Back or front tool

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15 Oct 2021 10:36 - 15 Oct 2021 10:39 #223191 by john11668
Replied by john11668 on topic Back or front tool
I do have two toolposts (see pic on page 1 of this thread)
It was my intention to operate generally in Back tool mode but to use front tool for parting and screwcutting.
It wouldn't be a hardship to go back to parting at the back , but there are advantages for RH threading to operate from the front.

So my threading code is

G76 P1.5 Z-12.5 I-0.001 J0.1 R1.2 K 1.85 Q0 H3 L0,
My start point is is specified in a prior line
G0 X10 Z10

So presumably I need to make my start point X-10 Z10 ?
I take it I would then have to specify the offset for the threading tool to take it it X-10
I am struggling to get my head around whether to also make the K value negative (-10 - (-8.15))  (nom diameter less minor dia) would give me K -1.85

Will take courage in both hands and give it a shot , with one hand on the red .button
Last edit: 15 Oct 2021 10:39 by john11668.

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15 Oct 2021 11:59 #223195 by john11668
Replied by john11668 on topic Back or front tool
Right , ! so I tried this .
System will not let me call up a K value less than J value and J value can not be less then 0 .
Without changing K value I cant seem to get it to work in reverse X

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16 Oct 2021 22:46 #223331 by andypugh
Replied by andypugh on topic Back or front tool
Not a solution to the problem, which looks like it needs working on, but perhaps you could use an inverted back tool for threading?

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17 Oct 2021 22:26 #223424 by john11668
Replied by john11668 on topic Back or front tool
I can do that indeed Andy.
Trouble is that it tends to lift the saddle . May try doing all from the front!
Just a shame there doesnt seem to be a solution otherwise .

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19 Oct 2021 16:22 #223574 by andypugh
Replied by andypugh on topic Back or front tool
I am sure that there is a solution. It's all just software and machinery, after all.

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20 Oct 2021 08:57 - 20 Oct 2021 09:01 #223641 by john11668
Replied by john11668 on topic Back or front tool
I am guessing you are correct in that Andy,
But a rookie like me is unlikely to be writing it .
I do believe that others are interested too

gangtool setup / backtools - Page 5 - LinuxCNC
Last edit: 20 Oct 2021 09:01 by john11668.

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20 Oct 2021 15:34 #223680 by Aciera
Replied by Aciera on topic Back or front tool
Any particular reason you're not considering switching between xz and -xz kinematics when using front and rear toolposts? I don't have a cnc lathe let alone one with two toolposts but it would seem to be worth a try.

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20 Oct 2021 16:22 - 20 Oct 2021 16:23 #223682 by john11668
Replied by john11668 on topic Back or front tool
Simple answer Aciera is that being a rookie I dont really understand what an XZ kinematic is and how to switch to a -XZ variant .

Any guidance would be appreciated . I would be happy to try anything.
Last edit: 20 Oct 2021 16:23 by john11668.

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20 Oct 2021 18:46 - 20 Oct 2021 18:52 #223700 by Aciera
Replied by Aciera on topic Back or front tool
The kinematic is basically how your 'joints' are mapped to your machine 'axis'. In your case of a lathe you have one 'joint' (ie motor) set up so the x axis moves towards the operator in the positive direction and the other 'joint' (ie motor) moves the Z axis perpendicular away from the spindle in the positive direction. As you have noted above, to reduce the stock diameter with the front tool the cross slide needs to move away from the operator in the negative direction. If you want to use the back tool the cross slide will need to move towards the operator.
One way to achieve thes would be, as has been mentioned, to change the sign of the x coordinates in the Gcode when the back tool is to be used. This of course requires the gcode to be processed accordingly.
Another possibility is to 'switch' the x axis' direction when using the back toolpost. This can be done using a user defined M-code ( in the simulation configurations that come with version 2.9pre M428, M429 are used for example).
So the configuration would start in, say, M428 (ie regular front tool mode so all Gcode commanding x in the negative direction moves the cross slide away from the operator into the stock) To use the back tool post you would call M429 to change the x axis direction and now all Gcode commanding x in the negative direction moves the cross slide _towards_ the operator also into the stock.
If you wanted to get fancy you might even be able to remap the M6 toolchange command to call M428/M429 automatically according to wich tool number is called.

There are a few things to watch out for with this approach though. For example you would not want to rehome the machine while in M429. Also moves in machine coordinates would need to be avoided and coordinate offsets would need to be handled wisely. Although that probably is true for all lathes with front and back tooling.
The other thing is the tool compensation which I'm not really familiar with on lathe setups, so that would also need to be tested carefully.

Note that this feature is only available in version 2.9pre 'master'.

[edit]
Usually I try to provide a simulation configuration to demonstrate switchable kinematics but I have only limited time at the moment to create a vismach model of a lathe. I'll see what I can do.
Last edit: 20 Oct 2021 18:52 by Aciera.

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21 Oct 2021 01:33 #223737 by joekline9
Replied by joekline9 on topic Back or front tool
I use Front and back tools a lot in the same program.
I put:
G10 L2 P0 R180 (Rotate around Z axis, Rear Tool)
before tool change for rear tool.
Put :
G10 L2 P0 R0 (Rotate around Z axis, Front Tool)
before tool change for front tool.
No need to mess with kinematics.

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