Re: [CAD_CAM_EDM_DRO] Was: Linear rail, accuracy in general

"John H." wrote:

> Hi,
>
> I noticed on this thread someone mentioned big high precision machining
> centres that have thermal management etc to keep them at real close
> tolerances at great precision. I remember seeing the working and result for
> how much a lump of steel extends for such and such a temperature rise and
> seeing how great it's effect was at these levels. But doesn't the wear on
> the machine's tooling contribute even more to inaccuracies? I mean the wear
> on a tool over tens of hours machining must be more than a few microns
> easily. Yay or nay? Please don't tell me the machine can factor in for tool
> wear? : )

On large workpieces, and large machines where the point where the nut drives
the axis is far from the actual location of the part, these expansions reach
levels of hundredths or even tenths of inches very easily. Workpiece
heating and tool and spindle expansion is even more a problem that
table and leadscrew heating, so these are the first places to look for trouble.

Tool wear is DEFINITELY compensated for. There are many schemes.
Probably the most advanced is using laser probes to gauge the tool size
every few minutes, or when each part or tool is loaded, and correcting the
tool size table. Other schemes have just plugged in new tool offsets every
so many parts, or every so many passes on the part. Another scheme was
using SCADA (which monitors the machine's performance) to gauge tool
wear by reading spindle horsepower demand, and correcting the tool offsets
based on how much torque the tool was absorbing. When the edge was more
worn, it took more HP, these forces deflected the tool more, so you had the
same effect as a smaller tool.

Jon