Problem Statement
Engineering design students and manufacturing hobbyists face significant challenges in manufacturing their designs or hobby projects as intended. Students, often constrained by limited budgets, frequently rely on 3D printing to create prototypes. While useful, these prototypes are not fully functional at scale. Hobbyists, in contrast, often invest in CNC routers, which provide spindle movement in the x, y, and z axes but are limited to machining relatively soft and flat materials.
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The ideal solution would be a CNC mill with spindle movement in the z-axis and table movement in the x and y axes, capable of machining hard metals into various shapes with high accuracy. However, such CNC mills are prohibitively expensive and typically found only in industrial setups.
Imagine a CNC mill that can machine a wide range of materials, comes in a kit design that can be assembled with basic skills, fits within a typical design workspace, and remains affordable for the target users. This project aims to design and develop such a CNC mill for a client, addressing the needs of hobbyists, DIY enthusiasts, designers, and engineers, ultimately benefiting the broader community of creators.
Design Working Principle
The design mechanism enables the autonomous changing of up to 3 distinct tool bits that are clamped within standard R8 collets. The general principle for how the autonomous tool-changing system should operate is explained below:
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The operator will load their desired tool set, by placing one directly into the spindle and the other two into the tool magazine, and will indicate which sized bit is in which corresponding slot.
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The spindle will machine a material using the first cutting tool held in the collet.
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When a different tool bit is required the table will move in the necessary X and Y directions to align the empty slot in the tool holder with the spindle.
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The spindle will move downwards in the Z direction and release the tool with its collet into the corresponding slot using the end effector mechanism.
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The Y position of the table will then move to align the spindle with the next tool bit, and the Z axis will move downwards until the collet is fully engaged inside the spindle.
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When the mandrel and collet thread make contact, the end effector will clamp onto the collet and the motor will rotate the mandrel mating the threads.
Magnetic Collet Holder
The tool magazine is composed of an aluminum plate of dimensions 5 cm x 20 cm x 3 cm with three 0.750” holes to allow clearance for both large and small tool bits. The holes are positioned 8.5 cm apart, with consideration of the diameter of the spindle, enabling it to engage with the selected collet without interfering with those beside it.
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Magnetic strips are placed around the edges of the circular holes and are attracted to the edges of the collet face. This being in combination with the chamfer on the front end of the R8 collets, slightly sinking into the holes, ensures the collets are able to be kept both upright and stationary.
Collet Being Picked Up/Dropped Off
X and Y axis move to position spindle above the collet. The spindle will then move downwards in the Z axis until the collet is fully contained within it.
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Then, depending on whether it is being picked up or dropped off, the robot end effector mechanism within the spindle is engaged or disengaged, respectively.
Robot End Effector Mechanism
In order to be able to automatically and reliably exchange collets, it was determined a new mechanism would need to be incorporated to loosen and tighten the thread mates between the mandrel and collet. To do so, this device must be able to autonomously clamp onto the collet and lock its rotation.
The use of a robotic end effector with two arms, situated in opposing slots on the spindle, can engage and provide the necessary radial force to lock the collet.
