Re: Look Ahead

[Jon Elson <jmelson-at-artsci.wustl.edu>]

William Scalione wrote:

> I am curious about the look ahead function in EMC. I was doing some engraving
> in aluminum today that consisted of lots of  small squares. The cam program
> made
> the toolpath from zigzags. Anyway, at about 40ipm the corners of the zigzags
> were
> nice and square but at 100ipm the corners were noticeably rounded. Is this a
> function
> of the look ahead or is this something else?

Yes and no.  What EMC currently does by default is to begin ramping to the next
move trajectory as soon as it reaches the point at the end of the previous move
where it would have to decelerate.  If the second move is nearly a continuation
of the same line, then it is GREAT!  If the second move is a right angle to the
first, it is a disaster!  It can cause quite serious rounding at higher speeds.
This is because the point at which it would need to decelerate gets further from
the specified endpoint the faster the machine is moving.

> I also noticed that when I was
> milling the
> table (made from mdf not metal) to be true with the cutting tool, it would not
> go all
> the way to the end of the x axis if I had a high feedrate. see example code
> below.
>
> X0Y0
> Z-.05
> X20
> Y.25
> X0
> Y.5
> X20
> Y.75
> X0
>
> At a low feedrate x would go all the way to 20 but at a high feedrate X would
> only go
> to 19.75

That's a quarter of an INCH?!?  Yikes!  But, it is not really that surprising.
Increasing the acceleration parameter will reduce this effect, but you need more
powerful motors to handle the more rapid accel/decel profiles.

When you expect this sort of problem to crop up, you can put a dwell
command there (G04) or use the exact stop mode, (G61).  This will slow
things down, as it requires a full stop on most moves, but it will make the
machine go all the way to the end of a move before starting the next move.
I use this when drilling PC boards with tiny drills, to make sure the XY
position has settled completely before starting to plunge the drill in, and that
the drill is fully retracted before moving X & Y to the next hole.

Jon