Lathe Levelling vs Live Center
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  1. #1
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    Default Lathe Levelling vs Live Center

    I finally got a chance to turn a 30” piece of stainless on the Celtic 14. I found that the end by the live center turns .009” smaller than the end by the spindle. From what I can tell, this likely means the lathe is not leveled. (Add shims under right front) Or...the live center could be off (center needs to move rearward)?

    How do you tell which is the case?

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    Start with a precision machinist level, once level you can think about adjusting tailstock if it needs it. Right now you are just guessing.

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    Quote Originally Posted by CJD View Post
    I finally got a chance to turn a 30” piece of stainless on the Celtic 14. I found that the end by the live center turns .009” smaller than the end by the spindle. From what I can tell, this likely means the lathe is not leveled. (Add shims under right front) Or...the live center could be off (center needs to move rearward)?

    How do you tell which is the case?
    Leveling one just DOES. No "tell" to it.

    Wear is the more demanding one.


    If it ain't brand new? The TS will be worn in several respects.

    Corrective measures well-covered "Right here on PM" and near-as-dammit universal to all makes of lathe. Also "non-trivial" as to time and effort, even if money not-so-much.

    Meanwhile?

    Learn the Old Skewl trick tens of thousands of hands had to use on "company" lathes we did not own and were not PERMITTED to correct, even if we knew how.

    Experiment with sliding a slice of cig-pack foil up to heavy kraft paper or even thin metal shim stock down along one side of the center as you socket it, and which "O'Clock position your one likes, different locations on the bed.

    That lets you MOVE the tip.

    Henri Rene Bruet even patented a version - "cheated" so to speak - by building adjustment right into the HBX's optional "capstan" tailstock at the factory.

    Pragmatic folk, the French can be.

    Or maybe just better at the fine art of being lazy than the global average?

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    For a quick answer to your question as to can the correction be made simply by offsetting the tail stock, the question of how consistent is the taper, must first be answered.
    You say the part is 30" long and tapers .009". Make marks with a sharpie every 1" along the parts length. .009/30=.0003 so, you should see the part grow/shrink .0003 every inch. If this is what is happening, then your machine is cutting straight, the tail stock must simply be adjusted.
    If however your measurements are erratic, for instance there is no change over some areas and a larger change in other areas, then the cause is much more complex. In that case is could be caused by bed twist, wear, head stock alignment, or most likely some combination.

    Either way, you really should level(align) your machine's bed anyway. Not only will it make holding tolerances easier, but will also avoid premature wear caused by misalignment. Also, it looks like your machine is of the type where the head stock can be swiveled, so head stock alignment should be checked as well as tail stock alignment.
    Do some searching here, this subject has been covered many, many times. Be careful though, not all the info out there is good.

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    On the top of what Derek said, if the issue is simply/mostly tailstock alignment, if you cut a 6" piece between centers (i.e. driven by a dog, not chucked in the chuck), you should observe a reduction of diameter at the tailstock very close to .009"

    Paolo

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    Er, you *are* turning it between centers, right? None of this idiocy of jamming the part in
    a chuck, and then putting a tailstock center in the other end.

    Right?

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    As has been suggested you have the cart in front of the horse turning with tailstock support without knowing both the condition of bed twist (leveling) and head stock alignment. An important question would be how much taper does your lathe turn when a rigid workpiece (at least 1.5" dia.) 6-10" long is held only in the chuck. If your lathe turns that test piece near perfectly then bed twist and HS alignment are likely very good, or at least working in concert with each other. If you then get taper on a long piece you can simply adjust it out with the TS. If you turn a taper on a piece held in just the chuck, you have other work to do before turning a long piece between centers.

    As was suggested by Jim, when using a TS it is most appropriate to turn between centers so the piece is not overly constrained. Funny things can happen when you mix chuck and TS support. The work will often change when removed from the lathe. That being said, plenty of folks use TS support when working from the chuck. In this case it is advisable to grip the work just on the end of the work with the last little bit of the jaws.

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    If bed is twisted and tail stock exact he would not see this as the tail stock would also be in the twisted position relative to carriage.

    Old school lesson...2 collar test to align tail stock then proceed.

    Sent from my SAMSUNG-SM-G930A using Tapatalk

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    Quote Originally Posted by Tony Quiring View Post
    If bed is twisted and tail stock exact he would not see this as the tail stock would also be in the twisted position relative to carriage.

    Old school lesson...2 collar test to align tail stock then proceed.

    Sent from my SAMSUNG-SM-G930A using Tapatalk
    Not sure what this means. He would not see what???

    Then proceed to what???

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    If the tail stock is exactly in alignment with head stock and turning between centers then the carriage would follow the twisted bed and the cut should be good, relatively speaking.

    Op reports a 0.009 taper turning between centers, that is what he is seeing.

    The photo is of a spool turned between centers to determine tail stock alignment.

    Adjust TS while making then use dial indicator after.

    The OP is observing taper between centers so the alignment of centers is simple first step.

    The tool can be made quickly from handy scrap.

    Once TS is in alignment then proceed with other adjustments such as leveling or twisting to compensate for wear.

    Lots of difficult to measure and correct things may be needed but simple 2 collar test should always be first if taper is between centers as it is way confirms centers are in alignment.

    If TS is high or low the 2 collar can compensate a bit but that is a higher lever of alignment.

