Adjustable Work Surface

December 12, 2024

Creating a height-adjustable work surface for use in high-impact, high-vibration, extreme temperature environments

The goal of this project was to improve the experience of workers sorting items in a cramped space. Before I joined this effort, a prior project had removed one of the shelves that the items were stored on and replaced it with a work surface at an ergonomic height. While the workers appreciated the workspace most of the time, on some days there were simply too many items in the area, and they reported missing the additional storage space that the shelves provided. To address this, we needed to design a work surface which was ergonomic in one position, but which could be reconfigured to allow for as much storage as there was before the change.

One of the first steps in this project was to determine our missions and design inputs, which would influence our design and provide the basis for more formal requirements later. Here is a simplified list:

Shelf/work surface weight capacity must be ≥ 100lb.
Minimize shelf/work surface weight--existing shelf weight is ~7.5lbs without hardware or supports.
In shelf configuration, system must accommodate 3 vertically stacked items of dimension: 508 x 711.2 x 596.9mm (20 x 28 x 23.5in).
It would be helpful, but not required, to register what position the shelf/work surface is in (up/down) .
Speed of adjustment is important--must not delay the workflow, but also must not move so quickly as to create a safety hazard.

No parts should be removed to adjust the height of the system, because they could be lost and not reattached.
The system will only be converted to the shelf configuration 1x per day for the busiest 6-8 weeks of the year, and it should not affect the functionality of the work surface for the rest of the year when the shelf configuration is not being used.
The system must be able to remain functional after withstanding shock forces of up to 20 Gs, and must not break in a way that creates a hazard when experiencing shock forces of up to 75 Gs.

As we began to understand the problem space, we performed market research to see how similar problems had been solved in industry. Here are a few of the devices and mechanisms we looked at.

I developed this system for evaluating the mechanisms that we were exploring based on cost, complexity, and user effort.

From there, we spent weeks concepting, down-selecting, and concepting again, as many of our goals were conflicting. How could we make the system as light as possible, but still strong enough to support 100lbs when experiencing shocks of up to 20 Gs? How could we make the system easy to raise and lower under no load, but prevent it from causing injury to the user if they tried to adjust it while it was loaded with 100lbs?

We tried to avoid a solution based on linear motion, because linear motion is generally more challenging to implement than rotational motion. But after much review, we decided to go with a linear-rail plus gas springs, as this was the best balance between all of our requirements

Here's a video of us thermoforming another part. Unlike the two in the previous photos, which were made out of clear PETG sheets, this one is made out of HIPS.

For the base of the night light, we designed and 3D-printed two parts to be the positives for our silicone molds. I took the lead on the base, shown in this photo, which held the PCB in place and supported the aluminum pipe.

My project partner designed the PCB cover. Originally, it was to be secured with bolts and heat-set inserts, but after casting, a press-fit was enough to hold it in place. However, through holes for the bolts are still in the cad and shown in this rendering. Instead of holes to allow access to the power and mode buttons on the PCB, the cover had tabs that transmitted force from the user into the buttons. To replace the battery, the user simply had to remove the cover.

We molded the PCB cover first so that we could test our process with our smallest part before moving on to the larger nightlight base. It was a good thing that we did this, as we had to troubleshoot several issues with our process. Our first cast part, shown in this photo, had a poor surface finish, lots of air bubbles, and completely lacked the through holes it was supposed to have for the bolts to secure it to the base.

However, by printing the mold form more carefully, using a vacuum chamber to reduce the air bubbles, and doing a 1-part mold instead of a 2-part mold, we were able to cast several more PCB covers at a significantly higher quality.

Based on what we learned from the PCB cover, we made some changes to the CAD for the nightlight base. Most significantly, we adjusted the design so that we could do a 1-part mold. We also added draft angles to the smaller holes and fillets to whatever edges we could to make separating the cast part from the mold easier. We had just enough silicone left to make our mold. In this video, my project partner is pouring the urethane into the one-part silicone mold. We used a popsicle stick to pop some of the surface bubbles just after the end of this video.

Once enough of our parts were cast and thermoformed for us to make two nightlights, we began assembly. The first step was to solder the LEDs onto the PCB. We used wire leads to attach them so that the LEDs could sit inside of the aluminum tube at the center of the pushpin while the PCB was contained in the base. We also sanded down our urethane parts on a belt sander to remove any artifacts left over from casting, and attached the cork to the base.

The next step was for us to cut out the thermoformed parts and attatch the two halves. We also sandblasted the interior of the clear sections to create a frosted effect. When we were ready to attach the halves, we used clips to hold the edges together while superglue dried, as shown in this photo. Once they were attatched, we used a spindle sander to remove excess plastic from the seam.

The final step was to put all the pieces together and turn it on! Here's a photo of both nightlights. You can see the seam that inspired us running down the center of the nightlights.

Engineering Drawings

Because our parts were made out of plastic and we weren't sure how well the molding and thermoforming would go, we designed them not to require tight tolerances to work in the assembly. But we did still have some critical dimensions, which are documented in these drawings.

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Bill of Materials & Cost Analysis

Here's a bill of materials that tracks the approximate cost of the two nightlights we made and the estimated cost for making 100 nightlights. The cost of labor is also calculated, and is based on an estimate for how long it would take a skilled person with access to the correct equipment to do the manufacturing and assembly.