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  • #31
    Originally posted by Forrest Addy
    If you are content with the finishes you get at present there is no reason for you to spend the money. But you weren't going to anyway; your mind was already made up.
    I would like to improve my finishes but nothing I have read has convinced me that converting to 3 phase should be my next step in that direction. I said right up front what my thoughts are on the single phase issue.

    FYI, classic chatter is a self-excited resonance like a bow drawn over a violin string. In a machine tool making chips some part of the cutting force loop (generally the tool or the work) deflects slightly unloads part of its strain, recovers, loads again, deflects, etc repeatedly, this raises a resonance whose frequency is governed by the spring in the system and the mass deflected (tool or work). Quite often chatter will emit a musical note easily heard over the machine's usual sounds. Drills often chirp but this is not chatter bourne of edge interaction with the work but the flutes scraping the hole walls. Noobs and those who should know better are apt to refer to as "chatter" any rhythmic pattern in the work surface, shadowing from congugate gear action, vibrations from motors, out of balance, rhythmic surge in the feed motion, tool recovery from interrupted cut deflections, and so on. In fact this is not chatter but gear telegraphing , phonographing, surge, etc. There are technical terms for most anything in industry. It's misleading and confusing not to use them properly.
    I scarsely mentioned chatter, except to say that the effects of torque variation should not be compared with chatter.



    Bodger you've done it again. You successfully raised an issue, made it contentious, lured people into choosing sides, started hares, fomented rebellions, raised straw men, and in general was agent provacateur for a topic usually eliciting yawns. In the end you state "well, I'm not convinced" and abandon the field to leave the combatants staring around in confusion. The CIA can use a man like you. Drop you into Iran and soon the whole country will be squabbling over non-issues.
    No doubt the CIA or maybe the NSA have a nice file on me but I doubt they will be calling on my services.

    Comment


    • #32
      Originally posted by The Artful Bodger
      I would like to improve my finishes but nothing I have read has convinced me that converting to 3 phase should be my next step in that direction.
      -Quite apart from any vibration issue, there's the fact that certain materials and certain cutters, notably various carbides, have optimal surface speeds and feed rates. An infinitely-variable VFD control makes it both easier to find that "sweet spot", and quicker to set it back to that speed when the next job comes up.

      Doc.
      Doc's Machine. (Probably not what you expect.)

      Comment


      • #33
        Originally posted by The Artful Bodger
        Thanks everyone for your contributions to this topic.

        Perhaps the effects of torque variations are detectable in some circumstances but having read the topic I have decided that I will not be spending money on a 3 phase motor and VFD in the expectation of being able to see a difference in the finish of my turned work.

        Well, you (and Evan) are only hurting yourselves, so go on bravely and suffer with your hair shirts, keep lashing yourselves with the cat-o-nine-tails........ WE are OK, and it won't bother us if you never use 3 phase.... some of us have seen night and day differences.

        (And I disagree somewhat with Forrest on chatter.... it has lots of variations... of course telegraphing isn't chatter, but chatter can look just like telegraphing. Forrest just hasn't used smaller more chattery lathes as much as some others who only HAVE them)

        PROVE it? To YOUR satisfaction?

        Why bother?

        Laboriously writing an explanation is time consuming and takes away from other activities.....

        And then the questioner dashes off a quick "well I'm not convinced, what about this other thing?" comment....... with the expectation that another long effort will be made by the damn fool writing.

        It is so easy to discount any argument and continue to stubbornly say "I'm not convinced, I don't believe it"...... it's like a game.... lead 'em on and make 'em work until they get tired this time, we'll get 'em again soon.....

        it only uses up willingness to answer legitimate questions, so it actually hurts everyone in the "community"..... if you do NOT want to know, just don't ask.

        it's just drawing water from the well and pouring it on the ground..... quit it, or otherwise the well will be dry for the next person.
        4357 2773 5150 9120 9135 8645 1007 1190 2133 9120 5942

        Keep eye on ball.
        Hashim Khan

        Everything not impossible is compulsory

        "There's no pleasing these serpents"......Lewis Carroll

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        • #34
          Originally posted by Evan
          It doesn't mean it is being transmitted by the drive train. Vibration can be transmitted via the static parts of the machine from the motor. It also isn't necessarily a problem cause by single vs three phase. There are many possibilities. However, if torque ripple were to blame then it should show up on all lathes that run single phase motors. It doesn't.
          It worked fantastic for me, the only thing that was changed on my lathe was the drive. If it was the 3~ power or the motor or both, makes little difference to me.

