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Saddle lift up when internally threading or boring

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  • Saddle lift up when internally threading or boring

    Last time I tried to thread something internally, every time the boring bar with the threading insert took a cut it would lift the saddle. This made it impossible to get a good internal thread.

    I don't understand why it results in saddle lift, to my mind it should be pressing the saddle down just as when turning an outside diameter or doing external threading.

    I also think it's probably not supposed to be able to lift at all, but it's there, you can lift the saddle a small amount before a stop prevents it going further. I wonder if when the lathe was new, this was precisely fitted so it would not lift? But does now. I've considered epoxying brass shims to the sliding area to prevent lift.

    Though I don't think it ought to happen in the first place. Any ideas?

    My lathe is a lot like a South Bend 9, modeled on that.

  • #2
    Its all about leverage. With a boring bar, the force is pushing down off to the side of the saddle, while a turning tool is usually directly over the saddle, or at least closer to it than a boring bar is. More distance, more the force has an effect.

    Shouldnt happen, no, the gibs should be tight enough to prevent that. Shimming the gib would be a good way to fix it. I had a similar problem on my lathe a while back, i just ended up machining new gibs out of some bearing bronze to tighten up the fit, made a world of difference

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    • #3
      it is possible that the cutting edge of the cutter is located below the axis of rotation of the workpiece

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      • #4
        Questions..........

        What size threads are you cutting and how deep a pass ??
        How big is the machine ?
        How far out is the bar hanging??

        The cutting force should be downward, but I have seen people thread with the insert upside down positioned against the back of the hole in which the forces would then be upward.

        JL..............

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        • #5
          Originally posted by pensioner View Post
          it is possible that the cutting edge of the cutter is located below the axis of rotation of the workpiece
          That's still downward force.

          JL..............

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          • #6
            I can't remember the answer most of those questions since I haven't tried to thread anything since around last christmas. But as I said the machine is a clone of a SB9. I always thread conventionally so the force should be downwards. I try and minimize stickout as much as is possible, I try and take as little a cut as I can per pass.

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            • #7
              If the saddle lifts at all your gibs are too loose. The force is down but leverage can cause it to get pushed up.

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              • #8
                The gibs (at the front) can't be set any better though. Non adjustable actually, but I machined them to get a closer fit. But when I adjusted them so there was no slop the carriage binds when it goes near the tailstock. So I am considering shimming the frame with brass shimstock to compensate the wear.

                Oh and I should specify, it lifts at the front only, the gib on the backside is adjustable and works fine, no lift there. So the saddle rocks a little when it lifts up in front.

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                • #9
                  Originally posted by macona View Post
                  If the saddle lifts at all your gibs are too loose. The force is down but leverage can cause it to get pushed up.
                  I'm thinking along those lines too. I'm not familiar with the model SB9 but if that saddle has some kind of hold down pads on the underside then try taking up play there by adding shims.

                  JL.............

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                  • #10
                    Most of the wear will be at the left hand end of the bed, so all you can do is tighten the gibs so it gets tight at the right hand end. You have already done this, so something extra will be needed. It will involve fitting at least a rear saddle lock which in conjunction with the front one can be adjusted to give a slight ammount of clamping in the portion of travel for the particular job. It would also be possible to fit locks at the right hand side of the saddle if it is prone to rocking. It would be possible to measure the rocking of the saddle on the ways if you were to strip it right down, and the centre part could be scraped slightly.

                    The Smart & Brown model A which I use at the museum has at least 0.025" wear at the left hand end of the bed, and a compromise had to be made with the gibs, also the rack is now lower at the left end, plus the leadscrew also slopes a little to even things up. Fortunately for us, the saddle assembly is very heavy, and I have never noticed it tipping when an overhanging boring bar is used.

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                    • #11
                      Originally posted by DennisCA View Post
                      ....................

                      Oh and I should specify, it lifts at the front only, the gib on the backside is adjustable and works fine, no lift there. So the saddle rocks a little when it lifts up in front.
                      Does it lift the whole width of the front?

                      Or does it lift the tailstock end only?

                      You probably have "rocking horse wear", which is very common. The ends of the carriage are worn more such that the V is wider at the ends. That makes them also looser, they are not down flat on the V-ways. So pressure on the headstock end will lift the tailstock end somewhat. (there will also be wear on the bed, a wide low area where the carriage runs the most.)

                      You can tighten gibs, but actually the only cure is to re-scrape the machine, at least partially. Tightening the gibs means that it will be too tight at the places that are not so worn on the bed, or if those are OK, then it will be too lose in the worn areas.

                      The actual wear may be different from what I describe, but it is almost surely the result of wear.

                      Your choices are to 1) deal with it. 2) re-scrape the carriage only 3) re-scrape or have the bed and carriage ground to fit.
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                      • #12
                        Originally posted by DennisCA View Post
                        The gibs (at the front) can't be set any better though. Non adjustable actually, but I machined them to get a closer fit. But when I adjusted them so there was no slop the carriage binds when it goes near the tailstock. So I am considering shimming the frame with brass shimstock to compensate the wear.

