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The machine tool coordinate system shifts before and after RTCP is turned on
- kanesl
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20 Jul 2026 11:33 #347872
by kanesl
The machine tool coordinate system shifts before and after RTCP is turned on was created by kanesl
Hello everyone, I'm a DIY enthusiast. My English isn't great, but I'll try my best to explain clearly. I'm encountering a problem while using LinuxCNC as the operating system for my machine tool. When I replace the existing kinematics system with my own custom-built five-axis kinematics system, the machine tool coordinate values change when I switch to RTCP mode at the current position, causing a system alarm because the machine tool coordinate system has exceeded its limits. My understanding is that enabling RTCP changes the workpiece coordinate system values, not the machine tool coordinate system values. I haven't been able to solve this problem yet.
Additionally, my machine tool structure is A (head), C (table), with the distance from the C rotation center to the machine tool zero point being (Xrc, Yrc, Zrc). The tool length is set to dt, and there is an offset between the two rotary axes with offset values of (Xos, Yos, Zos).
Additionally, my machine tool structure is A (head), C (table), with the distance from the C rotation center to the machine tool zero point being (Xrc, Yrc, Zrc). The tool length is set to dt, and there is an offset between the two rotary axes with offset values of (Xos, Yos, Zos).
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- Aciera
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20 Jul 2026 14:36 #347875
by Aciera
Replied by Aciera on topic The machine tool coordinate system shifts before and after RTCP is turned on
Make sure you test your kinematics for A=0 , C=0 first so you can confirm that the kinematic model returns proper results. Also make sure that the coordinate values switch between the same values when you change between trivial and RTCP kinematics.
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21 Jul 2026 02:57 #347898
by kanesl
Replied by kanesl on topic The machine tool coordinate system shifts before and after RTCP is turned on
Thank you for your reply. Currently, my kinematics has coordinate offsets of (Xrc+Xos, Yrc+Yos, Zrc+Zos) when A=0, C=0. As you mentioned, "the coordinate values remain consistent when switching between simple kinematics and RTCP kinematics." Therefore, it's certain that my kinematics might have a problem. I also have a few questions:
1. Is it necessary to use the tool tip coordinate system as the basis for deriving the kinematics?
2. I noticed an article on kinematics derivation, which I've attached. After deriving my machine tool kinematics using it as a standard, can I directly replace the XYZAC kinematics code in my code? Or do I need to do something else?
I will immediately put these two questions into practice.
Looking forward to your reply.
1. Is it necessary to use the tool tip coordinate system as the basis for deriving the kinematics?
2. I noticed an article on kinematics derivation, which I've attached. After deriving my machine tool kinematics using it as a standard, can I directly replace the XYZAC kinematics code in my code? Or do I need to do something else?
I will immediately put these two questions into practice.
Looking forward to your reply.
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21 Jul 2026 08:06 - 21 Jul 2026 08:13 #347909
by Aciera
Not sure what you mean by 'the basis' but in a machine kinematic model we usually want to find a transformation between the work coordinate system and the tool coordinate system so in that sense I would say it is necessary to use the tool coordinate system.
2. Note that the kinematic in the paper you refer to has both rotations on the work side (ie table rotations) while your setup has one rotation on the work side and one on the tool side. This means that both (tool and work) coordinate orientations can change with respect to the world coordinates ('world' can be seen here as the machine frame). I'll attach a different pdf that handles a very similar setup as yours. It is a bit more complex as it describes a nutating spindle joint but in your case the nutation angle would simply be 90°. There is also a second kinematic model that can be used for 'Tilted Work Plane' machining.
[edit]
There are a LOT of ways to get a kinematic model wrong so I would recommend using computer assistance to do the matrix calculations. You can find the corresponding jupyter notebook here:
github.com/Sigma1912/LinuxCNC_Demo_Confi...tary/xyzbc-sntr-docs
Replied by Aciera on topic The machine tool coordinate system shifts before and after RTCP is turned on
1. Is it necessary to use the tool tip coordinate system as the basis for deriving the kinematics?
Not sure what you mean by 'the basis' but in a machine kinematic model we usually want to find a transformation between the work coordinate system and the tool coordinate system so in that sense I would say it is necessary to use the tool coordinate system.
2. Note that the kinematic in the paper you refer to has both rotations on the work side (ie table rotations) while your setup has one rotation on the work side and one on the tool side. This means that both (tool and work) coordinate orientations can change with respect to the world coordinates ('world' can be seen here as the machine frame). I'll attach a different pdf that handles a very similar setup as yours. It is a bit more complex as it describes a nutating spindle joint but in your case the nutation angle would simply be 90°. There is also a second kinematic model that can be used for 'Tilted Work Plane' machining.
[edit]
There are a LOT of ways to get a kinematic model wrong so I would recommend using computer assistance to do the matrix calculations. You can find the corresponding jupyter notebook here:
github.com/Sigma1912/LinuxCNC_Demo_Confi...tary/xyzbc-sntr-docs
Last edit: 21 Jul 2026 08:13 by Aciera.
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21 Jul 2026 08:56 #347911
by kanesl
Replied by kanesl on topic The machine tool coordinate system shifts before and after RTCP is turned on
Aciera, thank you for your reply.
While waiting for your reply, I also crammed on five-axis kinematics. Your suggestions are very helpful. Using computer-aided calculations can indeed avoid many errors, and I've carefully read the article, hoping to understand it.
Looking forward to my good news.
Respectfully yours.
While waiting for your reply, I also crammed on five-axis kinematics. Your suggestions are very helpful. Using computer-aided calculations can indeed avoid many errors, and I've carefully read the article, hoping to understand it.
Looking forward to my good news.
Respectfully yours.
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23 Jul 2026 08:36 #347979
by kanesl
Replied by kanesl on topic The machine tool coordinate system shifts before and after RTCP is turned on
Hello, Aciera
After reading the paper you sent, I gained a lot of insights. Based on the method described in the paper, I successfully derived the kinematics that fits my machine tool configuration. Currently, it has been successfully implemented on my machine, and no other issues have been identified so far.
Lastly,thank you very much for your help!
After reading the paper you sent, I gained a lot of insights. Based on the method described in the paper, I successfully derived the kinematics that fits my machine tool configuration. Currently, it has been successfully implemented on my machine, and no other issues have been identified so far.
Lastly,thank you very much for your help!
The following user(s) said Thank You: rodw
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23 Jul 2026 09:14 #347981
by Aciera
Replied by Aciera on topic The machine tool coordinate system shifts before and after RTCP is turned on
Nice to hear, I'm glad it was helpful.
The following user(s) said Thank You: rodw, kanesl
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