Elephant Desk

This whole project originated from finding a solid chunk of melamine coated MDF that came from a university desk. The legs on it broke and our facilities management was throwing it out. I made some new legs out of 75mm square tube with 5mm wall. The finished table weighs 60kg and feels like it "could hold an elephant". I tried something unique with the tripod design.

It works well as it never wobbles, but you can't sit on the edge of the table on the one leg side. The whole table comes apart and packs flat so I can easily move, store and transport it.

One Day Build: Strong Indicator Arm

For a while now I have been using one of these ubiquitous Swiss made indicator arms. It works okay but I've never been fully happy with it. They have too many joints and too much wiggle for precision work. Even on the best units I have seen problems with the indicator alignment when indicating small holes. The dovetail isn't perfectly radial with the stem and it side loads the contact point.

I wanted a better, simpler solution. I came up with this single pivot arm inspired by some cheap indicator arms I've seen. The clamp plates are hardened A2 and the shank is 60 case Thomson rod. The spherical surface on the clamp plates were surfaced on a CNC and match the spherical indents on the shank.

The screw is kinda cool. Its a standard SCHS with a little plastic knurled knob pressed on. I didn't know these existed until one of the toolmakers I work with bought some a job. The clamp gives just enough friction on the joint that it wont be knocked out of place, but can easily be adjusted by hand.

The little spring on the front of the clamp is just there to hold the clamp plates on the shank and the dovetail open when the indicator is not on the arm. Right from the start the radial alignment wasn't perfect. This this design, that alignment is easily adjusted by grinding the faces of the clamp plates to affect the angle the indicator is held at. With a bit of fiddling I got the contact tip to less than 0.25mm radial misalignment. 

This isn't a perfect solution for every application but it works perfect for any time you need an indicator in a mill spindle. Going forward I would make a second one of these with a 10mm reduced shank for using in my small drill chuck.

Example of the suspect indicator arm. Photo from wttool.com

Adjustable Prismatic Square

This small square was my first ever toolmaking project. I wanted a small square to use when squaring blocks in the mill. There wasn't material available to make a one piece square, and I'm still unsure about how silver soldered machinists squares are ground on all sides after assembly. So, the base and blade would have to be two pieces, and why not make them adjustable. I didn't go for a traditional rectangular blade like often seen in combination squares because grinding such a thin part would be very challenging, and grinding the mating slot nearly impossible.

Instead I used a square profile blade clamped at 45 degrees. This allows for easy clamping with the clamp remaining withing the edges of the base. The clamp is spring loaded so it loosens easily. The blade is made from a M2 HSS tool bit and the base is 1095. Its a fun little quirky square. I didn't do any formal design for this; no sketches even. As a result the proportions are a little clumsy but that doesn't effect its use.   

Close up of clamp with blade in a middle position

Small relief for checking an outside corner

Stainless Steel Clean Room Pressure Vessel

This was a job for our micro-electronics manufacturing clean room on campus from about two years ago. The previous tank was made of 1.5mm 304 stainless and would balloon out with applied pressure. The clean room tech designed a new tank made with 6mm walls and asked us to build it. All the parts were cut on a water jet with a few machined fittings. The main labor in the project was all the welding.

Every seam was TIG welded inside and out. I first welded the bulkhead rim on to the front panel inside and out. This significantly bowed the plate, so I had to flatten in on a hydraulic press. From there it was standard fillet and butt welds, with many in the confines of the tank. The only seams not welded inside and out were the ones joining the final back plate to the main body of the tank. The bulkhead opening in the front was just too small to reach in and weld. The designer had thought of this and had made the back plate slightly larger to allow for a double pass of weld to attach it. After all the parts and ports were welded, the sealing surface of the bulkhead was milled to ensure flatness.

This was a really fun project because it challenged me to do more non-dominant hand welding. I was reasonably proficient using my non-dominant foot to control my pedal, but only had limited experience with welding left handed. In total I laid over 7m of weld, at least 2 of which was left handed.

One Day Build: Gale Crater Topographic Model

This project was inspired by a kick starter project I found where backers were funding large scale models of the moon (500 mm dia.). The models (which were castings of a high quality 3D print) looked stunning, especially when lit. Poking around out of curiosity, I found NASA has a substantial collection of 3D resources available here including 3D models of spacecraft, satellites, asteroids, comets, and Lunar and Martian surfaces.

I decided to carve a small model of Gale Crater, the current location of the Mars Science Laboratory. The model from NASA had a 3x vertical exaggeration, which I reduced to 2x. Made out of a scrap of PVC, the model is around 200x150 mm. It ran on a machining center overnight and took an estimated 16 hours. It was supposed to run faster, but motors on this machine are very low torque, so the feed rate is dynamically throttled to ensure the axes won't overshoot. All mesh editing and programming I did in Fusion360 and it ran on a Trak 2 Op.

