Re: [CAD_CAM_EDM_DRO] Threaded Rod WORKS!

> Select only a good piece of threaded rod. Leave the rusted ones
> at the store. If you use threaded rod on your machine, don't be shy
> tell them how it works! Accuracy, everybody talks about accuracy. What
> is everybody using their machine for. I never see any project being
> descussed here anymore.

Bob,

I can see that threaded rod may be a good way to start out in some
situations. In fact I probably wish I had prototyped my bearing
mounts by machining the ends of allthread, rather than the ballscrew
(which wasn't that expensive by itself, but can't easily be replaced
as the outfit had a $200 minimum order)

And many hobby machines may not need much accuracy, however there is
one aspect that I will call 'incremental precision' which they do
need. The primary reason I am building cnc machines is to make
calculated curves for musical instrument tooling. I did some of this
manually with a crude digital readout, not realizing that the readout
had a very small (maybe .002") amount of cyclic error due to the
unwise way I connected the encoder to the screw. This showed up in my
finished part as a very noticeable ripple superimposed on the contour
- it even transfered from the bell mandrel I had machined through into
the first few trumpet bells, until I carefully filed it out.

So, absolute accuracy may not be important for many applications, but
cyclic error can really ruin your day if you are machining curves or
even diagonal paths. Of course, cyclic error over a short distance
may be as likely to result from poor screw mounting as from errors in
the screw itself.

Chris Stratton


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Christopher C. Stratton
Engineer, Instrument Maker, and Horn Player
ccs@... 617 628 1062
http://web.mit.edu/~stratton/www/brassbuild.html