Re: [CAD_CAM_EDM_DRO] Re: Threaded Rod WORKS!

> Chris,
> What if the person has a 6" X 6" cutting area and the
> threaded rod is off by .005 or more over that distance. The
> machine will still be a learning tool and it can turn out great
> parts for RC cars and planes. Have you ever measured a handmade
> part. Looks great-works great but don't mic it out. I have
> been following lots of peoples progress. I see them selling
> their stuff off on Ebay after getting frustrated. To learn CNC you
> need a machine. After getting your feet wet you can then buy
> some expensive leadscrews. My guess is that at that point you
> will build a second machine using what you learned or buy one!
>

Bob,

The education value is of course there, and with carefull planning one
could probably replace the cheap screw with better ones as an upgrade.
However some of the problems can show up very quickly in certain
common uses of a machine.

If one only tried to use the CNC machine as an automated manual
machine - only feeding one axis at a time, doing circular bores with a
boring head, angles with a rotating vise, turning lathe tapers with
the compound, etc. Then yes you can produce good work with a machine
that has substantial absolute and cyclic errors.

The problem is that many people use the CNC capability to avoid
complicated fixturing by resolving angles and even arc segments into
X, Y, and maybe Z components. Then you get in trouble if the
increments in all these axis are not constant over their travel, and
in many cases (especially milling) also if you have backlash in your
screws.

The point of my story was that I thought I didn't need absolute
accuracy right away - didn't really care at first if my trumpet bells
were .020" oversize at 4" diameter. But having a cyclic error of just
a fraction of that superimposed nearly ruined the part, since curves
that were supposed to be smooth turned out wavy.

Chris Stratton