          Comment


          • #35
            If I were a younger fitter man so I could work from a grovel I would invite a few doubters to watch me start a cut in my lathe using the three phase motor at 60 Hz, disengage the feed back the tool away for safety, swap out motors (it would take a couple hours) resume the cut and in triumph point to the single phase motor signature in the feed marks on the work, then restore the lathe to three phase and point to the smooth superior finishes on either side of the corrugated looking finish resulting from a single phase motor. Same tool. same material, same feed, same speed, sdame depth of cut, - but different motor.

            And that's with the single phase motor mounted on the floor entriely unattached to the machine except by the V belt and the three phase motor direct mounted on the transmission.

            The disadvantage in the "single phase finish" is in part esthetic but the other part is practical.

            Esthetic: Nobody likes ugly. They like their work to reflect well on them and funny looking, unworkmanlike finishes do not enhance the shop's image. That's worth something but it's intangible and impossible to assess a cost for it.

            Speaking practically: In a smooth running lathe if you were turning a diameter on which a seal runs it would take about 30 secvonds to polish out the feed marks. If the lathe were prone to single phase vibration, it might take three to five minutes. Not a big deal but a PITA never the less. So what's a couple minutes? Maybe not much but sooner or later time accumlates into money.

            Move on to a ball bearing fit. Here you desire finish and an accurately sized, cylindrical fit on which to mount the bearing. Polishing does not improve roundness or cylindricity but if deftly done you can work from a small polish allowance to accurate diameters. It does add time.

            But why not go direct to size and bypass polish? Saves time and improves the fit. When your lathe is running smoothly, you can.

            More more than a few people, once they take a contrarian public position cannot be convinced against their will. In many cases public, physical, and undeniable proof can be set in front of a stubborn man and still he holds a position untenable to anyone with a shred of objectivity. Germ theory to the 19th Century medical community for example. To them incontrovertable proof and reprodicible results meant nothing. Preservation of their own rightness is everything, They would claim "My ship don't leak" as the water rises past their nostrils.

            So, I give up. You fellows with single phase lathes having finish problems stand on the line you've drawn in the sand.

            Those of you with single phase lathes having no finish problems probably wonder why the fuss. Indeed I do myself. I suffered disfigured finishes once, funished my lathe with a three phase motor and VFD and now my lathe produces work finishes rivaling those of a Monarch EE. Cause and effect.

            And for the record, I write to convey information I may have or can find to anyone; lurker, poster, or regular. The gadflies like the Bodger with their "convince me" atitude don't bother me.

            If there are a few I can help I take my reward in abstract satisfaction. Once in a while I get an eMail or PM from soneone I never heard thanking me for something I wrote that benefitted them. For me, that's the frosting on the cake.
            Last edited by Forrest Addy; 04-19-2011, 03:41 PM.

            Comment


            • #36
              Originally posted by Forrest Addy
              And for the record, I write to convey information I may have or can find to anyone; lurker, poster, or regular. The gadflies like the Bodger with their "convince me" atitude don't bother me.

              If there are a few I can help I take my reward in abstract satisfaction. Once in a while I get an eMail or PM from soneone I never heard thanking me for something I wrote that benefitted them. For me, that's the frosting on the cake.
              Which is exactly why I bother also...... but I have limits. One explanation, and some "clarification"....... if the doubters and "head gamers" persist, too bad, I am not playing that "game" longer than that.
              4357 2773 5150 9120 9135 8645 1007 1190 2133 9120 5942

              Keep eye on ball.
              Hashim Khan

              Everything not impossible is compulsory

              "There's no pleasing these serpents"......Lewis Carroll

              Comment


              • #37
                They say one picture is worth a thousand words so two are probably even better. I stayed up till 3:00AM doing these so do look at them.

                They show the relationship between the current in the motor windings and the torque being produced in both single phase and three phase motors. I do not claim absolute accuracy here, just an attempt to show the general relationships. The point is, the torque in a single phase motor goes to essentially zero two times in every cycle of the AC current in it's windings.



                This is essentially a 100% variation in torque that occurs 120 times per second. I can not offer direct proof that this will be transmitted by a belt drive to the spindle, but it would certainly be transmitted via a gear drive.

                On the other hand, because a three phase motor uses three phases that are out of phase with each other and all three of these phases contribute to the total torque, the sum of the three is always a positive number, never zero. In fact, the variation in torque is closer to 12 to 15 percent. Far, far lower than the 100 percent variation of the single phase motor.



                The mass of the rotationg load (spindle, chuck, and work piece) will certainly have a flywheel effect, but this will be the case in both single and three phase motors. So the difference in the motor's torque will still be a factor.

