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25-11-2024 #1
Hello everyone,
I'm new to machine design and have encountered some precision issues with my XYZ gantry setup for a dispensing machine. I am hoping to get your insights or suggestions for improvements.
Requirements:
Precision: The machine should place a dispensing head with 0.3mm accuracy.
Workspace: 400 x 400 x 250mm (X-Y-Z).
Mobility: Designed to be lightweight and have a small footprint so it can easily be moved by one person.
Current Issue:
There seems to be an X-Y error of approximately 1mm.
Design:
Drive System:
◦ X axis: single motor, belt driven.
◦ Y axis: single motor, belt driven.
◦ Z axis: single motor, timing belt rotates a lead screw.
Bearings:
◦ Y axis: Two 16mm parallel shafts with LMKL16UU bearings.
◦ X axis: Two parallel MGN12H linear guides, each with a single carriage.
◦ Z axis: A single MGN12H linear guide with one carriage.
Frame: Constructed from t-slot extrusions, with machined ends to ensure controlled length and perpendicularity.
Assembly: I used a large flat plate as an alignment jig during assembly.
I have attached screenshots of the CAD designs and photos of the assembly to provide a clearer picture of the setup.
I think it must be a problem with the squareness of the axes to the frame. The electronics/motors are working fine and each actuator is moving the correct distance (within 0.2mm).
Potential issues:
Frame Straightness/Flatness: The extrusions seem to have a flatness of 0.3mm or lower over their length when placed on a flat plate, but I'm
questioning if this is enough.
Structural Rigidity: The machine is rigid however if I push one side of the Y axis forward and the other backwards I can cause the y axis assembly to
yaw. Is the separation of the Y axis shafts too large for these bearings?
Alignment: Y axis bearings are not perfectly perpendicular; one side has about 0.3mm of clearance between the Y-axis and the endstop when homed. I dont think this alone could explain the 1mm error however.
I'm considering replacing the aluminium extrusions with custom machined plates to enhance precision, though I was trying to avoid this to reduce weight and cost.
I have seen CNC routers on here that are using aluminium extrusions which would suggest they are suitable for precision applications. Also it seems most consumer 3D printers use these aluminium extrusions so I thought they would provide enough precision for my application.
Can you see any examples of poor design or poor component/material choice which could be responsible for the precision issue?
Any thoughts are greatly appreciated. Thank you!
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