[Lemonmcgee1]

Hi,

I’m looking at purchasing my first CNC mill, which is a used Hardinge GX 1000 and I’m a little confused about the accuracy provided by the machine maker.

They state the following:

Accurate positioning accuracy ISO230-2
- Full stroke positioning 0.01

Accurate repeatability accuracy ISO230-2
- Full stroke repeatability 0.005

Ball-bar accuracy, example accuracy 0.004mm on X-Y plane

The machine I’m looking at has a x/y/z stroke of 1020/540/540 mm

Does this mean that the machine has a positional accuracy of 0.05 over the entire stroke of the x axis, for example? Does this mean that over an 500mm stroke in the x axis it has a 0.005 positional accuracy? Likewise, the repeatability would also be halved if the stroke was 500mm in the x axis?

Sorry if this is a dumb question.

Some clarity on this would be appreciated.

Thanks,
Ferdo

[peteeng]

Hi Lemon- accuracy is a complex area. I have attached the std for your info. The first figure 0.050mm probably means that 50um is the expected error for that axis. eg if you send the axis to a co-ordinate it will be within 50um of that position. same for repeatability. If you make a 100mm slot it will be within 5um each time but that cut maybe 50um from where it should be. Peter

accuracy is how close the tool is to its intended position. take the 100mm slot. If the slots width is important then repeatability is needed. If its position in space is important then accuracy is needed. If both are needed then you need a very precise machine
repeatability is how consistent the machine is every repeat part
tolerance is how acceptable the result is.