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New Plasma build question

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12 Sep 2019 21:45 #144991 by thefabricator03
Replied by thefabricator03 on topic New Plasma build question

But now Linuxcnc recognises a joint axis pair before homing, I often wonder why it does not just move those two joints in unison to allow jogging before homing. That would be more user friendly in my view then denying the jog request.

Whilst the machine may not be "square" at that point, moving both joints in unison is not going to break anything. Limit switches would still be active. Sometimes you just want to get the gantry out of the road to service the machine or something.



I am not sure how it is done but CandCNC have this feature in their CommandCNC software. You can move the machine without having to home. This allows you to move the machine to where ever you want it then touch off on you work piece then cut. It has the option to home the machine but it is not necessary for operation.

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12 Sep 2019 23:33 #145001 by dgarrett
Replied by dgarrett on topic New Plasma build question
If joint mode jogging of synchronized joints was allowed by
default there would almost certainly be immediate and
legitimate complaints about unexpected racking and damage of
gantry machines.

So, the default behavior that disallows such jogging is
intended for machines where homing switches are employed and
the machine is homed before running gcode.

Gantrys with two joints for an axis letter are not the only
situation where homing synchronization is required -- it can
be needed for setup of machines with parallel kinematics
(hexabots or Stewart platforms for example). Such machines
may have multiple synchronized joints - not just two as for
gantries. Any method that would provide joint jogging
for synchronized joints must also handle any non-trivial
(non-identity) kinematics that are supported by LinuxCNC --
not just gantries -- it gets complicated.

However, note that almost anything is possible with
LinuxCNC/Hal and in fact there is a provided sim config that
demonstrates how to configure a gantry with synchronized
homing and provide for both individual and synchronized
joint jogging:

configs/sim/axis/gantry/gantry_jjog.ini

The config multiplexes the ini hal pin that manages the
home_sequence for joints 1 and 3 (ini.N.home_sequence) and
uses the *realtime* MPG (wheel) jog pins
(joint.N.jog-enable,scale,counts) to provide a means to
synchronize jogging for joints 1 and 3. When joint 1 and 3
jogging is enabled, homing is prevented by the
motion.homing.inhibit pin.

Needless to say, providing these features is more
complicated than standard hal setups. The sim config
demonstrates a set of methods but does not include
additional provisions to lockout inadvertent button pushes
or operator error that could cause damage -- such provisions
are the system integrator's responsibility.

The sim config can be used for experimentation and test (no
hardware is required). The methods can be adapted to any
machine requiring the special features provided.

Refs:
github.com/LinuxCNC/linuxcnc/blob/2.8/co...ntry/gantry_jjog.txt
github.com/LinuxCNC/linuxcnc/blob/2.8/co...ntry/gantry_jjog.ini

Also note that the sim config can be modified to
allow operation without home switches by using
*immediate* homing, Ref:
linuxcnc.org/docs/2.8/html/config/ini-ho...ml#_immediate_homing
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13 Sep 2019 00:09 #145003 by thefabricator03
Replied by thefabricator03 on topic New Plasma build question
I agree that for a general setup for all types of machines it is wise to have homing mandatory,

But for a conventional plasma machine it does not make sense to keep homing at start up.

My machine is 4200x2500 frame size and when I finish with it I place the torch in the top left corner of the machine. When I turn on the machine I need to home but I have it set that I can home anywhere on the table and still jog. That stops soft limits working but worst case it I run into a limit switch. That is what they are their for.

I do not experience racking problems at all. Maybe that comes down to the machines rigidity, but I do not home the machine as per convention and my cuts still turn out square. I would notice as I align my large sheets (3000x1500) up to the edges of the table and when parts are cut there is still the same edge distance left across the sheets at both sides.

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13 Sep 2019 00:24 - 13 Sep 2019 00:27 #145006 by dgarrett
Replied by dgarrett on topic New Plasma build question

But for a conventional plasma machine it does not make sense to keep homing at start up.


And as mentioned, a machine can be configured that way:
...can be modified to allow operation without home switches
by using *immediate* homing
linuxcnc.org/docs/2.8/html/config/ini-ho...ml#_immediate_homing

Its your machine, your choice and LinuxCNC provides the mechanisms.
Last edit: 13 Sep 2019 00:27 by dgarrett.

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13 Sep 2019 01:10 #145011 by bevins
Replied by bevins on topic New Plasma build question

I agree that for a general setup for all types of machines it is wise to have homing mandatory,


I do not experience racking problems at all.


You will if you send one motor south and the other one doesn't move.

It will rack your world.

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13 Sep 2019 01:15 #145012 by thefabricator03
Replied by thefabricator03 on topic New Plasma build question
As long as the motors are working in tandem and linked together with one set to reverse direction it should not be a problem. I did learn that the hard way though :)

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13 Sep 2019 01:21 - 13 Sep 2019 01:21 #145013 by bevins
Replied by bevins on topic New Plasma build question

As long as the motors are working in tandem and linked together with one set to reverse direction it should not be a problem. I did learn that the hard way though :)


Well, this was about jogging before homing on a gantry machine. It is possible to move one side of a gantry without the other.
Maybe not by intention, but it is possible.
Last edit: 13 Sep 2019 01:21 by bevins.

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13 Sep 2019 03:23 #145023 by rodw
Replied by rodw on topic New Plasma build question

As long as the motors are working in tandem and linked together with one set to reverse direction it should not be a problem. I did learn that the hard way though :)


Well, this was about jogging before homing on a gantry machine. It is possible to move one side of a gantry without the other.
Maybe not by intention, but it is possible.


No I suggested that rather than to disable jogging, that it would be nice if each side of the axis was able to move in unison if jogged before homed. I still don't really agree with Dewey's comments here

If joint mode jogging of synchronized joints was allowed by default there would almost certainly be immediate and legitimate complaints about unexpected racking and damage of gantry machines.


If Linuxcnc knows they are a joint axis, then it could act in a different manner that still ensured the gantry was never racked by synchronising motion between each joint.

But I do accept what he's stated and the reasons for it and its good that he has taken the time to build a solution to this problem. AS people gain the confidence with LInuxcnc to control plasma machines it will become more of an issue. For example one user is building a 9 metre long gantry and it would take a long time to home the machine if all that was necessary was to move half a metre to gain access to an inspection panel or something.

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13 Sep 2019 03:29 #145024 by bevins
Replied by bevins on topic New Plasma build question

But I do accept what he's stated and the reasons for it and its good that he has taken the time to build a solution to this problem. AS people gain the confidence with LInuxcnc to control plasma machines it will become more of an issue. For example one user is building a 9 metre long gantry and it would take a long time to home the machine if all that was necessary was to move half a metre to gain access to an inspection panel or something.


I'm not sure I agree with that. It would take nothing to put a goto zero button that is to be used before the machine was shutdown.
I know, I have a 20 foot long machine in my shop in production and I have never had to home it from the other end of the machine.

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13 Sep 2019 09:48 #145040 by tommylight
Replied by tommylight on topic New Plasma build question
From my experience:
The way it is now is much better, no jogging before homing the machine.
Another plus for separate jogging of joints in joints mode is the ability to test the encoders or drives separately, no need to detach both motors from the machine. Already had that issue on a big industrial machine.
Personally, i can not find a reason for not homing the machine at the start of the work, it takes about a minute or two.
On the other hand, homing the machine anywhere is a very nice feature and sometimes it saves a lot of time during work, for instance if you already have a plate on the table and you need something else cut in a hurry.
And there are offsets, so this makes the last statement void.
So to recap, no jogging before homing is much safer for clients.
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