Interpolated vs. dogleg rapids.

[ckirchen]

Hopefully this will help prevent a crash on someone else's mill...too late on mine.

If you change parameter 315 "Rapid -> HS Feed" to 1, the mill will use interpolated rapids instead of dogleg rapids.

An interpolated rapid travels from point to point in a straight line. So, for an XY move, X and Y will both change continuously during the rapid. A dogleg rapid, so named because of its shape, travels at 45° to the X/Y axes until either the X or Y axis reaches its final value and finishes the move parallel to the remaining axis.

The potential for crashes comes from the fact that most CAM software calculates interpolated rapids so that they clear the workpiece, clamps, and fixtures. For example, in ProNC, if you specify .1" clearance, it will position the rapids so that they clear the workpiece by .1" at all times. However, the controller doesn't know that, so when it sees G00, it runs a dogleg rapid that may collide with the workpiece.

The only downside I have found so far is that interpolated rapids are slower. I think this is because the controller must calculate the position during the move rather than once the move is complete. Also, on the Haas, it uses smoother acceleration and deceleration, like on a G01 move. On short rapids, though, I don't think it will be a major factor, and the smoother acceleration seems to run nicer.

An obvious solution would be to force the CAM software to rapid to a clearance plane that clears everything, but I prefer to use interpolated rapids for two reasons: 1. the clearance plane that clears everything might be a lot higher than the clearance plane required for interpolated rapids, which results in wasted cycle time and 2. if I happen to forget to tell the CAM software to use a high clearance plane, the potential for a crash still exists.

Like I said, hopefully this will help someone else prevent a crash. I just wanted to put my findings out there.

Cheers,
Chris Kirchen

[nian_id]

Originally Posted by ckirchen An obvious solution would be to force the CAM software to rapid to a clearance plane that clears everything, but I prefer to use interpolated rapids for two reasons: 1. the clearance plane that clears everything might be a lot higher than the clearance plane required for interpolated rapids, which results in wasted cycle time and 2. if I happen to forget to tell the CAM software to use a high clearance plane, the potential for a crash still exists.

In most CAM programs you can override the rapids by setting a rapid speed. It will output the move as a fast G01 so the controller won't dogleg.

[ckirchen]

Originally Posted by nian_id In most CAM programs you can override the rapids by setting a rapid speed. It will output the move as a fast G01 so the controller won't dogleg.

True, but if I miss one of the overrides (which I have already done), the controller will revert back to dogleg and the potential for a crash or gouge still exists.

[dougtyler]

I tried changing setting 315 on my mini mill.
With HS rapid set to one the machine seems to have a more gradual acceleration and deceleration. I've never had a problem with the difference between the CAM rapid moves being different thatn the machine rapid moves.
I do, however, like the softer start and stop of the machine, with the option turned on.

[Mic6]

I'm diggin' that setting

[ckirchen]

I found a potential problem with using "RAPID -> HS FEED" and I wanted to make those who use it aware of it: http://cnczone.com/forums/showthread.php?threadid=82134

Chris Kirchen