Contouring and CVV was Re: DeskNC for Windows experience anyone?

--- In CAD_CAM_EDM_DRO@y..., "ballendo" <ballendo@y...> wrote:

> Jon,
>
> Since EMC uses the start-next-accel-at-point-of-current-decel
> technique for contouring, I am sure you are correct that unintended
> consequences would occur if trajectory planning "inserts" were
> applied.
>
> In true contouring, the simple algorithm above is not used.
> Aceel/decel value(s) is based on delta angle(s), length of entity
> (es),type of entities (arc,line), radius of arc(s), etc. All the
> while considering feedrate which will affect total distance needed

to

> stop axis motion (at any given time). So there is sometimes no
> decel/accel between blocks, and other times accel/decel may take
> place over several blocks.

If one were to combine the level of continuous contouring, either
that found in EMC or the ones found in say Master5 or Flashcut with
an automated Cam system that could also react to distance to and from
a specified angular change and an automatic feedrate change, would it
not be possible to fine tune the system response, particularly say to
a certain devilish corner in a family of parts (say corners on a
door) such that the performance of the hobby class system approached
that of the $20,000 hardwired box?

There are some distinctions that I have seen between various
implementations of continuous contouring. Constant velocity being
only one permutation, and not necessarily the "best". One problem
still to be adequately addressed is the one of the "crack the whip"
effect found when cutting a tight centerline radius with a larger
cutter. The effective feedrate is multiplied many fold and unless
the controller is using cutter comp there is no data to control the
actual feedrate around a hairpin turn. This makes cutter comp a
minimum requirement for 2D continuous contouring.

The numbers for an example are a 1/4 cutter programmed around a 180
degree arc of radius 1/32 at a feed rate of 5 inches per minute.

The length of the 1/32 radius is PI*D/2=.098, the length of the
actual cut along the edge of the cutter is PI*D/2=.392
The effective feedrate from the swift turn is 5*(.392/.098)=20 IPM

This is a fourfold increase in feedrate from a simple 180 degree
turn. Increase the cutter size and the effective feedrate will also
increase proportionally. Imagine the effects of taking a 90 degree
turn? If the 90 degree turn is replaced with a radius, it becomes
possible to make a turn without coming to a full stop, but the cost
is accuracy. If you could sense the corner coming and programmed a
lower feedrate, the radius needed to "make it" around the corner
without missing steps or exceeding the servo following error becomes
smaller and closer to true position, at some point even, within the
feature tolerance. This will of course vary by machine, material,
and system setup. It may be impossible to fully address it in the
control algorithms, as many of the variables are not part of the
conrol system. Having a hybrid control however would allow one to
adjust for the part, material, tool, or whatever external variable
without having to reprogram the contouring algorithm for every part.

Best Regards, Fred Smith- IMService
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