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Vibration
I will try to make this as short as I can. Not easy but I will try.
Vibration is your second worst enemy in precision measurement of parts.
Cleanliness and temperature are the first priority.
When your machine begins to move, it is free to move with six degrees of freedom within the constraints of the design. TX,TY,TZ,RX,RY,RZ. Those are the translations and rotations about the coordinate system.
Because you have considerable mass in the moving components, as the machine begins to ramp up to speed, it is trying to reach equilibrium.
That is why when you accelerate a car, you feel the force of the acceleration UNTIL you reach a steady state of velocity. If you accelerate around a corner, you have more than just one vector tugging at you. Your CMM does the same thing. This also occurs when you slow down. Mass in motion tends to stay in motion.
This occurs in calibration when you probe the sphere and have the machine changing direction to take the hits. If the machine is moving too fast and does not have sufficient prehit/retract distance, what happens is the machine does not get a chance to reach a state of equilibrium before it takes a hit.
So you get some numbers that are the probe offsets. Now you measure a part.
If you measure a part with the same speed and prehit values used for calibration, your results will be more consistent than if you calibrate at one speed and measure with another. In other words, you are minimizing the effects of vibration between calibration and part measurement.
It is still there but, within the machine's ability to measure, it is more or less the same and can be accounted for.
The larger the machine, the more "elastic" it is and the more it will move around. The ram is not unlike a very sensitive fishing pole. If you place it vertical and move it parallel to the ground, the base moves and the tip then catches up. That is exactly what happens when you rapidly accelerate a cmm.
In addition, the cmm also tries to rotate around the point where the torque is delivered to the drive mechanism. This happens on all three axes. So when you move your machine in 3 axes, there are a lot of things going on and THEY ALL CONTRIBUTE TO MEASUREMENT UNCERTAINTY.
If you change any of those values from calibration to measuring production parts, you are adding more uncertainty to the mix.
Fast moves and almost no prehit are cool to look at but for the most part do not save time if the results from the part measurement are suspect.
I used to do a LOT of rifle shooting and one of the most valuable things I learned was that when a shot is fired from a rifle, the barrel begins to describe a very small figure 8. As the bullet goes down the barrel it will leave the barrel somewhere on that figure 8. If the time in the barrel can be adjusted so the bullet leaves at a node ( where the motion up or down ceases for a millisecond before starting back down or up) the rifle will throw the bullet at the same place ( within limits ). The trick is to adjust the powder load until the rifle does what you want it to do. Slow the bullet down is usually what it takes. Careful adjustments of powder loads will help you find that sweet spot. Then you can adjust the sights to move the bullet from where it struck the target to where you want the bullet to strike.
If the strikes are all over the place, then you cannot make a valid adjustment.
Same way with your cmm. If you cannot measure the same part and get repeatable results, you cannot tell what is wrong or how to fix it.
If you increase the prehit/retract distance and give the machine time to let the vibrations die out, you should ( if everything else stays the same ) achieve repeatable results. If the part, measured on a bench is different than the part you measured on the cmm, then you look at the program and work on how the machine is collecting the numbers you are seeing.
If you have problems with cleanliness, temperature or vibration, these things will influence how well you can measure.
Having a part to measure over the short term and long term will allow you to see the magnitude of the variation influencing the machine. If you don't chart, then you have no way of knowing what the process is delivering.
The rationale of process control is that the "process is the process and will deliver consistent results until SOMETHING IN THE PROCESS IS CHANGED.
Remember this also, if first article parts are delivered to you to check that were run at 75% and the part is acceptable, that if the remainder of the parts are then run "balls to the wall" they are not the same.....the process has changed......nuff said..
I could ( but won't
) go on but you have a general idea of some of the things that have an impact on the results you get from using a cmm to measure a part. There is a lot going on and paying attention to the little things means you can focus on the big things and make significant changes that reduce the amount of scrap and rework.
Anyway, I hope this helps. Have a great day!