As a result of mesh reducing, the model has a beautiful, faceted effect

   

Modular 18650 Battery V3

This is another post in the series about designing an electric moped. This post mainly details the system architecture and high level design. I avoid delving into the calculations I used. This post also doesn't cover BMS design and programming. This may be detailed in a future post.

For designing the battery I focused on the maximum power draw required. For my intended use - running around town while at school - I'm not particularly focused on range. While I aim for a 100km range, I don't have enough information about my preliminary designs to know if this is possible. I began by calculating maximum power required. The motor will draw maximum power when accelerating so I decided on required power from my desired acceleration.

The calculations are kept simple, and ignored air resistance; I have calculated air resistance but it wasn't significant enough, especially during acceleration to warrant factoring it into the calculations. For my vehicle target weight, and wheel size, I decided I need to be able to supply a sustained 5kw.  To avoid needing an impractical amount of current, I aimed to keep the volts to amps ratio at 1:1. Because the micro controller I want to use has only 16 analog inputs, this means I can monitor at most 16 cells in series. Working this in to my target voltage/current ratio of 1:1, I settled on a battery topology of 16s20p. This gives a maximum power output 6.4kw at 5 Amps/cell, and a nominal power output of 2.9kw at 2.5 Amps/cell.

But enough of murky math, on to the design details. I decided I wanted a modular batter for several reasons. First was cost; I could start with fewer modulus and in case this whole thing didn't work out or I couldn't make it safe enough, I wouldn't be out a lot of cash in cells. If it did work, I could add more modulus later. Second was charging. Interfacing with a type 2 EV charger isn't particularly difficult, but it provides 240 volts, and the power supplies I would need to charge the battery would cost a lot, weigh a lot, and take up a lot of space. Charging on 120 volts wouldn't really make it any better as a high voltage and current would still be required.

The modular battery system allows modulus to be removed individually for charging. Its a novel idea and isn't practical for larger, or production vehicles. For a small moped, its a cool idea. I really like the physicality of removing modulus to charge them; it gives you a close connection to the bike. The BMS would signal to the rider what modulus need to be removed for charging. When a socket is not in use, it could be bridged with a jumper. The BMS could easily recognize the jumper and make appropriate adjustments.

The first sketches of the modular battery packs used a pivoting arrangement, with the loose end being held in by a rubber draw latch. This proved too complex for connectors between the pack and bike, as well as for waterproofing the modulus and the connections. The second idea was to use racks the modulus would slide into. The racks would be inside a waterproof enclosure with only 2 doors. This simplified connections and solved the waterproofing problem. This also made cooling much easier as forced air cooling in the battery box could now be used.

My first iteration of this design used mostly machine parts screwed together. I wanted all the module frames to be insulating and fire retardant if possible. End plates machine of polycarbonate spaced out by water jetted G10 spacers. This design was going to be expensive for fasteners, materials, and time. It also had stiffness issues, and would most likely suffer from cracking around the screws. After playing with it for a bit, I let it rest for a few months while I reevaluated.

First machined version of the rack mount modules

I don't remember where the idea came from, but I wanted to try the rack mount modules with 3D printed frames. I don't generally like hobby level 3D printing. Its probably from years of working with students who ask for help with poorly designed, printed parts trying to implement poorly thought out ideas. Note to the reader; drilling and tapping a 30% infilled part never works, no matter how many student groups try it. I wanted to try it anyway, what could go wrong. My first 3D printed design I stopped only halfway though; I really wasn't happy with it. I was still in the mindset of machining and weight saving; trying to make the parts and thin and light as possible.

First 3D printed version of rack mount modules 

You can probably see why I stopped. All the thin walls and tabs were too liable to snap off. It also would require a lot of support structure underneath due to a rim running around the underside. The model tree structure was also a mess and I had new ideas on how to structure it. I still didn't feel good about how I was designing it. Taking a pause, I thought about what I really was trying to do, not how I was going to do it. I started a new model with the goal of creating models as if these were going to be mass produced. What materials and techniques would be used then?

Some basic facts: it would be injection molded, it would most likely be out of glass filled nylon, it would most likely have torsional stiffness issues. These were the bases I needed. Attacking it again I got version 3.

Second 3D printed version of rack mount modules

I am very pleased with how version 3 came out, even though I already have improvements in my head. All the cells are parallel with each other, with spaces between for air flow. The contacts of the cells are soldered to copper bus bars under the front and rear covers. Both of these bus bars terminate in the tab on the front right of the module, where they attach to insulated copper pins that plug into sockets on the rack mounts. The tab is offset to allow two modules to oppose each other and use a central set of sockets.

Half the battery pack. Note how all the tabs nest together. 

All of these modules are connected in series

Layout of sockets on the rack mount

The connection pins are surrounded by a plastic tube to prevent accidental shorting. As these modulus are going to be removed regularly, its very important the modules are safe. The sockets on the rack mount have a unique shape. They allow for modules to be connected in series with appropriate polarity and allow the same packs to be used on either side of the sockets. I will most likely have a post in future with details and models of the sockets and rack mount. This central set of sockets would also have LEDs, one for each module to show the rider which modules have the lowest voltage and need to be charged next. The central sockets will also have spring contacts to connect to the module for temperature monitoring.

