G08 Advanced Preview Control

[ggborgen]

Gentlemen,
Can someone tell me what this G-code is used for and what it does?
Seen it use at the end of Renishaw probing macro #9820 Ren Stock Allowance
and it spark my interest. Thank You

[psychomill]

Post the macro so we can see how it's used.

I suspect that it may be shutting off G08 which is a control look-ahead function which can sometimes cause problems with macros especially with probing or other high number crunching sub programs....

I have a few different versions of #9820 but I'm not seeing it...

[ggborgen]

pychomill,
Here you go

[psychomill]

Somebody edited this program to turn G08 on (P1 = on, P0 = off). This is a FANUC look-ahead function for the control. I pretty sure Renishaw did not do this as Look-ahead is generally a known problem with macros, probing, etc. What this does is allow the control to 'read ahead' much further into the program before it gets there. It's an accuracy related subject and allows the control to "prepare" itself for different moves, comps, code strings, etc. Sounds like a good thing for a probe but it actually can have a negative affect due to macro calculations.

You don't generally see Renishaw shutting off any type of Look-ahead or other precision control (like G05, G5.1, G61.1, etc) because there's too many variations of this out there and how the strings are triggered. So shutting it off is the user's responsibility. In fact, you'll actually find that probe programs may contain quite a few dwells programmed (G4). This is to slow down the control and process to make sure you don't get any math errors. Sometimes you'll even see several dwells back to back.

If you want to know more about G8 (or G5, G5.1, etc)... do a search through this forum. There's tons of posts and discussion on the subject.

For whatever reason, someone typed in the G8 to turn back on in the probe macro instead of say the part program itself.

[sinha_nsit]

I think dwell does not shut off look-ahead feature. Macro calculations would still be done, till the next NC statement.

[psychomill]

I did not say it will shut down look-ahead.....

I stated that it is to "slow" down the control. This is in order to ensure the execution of calculations and/or statements...

[stevo1]

Psyco is correct as I have had to do this on a few occasions. In some of my macros when using a system variable in a loop they system variable would not update fast enough for the program. In the program a calculation would change the system variable and in the next line of code I would check the status of it but because I was checking it in the next line it would not be updated and it would blow by the check. I had to add a dwell right before the check line to give the machine time to update the system variable.

Look ahead has burned me many times in some of my extensive macros. It’s easy to over look as the problem and has caused many head pounding to control monitors before figuring it out . Few other things that do this is the “rounding point math” macro variable reads one thing but calculates another.

Stevo

[sinha_nsit]

I had the following example in my mind, where the idea is to keep an output device on for five seconds, and then restart the program execution:
#1100=1;
G04 X5;
#1100=0;
This does not work. #1100 immediately becomes zero. The device would switch on only momentarily.

Illogical rounding is something I never expected. For example #0.5=1 stores 1 in #1 on Fanuc controls!

[ggborgen]

Gentlemen,
I have also seen dwells written like this:

#3001=0
WHILE[#3001LT9000]DO1
END1

Is there a reason to do it this way?

[stevo1]

#3001 is a timer and can be written as a dwell. The WHILE statement is more or less used to repeat a section of code for programming a specific amount of times. This is actually kind of a smart way to do it because IIRC the program will not read outside the END line until it is satisfied so it should eliminate any issues with the look ahead.

To answer your question, No I have not seen a dwell written this way before. When it comes to programming there are so many ways it can be written.

When grooving on our lathes you always wanted to dwell in the bottom of the groove so you got a flat bottom. Some people just program the last few thousands at a slow feed but I would program the amount of dwell needed based on machine position and spindle speed.

G4X[#5041/#4119*10]
This would give the proper amount of dwell based on all the factors. This way it would not dwell to little or too much.

Stevo

[ggborgen]

G4X[#5041/#4119*10]
This would give the proper amount of dwell based on all the factors. This way it would not dwell to little or too much.

Stevo[/QUOTE]

Stevo1
If programmed in G96 is #4119 programmed spindle speed or actual?

[stevo1]

First off I want to apologize that the factor of *10 is suppose to be *15. This is not exact as I had stated. If you want to get exact numbers you have to do more calculations.

To answer your question the #4119 is the programmed SFM so if you take the current diameter / the SFM*15 it will be pretty much there within a few milliseconds.

As for calculating it out exact you will have to calculate like so with 200sfm at a 31” diameter as an example.

G4X[60]/[[#4119*3.82]/#5043]](=2.43455)

G4X[#5043/#4119*15](=2.325)

Stevo