Re: [CAD_CAM_EDM_DRO] Re: Interplotation?

Ballendo,

Thanks for the descriptions of interpolation. They do indeed sell
ech-o-sketches here. You said in the very beginning of your reply that if I
was to imagine a machine with just the X and Z axis then turning one of
these knobs would move the pen in four directions. Wouldn't it be turning
both the knobs at the same time? Turning just one will only move it along
one plane so it would only have two directions.
I have absolutely no idea what parabolic interpolation is all about but
I will have a guess and you can tell me how many miles I'm off. Is it that
the Z plane can also be tipped on it's axis to form curves that are vertical
to the bed. I only remember a parabola as being a curved line on a graph or
a mirror for a lamp. And that you can draw one by keeping the pen at a
distance that is shared between the two points but is always the same (The
draw the elipse with two pins and some string trick). So would parabolic
interpolation be that the Z will tilt so it will be able to reach under the
top of a curve? I'm thinking of something like a mirror from a torch on the
bed and it's standing vertical so there is an underside to it that is hard
to reach, it curves under itself. Anywhere close?

John

----- Original Message -----
From: <ballendo@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Monday, February 19, 2001 2:27 AM
Subject: [CAD_CAM_EDM_DRO] Re: Interplotation?


> John,
>
> Imagine you have two knobs(In our CNC machines these are hooked to
> motors, which are computer controlled). Turning one knob moves
> a "pen" up and down. Turning the other moves the "pen" left and
> right. (like an etch-a-sketch. Do they sell these in the UK?) So, you
> can "draw" lines in four directions by turning only ONE knob at a
> time.
>
> INTERPOLATION is what happens when you turn BOTH knobs at the same
> time.
>
> In cnc, there are 3 basic forms of interpolation; linear, circular,
> and helical. CNC machines also have THREE "knobs", called x,y, and z.
>
> In LINEAR interpolation, the knobs (motors) are turned in such a way
> that STRAIGHT lines are formed. (As Alan mentioned, in all
> directions.)2D linear interpolation means we can "draw" straight
> lines of any angle on a flat plane.
>
> Unlike the example with only two knobs above, many machines can ALSO
> move 'toward or away' from the 'left/right/up/down' plane. 3D linear
> interpolation means we can can "draw" straight lines in any direction
> in space (using the toward/away axis as well as the left/ right/ up/
> down).
>
> CIRCULAR interpolation means we can move the 2 knobs (XY usually,
> some machine controls allow any TWO of the three available) in such a
> way that we "draw" circles and/or circular arcs. This is usually "a
> step UP" from the ability to do linear interpolation only.
>
> You can see that it will be more difficult to turn the knobs to make
> a circle, than it will be to make something that LOOKS like a circle,
> but is REALLY a bunch of straight lines, done with linear
> interpolation.
>
> The reason is that creating a line requires only that we set the
> correct RATIO of feed between the knobs, and turn them for as long as
> needed to make a line of the proper length. Once we start, there are
> no changes to the ratio. Circles, on the other hand, are CONSTANTLY
> changing the RATIO of movement between the two knobs! And EVEN the
> DIRECTION of the rotation! This is why early cnc controls only
> allowed circular interpolation 90 degrees at a time. (Each 90
> degrees, the knobs will only have to turn ONE direction; at the start
> of the NEXT 90 degrees, AT LEAST ONE will need to CHANGE direction.
>
> HELICAL interpolation combines the two previous forms. Two knobs are
> turned such that they make a circular motion, WHILE the third knob is
> turned to make a linear movement. This combination will create a
> helical movement through space.
>
> Another form of interpolation which is usually only found on the most
> expensive machines is PARABOLIC interpolation... Can you tell me what
> this would be? :-)
>
> Hope this helps?
>
> Ballendo
>
> > > But
> > > what exactly does it mean in milling? That the software / mill
> can put all
> > > the axes' together into a set of 3D dimensions? Or is it
> something to do
> > > with how it controls each axis relative to the others?
> > >
> > > Thanks,
> > > John
>
>
>
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