Thursday, August 13, 2009

Lost Foam Casting 2 - Foam, Surfaces, and Sand

What foam to use? Almost all foam sold in building supply stores contains fire retardant. The right property for home insulation but the wrong property for casting. The fire retardant makes it hard to cut with a foam cutter and increases the thermal mass which in turn increases the risk of a short pour; i.e. a pour where the aluminum freezes before vaporizing all the foam.

Styrofoam pieces sold at craft stores work fine however the grain is coarse and the price is wrong. The best foam is from packing materials such as boxes that contain appliances, electronics, or computers - fine grained foam, no fire retardant, free.

Surfaces and sand: I like foam casting because it is quick. Make the piece, plonk it in some loose sand, pour. This, however, leaves a rough surface with embedded particles of sand. I like this surface for some of my art but it is terrible for anything that will be machined.

The topic of surfaces depends upon the sand. Using fine sand such as petrobond or green sand will produce a surface that matches the foam surface.

When you use a tool such as a grinder, rasp, or even sandpaper not only does it leave foam dust everywhere but the surface is coarse. A hot wire tool such as a foam cutter or an inexpensive soldering iron seals the surface as it melts foam. Other techniques I have read about, but never tried, are dipping the piece in molten wax and coating with latex paint. Reportedly these leave a smoother surface however as these materials get exposed to molten aluminum they burn.

Wednesday, August 12, 2009

4 holes, 4 pieces of steel, 4 machinists

I am making a kind of tool which consists of four bars of steel, each with four holes drilled in it. The holes do not need to be precisely located but they do need to be on center and straight. 4 holes, 4 pieces, no problem.

According to the book, the proper way of drilling a hole is to use a spot drill, then to drill a relatively small diameter pilot hole, and then to drill the full diameter hole. There will be a machine screw put in the hole so I also need to drill a countersink so the top of the screw will be flush with the bar of steel. Let's see -- 4 holes, 4 pieces, 4 drill bits - that comes out to 64 separate operations.

I made a pilot piece and discovered it is a pain to change from spot drill, to pilot drill, to full size drill, to countersink. The other way is to do all the spot drilling, change to the pilot and do all of them, then do all the full size drilling, and then all the countersinks. This too is a pain because it takes awhile to keep lining up each operation on center. And there are 64 operations.

Which is faster - or might there be a different and better way? The books, at least the ones I have, are no help for this basic a question. It takes experience - and so I turned to my favorite machining forum on the web. One reason it is my favorite is that if I am polite even the dumbest question is answered.

I asked, and I got four different and better alternatives to the two methods I had inquired about. With one basic question I received insights that will serve me in my future machining. Throughout my adult life I have learned to value experience and here was yet another proof.

This started me musing on book learning vs. experience, and how this particular body of knowledge is vanishing. Manual machining is almost dead due to computer controlled machining. My lathe is sixty years old and works fine now but in another forty years it will either be scrap or be in a museum. As for today's machinery - which are almost all too big to fit in a home shop - there are already 3D computer printers that print solids. It is fairly easy to see that printing metal, nanotechnology, or something else will whisk away today's computerized machining equipment if not within twenty years, certainly within forty years. Manual machining knowledge is destined to become a tiny niche, much the way there are a very few people who preserve the knowledge of making wooden wagon wheels.

Machining is very interesting to me, and I am so appreciative of experts who share their knowledge, it makes me sad to think of the future. Then I realize this is not knowledge of eternal truth, it is knowledge of the real and material world. Its very nature is to change, perhaps to die, perhaps to morph into something else. The proper response is not sadness but gratitude and enjoyment and even as I type this I am back in that place, in those emotions.

This weekend it's back to the shop. One of those new suggestions would reduce the 64 operations to 20. Hmm....

Friday, August 7, 2009

Lost Foam Casting - 1 Foam Cutter

All of this treatise on lost foam casting of larger pieces will begin with the same title so, if you are not interested, you can skip these blogs.

The first thing one needs is a foam cutter. A hot wire is superior to cutting, sanding, or grinding foam to a desired shape. It leaves a clean, sealed edge The only downside are foam vapors which can not be good for one's health so I wear a mask when I cut.

