Re: Question about angular and linear feedrates

--- In CAD_CAM_EDM_DRO@y..., "Dan Statman" <dan.statman@r...> wrote:

> Fred,
>
> Of course you are absolutely correct when taking into account true

4-axis

> 3-D work. But given the fact that low end controllers are not

handling the

> mixing of linear and rotary axes very well by using the linear axis

feedrate

> exclusively and then spinning the rotary axis at a value to keep

the move

> synchronous, which may be far in excess of a reasonable value,

don't you

> think it would make sense to limit the speed to the SLOWER of the

rotary

> feedrate or the linear feedrate.
>
> This obviously assumes that the feedrates are selected with

knowledge of the

> radius of the rotating part. By selecting the feedrates with this
> knowledge, I then do not have to have the control program know the

radius.

> and the only downside is that sometimes the part moves slower than I
> intended. This is not nearly as detrimental as moving faster than I
> expected. If it moves slower then the part takes longer, if it

moves faster

> the cutter breaks and the part is potentially ruined.
>
> Dan.
>
> ----- Original Message -----
> From: "imserv1" <imserv@v...>
> To: <CAD_CAM_EDM_DRO@y...>
> Sent: Sunday, August 04, 2002 9:10 AM
> Subject: [CAD_CAM_EDM_DRO] Re: Question about angular and linear

feedrates

>
>
> > --- In CAD_CAM_EDM_DRO@y..., "Dan Statman" <dan.statman@r...>

wrote:

> > > The points you state are obviously correct in that the

controller

> > has no way
> > > of knowing the radius of the spinning part to determine a linear
> > feedrate
> > > based on the rotation feedrate. My contention is this,

wouldn't it

> > always
> > > be a better compromise if when the feedrate is set to 1.0 then

the

> > linear
> > > axis should never move faster than 1.0 ipm and the rotary axis
> > should never
> > > move faster than 1.0 degrees per second, and the non-limiting

axis

> > would be
> > > adjusted so that the move is synchronized.
> > >
> >
> > Think in 3D rather than cylindrical surface only. Ther is no

reason

> > that the controller cannot be aware of radius.
> >
> > Assume you have a rotary axis parallel to the X. This would be
> > termed an A axis. You can vary the Z, and A simultaneously.

Don't

> > change the X at all for this example.
> >
> > You command a series of Z-A movements (Z=0 at centerline of A

axis).

> >
> > Z1 A90
> > Z0 A180
> > Z1 A270
> > Z2 A360
> > Z1 A90
> >
> > This forms a 1/2 roundish kind of part. You might of course want

the

> > inner area to even be concave and the Z=0 at 180 might actually go
> > negative. If you apply a single feedrate in degrees per minute,

or

> > degrees per second, you will find that this contour in very

difficult

> > to cut, with slow cutting near the center and fast rotation near

the

> > outer diameter of the part.
> >
> > If you now add in some X movements as well, the problem is further
> > compounded, but you finally see the whole picture.
> >
> > The solution for rotary 4th axis contouring is for the controller
> > software to evaluate each movement as a series of linear
> > approximations and apply the programmed movement in inches per
> > minute, with a limit of the actual maximum feed rate possible on

any

> > of the 3 axes, including the effects of the radius, as defined by

the

> > Z height. While it's true that there is also a Y component

available,

> > there is no supported software that will actually utilize that
> > component in a low cost product(yet). Therefore if the 3 axes

were

> > processed in the controllers (X-Z-A), it would result in a very
> > usable solution for the hobbyist
> >
> > Things to watch out for:
> >
> > 1) Some controllers cannot make a straight X or Z movement when in
> > rotary mode as there are no degrees defined in the move block, and
> > they do not maintain a linear and a rotary feed rate.
> >
> > 2) Some controllers will only program the radius at one size. Any
> > significant Z movement off that predefined cylinder will create
> > wildly fluctuating real feed rates at the cut.
> >
> > 3) Some controllers will pick the slowest axis no matter what.

and

> > then do not compensate for Z variation if the rotary is the

slowest.

> >
> > 4) Some controllers cannot differentiate between linear and rotary
> > movement at all and require the user to scale his axis and feed
> > rates, and even though the rotary is a function of the Z height,
> > apply all feed rates linearly.
> >
> > I wrote a program that post processed 3D linear G-code into 3D

rotary

> > axis with proper feed rates in Degrees per minute(360.999) for the
> > entire envelope I have defined. The theoretical movements and

speeds

> > were correct, but none of the low end controllers were capable of
> > actually processing the data. (lack of 4th axis coordination,

poor or

> > no continuous contouring, etc) This was before EMC and it may

now be

> > possible to process the data correctly, particularly since EMC

pre-

> > processes the entire file, and with a quadrupling of processor

speed

> > since I wrote the program 4 years ago.
> >
> > Best Regards, Fred Smith- IMService
> >
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