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Need an accurate grid for backlash comp.

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  • #16
    That too

    Originally posted by Paul Alciatore
    All of the above is good, but backlash is only one type of error that can occur with an X-Y table. If the gibs are loose, the table can also rotate. This can easily produce 5 or even 10 thousanths of error in a position. If your screws are in fairly good condition, this rotation can easily produce a lot more error than the screws do.

    With readouts of any kind, rotation of the table can still produce errors even when the readouts are dead on. For one thing, the readout's scales are almost always mounted at an offset from the center of the table or the axis of the spindle. Even a pair of absolutely perfect scales would only guarantee that "perfect" accuracy at the point where the two scales intersect as viewed from above. That's the point they are actually reading, not where you are drilling or milling. If the table can rotate, it will and your tool will be displaced from that "perfect" location by some error amount.

    Proper adjustment of the gibs is very, very important for the best accuracy on any machine. For the best accuracy, any axis of movement that is not actually being moved for a particular cut, should be LOCKED DOWN - TIGHT. This is easier to do with a manual machine as a CNC one would need frequent pauses for operator attention when making most parts.

    And there are other sources of error in most machines.
    Thanks Paul.

    That nicely rounds it off and sums it up.

    That stuff that you posted re CNC and the non-locking/clamping of the non-operating slides is what has bothered me with CNC as it flies in the face of traditional practice.

    Mustn't forget the quill in the case of all vertical mills, the column clamps on a column mill and the knee clamps on a knee mill.

    I am surprised and disappointed at what lateral movement there is between the clamped and free conditions/movement/location of slide-ways (quills included). I expect that a lot of people will assume that it must be OK and don't check.

    It is an interesting exercises to say cut a face along the "X" axis (with the "Y" axis clamped) and take a medium-fast cut bot conventional and climb milling and read off a dial indicator reading lateral movement. It's quite a bit actually, and as said no DRO will find or identify it.

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    • #17
      Evan is right as usual... using glass scales for feedback requires a _lot_ of mechanical stiffness between motor and scale; any looseness anywhere (ballscrew wear, gibs, connector torsion, etc) will cause resonance if the gain is high (needed for good positional accuracy under load and for multi-axis sync). One useful technique as clutch pointed out is to use both.

      One company I worked with back in the 80s used digital encoders on their very accurate positioning stages; once the stage was close they would use A/D converters on the two raw analog sine waves from the encoders and subdivide that signal into 255 positions. They're (Anorad) is still using this technique with even higher resolution, I see from their website. We built a 30' long CMM using equipment from them and others; had to error map the thing using laser interferometers and a rather complex forward kinematics model..... it took months to get it to work right....

      - Bart
      Bart Smaalders
      http://smaalders.net/barts

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