In today’s market CMM accuracy is usually stated as ISO10360-2. This is the statement of volumetric accuracy that is given dependent on the distance of the measurement you are making. The statement is usually done in a simple formula such as accuracy equals X.X + L/XXX.

The first part of the formula is a constant and should be considered in every measurement taken regardless of the distance traveled. The following are a couple of examples

Part Length is 500 mm, machine accuracy is stated as 2.4 + L/250

Substitute in the distance being measured in place of the L

2.4 + 500/250

2.4 + 2

4.4 microns

0.00018 inches approximately

Part Length is 1000 mm

2.4 + 1000/250

2.4 + 4

6.4 microns

0.00028 inches approximately

Some companies state the accuracy formula differently in the second term to make the accuracy seam better an example would be 2.4 + 4L/750. The formula would work out as follows

2.4 + 4L/750

2.4 + 4*500/750

2.4 + 2000/750

2.4 + 2.67

5.07 microns

0.00022 inches approximately

If you look at the two ways of presenting the accuracy 2.4 L/250 may not look as accurate at first glance as 2.4 + 4L/750, but if you work out the formula it shows that this is not true

Just an additional note some companies, will calibrate a machine quoted to ISO to the old B89 standard, there is a significant difference in the measurement testing. ISO requires you use certified length devises such as Step Gage or Gage Block, B89 uses a ball bar that is of one uniform length. Additionally, by measuring spheres instead of steps you average the probing error with the ball bar. If the machine is sold to ISO you should insist that it calibrated to the ISO standard to ensure that it meets the requirements

There are other parts of the standard that we are willing to discuss such as ISO 10360-4 and 10360-5 please contact sales@mastersprecision.com

The first part of the formula is a constant and should be considered in every measurement taken regardless of the distance traveled. The following are a couple of examples

Part Length is 500 mm, machine accuracy is stated as 2.4 + L/250

Substitute in the distance being measured in place of the L

2.4 + 500/250

2.4 + 2

4.4 microns

0.00018 inches approximately

Part Length is 1000 mm

2.4 + 1000/250

2.4 + 4

6.4 microns

0.00028 inches approximately

Some companies state the accuracy formula differently in the second term to make the accuracy seam better an example would be 2.4 + 4L/750. The formula would work out as follows

2.4 + 4L/750

2.4 + 4*500/750

2.4 + 2000/750

2.4 + 2.67

5.07 microns

0.00022 inches approximately

If you look at the two ways of presenting the accuracy 2.4 L/250 may not look as accurate at first glance as 2.4 + 4L/750, but if you work out the formula it shows that this is not true

Just an additional note some companies, will calibrate a machine quoted to ISO to the old B89 standard, there is a significant difference in the measurement testing. ISO requires you use certified length devises such as Step Gage or Gage Block, B89 uses a ball bar that is of one uniform length. Additionally, by measuring spheres instead of steps you average the probing error with the ball bar. If the machine is sold to ISO you should insist that it calibrated to the ISO standard to ensure that it meets the requirements

There are other parts of the standard that we are willing to discuss such as ISO 10360-4 and 10360-5 please contact sales@mastersprecision.com