                        Oh and I should specify, it lifts at the front only, the gib on the backside is adjustable and works fine, no lift there. So the saddle rocks a little when it lifts up in front.
                        In an internal threading cut there's two pressures. One is the downward load on the top of the cutter due to removing the metal. The other is any radial pressure at the cutting tool needed to hold the cutter into the cut. USUALLY the radial pressure is quite light. So I'm thinking that something is off with the cutter geometry that is causing a binding in cut and this is increasing the radial pressure. And since that radial back pressure is higher than the support points at the bed and the lathe is fairly light it is levering the carriage up and back and showing up as lift at the front.

                        To me this suggests that part of the clearance flanks on your cutter are rubbing. Otherwise the downward pressure on the top of the cutter would be easily enough to hold the carriage down. You could either increase the clearance angles so it moves the heel back as shown in this sketch or you could use your regular angles but then grind in sharper angled secondary bevels below the short upper part. Sort of like the secondary relief on a drill bit to avoid the trailing heel of the end faces dragging and stopping the drill from cutting.

                        Click image for larger version

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                        Also keep in mind that the leading flank just under the cutting lip needs a few more degrees of angle than the trailing flank so that it maintains the cutting clearance angle as it moves forward far more fast than any regular cut. I think a lot of us forget or normally get away with using regular clearance angles on a lot of threading jobs. And that's fine for finer threads on a larger diameter. But we can end up rubbing the heel of the leading flank on coarser threads and smaller diameters.

                        Click image for larger version

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                        You can test this easily enough by using the threading tool to cut a "relief" on the front of the same size hole where you just ease the cutter into the work even at full thread depth. If it cuts a clean fine chip using very light pressure and does not need a bit of pressure to start the cut then the cutter is keenly sharp. But if the bar does flexes or doing this lifts the carriage then the cutter dull. If it cuts fine for this slow fed relief cut but we know it lifts the carriage under threading load check to see if there's any sign of the heel rubbing on the lower edge of the flank.

                        Also note that just removing part of the lower flanks won't avoid this. The primary flank clearance angle just below the edge still needs to follow what is shown in the diagram. You can still get this heel (or elbow?) rub even if you grind away a more aggressive secondary clearances to deal with the small diameter internal threading. What works for the very slow feed spiral when cutting such a relief isn't at all close to what it needs for the far faster thread cutting spiral. So that can cause a high back pressure in the bar when doing a threading pass. And it probably leaves a very chewy looking thread at the same time.

                        The other thing that will cause a lot of pressure to force the cut is if the cutter is not free cutting but needs some pressure to start and stay under the chip. Is the cutting edges dead sharp so that it takes off whisker hairs of metal at the slightest touch or does it need some pressure to push into the metal? If you're using a carbide insert perhaps it's become chipped on the very end? Or it's one of the styles with a "durable" edge that needs considerable pressure from a big machine using a burly size bar? Or it's been some time since you touched up the grind if it's an HSS cutter?
                        Chilliwack BC, Canada

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                        • #13
                          I've used the chinese carbide inserts, been meaning to try my own HSS instead. I could try and regrind the carbide insert first.

                          It's probably what you say, something wrong with the setup.
                          DennisCA
                          Senior Member
                          Last edited by DennisCA; 04-21-2021, 02:55 AM.

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                          • #14
                            Originally posted by J Tiers View Post

                            Does it lift the whole width of the front?

                            Or does it lift the tailstock end only?

                            You probably have "rocking horse wear", which is very common. The ends of the carriage are worn more such that the V is wider at the ends. That makes them also looser, they are not down flat on the V-ways. So pressure on the headstock end will lift the tailstock end somewhat. (there will also be wear on the bed, a wide low area where the carriage runs the most.)

                            You can tighten gibs, but actually the only cure is to re-scrape the machine, at least partially. Tightening the gibs means that it will be too tight at the places that are not so worn on the bed, or if those are OK, then it will be too lose in the worn areas.

                            The actual wear may be different from what I describe, but it is almost surely the result of wear.

                            Your choices are to 1) deal with it. 2) re-scrape the carriage only 3) re-scrape or have the bed and carriage ground to fit.
                            I don't think it's this because I can tighten the only carriage gib the machine has that is adjustable, which is on the rear, and it doesn't cause a bind over the travel of the bed when it's adjusted to be tight. And the gib lifts over the whole front when it happens.

                            The thing on the front isn't really a gib, it's more like a small tab that only seems to prevent it from jumping up all the way. I am not sure if the builders meant for this side ot be held down properly, maybe they thought it would be enough with a rear gib. Maybe this tab was only to prevent it from moving too much. Seems it has such little surface area it cannot be a gib meant to slide against another surface.

                            It's in the plans to rescrape the machine at some point though.

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                            • #15
                              If you use carbide inserts, then the ones designed for stainless are sharper than for steel, and those for aluminium are as sharp as HSS. The aluminium grades can be used on steel with lighter depths of cuts. They can cut at depths of 0.0005" for finishing and are really useful on steel if used gently. The lighter cuts will be less likely to cause the saddle lift.

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