gangtool setup / backtools

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30 Sep 2012 13:17 - 30 Sep 2012 13:47 #24729 by gmouer
I am just wrapping up a retrofit on a American Way lathe, it is about identical to a Omniturn GT75 type machine and also very much like some of the Hardinge gang tool lathes.

My question is about how front and rear turning tools are normally handled in linuxcnc. The direction of X feed is opposite for front and rear tools in relation to the tools moving further into the work. I searched around for info on this subject but pretty much came up blank. There was some posts concerning reversing spindle rotation for backtools which is also necessary.

I'd appreciate any info on how front/rear tooling changeovers are handled from both a G code programming standpoint as well as setup within linuxcnc .(such as Hal, ini, tooltable, classicladder techniques to handle front/rear tooling)

Thanks
George
Last edit: 30 Sep 2012 13:47 by gmouer.

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30 Sep 2012 15:00 #24739 by andypugh
gmouer wrote:

My question is about how front and rear turning tools are normally handled in linuxcnc.


I have a feeling that they are rare enough for there not yet to be a "normal" approach.

I will say that the obvious way to handle gang-tooling (of having more than one tool in the same tool pocket) is a non-starter in the current tool table format on the code level.

LinuxCNC seems to understand tool orientations, but I have not tried a back-tool number to see if this has any effect other than graphically. I am also not sure if it _ought_ to change any behaviour.

I can actually see arguments for showing all tools on one edge in the G-code preview. (otherwise imagine a 3-tool lathe, or a 4-tool)

So, the problem then seems to be that the back-tools are likely to move completely the wrong way under G-code commands.
It might be possible to set those tools up on a virtual U-axis, but the U-W plane does not support curves, so this would be awkward.

I think that it is probably possible to make something work, but an ideal solution is likely to require changes in LinuxCNC. Perhaps we (meaning you) need to write a detailed specification of what the behaviour ought to be?

One improvised way to make it work might be to create a modified kinematics file that switches the sense of the X axis when tool number is higher than a threshold...

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30 Sep 2012 18:05 - 30 Sep 2012 18:08 #24754 by gmouer
Ouch, that reply is more scary than I expected !! I know BigJohnT did a HCNC retrofit, maybe he has some ideas on the subject.

I did look over the docs on the tool table and noticed there were tool types for approaching from the back and front of the work, type 4 and 8 for example. It would be interesing to know if such different types change the behavior of linuxcnc in any way as far as X motion.

George
Last edit: 30 Sep 2012 18:08 by gmouer.

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30 Sep 2012 18:23 #24755 by andypugh
This chap seems to be handling front tools for turning and back live tooling:

But it is anyone's guess what the G-code looks like.

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30 Sep 2012 19:29 #24760 by andypugh
Actually, after messing about with the simulator a bit, it isn't too difficult.

You need to program negative diameters for the rear tool post, but as long as you do that, it just works fine.
You also need to explicitly use M4 for the spindle too, CSS mode doesn't seem to invert the spindle speed for negative diameters. (This makes some sense, as facing cuts very often go past centre, and you probably don't really want the spindle reversing from full-speed forwards to full-speed reverse at that instant.)

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30 Sep 2012 19:58 #24764 by gmouer
Thanks so much for the help. Its preliminary yet, I was just doing some fine tuning on the servos today. I have to make up some tool holders next.
This info might be something to include in the lathe section of the docs, surely its going to come up again.

Thanks again

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01 Oct 2012 09:36 #24777 by ArcEye
Hi

The situation for lathes is not ideal, but workable.

I have a slant bed lathe, so all my tooling on the ATC is back tooling.
I made a gang plate to take holders, front tooling, but seldom use it.

There is no problem using either back or front tooling, only mixing the 2 in one program.
If you want to start in front tool and change to back tool part way through the program,(or vice versa) negative workpiece values and suitable tool table offsets are all you can do.

With the normal back tooling I home at full X height above the spindle then set G54 X to the billet diameter. From then on you are working in positive X units, in my case from 280mm down to 0.

The most common operation that would require front tooling is threading, and I have got around that by making an upside down tool-holder for the ATC and using that with the spindle in M3.

The bugbear really is that the Axis display always shows a front tool plot.
In theory you can change to [DISPLAY] GEOMETRY=-XZ but this leaves the dimension bars and XZ intersection lines orphaned.