There are tons of plans for cutters on the web and the prevailing aesthetic seems to be to make them as much as possible with scrap from your shop. You need a power source, a wire that can get hot, and a way to hold it together.

Power source: A/C is dangerous although some folks use bell transformer. I had a computer power supply that had enough power but shut off as soon as I connected it. My theory is that it was smart enough to sense a short circuit and shut off to not damage the computer. I wound up buying an inexpensive battery charger that puts out 10 amps at 12 Volts. Beware, the more expensive automatic shutoff models do not work because you can not control them. Get the less expensive fixed charge model that produces at least 8-10 amps. I have seen very inexpensive models that put out only 2 or 4 amps and they are not enough.

Wire: Since I weld, I have MIG wire available. Inexpensive, easy to replace, lasts a surprisingly long time. If you do not weld you can get a small spool at Home Depot. I use .023 or .025 wire; .030 works but needs more power to get hot enough so I prefer thinner wire. You do not need expensive wire such as nichrome.

Hear Control: A light dimmer works fine with the battery charger.

Assembly and wiring: As the wire gets hot, it will stretch. In order to keep it tight, the top arm of the cutter is on a pivot (a bolt) and there is a spring at the back to keep it pulling upwards. The bottom picture shows this assembly and a black alligator clips from the battery charger. The red clip is connected to the bottom of the post to a piece of lamp cord that runs underneath the base.

Lamp cord runs from the top out across the arm, and is wrapped around a bolt - you can see it in the top picture. The MIG wire is twisted on to the other side of the bolt. At the bottom there is a hole in the base for the MIG wire to pass through to connect around another bolt to the lamp cord.

To provide clearance for the wiring on the bottom, I glued some wood strips to raise it about an inch.

Bill of Materials: scrap wood; four bolts and nuts; battery charger; light dimmer; outlet box for light dimmer; about 4' of lamp cord; plug; MIG wire.

Design Feature and Flaw. In the middle picture you can see that the top arm is angled. The spring is off to the side so it tilts the arm. As a result the wire is not perpendicular to the base. I will fix this some day but it is only an issue when cutting a very thick piece of foam.

The feature can be barely seen in the bottom photo - a line of holes extending at 1/2" intervals from the wire to the front of the base. These holes are just big enough for a small finish nail. When I want to cut a circle, I run the nail through the center of the foam and put it in the appropriate hole. Turn on the power, rotate the foam, and you have a nice disc of foam.

Sunday, August 2, 2009

Casting failure on the way to success

I did a lot of work in July and took lots of photos with the idea of writing about my adventures with lost foam casting. There is a lot of good material in the backyard metalcasting forum however I seem to have gone beyond what others have posted about -- although I am sure others have succeeded at what I am attempting.

Now that the hook is baited, what am I attempting? To make bigger castings. My goal is to be able to make a piece with as much aluminum as I can handle myself - about ten to twelve pounds.

With small pieces, lost foam is a piece of cake. You make the foam piece, hot glue a sprue and a vent, plonk it in some loose sand, and pour away.

When the piece gets bigger, everything changes. There is too much thermal mass for the molten aluminum to vaporize the foam. Acetone dissolves foam but then the loose sand collapses into the void. The first technique I tried was to put the piece in petrobond sand, ram it up, and then drip the acetone. Acetone and the vapors are very flammable - probably explosive - so I do this the day before pouring so the vapors have a good time to escape. . There is a flask with petrobond in the photo above sitting in the wagon and you can just see the red color of the sand. No explosions so far

When the piece gets bigger than the flasks on hand the options are to build a bigger flask or do something else. My pieces are often odd shapes, my petrobond is limited, my space for flasks is limited, so I opted for the something else - investment casting. This is simply investing, i.e. coating, the piece with a liquid material that becomes solid. Contemporary investment materials are expensive and require firing in a kiln. The process is designed for lost wax rather than lost foam so it will not work. As a result I have been trying a material that worked for sculptors for centuries - sand and plaster. In the photo above I am pouring a small piece in this investment.

So far the investment casts have mostly failed, however I am learning from each failure so it has been a positive experience. More to come.