In the Post Processor, what are the units for *RAPID_FEED*?
In the Post Processor, what are the units for *RAPID_FEED*?
MitchB,
Rapid Feed is what ever your CNC Controller has been set to mm or inch.
It is the same as G0 so it will go Max Speed as set by your CNC Controller.
Regards,
Mauri.
Thanks, but I don't think that's it. *RAPID_FEED* is used only by the post processor to calculate the time spent doing rapid feeds. It is not used in the gcode. I set it to 2 and the result suggests that the units are unlikely to be in in/min since the resulting time is way too high.
It would sure be nice if this was in the documentation.
MitchB,
Sorry I did not explain myself a bit better.
Example from my Post for Mach3:-
*RAPID_FEED*
4000
*FEED_UNIT*
F"feed"
*FEED_SCALE*
1.2
"The RAPID FEED, relates to your machine maximum F Speed Velocity so if it is 4000 then that is what you put there"
Regards,
Mauri
MitchB,
The only way you can get a reasonably accurate time is to run a long G-Code on your CNC Machine and note that time down.
If all your CNC Machine Velocities (RAPID_FEED) on all Axis are the same then that is best for Time accuracy on madCAM Post.
Use the FEED_SCALE to then fine tune the Post Processing Time until it matches the CNC Machine Time.
This will be close to each Post Time vs Machine Time, but unless you have Linear Motors they will never be the same.
Remember you have friction as your enemy and that can change at any time.
Hope this helps.
Regards,
Mauri.
Thanks for persevering. I have a small Taig mill. My cutting feed rates are in the range of 10 inches/minute. Rapids are around twice that. I have set *RAPID_FEED* to 10 and *FEED_UNIT* to min (but this variable doesn't make a difference). The Post Processor says my part's cutting time is 12 minutes and the rapid traverses are 3 hours. Clearly there is something I am not understanding.
One thing I don't understand is your use of *FEED_UNIT*. The documentation says "*FEED_UNIT* // Set the unit for feed. min or sec. (this is only used for time calculation)", but you are using a different value.
I don't need anything close to accurate. I would just like the rapid traverse to be something that makes sense - like 2 minutes.
MitchB,
I use mm on our equipment.
You need to put the Velocity of your CNC Mill for *RAPID_FEED* it uses the same value for all axis.
So if it is 20 then put in 20.
*FEED_UNIT* use F"feed"
*FEED_SCALE* start with one then increase the value until you get the correct time.
Below is what I have on my Mill.
*RAPID_FEED*
1000
*FEED_UNIT*
F"feed"
*FEED_SCALE*
1.47
Regards,
Mauri.
With the following settings, I get 12 minutes of materials cuts and 2 hours, 54 minutes of rapid traverses. The gcode has feedrates of between 2 and 10.
*RAPID_FEED*
10
*FEED_UNIT*
F"feed"
*FEED_SCALE*
1
My current theory is that *RAPID_FEED* is in millimeters/minute. Maybe the intention is that *FEED_UNIT* and *FEED_SCALE* be used to convert to inches/minute if that is required, as in my case. The documentation is not enlightening.
MtichB,
This is a timing of a G-Code Post in Mach4 with my settings and it compares with the actual cut time on our Router Table.
Tool changes: not specified in post processor
Rapid traverses: 1 minutes 58 seconds
Cut in material: 53 minutes 42 seconds
Total machining time: 55 minutes 40 seconds
If you want accurate time you cannot make changes o your CNC Machine settings otherwise the time will change.
Example if you use Mach3.
Making setting changes to the following will change time compared to your Post time.
Velocity
Acceleration
CV settings or Exact Stop
Even Mechanical like tightening the Gibb's.
*UNIT_FEED* F"feed" This is you G1 F Speed in your generated Post, it uses it to calculate the time (you may have a different G1 for the Z Axis as compared to your X/Y, so his takes care of that).
*FEED_SCALE* 1 or greater is a variable used to adjust the time due to the possibility of X/Y/Z/A not all having the same Max Velocity (Rapid Feed) settings on your CNC Machine, also takes care of your acceleration being slower than the ideal as well as friction, CV or Exact Stop settings.
I do believe that these three settings used to calculate a more accurate time work just as well in both Imperial ad Metric systems.
All CAM programs have them in some form or another and they are used to approximate the cut time for you, however all have to tuned in and then only work if you do not change settings on your CNC machine.
Regards,
Mauri.
I'm not comparing the time the post processor reports to actual machine time. I'm comparing the reported rapid time to the reported cut time of my part. It's telling me my rapid time is TEN TIMES the cutting time. This is clearly completely wrong. It has nothing to do with the actual machine.
I appreciate the tweaks you suggest to reduce the error plus or minus a few percent but I'm trying to understand why the post processor rapid time is off by orders of magnitude.
MitchB,
What Post are you using Mach3?
If so what is your Motor Velocity settings in Mach3, eg. 20 inch/min?
If that is the number then *RAPID_FEED* must be 20 not 10.
Normally G0 is faster then G1 in most G-Code, you may get circumstances where they are equal but never less.
What is your Max and Min G-Code F speed 2 to 10 inch/min?
Regards,
Mauri.