I have found that I mentally adapt the tool plot, it is not actually any different, just shows the underside of the workpiece not the top.

G2 and G3 can be fun, but I use a sim based upon my actual lathe config, so that I see what I am going to get in the workshop

regards

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01 Oct 2012 12:17 #24781 by gmouer
It appears that linuxcnc's ability to work with front and rear tooling together basically does not exist. Front and rear operations are commonplace on gang tool type lathes. Rear mounted parting tools are a common mod on smaller lathes as well. It looks like the only present solutions are crude work-arounds that would certainly not be friendly with cad/cam use. I thought I ran into a patch for the toolpath display that would correctly handle back-tools but I could be wrong.

As a newbie to linuxcnc, coming from mach, the machine retrofit went very smoothly. The mesa boards and hal made interfacing hardware just about limitless. the learning curve was steep but well worth it. I never dreamed that there would not be provisions for front and rear lathe tooling. I guess there is no option but to use some work-arounds like others have.

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01 Oct 2012 12:50 #24797 by andypugh
gmouer wrote:

I thought I ran into a patch for the toolpath display that would correctly handle back-tools but I could be wrong.

I think I recall something similar too.
As ArcEye says, you can use GEOMETRY = -XZ to flip the display, but that's only a partial solution, and isn't much help for lathes with both front and back toolposts.

As a newbie to linuxcnc, coming from mach, the machine retrofit went very smoothly. The mesa boards and hal made interfacing hardware just about limitless. the learning curve was steep but well worth it. I never dreamed that there would not be provisions for front and rear lathe tooling. I guess there is no option but to use some work-arounds like others have.

Well, the big difference compared to Mach is that if you can decide how the system should handle dual toolposts then you can either change LinuxCNC yourself, or persuade someone else to do it (or, in theory, pay a commercial coder to do it).

I suggest a discussion here, and on the users mailing list, followed by drafting a specification for how it ought to work, then submission as a feature request.

The first issue that springs to mind is how to tell the system that any particular tool is on the rear toolpost or the front one. Preferably without changing the tool table format. (additions to the tool table format in the current version are not possible, but are very likeley to happen in LinuxCNC3, but that is likely to be years away). My first thought is that a negative number in the tool orientation column could be used.
Checking the CANON_TOOL_TABLE struct, orientation is an int so can be negative.

Questions to answer:
Does an other-side tool display on the same side, or the opposite side to the normal-side tools?
Does an other-side tool automatically reverse the spindle?
Does it automatically invert the X axis? If so, how does this interact with touch-off and tool table offsets?

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01 Oct 2012 13:20 - 01 Oct 2012 13:22 #24798 by gmouer
Some discussion and then the addition of some support would be the ideal solution. Unfortunately, I am brand new to linux and linuxcnc, so I am not really capable of carrying on a intelligent discussion on linuxcnc subjects. I could certainly do some research into how this matter is handled in other controllers/machines.

I did look at the omniturn GT75 manual, a lathe which is for all purposes identical to mine. Page 3.6 mentions entering a negative number if the tool is on the backside ( www.omni-turn.com/pdf/Sec3%20Tool%20Offsets.PDF) I will have to read those manuals further to see how they handle the whole matter. I have a friend that operates cnc swiss lathes for a living, I will ask him how they handle front/rear tools on those machines.

My guesses to some of the questions?

No, the spindle should not reverse automatically. My lathe can do a M3 directly to a M4 without a stop BUT some machines may not like that a bit !
My guess is that the spindle should be handled during the toolchange, stopped and restarted reverse if necessary. Reverse rotation would not always be necessary, such as in the case of a rear parting tool, mounted inverted. Possibly the reversing method could be something in the machine definitions within the machine ini file

The toolpath display should show the tool approaching from the opposite side of the part for a rear tool. ( in the tooltable docs, the type 8 tool is rear approaching, not sure how this is handled presently)

I am already getting the impression that negative values of X "might" be the standard for rear tools. Again, further research needed.

Hopefully others will respond with their thoughts on how front/rear tooling, and/or gang tooling should be handled by linuxcnc.
Last edit: 01 Oct 2012 13:22 by gmouer.

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