Reverse

[Whacko]

I did some searching on the wiki, and in the documentation, but I don't seem to find a function to reverse the machine position along the program path. This is usefull and essential for applications like flame cutting and plasma cutting if you lost the cut, hit the pause, and need to backup to the point where the cut was lost, so you can restart the cut. I've set up a machine and ran it sucessfully with EMC2 on UBUNTU 804 dist, and it is impressive. It has the edge over Windows as far as realtime is concerned. Is there an option for running in reverse?

Whacko

[SpeedsCustom]

I have been trying to figure it out too....Or fast forward to. I can't seem to do it, and I have been using it for about 9 months.


-Jason

[acondit]

There has been a lot of discussion on the irc and in the emc user mail about restart from a point. I don't recall exactly how it is done (and I believe that people are still trying to figure out how to improve it) but there is some ability to back up to some point in the code and restart (this is not the same as running the code in reverse).

Some of the issues are turning on spindle, coolant (or in your case the plasma torch), inserting the proper tool and setting the state of various variables that would have been calculated had the program ran to that point.

Alan

[Big John T]

There is no "reverse" in EMC. There is "start from line" and many improvements have been made to that lately. In 99% of machines you would not want to "back up" without manually raising the tool from the work piece, shutting off coolant etc. The exception being devices like plasma that are not in direct contact with the work piece. In any case you position your tool to the sensible position and select the line you wish to start with and do a Set Next Line from the menu. Then run.

http://www.linuxcnc.org/docview/html...is.html#r1_3_1

As always practice this without a tool in place so you understand the results.

John

[chester88]

If you want that feature (I agree that would be handy) you should add it as a feature request at http://sourceforge.net/projects/emc/ or inquire at the maillist for emc:
emc-users@lists.sourceforge.net

[skullworks]

IIRC this was requested long ago. It was one of the needed functions for EMC2 to be adaptable for Wire EDM use. It has been a back burner issue since because there is not a lot of users attempting wire EDM conversions and other features have a more broad appeal and usefullness for the average user.

To put this in prospective this issue was looked at before rigid tapping and many of the other new great features that have been added to EMC2. Several M functions and other utility were added at that time. It will come in due time.

Another point to consider is that most of the people who write the code for EMC are doing so without a real machine testbed (of the exact type) for what they are writing. While I say this in jest - It does have a point. We have "the blind leading the blind" and without users suppling good trouble shooting feed back it often takes awhile to fine tune some issues or features.

[Whacko]

I can implement this via an embedded aproach, recording the toolpath in a memory buffer, and do reversing out of the EMC control, but obviously this would be a great feature if it would be included in the EMC environment. It should be possible to add some threads that would do the backtrace buffer but it would add some overhead to the realtime kernel. I know about the "run from here" feature, but it's cumbersome in especialy a plasma or flame cutting application, since the reverse is mostly needed in a block. If the cut is lost into a line, it is difficult to restart at the start of the line due to the plasma arc stable input which will not be true on the section which has been cut, and a lot of plasma machines will timeout if the arc transfer is not sensed. A work around is to put a piece of scrap over the section that has been cut, but sometimes this causes the scrap to be welded onto the work. Any ideas?

Whacko

[Big John T]

If you loose your arc often enough that this is a big problem perhaps you should look at your plasma hardware. Not only would you need to start at the point of failure but you also would have a lead in move to figure so for a plasma application you would really have to start from scratch up to the next segment.

John

[Whacko]

Hi John,
No problems, it happens. Not often, but it does. And when the cut is in the middle of a huge nest, on an expensive 316 SSteel plate, the neccesity of a reverse function becomes apparant. The lead in is really only neccesary for the pierce, as piercing creates a bad cut area, so to restart at the lost cut point is same as edge start, no need for piercing. The hardware is Hypertherm machines, the best quality machines, we are channel partners with Hypertherm and have sold many machines for the CNC mechanized market. We manufacture CNC flatbed machines and used to fit Burny controls, got disappointed with them(price and service), moved to Wescan (Linatrol) and ended up using PC based systems. Mach3 works very well except for the realtime issue.

Regards,
Whacko

[Big John T]

So if you did a start from line and had a button to interrupt the torch start until you got to the point where you needed the torch to start would that work? BTW, start from line has been improved in EMC as far as showing the last line to run and some other things I forget at the moment.

When the arc stops are you stopping the torch movement? I know on my Hypertherm 1250 I have a arc good contact... I use it to wait for the arc before starting movement. I assume without looking that it could be used to stop the movement as well and perhaps set off an alarm.

I can understand your desire for this now as you are selling machines and need the best solutions so you can sell more machines.

John

[Whacko]

Yes, the arc good signal does stop the motion but the torch ends up futher down the line due to the ramp down of the motors or else an exact stop will cause some steps to be lost on the stepper gantry. On the servo systems it is less of a problem, as the loop is closed and the servos will back up the overshoot. One can effect a restart by a few methods, as the one you mentioned or with a piece of scrap over the plate so the arc good signal will become effective.

I haven't sold a machine with the EMC on yet, I'm still getting to know the system and getting into the development side as I've only recently taken the plunge to Linux. I have built and sold many machines with the Wescan and Burny controllers, and three with the Mach3 interface. I suppose I'm a bit late on my move to Linux, but rather late than never as they say.
The 1250 is a great machine. We build machines including the Powermax range up to the mechanized plasma's like the HPR260 and the Max200's . I have a RND machine set up as a gantry with steppers and EMC and the new Powermax 45 mech system. This is going to be my experimental system until I'm comfy with Linux and EMC. It is running and I'm getting some great results.
I use my own height control and it does the initial height, pierce height etc. Then goes over to arc height control. I'm looking at the development side as I need a comparator to check the actual feedrate agains the commanded feedrate so I can have a signal to inhibit the arc height control when the feedrate slows down for corners and small lines and arcs. Someone might have implemented this in EMC already but I havent seen any docs or threads here about this. Do you know if someone has done this already? One don't want to reinvent the wheel.

Best regards
Whacko

[Big John T]

Originally Posted by Whacko I use my own height control and it does the initial height, pierce height etc. Then goes over to arc height control. I'm looking at the development side as I need a comparator to check the actual feedrate agains the commanded feedrate so I can have a signal to inhibit the arc height control when the feedrate slows down for corners and small lines and arcs. Someone might have implemented this in EMC already but I havent seen any docs or threads here about this. Do you know if someone has done this already? One don't want to reinvent the wheel. Best regards Whacko

I use a floating head with a micro switch to "find" the material top. Seems to work well for me. I think Carl and Jarl did that with a comparator in EMC.

http://www.dallur.com/index.php?id=130

and

http://wiki.linuxcnc.org/cgi-bin/emc...Height_Control

I started to do that but the results with my floating head were fine for me. Anyhow it is easy enough with HAL once you get the hang of it to do pretty much what you want.
Speaking of small lines and arcs are you using G64 Pn.nnn for your path control? For high speeds like plasma with artsy fartsy stuff with a zillion lines/arcs it really makes a big difference on keeping the speed up.

John