James,
Didn't see the link yet, but usually you trade one source of error
for another. The probnlem with rotary-used-linear machines is that as
the distance from the pivot changes so does the mech.
advantage, "step" size, etc. And trying to make a linear move near
one of the pivots requires a nearly instantaneous change in direction
and requires acceleration far in excess of what most inexpensive
drives can achieve...
There's more, but these are the bigge's
Ballendo
In Homebrew_PCBs, "James Newtons Massmind" wrote:
>As far as "error components" goes, has anyone looked at reducing the
>number of joints required as a means of reducing error? And
interesting idea is that of using only two joints in a radial system
rather than the 4 or more required in a linear system.
> http://www.massmind.org/techref/idea/lccam.htm scroll down
> http://freeandeasy.sourceforge.net/ at the end of the paragraph
about
> "prototype XY table"
> http://freeandeasy.sourceforge.net/rotary.php
>
> Roller-skate bearings top and bottom on the main shaft were sloppy
(as
> expected) but when they are pressed towards each other, the
compression
> takes all the slop out and the result is very good. It reminded me
of what
> you were doing, but I don't think it is really the same.
>
> ---
> James Newton, massmind.org Knowledge Archiver
> james@m... 1-619-652-0593 fax:1-208-279-8767
> All the engineering secrets worth knowing:
> http://techref.massmind.org What do YOU know?
>
>
>
>
> Message: 21
> Date: Mon, 14 Jun 2004 01:53:10 EDT
> From: JanRwl@A...
> Subject: Re: bit cleaning while drilling PCBs
>
> In a message dated 6/13/2004 7:52:33 PM Central Standard Time,
> dg140@f... writes:
> How did you account for backlash in your homebrew CNC rig?<<
>
> You DID see the photo in the PHOTO section under "Jan Rowland"?
>
> I built that in '84, and was feeling CHEAP, so I did NOT spring for
the
> preloaded Turcite nuts sold by Ball Screws and Actuators, San Jose,
CA. I
> bought
> the "Single" Turcite nuts which have a 15/16"-16 threaded end for
fastening
> into the carriage or flange or whatever. I then carefully
bored/tapped a
> hole
> for 6-32 cap-screws through one side, and then I band-sawed through
that one
>
> side with the screw-hole so that inserting and tightening the screw
would
> "reduce" the I.D. This would tighten-up on the screw nicely, and
obviously
> a "feel"
> for HOW-tight was necessary. It worked, and has not been a problem
needing
> re-adjustment. These nuts are also more than twice as compact as
the
> (spring-)
> preloaded nuts, and MUCH less expensive. These screws are 7/16"
dia.
> 1/8"-pitch, 4-start. Thus, one turn = 1/2" movement. So each step
of the
> axis
> steppers is 0.0025". Thus, I had to write my "Coordinate recording
program"
> to
> round-off each X and Y typed in to the nearest 0.0025" (not
0.001"). I
> realized
> how cumbersome this would be (got USED to it!), but wanted the 1/2"
lead for
>
> SPEED of operation between holes.
> What about those PCB drilling machines, what do they do for
backlash and
> other error components?<<
> Of course, all lead-screws on such professional CNC $100,000
machines are
> covered with bellows ("way-covers", etc.), so they are not viewable
by a
> visitor
> not willing to exhibit violence. I am however POSITIVE they are
preloaded
> ball-screws! AND, the screws are surely "precision ground", not
the ROLLED
> type
> also available.
>
> "Other error components?" There are MORE? Uh . . . I'd guess
MUCH care
> and
> careful workmanship by folks who can READ (if you get what I mean!).