    Sent from my SAMSUNG-SM-G930A using Tapatalk

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    Thanks guys...lots of food for thought. I'll start by double rechecking the levelling and move on from there. In following up on Derek's question, "yes" I am turning between a chuck and tailstock center. The way I cut my part true was to divide the .009" into the 30 inches and used the DRO to increase the cut directly proportional to the place along the bar. I came in about 2 inches early at the spindle side to allow for the chuck support on that end. So...it appears the error is proportional to the place on the bed.

    I have not noticed a difference in cut using the live center within 10-12" of the spindle, but then I have not done any precision work in that configuration yet. I will have to experiment further. I'll also have to order a center for the spindle, as I don't have one now.

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    QT:[ far- end by the live center turns .009” smaller]
    *I would never turn a 30" part to find the lathe error.
    One can run an indicator on a live center. (do that and report back to PM)
    One can measure a part , then come to touch the part with his bit or to a feeler gauge to see the expected error.
    One can touch off an indicator on the part to see expected error.
    Very common in grinding to feel a part before touching it with a wheel.
    Some times a grinder hand has only a few thousandths or tenths to take so cant have error in the first pass..
    *The same should be done on a lathe...and 9 thow is like a mile, one can see that with eyeball.

    QT:[this likely means the lathe is not leveled.] *Don't run out and buy a $400 level
    No, it could come from a number of things being wrong. tail not in line, bad tail live center. chuck end running out and tail perfect on a dead center, tail not centered or on height, tail center set at close to the chuck and bed not the same far out, plus a few more things might be wrong..Even the tool holder can turn and make a part get small ended...

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    [QUOTE=Tony Quiring;3477931]
    Op reports a 0.009 taper turning between centers, that is what he is seeing.

    Nope. He's holding one end in a chuck and one end in a tailstock center. This never works well
    if you are wondering about your lathe's accuracy.

    He's going to need one of those super duper levels, a laser, and a taught wire thing probably to
    sort this all out.

    Otherwise just turn the part between centers like they've been doing since before dirt.

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    Quote Originally Posted by jim rozen View Post
    He's holding one end in a chuck and one end in a tailstock center. This never works well
    So long as the chuck isn't wandering about on the spindle, I don't see the relevance of the headstock workholding after the first cut. If you need absolute concentricity, like when transferring a shaft between machines, then centers all day long. But if your cut depth exceeds the runout of the chuck I don't see the advantage.

    I'd lean toward an error in the tailstock.

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    Once I had a car. It was a nice car but it pulled to the right when it was going down the road, SO, I decided that in order to fix that problem I should put a huge tractor tire on the front left wheel. It solved the problem alright, but the left side of the car was really high and it made it difficult to get in, SO, I bent the frame so the car sat level. But then, the tire was rubbing on the fender, so I cut it away so it cleared. That worked great too, but it started pulling to the left when braking. I had the solution, I disconnected the left front brake. To keep things balanced, I also disconnected the right rear brake, you have to do these things in a pair ya know.
    I'd still be driving that car if I hadn't rolled it a week later in the auto parts store parking lot. I was going there to get some heavier springs that I needed to stop all the vibration.
    A while later I was talking to a guy who said it probably would have been simpler to diagnose the original problem by using a series of logical steps, in a prescribed order. He suggested checking the tire pressure first, then having the alignment checked, and some other stuff. I stopped listening, clearly he was an amateur and didn't know what he was talking about.

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    Quote Originally Posted by Cole2534 View Post
    So long as the chuck isn't wandering about on the spindle, I don't see the relevance of the headstock workholding after the first cut. If you need absolute concentricity, like when transferring a shaft between machines, then centers all day long. But if your cut depth exceeds the runout of the chuck I don't see the advantage.
    In a perfect lathe you're correct, but there is no such thing as a perfect lathe, even the best are somewhat off of perfect, just different amounts. Therefore gripping a part (tightly/deeply) in the chuck and placing the other end in the center requires a number of things to be perfect such that there is not a cyclical bending force imparted into the part by the lathe. This bending force depending on how large might create an hourglass or egg shaped part vice tapered depending on the condition of things.

    If you are going to turn a longish part between chuck and center, how do you drill the center hole, choke up and drill it first, then pull it out and rechuck, or hang it out and drill it hung out?

    If you are going to turn between chuck and center you must be mindful of what might be taking place.

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    Harsh reply removed.

    I understand this is less than ideal, but I don't understand why it doesn't work.

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    Quote Originally Posted by Cole2534 View Post
    I understand this is less than ideal, but I don't understand why it doesn't work.
    For an object lesson talk to CJD, the person who started this thread.

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    Quote Originally Posted by jim rozen View Post
    For an object lesson talk to CJD, the person who started this thread.
    Except that hasn't proven to be the issue, and thus my Q...

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    There is nothing inherently wrong with holding a work piece in a chuck and supporting the other end with a center. This is done every day to get jobs done, and in some cases, I'm thinking of very large work pieces, the only alternative. Care must be taken to be sure the shaft is aligned to the center, not influenced by the chuck and pushed to one side causing the center to bend the shaft, or some part of the machine, trying to keep it on center.

    I do not think that the fact that the OP is using this method is the cause of the error he is seeing. Because he has reported back that the taper he was producing was very consistent(meaning the part grew the same amount over a given length anywhere on the part), I suspect the issue could have been resolved simply by offsetting the tail stock the needed amount, but that's a guess.

    As far as evaluating the condition of a lathe goes(head stock/tail stock alignment, bed straightness, ect), holding something this way is not a good idea. It will create more questions than answers.

    It still amazes me that this topic has been covered as many times as it has, the correct and only logical procedures have been explained by people who do this stuff for a living, and yet so much misinformation is constantly regurgitated.

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