Stefan,
If your cnc machine was set up as Dave suggests, you'd use a couple
of its "normal" features to make this easy:
1)Incremental motion, which is G91 (considers previous/starting point
as zero each time)
2)Canned cycles for "drilling" G81 (this makes your cnc file just a
set of locations, as each time the G81 is called, a series of moves--
designed for drilling-- are made. We wouldn't be drilling, but with
some creative use of the canned cycle parameters, we would achieve
the desired motion--as Dave already described.
If you add the ability to use Macros--which my cheap cnc machines DO
have--then it gets even easier. A macro is just a set of instructions
that YOU (or anyone) creates and holds "together" under one code,
like M345.
Now the dispensing can be done using either of these methods; you
will just have more control if Macros can be created and used. Not
all cnc machines have them...
So you would set up the canned cycle drill params for different
sized "dots"--same with macros--. The difference is that with macros
you'd have a DIFFERENT macro for each line, dot or even entire SMD
package that you want to "lay down" paste for. With the drill cycle
(s), you'd have to change the parameters for each set of dots of a
given size. Then do the next size, and so on...
Dave left out one critical part, which is that the stepper driving
the paste plunger needs to reverse a bit after each dot is placed.
This keeps a "string" from hanging between dot placements. (Any one
who's ever used a caulking gun knows what I'm talking about<G>) The
drill cycle "retract" does this for us.
Hope this helps,
Ballendo
PS. You will actually be using TWO drill cycles per dot. One for the
paste syringe, and one for the z axis movement. With macros these two
can be combined into ONE macro call. And the rest of the file, as
written above, is just the xy coords of your dots. (The canned cycles
are "persistant" until something else is called, so the desired
action takes place at each defined XY location.) My machines will do
this.
PPS. For this "screw drive" method of dispensing to work well, you
need a stiff syringe, and a SOLID mount for it. Because "flex" plays
havoc with dot size, as the syringe pluger moves downward. At the
top, the entire syringe tube can flex to absorb some of the downward
movement without dispensing; so as you get lower the dot size can
increase. Fortunately we have another couple "normal" cnc functions
which we can use to deal with this, which are tool offsets, and tool
wear offsets. Creative use of these will acount for the problem
mentioned.
And the "other" choice is to use a dispenser from (or modeled after)
those industrial units sold by EFD, and others...
--- In
Homebrew_PCBs@yahoogroups.com, Stefan Trethan
<stefan_trethan@g...> wrote:
>
> >
> >
> > This is one of those things you find out about machine control.
> >
> > with some practice, you find that by being 0.5mm from the pcb, and
> > starting the paste to flow and presising the syringe 0.1mm, that
some
> > exact amount is dispensed.
> >
> > this will be repeated on every occurance.
> >
> > One thing that comes to mind is the surface adhesion. If the
paste
> > does not adhere to the surface, it will not stick and the next pad
> > will get double.
> >
> > But, each of these things are things you can control.
> >
> > Dave
> >
> >
>
> You basically say whatever the machine does wrong, it does it wrong
the
> same way every time, right?
>
> What about the software, how do i get the file i need to feed the
cnc?
> No point in building a expensive cnc machine and then it is
impossible to
> get the right file.
>
> I reckon the "generating the file" thing would be even harder if
one
> thinks of pick and place?
>
>
> ST