I at first planned to use RTDs of some kind, but this would require additional analog inputs on the micro controller. Thinking about how I would process these inputs, I realized all I would be doing is monitoring them in reference to a threshold value. I don't need a micro controllers for this, I can just use digital thermal switches wired to digital inputs. Only two contact are needed for multiple switches to be wired in parallel. If any run over temperature, the circuit will close causing a system shutdown.

The high current pins with their protection. The two small holes will receive press fit copper contact for thermal switches

 The front and rear frames are deeply ribbed to improve their area moment of inertial to reduce deformation. I also tried to make the connecting surfaces between the two frames as large as possible to increase stiffness. The two halves are held together with 6 screws specific for plastic. The square grooves on the outside edges are for the rack mounts. 6mm rails will be on the racks and the modules will slide into them. The filled in sections near the corners on the rear frame in the photo below is to allow for a place for the cover plate screws to attach. I also predict the corners will get abused a lot, so I made their walls thicker.

Rear frame. Note deeply ribbed and crossed base section

Front and rear frames assembeled

The copper bus bars are distinct to each side. To run power from the rear from to the front frame where the connectors are, a thin piece of copper runs up the bottom of a rail groove. This is fairly well sheltered so I am not worried about shorting. I also plan to cover this run with a thick piece of Kapton tape just to be safe. The batteries will be soldered to the copper bus bars with small jumper wires. The jumpers will be sized with a fusing current around 5-6 amps. This is a safety measure in case any cell is over current. The wire will melt and disconnect the cell. This is similar to how Tesla builds their batteries. The bus bars will just be held in place with some hot glue or CA glue. On the front frame the two bus bars get pretty close to one another. There is a least a 5mm air gap. I'm not worried about this because the bus bars wont move, and there is at most a 4 volt differential between these bus bars. There is very little chance of anything happening.

Upper bus bar showing the termination points in the lower right on the tab

Detail showing rear bus bar running up the bottom of a rack mount groove 

The front and back covers serve just to cover the bus bars and are held on with smaller versions of the designed for plastic screws. Fasteners will cost a lot for these modules. I already know how I'm going to eliminate 4 from the next design iteration, but I refuse to save money by buying phillips or slotted head screws. I won't do it. All in all the pack is very light, weighing in at 1.47kg, with only 80g of that being the plastic frames and copper bus bars.

Like I mentioned I already have ideas to make this design more reliable, use fewer screws, and have easier to assemble bus bars. I also need to incorporate a handle somewhere to allow easy removal from a slot in the rack. I also worry the tab holding the high current pins will break, but talking to manager of our maker space, he assured me there is little worry of it breaking. The next design will include the basic structure of the central sockets as well as the battery packs.

One Day Build: Carbide Foot Depth Mic

When cleaning the shop where I work one day, I found an old Tesa micrometer head. It was very unique as the sleeve was graduated only in 0.1 inch increments, not the traditional 0.025 inch. The thousands indication was instead done with a set of rotation indicators behind a window that ticked off every 5 thousandth. The micrometer was also graduated in 0.0005 inch increments with a 0.0001 vernier. Its a very beautiful and high quality tool but it had be abused and left in disrepair; the locking screw was missing, it was quite dirty and gummed up. I took it into my care and after a disassembly, cleaning, adjustment and a homemade replacement part, it was good as new.

Now what do I do with it? I have a very accurate outside micrometer already. I decided a depth micrometer would be very useful, especially when grinding thin parts on the surface grinder. Even though this micrometer head is intended for outside use, you can still measure depth by subtracting the read measurement from 1. The issue with using it on the grinder is it will stick to the chuck. Simple solution; use carbide.

The body of the is foot hardened A2. It was all machined out of my head, no drawings or CAD. I tried brazing the carbide blocks in but we just don't have the right type of flux where I work and I ended up having to remake the steel part. As a substitute for brazing I glued the blocks on with Loctite 380. All sides were ground after the glue cured.

The micrometer head is held in with a split pipe thread bushing. With pipe threads being tapered, if you cut a slit in a male pipe fitting, the fitting will collapse as you tighten it. After grinding I very carefully trammed a milling machine and took a skim cut on my fixture plate. I clamped the foot down flat to the fixture plate to ensure the bore I put in the bushing was as close to square to the bottom face as possible. Under slight pre-tension, I bored the brass bushing to fit the micrometer head.

Assembly was straightforward after that. I set the micrometer head within .005 inch in the bushing, then used the sleeve to zero it. The tool has a very consistent repeat reading. I checked the accuracy with gauge blocks and the reading is spot on. The issue I did find was the foot has a .0006 inch convex bow in it. This changes the micrometer reading depending on which side of the foot you apply pressure to. I want to fix this in the future but I can only see doing this through lapping. I don't have any lapping equipment right now. However, under normal use the tool still is very usable, and most importantly doesn't stick to the mag chuck when grinding.