Hilton
Vibration is your second worst enemy in precision measurement of parts.
Cleanliness and temperature are the first priority.
When your machine begins to move, it is free to move with six degrees of freedom within the constraints of the design. TX,TY,TZ,RX,RY,RZ. Those are the translations and rotations about the coordinate system.
Because you have considerable mass in the moving components, as the machine begins to ramp up to speed, it is trying to reach equilibrium.
That is why when you accelerate a car, you feel the force of the acceleration UNTIL you reach a steady state of velocity. If you accelerate around a corner, you have more than just one vector tugging at you. Your CMM does the same thing. This also occurs when you slow down. Mass in motion tends to stay in motion.
This occurs in calibration when you probe the sphere and have the machine changing direction to take the hits. If the machine is moving too fast and does not have sufficient prehit/retract distance, what happens is the machine does not get a chance to reach a state of equilibrium before it takes a hit.
So you get some numbers that are the probe offsets. Now you measure a part.
If you measure a part with the same speed and prehit values used for calibration, your results will be more consistent than if you calibrate at one speed and measure with another. In other words, you are minimizing the effects of vibration between calibration and part measurement.
It is still there but, within the machine's ability to measure, it is more or less the same and can be accounted for.
The larger the machine, the more "elastic" it is and the more it will move around. The ram is not unlike a very sensitive fishing pole. If you place it vertical and move it parallel to the ground, the base moves and the tip then catches up. That is exactly what happens when you rapidly accelerate a cmm.
In addition, the cmm also tries to rotate around the point where the torque is delivered to the drive mechanism. This happens on all three axes. So when you move your machine in 3 axes, there are a lot of things going on and THEY ALL CONTRIBUTE TO MEASUREMENT UNCERTAINTY.
If you change any of those values from calibration to measuring production parts, you are adding more uncertainty to the mix.
Fast moves and almost no prehit are cool to look at but for the most part do not save time if the results from the part measurement are suspect.
I used to do a LOT of rifle shooting and one of the most valuable things I learned was that when a shot is fired from a rifle, the barrel begins to describe a very small figure 8. As the bullet goes down the barrel it will leave the barrel somewhere on that figure 8. If the time in the barrel can be adjusted so the bullet leaves at a node ( where the motion up or down ceases for a millisecond before starting back down or up) the rifle will throw the bullet at the same place ( within limits ). The trick is to adjust the powder load until the rifle does what you want it to do. Slow the bullet down is usually what it takes. Careful adjustments of powder loads will help you find that sweet spot. Then you can adjust the sights to move the bullet from where it struck the target to where you want the bullet to strike.
If the strikes are all over the place, then you cannot make a valid adjustment.
Same way with your cmm. If you cannot measure the same part and get repeatable results, you cannot tell what is wrong or how to fix it.
If you increase the prehit/retract distance and give the machine time to let the vibrations die out, you should ( if everything else stays the same ) achieve repeatable results. If the part, measured on a bench is different than the part you measured on the cmm, then you look at the program and work on how the machine is collecting the numbers you are seeing.
If you have problems with cleanliness, temperature or vibration, these things will influence how well you can measure.
Having a part to measure over the short term and long term will allow you to see the magnitude of the variation influencing the machine. If you don't chart, then you have no way of knowing what the process is delivering.
The rationale of process control is that the "process is the process and will deliver consistent results until SOMETHING IN THE PROCESS IS CHANGED.
Remember this also, if first article parts are delivered to you to check that were run at 75% and the part is acceptable, that if the remainder of the parts are then run "balls to the wall" they are not the same.....the process has changed......nuff said..
I could ( but won't
) go on but you have a general idea of some of the things that have an impact on the results you get from using a cmm to measure a part. There is a lot going on and paying attention to the little things means you can focus on the big things and make significant changes that reduce the amount of scrap and rework.Anyway, I hope this helps. Have a great day!
Hilton
Taking care of the little things is what makes fixing the larger issues, possible...
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