Quarantine Cross-Section
ME203: Design and Manufacturing
Stanford University - Fall 2020
Design project for Product Realization Lab course at Stanford University. All components were originally design or selected from a vendor (McMaster Carr) for the CAD assembly. Design in SolidWorks.
Nothing screams quarantine like slicing into your freshly baked sourdough and being beholden to a cross-section you have toiled for since you first mixed flour and water in a jar.
I personally got into sourdough about a month before shelter in place orders began in the bay area. I wanted a product that really enhanced the entire ceremony of slicing through the crispy crust of a rustic sourdough loaf, and I think that is what I have here. The hardwood teak board serves as an organic but very aesthetic pedestal for your sourdough. The mechanism that guides the blade path consists of a miniature drawer slide, a slotted linkage, and a shoulder screw: allowing for translation and rotation of the blade. Other details are explained below.
SolidWorks motion study with partially constrained motion of the bread knife component. Linear sinusoidal motor pushes the knife in the fore-aft direction and a linear fixed distance motor drives the blade downwards in the vertical direction.
SolidWorks exploded view tool to visually check how all fasteners and sub-components come together on the bread slicer.
Ideation Sketches
Initial concept sketches for a manual bread slicer. This initial design was inspired by a antique bread slicer restoration video I watched on Facebook. While of course all one really needs is a simple cutting board and a bread knife to cut off slices of bread, I love the idea of making a ceremony out of the whole affair using a manual bread slicer.
Prototyping
The teaching team provided us with corrugated cardboard for physical prototyping of our projects. I used this as an opportunity to better plan the mechanism that controls the blade path: a single revolute joint with the blade rigidly attached to the shaft.
On the right is a quick demonstration on how the blade “moved through” a bread loaf.
After constructing this prototype, I decided I needed to make some changes to the location of the blade on the bread slicer as well as the degrees of freedom / joints I would have associated with the blade and the blade path.
Iterations of CAD
First iteration of CAD. Key design change from the prototyping stage is the blade support mechanism. Rather than use a single revolute joint at the read of the board, a multiple degrees-of-freedom slider is featured. A miniature drawer slider is mounted on the side of the board, which is connected with shoulder screws to a slotted linkage, which is finally attached to the bread knife itself.
The second iteration of CAD focuses again on the mechanism which guides the blade path. The blade has moved from the side of the board to the center. A trough is also dug into the centerline of the board to allow for the blade to travel past the bottom of a loaf of bread. Cross-sections of this iteration are shown below.
A sagittal cross-section of the bread slicer (left) shows how small magnetic blocks are used to attach the aluminum legs to the teak wood board.
A frontal cross-section (right) reveals the negative spaces cut out on the underside of the board. The magnetic blocks, aluminum legs, and slider mounting block all sit within these recesses.
Final CAD for Scaled-Up Manufacturing
For the final iteration of CAD - I was required to take a look at any plastic or metal components of my assembly and redesign them for large scale manufacturing processes. For the initial iterations, any metal components featured in my bread slicer were designed to be machined on a knee mill in the Product Realization Lab’s metal shop.
Overview of Manufacturing Processes
304 Stainless Steel Handle Supports - Stamping and Forming
These components attach the teak wood handle to the end of the blade. The outer perimeter of the steel handle supports would be stamped out from a 1/16 inch sheet of 304 stainless steel. Then, the desired form is achieved by bending the blanks in two locations. Using the SolidWorks sheet metal tool, I converted the stamped blank (left) to a handle support with appropriate geometry (right). As advised by our class notes, the bend radius for a hard metal such as steel should be 6 times larger than the thickness of the metal: which is satisfied with a bend radius of 0.394 inches.
356 Aluminum Legs and Slotted Linkage - Sand Casting
These components have uniform thicknesses throughout and feature geometries more compatible with additive manufacturing (e.g. sand casting). It is somewhat noticeable on the CAD screenshots; draft angles of 2 degrees have been added to the external surfaces of these parts to allow for release from the molds. The sketches on the right are rough visualizations on how the negatives spaces would be situated in the sand molds for casting.
304 Stainless Steel Bread Knife - Stamping
Similar to the blanks that would be stamped out for the handle supports, the outer perimeters of the the bread knives would also be stamped out from a roll of sheet metal 1/16 inch thick. For the serrations, I was unable to find a standardized way to produce that feature on a mass-production scale. After reviewing a few workshop videos on knife making as well as videos from the show How It’s Made, I proposed that an angled belt sander could be used to grind in the angled serrations onto each knife blank (see sketch on right). Another approach could involve stacking up several knife blanks in a work holding jig and use a large grinding tool to serrate several knives at once - though, some amount of angled grinding would still be necessary.
Teak Wood Components
Unfortunately, wood manufacturing / carpentry processes were not a focus of the course. However, I am somewhat familiar with carpentry tools and techniques from assisting my father on his projects throughout the years. The teak handle could be easily turned on a wood lathe. No preemptive threading would be done on the holes on the sides - the screws could bite into the wood easily enough. Any recesses, or mortises, could be done using a router with a square corner bit. The chamfers on the edges of the board are of arbitrary size, so those could also be machined using a 45 angle router bit, planar, etc. And the mounting block is a simple rectangular geometry and could easily be done with any type of table saw.