                My drawings show simplified motors and as the notes on them state, real world motors generally have more poles on the rotor or stator or on both. But this does not in any manner negate the conclusion. More poles would simply produce motor that rotate slower. The sequence between adjacent poles would be essentially the same and the resulting torque would vary in much the same manner. The motor's sped will drop more due to the load when the torque drops to zero than when it only drops by 12 to 15 percent.

                As for the vibration of the motor being transfered to the lathe via the mounts, that could also be a factor. However, a three phase motor can vibrate just as much as a single phase one and with a well balanced motor it may not be as much of a factor as the varying torque. I suspect that the variation in the torque is a much larger factor in this situation.
                Paul A.
                SE Texas

                And if you look REAL close at an analog signal,
                You will find that it has discrete steps.

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                • #38
                  Those a good diagrams Paul and they certainly demonstrate the variation in torque inherent in a single phase motor however they only show what is happening in the magnetic fields and do not show how much these variations are smoothed by the inertia of the spinning rotor.

                  Comment


                  • #39
                    Originally posted by The Artful Bodger
                    Those a good diagrams Paul and they certainly demonstrate the variation in torque inherent in a single phase motor however they only show what is happening in the magnetic fields and do not show how much these variations are smoothed by the inertia of the spinning rotor.
                    No, they don't. But that would be a similar factor for both types of motors. So the single vs. three phase torque difference remains even with that smoothing and any other factors.
                    Paul A.
                    SE Texas

                    And if you look REAL close at an analog signal,
                    You will find that it has discrete steps.

                    Comment


                    • #40
                      Actually, Paul's diagrams don't show what is happening in a single phase motor. A single phase motor is modeled as having two rotating fields in opposite directions. The first is the forward field and it exhibits a phase lag to the input AC. The second field counter-rotates with an additional phase lag to the first. The difference between the two rotating fields is the slip angle and is responsible for the generation of torque. Neither field is synchronous to the input AC and two of the three are always out of phase at any moment in time. Because of this the actual magnetic fields are never all zero at any moment in time. To further mix it up most modern motors have skewed rotors to reduce torque ripple. On top of that many machines have capacitor start-capacitor run motors which generate a large phase difference in the induced field and consequently have very little torque ripple even under heavy load.
                      Free software for calculating bolt circles and similar: Click Here

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                      • #41
                        So how do the thorists here account for the finish problems that disappear when a single phase motor is replaced with a VFD and three phase motor.
                        Last edited by Forrest Addy; 04-19-2011, 09:55 PM.

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                        • #42
                          easy - that's just proof that coincidences do happen

                          Comment


                          • #43
                            Originally posted by Evan
                            many machines have capacitor start-capacitor run motors which generate a large phase difference in the induced field and consequently have very little torque ripple even under heavy load.
                            While the "many machines" is subjective hand-waving without a shred of actual "proof"..........

                            At least there is a fact contained in that, which is that a RUN capacitor motor at least MAY have "very little" torque ripple. it isn't a guarantee.....

                            And if you "model" a 2 phase motor, which is what a "run cap" motor effectively is, you will find that the actual torque ripple MUST BE larger than with 3 phase..... All you have to do is overlay the two phases and see how 'deep" the "current notch" is....... because current is what torque comes from...... if the current ever goes to zero, the torque at that moment must be zero, and the minimum torque corresponds with the minimum input current.

                            I would suggest a survey, as most I see are apparently cap START at best......... with a very few run-capacitor types. More european machines may have run capacitor types.

                            And the term "very little" is subjective, needing a number put to it to be a valid "proof" of the argument.

                            However, Evan, at least, is putting forth possible reasons, and so is deserving of comment from those so inclined, where a simple... "you ain't convinced me of nothin, youngster" is "cheap" and deserves only the "cheapest" of replies, if that.

                            So.....

                            There is ONE winding in a single phase resistance-start motor...... the stator. Taking the simplest type of single phase motor, which is a 2 pole motor......

                            Then we have two magnetic poles, and a single winding (during "running") with two wires.

                            In the simplest possible argument, we have 2 poles which reverse polarity..... IF THEY REVERSE, there MUST BE a time during which the net magnetic field is zero, or so near to it as to make no difference. We also have two wires, which likewise reverse polarity of current, and THEY can reverse by no other means than passing through a zero current state, during which the induced magnetic field is zero or essentially so (since current produces the field).

                            There is NO OTHER WAY in which a "north" pole can become a "south" pole, other than passing through a state of essentially no net field, since the two pole states are opposites.

