Its been a while since the last update. Apologies for the gap but had to take care of the usually boring work that pays the bills. Hope you all had a good Christmas and New Year. Hopefully 2013 will be the year we finish up the first bike.
In anticipation of that goal, and the convienent timing of a friend having access to a low cost FDM prototyping machine, I am shifting focus to make some chassis parts. With a majority of the front end parts complete and the swingarm only needing a fixture to be welded on, completion of a rolling chassis is mostly being held up by lack of a chassis.
There’s a bunch of tubing, 3 billet parts, and a 4 castings. The tubes and billet parts are easily taken care of in my shop but the cast parts need to be outsourced. For the engine castings I contracted Harmony Castings, an experienced mold designer and fabricator to make the patterns as these were very complex parts with multiple cores. These chassis parts are much simpler parts with only a 2 piece mold needed. It is something I felt could be done in-house. As with all molding and casting operations the cost of the patterns is the lion’s share of the price. If I were able to make the patterns myself then the castings could be produced at a very reasonable cost.
The engine crankcases were made with a modified sand casting technique and cast in aluminum in keeping with the current best practices for engine crankcase production. These chassis parts will need to be in steel to be welded to the steel tube members. Since the parts are smaller and don’t have any coring needs I chose the investment casting process. This is one of the oldest metal casting techniques in existance and makes use of an expendable wax copy of the part and an expendable cermaic mold. The investment casting process has several advantages comapred to other casting processes:
- few geometry restrictions
- near net shape minimizes machining needs
- thin wall capability
- high casting tolerance
- high casting integrity
- high quality surface finish
The investment casting process is a multi step casting process that takes wax models of your parts which are then covered with a ceramic coating (the ‘investment’) that is then cured and fired to melt out the wax and produce a hollow ceramic shell of which the hollow internal volume is the part that you want to make. This hollow ceramic shell is then filled with metal, allowed to cool, and the ceramic shell broken away to reveal the final metal part. Investment casting is used extensively in jewlery making and many industries where small near net shape castings are needed. The first step is to have wax copies of your parts made with appropriate scaling to accomodate the various shrinkage ratios in each step. For making one part you can machine or carve a wax copy of the part you use but then since you have only one wax you only get one cast part. To get multiple parts you need some sort of mold to cast the wax to make multiple copies. If you have high volumes of parts you can make an aluminum mold that you inject the wax into. For the low to medium production volumes, where these parts are, there are some lower cost but still high quality options. The most common is to make a silicone mold around a scaled master copy of your part. The flexible silicone mold can easily accommodate no draft, undercuts and very small molding details. Since the silicone molding process is done at room temperature and pressure the master pattern does not need to be made from a strong material. It can be wood, foam, or…. plastic.
Enter David Celento, a fast 125 rider, architect of the highest caliber and all around great guy. I met Dave years ago at a USGPRU event at Jennings GP Raceway in Florida. Up to this point in my life I had never seen matching Grand Prix motorcycles painted in seafoam green.
Besides being bike color-challenged him not being too pissed that I had glancing contact with his wife on the track (she’s a 125 racer too!) convinced me he was a stand-up kind of guy. He and the missus drove from track to track with a beautiful Airstream trailer that we spent many a Saturday evening drinking wine and bench racing in. He now has access to a low cost rapid prototype machine that will enable us to create the patterns we need from an ABS-like plastic. The Makerbot rapid prototyping machine he has is in need of some dialing in and boy do I have some parts that can help out!
The Makerbot printer is one of the latest in a line of partial DIY rapid prototyping machines that have become quite competent if set up properly. For an approximately $2000 sale price the bang for the buck is very high.
If not set up properly they can make a mess.
But when the machine is properly set up the results are impressive:
Now that the machine is set up properly and making plastic parts the next step is to spray it with a few coats of filling primer, sand it all smooth, then spray it with a gloss clear coat to seal the surface with a very smooth finish. The part will be cast in silicone and any scratches or fill patterns will be accurately replicated in the silicone then the wax then the metal part, so in order to get a very smooth final part finsh we need a very smooth master surface finish. Peter will be helping me out with the sanding and painting process.
Once the parts get here I’ll post some pics of the finishing and silicone mold making process. I hope to have a productive few months ahead of me with the intent of having a rolling chassis ready by the beginning of August.
That’s it for now but more is coming soon.