Re: [CAD_CAM_EDM_DRO] Re: What makes a machine CNC ready?

In a message dated 6/30/2006 8:23:07 A.M. Central Daylight Time,
lcdpublishing@... writes:

Why would that ball screw fail in 5 years? I have never seen any
references published that state the life span of a ball screw is 5
years, 10 years, or 20 minutes. The only references I have seen are
first hand on industrial machinery - not the same as the rolled
screws but at least it is a comparison. Anyway, there are machines
with ball screws as old as 30 years still running that I am aware
of. There are also ones I have seen fail in about 6 to 7 years -
usually on machines where cast iron is cut. But I have never seen a
published specification stating that a rolled (or other) ball screw
will fail within 5 years under heavy, light, or moderate use.

Perhaps you could provide use with those references?

Thanks

Chris



Chris:

You are absolutely correct. I install, service, etc. industrial cnc
woodworking machinery. I have machines that I installed back in the mid 80's that
are STILL running with the original ball screws in them. I also have other
CNC machines that do not use servos but simple asynchronous 3 phase AC motors
and they use the metric ACME type threads, these wear out pretty often if they
are not oiled frequently. Of course the MTB puts an automatic oiler on the
ACME lead screws but once the oil from the factory is gone, the end user
rarely bothers to re-fill the reservoir, thus the screws go bad. Now, with ball
screws, I have seen machines that actually sat outside, poorly covered for a
year or more with ball screws on them and I have been hired to install them,
the one thing I have not had to replace are those ball screws, they seem to
still be fine. Also, there is NO economical way to completely protect ball
screws from wood dust in a factory plus the customers usually never replace
what protection is there and, again, these ball screws still function well
within the tolerances of the machines. Having said that, if you are cutting metal
and want to hold the tolerance to, say, less than .001", protection would be
needed. Also, metal might be more damaging in some cases. All of the ball
screws with which I have worked have wipers on the end of the re-circulating
ball nut that keep the actual screw cleaned off.

One note regarding woodworking machinery, the tolerances of the CNC machines
don't need to be more than +/- .005" as a general rule as wood just won't
hold anything that close, moisture and temperature has a huge effect on the
dimensions. Most of the woodworking machinery is therefore designed to actually
hold tolerances around +/- .001" so there is a bit of a "safety" margin to
obtain what the customer wants.

It all truly does go back to the application. If you want true accuracy,
dependability, reliability, and are going to be using the machine day in and
day out for 8 or more hours per day, you need to use ball screws. On the other
hand, if you are going to be using the machine for short times and maybe not
even every day, like a hobby machine would likely be used, you can save some
money with the more traditional lead screws and nuts such as ACME threads.

As previously stated, application does drive selection and design

Randy A.


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