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Pippin' is easy...lathe diagnostics question

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  • Pippin' is easy...lathe diagnostics question

    So moved on to experimenting with facing cuts today, that pip in the center slowly but surely disappeared, though I begin to understand the issue with some tool holders and their adjustment
    (the knurled nut/wavy washer/hollow hex bolt issues).

    Perhaps more importantly, the face is not quite flat...with a straight edge the closer you move to the center, it begins to very slightly rock (and I did notice it required a bit more effort to turn the handle...ahh, the value of a manual lathe to learn on ).

    This leads to a more general question, is there any sort of diagnostic process (flow chart like?) to sort out issues like this? Or is it just a single cause that a noob has to learn about? e.g. could slop in the cross feed nut account for this slight crown towards the middle...not sure that even makes sense but also as an example of where to look first?

    Got and have read both HTRAL and the "Lathe Operation" (Atlas book) and while they have both been extremely helpful, I don't recall anything of the "if this happens, then you do this" nature.

  • #2
    Put a piece of metal in your chuck,about 1 to 2" in diameter,and 6" sticking out from the chuck. Take light cuts on it to slightly reduct its diameter. They need to be light,and your tool needs to be sharp. HSS is best for sharpness. Take a few extra passes without advancing your cross slide. Measure the diameter of the cylinder you have turned. If there is a difference in diameters,your headstock needs alignment.

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    • #3
      Facing

      Most likely the carriage moved a little to the right unless you had it locked. Most lathes will make a very slight concave facing cut. A sharp HSS tool is good advice. If using a carbide tool, don't go past center, as the material is coming up on the far side of center and will often chip off the cutting edge of the carbide. If possible, I will put a center drill hole or do a drill operation first in order to stop the cutting tool in the hole and not have to worry about going exactly to center.

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      • #4
        Pushing away may be the problem, but have a look post #175 in the link for a better test.
        http://www.practicalmachinist.com/vb...13/index5.html
        The parallel is set-up in the 4 jaw such that when the spindle is rotated 180* the indicator doesn't move, in other words a 0-0 reading. That should be done at the end of the parallel. After that has been achieved, traverse the cross slide across the parallel. Ideally the indicator should read 0 to .0005" concave. Make sure you have a 0-0 reading across the width of the parallel, otherwise you may get false readings.
        If you get the desired results, then I would look at the push away situation. If the readings show a convex situation, then you have an alignment problem.
        Harry
        Last edited by beckley23; 08-04-2011, 11:12 PM.

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        • #5
          Thanks for the link...and I even understand the "why/how" of the test.
          It will take a few days to get done, mostly to either find or purchase new parallels.

          With it in print in front of me, I too suspect movement to the right [though I knew to lock movement down as much as possible, don't know why I didn't ]

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          • #6
            Using a magnifying glass look at the cutting edge of the tool and also look for rub marks on the tool below the cutting edge. If it is not sharp it will tend to push away from the work. Likewise if there is not enough relief it will push away from the work.

            Bob

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            • #7
              When facing, the closer you get to center, the more the tool will want to push away. You might find that you're turning a slight concave, then it goes high in the center. For a better evaluation of the degree of concave, use very sharp tooling with significant cutting edge clearance, and get it bang on height-wise. Take a few spring passes, with the carriage locked and the crosslide and compound gibs snugged.

