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« on: December 15, 2019, 11:24:08 AM »
hyildiz,
First I must say, in a positive manner, that I appreciate your quest for perfection which is desired
but never achieved. So here is my reality check!
There are TWO standards relative to this thread on positioning error. One is electronic and one is
mechanical and idealy one should agree with the other. The electronic standard (ie; closed loop
and servo related ) I am going to leave to those folks that have in-depth knowledge of that
"system". Doubt you any way to confirm the electronic standard to a traceable basis.
The mechanical standard becomes some "value" based on testing and how the testing is done,
ability to measure accurately, etc, etc, etc. Nothing is perfect, accuracy is a matter of degree, and
the "value" is will represent the "system" of total components.
Thus the real end quest, practical end product of your efforts, will be a "system" that provides for
some expected tolerence of that machines capability. Note machines capability, not machined part
final tolerence, since that is different matter.
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KNOW WHAT YOU HAVE
1.Should have manufacturers spec " Taiwan made single nut ballscrew directly attached to a servo
motor with coupling, ball screw grade, etc. Exactly what kind of coupling is used?
2.Torque specs for whatever relates to installation.
3.Calculated inertion etc required to move the 900# + table. There are programs that can be used
to determine axis system loadings.
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UNDERSTAND
1. All "systems" have backlash / play in movement, for both static and dynamic loading, since if
there were NONE the axis could not move. Just a matter of degree.
2.Know what you are actualy measuring. It can get difficult to isolate components of a system
to see how each component adds to the total.
3. You must be methodical, focused, meticulous as heck, for what ever you measure.
4. The smaller the measurement gets the more difficult it gets to acquire meaningfull data.
Other influences come into play when you get to the micron levels.
5. Do you have the calibrated tools to aquire the data and what are their specs.
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MEASUREMENT - for each axis
1. When you measure a screw you should actualy "profile" the screw over intended table
movement. For 30", with initial backlash removed, record measurements in 0.1" increments.
It's done for both directions of travel. Say you have a spec for a ground screw and it says
"0.001" / foot". What your interested in is how linear that screw is. I have seen all kinds of
variations, such as, it was very linear, or had a section / sections that varied, the reverse
direction profile varied from the other direction profile, but, overall it did meet meet the
spec. No screw is "perfectly" linear. A graph of measured movement relative to distance will
show the non liearity. The above is a PITA to do, rather quick and easy for me since I do it
opticaly.
2. Now note that depending on how the screw and nut is mounted there is play. How will you try
to measure that component of the system? For setting preload, a rationalization could be, to
have the static load condition set such that an measured applied torque minimizes play and
allows for the table to just move. Just making a point that a micon here and there for every
component can up quickly. How dose calculated force to just move the axis compare to
your measured force? Maybe one wants to minimize usage of available motor torque to move
the table so that gained torque is available for actual machining.
3. Inline solid coupling of the screw to a motor eliminates non linear movement of the screw due
to motor shaft rotation, BUT, depending on how it's installed, loads may be thrown back into
the motor. I had a bellows type that overtime started to add backlash to the system and the
backlash was not repeatable. Just another component which can affect measured table
movement.
4. Motor construction varies and can add varied / non-repeatable linear movement depending on
how installed or made.
5. 1 micron = 0.00003937" Human hair is approx 0.003" diameter. So 5 microns =0.0002"
Post collecting all the data, and in consideration of the data, you will rationalize to some practical
value of the mechanical standard.
HYILDIZ, Have fun and Happy Holidays,
RICH
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