Hi there
Say i put an indicator on my lathe turret and put its needle on the face of a turned part’s face While still clamped in the jaws.
Now i rotate the chuck by hand and move the indicator in the x axis also, for the part its in the radial direction.
What am i measuring?
My current assumption is total runout against the jaws axis.
I ask this because in the drawing of the part im required to measure flatness in clamped condition so if im correct, that total runout contains flatness in it
I'm guessing you have to move the indicator past the centerline in X.
That way you can prove or demonstrate that your X axis is truly perpendicular to the spindle axis.
For example if your X axis was not perfectly orthogonal to the axis of the spindle then you would cut a very shallow taper / dish into the face of your part. [Or if out of square the other way then a very shallow "pointy" cone / convex feature. (No crown -ing ).
Normally if you travel the indicator on the x axis towards the center the reading should be near zero but as soon as you
cross the center line and axis of the spindle if your X axis is not square then you will pick up readings that correspond to the
mirrored angle difference that the X axis is out of square by.
If you draw it out you'll see what I mean.
Otherwise you are just measuring the angular wobble in the spindle bearings ??? lol. (which on a lathe should be near as damn zero as can be.). [Similar sort of test would be good for 5 axis trunnion to check angular wobble on a C axis rotary (mainly a bearing alignment issue) ].
I guess it also proves that the part was not removed from the jaws at any point or shifted in position (ever so slightly) in cut.
That would basically prove that the turned face is flat AND the X axis is orthogonal to the spindle, as both conditions have to met for a "flat face" … (But obviously you have to cross the centerline as otherwise your indicator is just traveling perfectly parallel to the cut face even if the X axis is out of square to the spindle axis.).
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OP's question sounds like one of those
Joe Pieczynski
posed machining related brain teaser's / conundrums on youtube.
[In summary] IF you cross the center line along the X axis... It can show/Prove...
i.) The X axis is square to the axis of the spindle.
ii.) The part is indeed "Flat".
iii.) The part has not moved (in the slightest degree) in cut or from any other operations.
iv.) The part for sure has not been removed from the chuck (most likely) as there is no angular wobble on the face of the part,
e.g. wrong set of soft jaws that got accidentally swiped or swapped etc. (not that one could not spend a long time fiddling with the part to somehow true it up again, but if you have milled features that need to be orthogonal to the face it would take some skill + knowledge to really get it right especially for a longer part.).