                            During the time in which the net stator magnetic field is zero, or essentially zero, there can be no torque input to the rotor, because the only coupling between them is the magnetic field, and no field is imposed on the stator by teh coil.

                            Any field in the stator iron from the rotor currents at that time represents a LOSS of energy, slowing the rotor.

                            Lets take a slightly different approach..... "energy".

                            Electrical energy is the product of instantaneous voltage x instantaneous current, integrated over time. obviously, for a product to be non-zero, both the inputs must also be non-zero.

                            if there is any time during which the current in the stator is zero, and we know that there MUST be such a time with a standard single phase motor, then there can be NO energy input to the system during that time.*

                            if there is no energy input, then input torque must be zero or negative also, representing "no torque" or "losses" respectively.

                            For teh situation to be otherwise, we essentially have to violate simple and easy-to-understand physical laws.

                            As for the magnetics and machine operation having no relation, well I don't know where the poster supposes the power and torque to operate the machine comes from other than the magnetic fields in the motor..... And the motor can accelerate rather quickly (see a previous post for a quick estimate) so arguments based on the "rotor mass" are highly questionable at best...... but he has put forth no argument of any substance, and so gets no further answer.

                            Suffice it to say that there SHOULD be a difference per theory, and most people trying it actually SEE a difference, in finish and general machine operation, so the best "proof" is giving it a try..... but if you are satisfied with the inferior finish from single phase, then you are evidently just not a "3 phase customer"....... and there's nothing wrong with being less fussy, is there?


                            * In a run-capacitor motor, energy is stored in the capacitor, and is available to the motor during the "zero" of input current.... which can provide at least "some" torque input at all times. The exact amount of "torque support" given by the capacitor depends on the motor and load.
                            Last edited by J Tiers; 04-19-2011, 10:48 PM.
                            4357 2773 5150 9120 9135 8645 1007 1190 2133 9120 5942

                            Keep eye on ball.
                            Hashim Khan

                            Everything not impossible is compulsory

                            "There's no pleasing these serpents"......Lewis Carroll

                            Comment


                            • #44
                              My Asian 12x36 has a capacitor-start capacitor-run motor and considering it came from China with it I assume there are indeed a great many lathes with c-s c-r motors.

                              No one doubts that the single phase motor produces less constant torque than a three phase but what no one has been able to show is how much, if any, of these torque variation actually make it all the way to the chuck.

                              Furthermore, everyone seems happy to accept that a change in torque causes a change in surface finish, and I am among them, but I am not convinced that the miniscule changes we are talking about cause visible effects.
                              Last edited by The Artful Bodger; 04-19-2011, 11:05 PM.

                              Comment


                              • #45
                                In a run-capacitor motor, energy is stored in the capacitor, and is available to the motor during the "zero" of input current.... which can provide at least "some" torque input at all times. The exact amount of "torque support" given by the capacitor depends on the motor and load.
                                As you well know but are refusing to acknowledge, energy is also stored in coils. Since the motor is not in phase with the AC input then neither are the minima of the fields. Also, the time of zero voltage is not the time of zero current. How can that be?

                                There is NO OTHER WAY in which a "north" pole can become a "south" pole, other than passing through a state of essentially no net field, since the two pole states are opposites.
                                Quite right. However, there is more than one pole in an induction motor and they don't all pass through zero at the same time. That is why they make rotors with skewed poles to take advantage of that fact.

                                As for the magnetics and machine operation having no relation, well I don't know where the poster supposes the power and torque to operate the machine comes from other than the magnetic fields in the motor..... And the motor can accelerate rather quickly (see a previous post for a quick estimate) so arguments based on the "rotor mass" are highly questionable at best....
                                The cycle to cycle interval at 60 Hz is 16.7 milliseconds. The transition through zero is essentially instantaneous and so is the time that the voltage is zero. I don't think the rotor is capable or responding to an instantaneous event let alone even a full cycle of only 16.7 thousandths of a second. That is why we have the concept of root mean square voltage.

                                Suffice it to say that there SHOULD be a difference per theory, and most people trying it actually SEE a difference, in finish and general machine operation, so the best "proof" is giving it a try..... but if you are satisfied with the inferior finish from single phase, then you are evidently just not a "3 phase customer"....... and there's nothing wrong with being less fussy, is there?
                                Examples of two very different finishing operations. The first is crap India made black iron pipe. The spots are coolant.



                                This second image is a setup that should ring like a bell if anything will. It is a thin wall aluminum drum that I skimmed by about .010 to clean up some scratches. Any suggestions on how I could improve the finish?

                                Free software for calculating bolt circles and similar: Click Here

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