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Robot Machining Symmetry Problem

#1
Hi, hope you are having a good day.

I am encountering a very weird problem while trying to machine shapes with a Motoman HP20D robot with a tool attached as the end effector.

I've calibrated the Tool TCP from the robot, and inputted the values as the TCP Frame.

I've also calibrated a frame in RoboDK utilizing the three point method.

I am machining a pawn from two different sides in Fusion360, I am attaching the Fusion360 file below.

https://a360.co/4avf2ox

After transferring the machining path via the RDK plugin, i transfer the files onto the robot and machine the pawn, but the sides do not match (see pictures below)

The offset is constant in most cases, around 3mm

I've not been able to identify the reasoning behind this; I am assuming its some conflict between fusion/RDK.

I've attached the project file below.

Thank you for your assistance!


Attached Files Thumbnail(s)
WhatsApp Image 2024-05-17 at 17.00.30.jpeg   

.rdk   mt3.rdk (Size: 4.45 MB / Downloads: 255)
#2
I do not think this is an issue of Fusion or RoboDK.

Achieving a high accuracy with bigger robots can be challenging. Industrial robot arms are highly repeatable but not accurate, the nominal accuracy of a robot depends on the robot brand and model. In your case, 3 mm seems a bit high but not enough to discard kinematic accuracy as the culprit.

Have you tried with different parts?
Is the 3mm error constant in a single direction or does it vary?

RoboDK Calibration can be used to calibrate the robots, a measurement system is required to calibrate a robot. You can read more about this in the following link:
https://robodk.com/doc/en/Robot-Calibrat...acker.html

You can see a sample report of our accuracy measurements/improvements here:
img34.png   
#3
Hi Alex, thank you for your reply.

I was afraid the accuracy issue was the case, but was discarding it because the inaccuracy was so big.

I monitored the positions of the tool in RDK and it seemed accurate as well, so I thought it probably was an issue with something after RDK, but I was not able to figure out the reason so I decided to ask here.

This always happens when we are machining from the sides; I also believe it is only the right hand side of the robot that is not accurate. I tried to reteach the tool, change the machining CS origin etc. The offset got smaller on one side, but never disappeared.

I also was able to machine and finish a 2/3 sphere shape, the shape came out perfectly fine even though I used multiple operations and strategies.


If we can find a way to define a machining strategy without changing the Tool Orientation feature in Fusion, we might just be able to machine our part.

It might be that our cutting tool length deviates slightly when the robot rotates around its 4th axis to machine from the opposite (tool orientation 180deg) side.


Thank you for the advice, I will keep digging, and update this post if I find a solution as a reference for those who encounter similar problems.
#4
Robot accuracy is the main issue as Alex described, and 2-5 mm is something you can expect with a typical robot machining setup.

If you didn't move the part when machining the 2 sides, you may be able to obtain more accurate results by improving the tool center point (TCP). You can calibrate your TCP with 10 points or more to understand the errors you can get with your robot setup.

You can find information about how to better define your TCP using RoboDK here:
https://robodk.com/doc/en/General.html#CalibrateTCP
  




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