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