Bong!

9 Nov, 2009

Project 8: Casting And Molding

Rubber Percussion Mallets

The Project:

Having made a gong earlier, I have had no mallet for it. I also got through the design and initial fabrication of a xylophone and could use some mallets for that when it is finished too. For casting and molding, I decided to make rubber and plastic mallets.

Cad-ing the mallets- Blender SucCess!:

I'm doing well with Blender. It is what I have been using and I was hoping to use it again for this CADing. By now I've learned how to set dimensions and location (keyboard N- Transform Properties). This overcomes the more qualitative style of Blender allowing it to become a measured CAD tool. It also includes rotation and is nice and simple This week saw me overcome two of my bigger gripes, that STLs didn't save and that Boolean operations didn't seem to work. Turns out with STL, unlike other exporting functions, it only saves the selected objects. Simple enough, select your meshes (keyboard B lets you select using a box) and save away. Booleans (keyboard W, or through the SPACEBAR menu) probably used to confuse me because it doesn't modify your present objects, it actually makes a new object. This means that executing the boolean operation you can't actually tell anything has happened until you move your old objects out of the way. This is particularly the case if you are using solid textures. For this reason, I'd recommend working in the wire view for bools. Once this persistence is understood, it is conceptually simple. I did find the persistence of the original objects sometimes frustrating. I'd end up with multiple copies of things that would confuse later instructions. At the same time, it wasn't destructive. One last note, occasionally with some of the deleting functions they aren't 100% exact. You get some slight lack of overlap which technically shouldn't be there but does. I found this most often during difference operations. But it is really easy to use the EDIT view to select individual verticies or edges and delete them.

On a side note, before I figured out the issue with STL saves, I tried exporting into vrml 1 and dxf and converting in Rhino. In Rhino, the vrml imported distorted and the dxf wouldn't allow me to export to STL. I kept being told there was no surface object even though from what I could tell, it was a surface. And a closed one too. Odd.

Cleaning up unwanted verticies (above) and Wire textured Mallets (below):

With use of these newly understood functions, I picked two of the more oddly shaped mallets and happily cad-ed away. I designed the mallets based on standard commercial offerings and good structural integrity. These were going to be two part molds and I wanted the handle to go past the joint. This somewhat complicated the mold design but was feasible. It also made for a nice fit. Lastly, I added notches so that my two mold sides would be more stably joined.

STLs for Two Styles of Mallet Head

Time To Mill:

Since I had my STLs, I first went for the Modela and with the Modela Player. Initially I started with one of the thick blocks making the 2.5” gong mallet. Aside from problems with the Modela player, I quickly realized this wouldn't work. The design included a cut over 1” that was a straight edge to the surface. There is no way the modela could achieve that. The big block meant only roughly an inch clearance between the bit housing and the block which means less than an inch of downward travel before the tool itself is against the wax. If I had soft edges, this wouldn't be too bad., but I had hard edges. I wasn't going to get the depth I needed.

I removed the big gong mallet and just cut the small on. If I have time to mill the bigger mallet on the Shopbot it would be great. Unfortunately, there are a couple things you need to know beyond just the shopbot to mill using it and I haven't yet learned.

Turned out the Modela Player software was really easy. I made one mistake initially where I didn't parse the difference between the draft cut and the fine cut. The draft cut is done before the fine cut. I tried skipping it and the modela just plunged into the block. Thankfully I was watching and stopped anything bad from happening except random holes in the wax. My confusion on this was compounded by a backlog of incorrect prints to the modela. I'd try and clear things only to have the Modela start milling an un-wanted too path. I thought I had tried a draft run but really was just going through an old fine cut prints. I watched a lot of air milling till the existence of the print queue hit me and I managed to get everything cleared. Even then, it really didn't want to let go of that last job. Thanks John for calling me with the tip that the draft setting was definitely the issue.

When I was having trouble with Modela player, I decided to switch to CAD. I used the stl2png script without too much issue. It was slow (~25 min) to convert my STL but otherwise fine. However CAD.py was taking exceedingly long to contour it even with the basic settings. I don't know what the problem was but I was glad to be able to abandon it.

Using the Molds:

I used a ball end for milling as the surface that was going to be used for the mallet was rounded. As a result, my fit had to be tweaked. I used a chisel and knife to clear away uncleared corners outside the mold so that the overlaps would fit together. I am quite happy how the molds turned out. They fit together nicely. I added a small escape hole made by hand drilling with an 1/32” end mill.

Apparently I am also going to need to learn how to fill the molds better. My rubber mixed very nicely but I ended up with one giant air hole which must have come from trapped air when I smushed the two sides of mold together. I did see air escape from the escape hole. This air was on the opposite side. Interesting... Well, the mallet is a little too soft anyways.

While I was at it, I also molded my Media Lab key again. 3-D printing didn't go so well, lets try more traditional methods. David Cranor has also been playing with key molding. At present we have a low temperature melting metal key that fits into the door socket. My ML key being in a mold when I tried this, I didn't try to twist. Cranor made his office key earlier and it broke. The higher temperature metal would be good but I don't think our silicone goes over 270 which isn't high enough. Lost wax casting it is...

Vacuum Forming- Mold making the Fun and Easy Way!:

It doesn't get much easier than the vacuum form. Which is why I have a completely deformed piece of PET G trash. It you wait too long, the PET melts apart. The goal was to make an egg holder. Yes, I know that's what an egg carton is for. But my house has chickens and ducks and sometimes you want to let them out and grab the eggs fast. The egg cartons live in the fridge and you don't put dirty eggs in the fridge. Eggs tend to roll off counters and I often find myself wondering, “where can I safely leave these eggs?” Now I have a butterfly egg holder.

“Vacuum Form Me!

I'll make a tasty food mold of the masculine Ideal.”

Sorry Superman, you're too big!

(Warning: Do not attempt this with chicken eggs. Eggs were damaged in this process. But they were duck eggs and are stronger shelled than chickens.)