Cast BBQ Wheels

Welcome to our very first Instructable. We hope you enjoy our project and feel inspired after reading through our great weekend endeavor. Perhaps even you learn a thing or two – we did for sure!

In this project we used our interest in manufacturing to repair one of the most important pieces of equipment any household should have – the BBQ.

After a brutal, long and cold winter it is finally time to bring out the BBQ. To our despair the manufacturing quality of the BBQ – branded Jamie Oliver was not designed for the Norwegian winter conditions. So, while giving it a thorough cleaning, one of the wheels decided to disintegrate... This is obviously unacceptable, and an emergency engineering team were immediately summoned.

The solution was simple enough – metal is stronger than plastic, and there is only one proper way of making wheels – casting them in aluminum. And since we already had a backyard foundry up and puffing this became an obvious solution.

We used 3D modelling software to design a new wheel and casting inlets. A 3D printer was used to make casting models / master models and a good amount of elbow grease were used to finish the models before being ready to be casted.

Here is what you need to get started:

• Design software: we used Fusion 360, but almost anyone will do. Tools and machinery:
• 3D printer
• Miter saw
• Metal bandsaw or Hacksaw
• Metal Lathe (or access to one)
• Metal melting furnace
• Sandpaper
• Filler - for bodywork repair
• Casting flask
• Foundry sand
• Rammer
• Slip agent
• Protective gear
• Crucible
• Pouring tongs
• Skimming tool
• Brass brush
• Soft brush

Step 1: Designing

Equipment list:

• 3D CAD tool
• Measuring tools

The first step in designing new wheels is to take necessary measurements. Some measurements are more important than others, like the outer diameter of the wheel. If this would be smaller or larger than the original, the sausages would roll of the BBQ!

To design the wheels, we wanted to try Fusion 360. The software was easy to use for first-time users, as we did not have any experience using the software. The whole design process did not take any longer than 15 minutes. We should mention that we have a lot of experience in using other engineering design software, but the user friendliness of Fusion 360 makes it easy to figure out.

When designing parts for metal casting there is a few things that is important to remember. We are casting in sand using a master model to make the cavity to pour the liquid metal in. To be able to remove the master model form the casting sand, it is strongly recommended to make a release angle on all faces in the direction of pull, just like your kitchen glasses. The angle should be around 6 degrees. Sharp corners are clever to avoid, so we added large fillets where we could. The fillets on the spokes looks a bit odd, but the effect on the final part was just as we wanted it. On the backside of the model we put two holes for alignment. We initially intended to make a split model and assemble to halves in the mold, but we decided that one half was enough.

One of the coolest features in Fusion 360 is the 3D-print output. This automatically converts the model into STL and open the slicing software.

Step 2: 3D - Printing

Equipment list

• 3D printer of any kind
• PLA or filament of your choice
• Bodywork filler
• Fine grit sandpaper

To print our model, we used Slic3r as slicing software and our Prusa I3 MK3s to bring parts into physical objects. The pattern part doesn’t need a fine resolution, we planned to use filler and sandpaper to finish the part anyway. A coarse layer height makes it easier for the filler to bond and drastically reduces the print time. We used PLA to make our models.

Settings:

• Standard PLA
• 0.2mm layer height
• 2 perimeters
• 10% infill

After the printed models were printed, we covered them with a thick layer of bodywork filler and let it cure a couple of hours. We then sanded it to a smooth surface with fine grit sandpaper.

Step 3: Preparation of the Casting Flask

Equipment list:

• Casting flask
• Foundry sand
• Rammer
• Slip agent
• Soft brush

Preparation of the casting flask is in short about making the physical space for the liquid metal to fill and then solidify in. This is done by ramming specially made foundry sand around the 3D printed model creating an impression in the sand for the metal to fill. This impression needs to be free of any debris and contaminations.

First the template is placed facing up in the bottom flask and a slip agent is applied to its surface. This is to help free it from the sand after ramming. The inlet is printed in grey PLA and is placed a distance away from the model and shaped to avoid turbulence when pouring.

Then it`s time for packing the sand around your model. Casting sand differs from regular sand in its composition. It is made specifically so that it keeps its shape after you apply force to it. This is done with the rammer tool. The process goes like this; fill some sand and ram it tight, then fill some more sand and ram some more and continue until the flask is full.

After finishing the bottom flask, a fine brush is now used to remove any loose particles. Then the surface is coated with a slip agent to help facilitate separation of the top-and bottom flask later on.

