Accuracy vs repeadability

[baldysm]

I'm in the process of finding and eliminating CNC lathes that might fit my needs.

One of the things that confuses me is that accuracy and repeatability specifications are not the same.

What's the difference?

For my stuff (particularly threading), I need to tweak things to get it just right. Then I want to do the production run, and have them all the same.

Thanks

[rustamd]

here, these pictures are from wiki article about accuracy:

This one is accurate, but not repeatable:

http://upload.wikimedia.org/wikipedi..._precision.svg

While this one is not as accurate, but very repatable:

http://upload.wikimedia.org/wikipedi...w_accuracy.svg

[Geof]

Accuracy is how close the machine can come to meeting a defined standard; if you tell the machine to move 1.0000 inches how close does it come to moving that exact distance.

Repeatability, also known as precision, is how close it stops to the target coordinate; if you tell the machine to move to the point X-5.0000 inches how close does it get.

Because any move has two end points the accuracy has at least double the error of the precision; there is an error in where it starts from and an error in where it stops.

Accuracy also depends on the quality of the ballscrew and it is possible to have very good precision and very poor accuracy. But it is also possible to correct for this by comparison with a master standard provided the machine has good precision. The machine might stop in the 'wrong' place but if you know how far out it is and it stops repeatably you can use a correction factor. This is what 'mapping' the ballscrew does.

[baldysm]

Think I understand now.

Accuracy is how close it comes to moving a specified distance.

Repeatability is how closely it gets to a specified position.

So if consistentcy is my goal, repeatability is the number I should be looking at.

Thanks guys.

[blackc1]

Accuracy is how close it moves to your specified position, Repeatability is if it moves to the same place every time, that is if it is off .0001 inch per inch moved does it move to 5.0005 inches every time because you can program the difference if it is repeatable.

[JMcDonald]

Imagine you are playing a simple game of darts, just going for the bullseye.

If, after several throws, all of them are bunch in a 1 inch circle, but 12 inches to the left of the bullseye (big dart board :P), you have good precision, but poor accuracy.

If, after several throws, you find that your darts are scattered within a 4" circle that is centered roughly on the bullseye, you have pretty good accuracy, but poor precision (I.E. inconsistant).

I think that in the world of CNCing, precision is more important for producing multiples, while accuracy is more important in prototyping. For producing multiples, you can slowly factor out errors in accuracy, as long as the machine is "precisely" inaccurate (that is, it is off by about the same amount each time). For prototyping, you would be more concerned with the machine making exactly what you programmed it to, than if you can make exactly the same part over and over.

Though, I can't imagine a machine actually being accurate without being precise.

[JROM]

every machine ever built has small defects in its parts. Ball screws after all are made on a machine with errors of its own. So in its raw form all CNC machines have errors built into them. The way this is handled is in the control with Pitch error compensation. By using lasers or a ball bar you can test the machine for location at any point in its travel.
If there are errors in a location that repeat a value can be entered in the pitch comp table in the control to correct the mistake. That is what makes the machine accurate.