Overview

Designed the rotational structure of a rotatable toaster using Solidworks. Incorporated rotor spring, bearing, solenoid and shafts to achieve 120 degree rotation that can be locked at two position.

The final design followed the form factor depicted in the morphology process. Four main components are identified in the figure: electronics housing, toaster housing, support housing, and the shaft and lock module. The electronics housing houses the control boards and power converters,while providing rotation support for the toaster housing with the supporthousing via shaft linkage. The heating of bread is achieved by the toaster housing with heat coils within. The shaft modules either with or without locking module are placed within electronics housing and support housing,offered as an enclosed package to achieve rotation, and locking of the toaster housing.

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Overall exploded view depicting major components including shaft & lock module, electronics housing,support housing, and toaster housing.

Locking Mechanism

As mentioned, two types are shaft packages are implemented in the design as shown in the Figure. The shaft and lock module incorporated a customized shaft adapter so that the groove can be used to guide the solenoid, with holes constraining the shaft movement to two fixed positions. The shaft and lock module are placed within the electronics housing for better wire routing, while removing the need to have wires going from the toaster housing to the support housing.

An exploded view of the shaft and lock module is shown in the figure. A rotor spring is positioned on each side of the toaster to provide balanced torque rotating the toaster housing to the tilted 120º position, the mounting position of the rotor spring. A ¼” diameter customized shaft is used to deliver the torque from rotor spring to the shaft adapter, which eventually drives the shaft and the toaster housing. A bearing will be mounted together with the rotor spring within the spring and bearing enclosure, to provide rotational friction rather than surficial friction. The enclosure packed several components into one, while preventing dust and oil from external environment.

A lock screw with inward drafted thread will be used to mount the shaft to the toaster housing. As demonstrated in (a), the oversized screw will expand the ∅1” hollow shaft with cutout so that radial force will be applied to the wall of toaster housing, securing the connection between the shaft and the toaster frame. Loctite will also be applied to the highlighted blue section to prevent thread loosening throughout product lifetime.

The shaft adapter will connect the solid ∅1/4” inch shaft to a hollow shaft, allowing wires to route through the gap between two shafts via the customized hole on the shaft adapter as shown in (b).

The rotor spring will be placed within the shaft enclosure as shown in (c), with the hook of the spring latched onto the housing. The housing provides self-locating feature for easy assembly besides the enclosure feature for reliability.

The linear solenoid is responsible of the locking mechanism, and directly determines the movement of the toaster assembly. In the figure, the relative motion of the solenoid and the shaft adapter with groove with respect to the motion of the toaster assembly is shown. The pull-type solenoid will retract once signal has been sent, enabling free rotation of the shaft as the lock tip will be touching the groove surface, shown as step (2). The torsional spring will provide torque rotating the toaster housing to the 120º position where the already pushed out solenoid will push the lock tip into the designated hole, preventing further rotational movement. The speed and resistance of rotation can be adjusted by the surface texture of the lock tip, with rotor spring providing sufficient torque with respect to the rotation angle.

Once the toaster housing has been flipped back to the spring-neutral position as depicted in (3), the user will interact with the buttons to release the toast-completed bread via releasing the electromagnet-controlled spring which exerts forces compressing the bread. The user would re-engage with the user interface to unlock the shaft and flip back the toaster housing to the upright position using the handle, at which time the solenoid has been retracted again to enable free rotation.

Tolerance Analysis

Tolerance analysis has been performed on critical component interfaces. The interface of shaft adapter with groove, the solenoid, the lock tip, and the electronics housing will be demonstrated as shown in the figure.

By taking account of the tolerances of custom components as well as commercial-off-the-shelf (COTS) components, a tolerance stackup can be provided as shown in the table. The F/R (failure rate) generated by root-sum-square (RSS) method has been provided at the bottom right corner, indicating statistical failure rate in assembly.

An RSS F/R of over 7% upon the first calculation is hardly acceptable. The assumed tolerance of the height of the solenoid is identified as the largest contributor to the overall F/R by referencing the percentage contribution to tolerance with respect to each element in the stackup. Since this tolerance value is hypothetical given the reference drawing on McMaster Carr (Supplier), a more practical estimate is made to reflect a tighter tolerance control, which is then reflected in Table 6. The updated tolerance stackup has a F/R of ~0.75%, widely acceptable in the case of mass production of this toaster. The same identifying and adjusting procedures are applied to all tolerance analysis scenarios to minimize the F/R upon final assembly. The calculation help suggest the optimal tolerance value for key dimensions of custom components.

The table below provided an overview of 6 tolerance analysis performed for the design. The RSS tolerance failure rate is attached to each tolerance analysis interface, while comments are provided to indicate the performance of the interference. Of the 6 analysis, (4) has a drastically higher F/R, as the goal is to have an interference fit so that the expansion of outer shaft can be achieved by the drafted lock screw to provide axial fixature force on the toaster housing, as illustrated in detail figure (a). All detailed tolerance analysis reports are provided in the last section.

DFM for Machining Components

Five key components within the shaft and lock modules utilized CNC machining or turning to complete. These components played a critical role in torque transmission throughout the assembly, made from metals including stainless steel and aluminum to achieve higher strength and durability throughout life cycle. Of the five components, only the shaft adapter with lock groove, which directly controls the shaft movement together with the lock tip and linear solenoid, requires over 4 machining processes. The rest of the components can be milled/turned within 4 steps.

The lock tip will be machined using a CNC lathe from a single solid rod as shown in the figure. After lathing the drafted section via rough cut, curved fillets and contour will be cut with smaller diameter tool bit to achieve smoother material finish. The rectangular cutout will be machined to fit the clevis rod on the linear solenoid. The hole will then be cut as the finishing step.

The ∅1” hollow shaft will be machined directly from the COTS component of internally threaded hollow rod. The raw material will be threaded at the specified outer diameter and inner diameter, without the need for lathing. Two perpendicular machining cuts will be made as suggested in the figure. The cutout is required for the section of the shaft to be expanded when locknut is inserted.

The ∅1/4” solid shaft requires four steps to complete as demonstrated . With lathing at the initial two steps, the OD of different sections will be set. The slot on one side of the rod will then be machined before the threaded section has been machined.

The shaft adapter with lock groove will be machined from a solid cylindrical rod as shown . Without the need for lathe, the block can be machined upright to the contour geometry, then cut out the groove section. Two locking holes will be cut, with a following finishing step that generate the fillet and radius. The hole for ∅1/4” shaft mounting will then be rough cut before threads are placed for both ∅1” OD and ∅1/4” ID. The cutout hole for wire routing will be completed as the finishing cut. De-burring will be required to avoid burrs damaging wiring when assembling the shaft module.

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The shaft adapter for the support housing would require much less steps compared to the one with lock groove. As shown, the entire component can be lathed out with set OD and ID. The threads will be made as the finishing step.

Overall, all machine components incorporate the DFM mindset for relatively simple machining cut processes, without the need for customized tool bits or high-spec CNC machines. Almost all components can be completed with a 3-axis machine, while the shaft adapter with groove might be benefited with a 5-axis to save the time of switching fixtures. Multiple units of the same components can be machined together to save machining time per batch.

Tolerance Stack Up Appendix