by BCRider View Post
                            Based on what you posted about the wear and how when set back towards the tail it binds that wear on the bed is always going to be matched by wear on the saddle. So JT's "saddle wear" issue is certainly present to some degree.

                            I'm still thinking that this is mostly about a relatively light carriage and apron combined with an insert that needs more pressure to do the cut than it might if you used one intended for aluminium (the keener edge and better relief angles for a more aggressive bite) or used a keenly fresh off the grinder HSS cutter. I know that even fighting with a cheap insert cutter how once the nose crumbles even a touch how much more pressure I needed to use at the handwheel. And how much I needed to use some fairly high magnification to see the damage.

                            That or I'm still wondering if you tried to use an OD threading insert on the boring bar?
                            No it's a chinese internal threading bar and insert. I've been meaning to make my own and beefier boring and threading bars.

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                            • #44
                              Here is what i woukd try....mount a stout boring bar with about 6 inches stickout.. mount dial indicator from carriage to ways. Different locations..... in each one ..grab boring bar with both hands, pull/push up and down and sideways .... and see what the dial tells you..

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