              I've read that you do want some degree of concave as that is better for parts that have to sit together without rocking. A completely flat facing would be ok, but any convex would likely not be ok. Whether or not a manufacturer takes this into account when machining the carriage, I don't know, but of course it is important to have the headstock axis parallel to the bed- if this leaves you turning a slight concave- be happy.
              I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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              • #8
                Originally posted by gwilson
                Put a piece of metal in your chuck,about 1 to 2" in diameter,and 6" sticking out from the chuck. Take light cuts on it to slightly reduct its diameter. They need to be light,and your tool needs to be sharp. HSS is best for sharpness. Take a few extra passes without advancing your cross slide. Measure the diameter of the cylinder you have turned. If there is a difference in diameters,your headstock needs alignment.
                Please do NOT RECOMMEND headstock adjustments after the most BASIC of tests without massive instruction on how to do it just right, And the fact that its much more likey to be bed twist then headstock alignment if the lathe has not been set up right.

                the last thing a noob should do is try and adjust the headstock alignment. And by last I mean 'never untill hes no longer a noob'
                Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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                • #9
                  If your lathe is "normal". it was set up at the factory to always cut a slight concave surface when facing.

                  Al

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                  • #10
                    Locking the carriage down appears to have solved the "crowning" of the facing...I still think something is not quite right however...I took more note of the "how" of the cross slide movement and it feels like it takes more feed pressure from the 4 o'clock to 10 o'clock positions (no power cross, manual feed pressure as in effort to turn by hand).

                    The whole thing appears to be turning a taper...been brushing up on "solutions" so I guess at the top of the list soon is seeing how much run out the chuck imparts, double checking the crowning of the face via method suggested previously, checking level of lathe (not done since complete final assembly so...) then fiddle with tail stock end adjustments.

                    Edit Sat morning: you know how you have those moment of lucid thought just when you wake up?
                    While it dawned on me what the overall "goal" here is...to make one of those cylinder squares that is within your needed tolerances each and every time and then, if needed, to bore it out so that the removed "space" is now within a similar set of tolerances...with clearly any desired taper being the exception
                    Last edited by RussZHC; 08-06-2011, 09:32 AM.

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                    • #11
                      Now, at least within reason...

                      Spent most of the afternoon and early evening measuring and tweaking. Not saying its entirely "beat" but IMO much better than it was...
                      vertical lift of spindle = .0014 (so about double the small suggested by SB for similar lathe w bronze bushings/bearings)
                      run out chuck mounting plate edge = .0003 (doesn't get to .0002 either side of the zero)
                      edge of chuck body proper = .004 (.002 either side of the zero) [the more I look at it, now that I have a few to compare it to, this chuck is the more "beat up" of the older used ones; I think the 4 jaw will be much better esp once I learn to use/adjust it properly , next on the list]
                      chuck face = .003

                      used test suggested by beckley23 (above, and thanks again by the way, have enjoyed reading all your threads on PM...lots of info for a noob even if I can't use/fathom a fair percentage and the movement along the parallel is concave and falls somewhere around .00075 (could not find really fine dial...more than half an increment and could still see space between edge of indicator arm/hand and next increment)

                      read up on adjusting taper "out" and its now at the point where a single sheet of notebook paper gets stuck about 2.5" along the 4" blade of engineers square...may try and get it better but will let it "settle" first and decide how much "fussing" I want to do...

                      Thanks all for the suggestions and help

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                      • #12
                        Originally posted by Al Messer
                        If your lathe is "normal". it was set up at the factory to always cut a slight concave surface when facing.

                        Al
                        How could that be normal? A lathe is designed to make square and parallel cuts. I can't even imagine how that would be "designed in".

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                        • #13
                          Hi,

                          Because there is no such thing as perfect, many lathes were and still are made to face just a little bit concave. This ensures that if you set it up on end, it will stand upright without a wobble. Normally this little bit is around .001" or less depending on the diameter of the piece in question. This very seldom bothers the final part in any way.

                          dalee
                          If you think you understand what is going on, you haven't been paying attention.

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                          • #14
                            Originally posted by dalee100
                            Hi,

                            Because there is no such thing as perfect, many lathes were and still are made to face just a little bit concave. This ensures that if you set it up on end, it will stand upright without a wobble. Normally this little bit is around .001" or less depending on the diameter of the piece in question. This very seldom bothers the final part in any way.

                            dalee
                            Could you please explain how this is achieved?

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                            • #15
                              I would love to know as well.

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