Placing the top flask and the templates for inlet and riser. One of riser`s jobs is to apply pressure to the liquid metal as it solidifies. This helps pushing the metal into cavities and small details. It also allows trapped gas bubbles to escape and to let contaminations float to the surface and away from the cast. Therefore, the riser should be the last part of the cast to turn solid and is thus often insulated.

After sand is filled and packed into the top flask, all plastic templates are removed. Slip agent and slip angles helps this process go smoothly and what is left is the imprint for the metal to fill. A metal pipe from an old vacuum-cleaner is used to make holes for the inlet and outlet in the top flask.

Inlet and riser is made from gutter pipes and placed on their respective holes on the top flask. To avoid liquid metal overflowing when pouring it is practical that the inlet and riser has the same height. Since rules are there to be challenged the riser is not insulated for this cast.

Step 4: Melting Down Aluminum and Pouring

Equipment list:

• Protective gear
• Furnace
• Crucible
• Pouring tongs
• Skimming tool
• Brass brush

Now for the melting and pouring part and let`s start with safety: Use protective gear when melting and handling molten metal. E.g. for this pour the following gear was used: leather apron, protective goggles + face mask, leather welding gloves, woolen socks and crocks.

Avoid liquids near the melt. Because of the high temperatures involved, a liquid will in general instantly transform into gas if it comes into contact with the melt. E.g. water will expand in volume roughly 1600 times, which in worst case can lead to explosions with hot metal flying everywhere.

Pre-heat any equipment that will come into contact with the liquid metal. E.g. the skimming tool used for removing the dross.

For this cast a propane-fired furnace is used to heat the melt to a pouring temperature of around 750-800 degC. Bricks are used as counter weight in case the casting flask starts to separate during the process.

Aluminum is poured until the surfaces of the inlet and riser is at the same height. In theory the riser should be the last part of the cast to solidify. In this particular case they had identical geometry and insulation but the cast still turned out good. So; no rules without exception. Also, if the liquid surface in the riser starts to sink you can always apply more aluminum to keep the pressure.

After the pour comes the waiting. Cooling will depend on the size of the model cast, remember that you start out with 750++degC so it will take some time. Enough time to eat some burgers and have a beer. When the metal is sufficiently cooled and you just cannot wait any longer, the flask is separated and the results revealed. The slip agent mentioned before ensures a clean separation of the sand from the top and bottom halves.

With a tong the cast is removed from the sand and cleaning ensues. Burnt sand is discarded while good sand is saved for next time. Some water and a carefully applied brass brush is used to clean the sand off the aluminum.

Step 5: Machining to Final Shape

Equipment list:

• Lathe
• Saw

We now have a clean model which is almost ready for machining with only one thing left; removing the inlet and riser. Removing inlets and risers after casting is part of the process since they become integrated into the cast during solidification. For this you will need a saw capable of cutting metal.

After the casting is done, excess material from e.g. the riser and inlet is removed. We used a metal bandsaw for this, but a hacksaw will suffice just fine. The part is then moved to a lathe for machining it down to final shape and a hole for the axel is drilled. Since we designed the wheel with a draft angle we need to careful when machining the backside, it does not grip very well in the chuck. We machined it in three steps;
1 – backside to remove riser, straightening the backside and cutting the outer diameter halfway down the wheel.

2 – flipped the part and aligned the outer diameter true. Machined the remaining of the outer diameter. Turning the front face of the wheel and drilling the center hole for the axle.

3 – the last step is breaking all the edges, outside and inside, front and back. We decided on a 2mm chamfer on all edges.

Aluminum is easy to machine. A sharp bit and high rpm on the chuck makes cutting chips to a nice experience. Using alcohol as a cutting fluid makes for nice shiny surfaces.

Aluminum wheel upgrade for the BBQ is ready to be mounted. Perfect fit. Now lets get some burgers!

Step 6: Final Words

In conclution we are very satisfied with the end result. Although it might have been easier, faster and cheaper to order a spare wheel, the effort was totally worth it. These wheels will certainly not fall off! They might be a bit overkill compared with the rest of the construction, but we`ll take care of that in a later Instructable. And, Mr. Jamie Oliver, when you read this - you are an awesome chef, but could need some training in manufacturing. Please contact us for advice!

Thank you for reading this Instructable, hope you enjoyed it. We enjoyed making it!

Best,

Svein, Pål Idar, Terje