Lost Foam Castings

[vtsteam]

Just thought I'd post some pics of lost foam patterns and casting I have made. The intent was tto produce a homemade linear bearing housing for a simple CNC turn mill. Similar to the linear bearing housings illustrated by Dave Kush, a lost foam casting pioneer at his buildyouridea.com.

The drawing (in Google Sketchup):

[vtsteam]

Using a plugin to produce a .dxf file output in Sketchup 7 (free edition, and run in WINE on Puppy Linux) I transferred the profile to Jedicut (also free, and running on WINE) and cut the foam patterns on a CNC hot wire cutter I built, using an old 4 axis  HobbyCNC controller and steppers.

Here are the foam patterns:

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The patterns were attached to a split sprue -- trying out the Dave Kush methods:

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Then the pattern was dipped in wallboard compound (sheetrock mud) diluted to a yogurt consistency, and then hung to dry. In summer Vermont weather this takes 1 to 2 days for complete drying. Unfortunately this can't be done in a kitchen oven, since the foam might melt.

[vtsteam]

The bearing housing pattern was placed in a large tin of loose sand. The 4 sprues were cut off short, and a hinged split riser, similar to Dave Kush's was placed on top of the sand. A hot wire was poked down the sprues to clear them a little in advance of the pour. The aluminum was poured quite hot compared to my usual practice, because the sections were long and thin, as were the sprues. In addition, the heat must do the work of flashing the polystyrene to gas, and getting rid of the gas bubbles before solidifying.

The first casting was shorted, and a second was poured with even hotter metal, successfully. I don't have photos of the pour, as I was working alone, but here is the finished casting from the second pour.

[vtsteam]

This is how the housings would be fitted on the mill ways. The photo actually shows an earlier (pink) version of the patterns without the extended leg.

[Noitoen]

That's hot  I mean cool. Must use my yard space to try out some casting.

[PekkaNF]

"CNC hotwire cutter"  Is there absolutely anything you haven't tried yet? maybe square dance 

How much do you need to scale the foam plug to counter shrinkage?

Pekka

[vtsteam]

Thanks Noitoen. Let us know how it turns out!

Pekka, thanks! re. shrinkage I'm not sure -- do you mean the metal part shrinkage after casting or the foam shrinkage when hot wiring?

1.) The metal shrinkage will be the same as it is for any type of casting.

2.) The foam shrinkage is generally called kerf in hot wiring (as in sawing). It is actually a combination of evaporation and shrinkage away from the hot wire, which in CNC is a result of radiation from the wire, not contact. with the wire. In hand cutting, contact often results, however, because it is nearly impossible to do that slowly and uniformly enough at low enough heat to achieve non-contact.

Usually you set the kerf allowance in the CNC program (like Jedicut, which I use) -- and it stays pretty consistent for a particular brand of foam and foam type and heat setting. But not absolutely so. Since foam is cheap, I generally cut a test part, first, and if that needs some adjustment, I increase the kerf allowance in the program.

btw, I wrote a spreadsheet hot wire cutter design program a while back for determining what size wire to use with a particular power source and bow length. You can juggle values around to work out a rig for cutting foam with what you have on hand for electrics, or wire. That works for a hand held bow or a bow driven by CNC, and works for virtually any power source, from batteries to computer PSUs to wall warts.

My own weapon of choice for cutting foam is an old style "dumb" 12V auto battery charger, connected through a router speed controller to mains. My wire is single strand stainless steel salt water fishing leader material .011" in dameter.

If anyone wants the hot wire power supply spreadsheet program, I'll post it here.

I always think it's important when discussing foam cutting details to warn people to cut ONLY polystyrene foam with a hot wire, if you intend to try it.

Polyurethanes and polyisocyanurate foams (usually tan or green here in the US, but possibly other colors elsewhere) will yield poisonous fumes if cut with a hot wire.

[PekkaNF]

Both

They will compound. I have done some manual hot wire cutting for model aeroplanes (had to learn build them as fast as I trashed them). I never came proficient on building nor controlling the planes...but I just made eneough trash to learn something. I have also cast low temp metals after pattern and learned very early to make a little oversize pattern. I heard that patternmakers had some rulers that were scaled few % oversize to produce just right dimenssions after casting.

Your projects are really fasinating and indicate how much proficiency you have accumulated by doing all sorts of things.

I wish I had your spradsheet program 15 years ago. I build one SCR primary regulated PSU that offers someting like 2 to 27 V out. Did some experimetal cuts, until "whiskers" looked about right and then I knew the right current. I had several bows made of extruded carbon fibre tubes and bigger ones from glas fibre skining poles. I aqquired Nichrome wire from Mexico, Germany and finaly from Finland...I used a lot one type of yellowish styroxfoam that had really hard skin, after Dow chemicals started using recycled material on their blue foam. The recycled was very good for building, but it had some hard granules that left a dent on othervice perfect surface. The hard skin variety was sometimes very good, when a thin wing section was cut out of it and kerf was substituted with glas fibre cloth/epoxy/delaminating strip and weighted down for gluing. But if I cut too close to it or left it into core it would bow.

Pekka

[vtsteam]

Oh, well Pekka, you seem to have a lot of experience cutting foam!

I have had the part bow if cutting too near an edge with a thin flat part, as well.

It's similar to machining only one side of a slab of cold rolled steel -- there are built up stresses near the edge of the foam plank as it comes from the factory.

The sequence of cuts using CNC can also affect the shape -- a few hot wire boards have heat control, and most foam cutting programs can automatically adjust that for the direction of the cut, material, etc.

I have heat control on my old HobbyCNC board, but I can't use it. I think I have a bad IC, and the board is considered "legacy" and unsupported therefore. That's what happens if you buy a kit and don't build it for a few years. 

[Noitoen]

I would think that, the best way to control the hot wire would be with a constant current power supply. The temperature would "self regulate" itself, independent of the material thickness or density.

[vtsteam]

Well Noitoen, different foam materials and densities, different cutting speeds, travel stops and reverses, cutting directions (horiz, vert) different wire materials, diameters, power supply ratings, etc all play into it.

Variable supplies are best, and in CNC the wire heat is controlled by direction and speed. For instance, cutting vertically upwards widens a kerf from excess heat. And at a trailing edge of a wing, the wire reverses direction near a thin section -- often creating too much heat locally. Regulation of a power supply, either constant current or voltage isn't a big requirement. Getting gear into the ballbark is. And being able to vary by cut position and direction is.

Actually the program I wrote assumed the closest thing to a constant you can approach is watts per unit length of wire. No matter what wire size or material, or power supply type, bow length, etc. Watts per inch (or cm) is a relatively constant number for cutting a similar set of materials. This programe was aimed at determining what to buy or make, given the gear you already have on hand, and what length bow you can accommodate with it.

As an example, a person has some .020" steel mig wire and a 12 volt car battery to supply a hand held bow -- he/she wants to know how long a bow is required to get them in the proper heat range for cutting foam. That's what my program was designed to answer -- given some variable set, what is needed to get them in the ballbark for cutting foam.

In that example, if the bow length required was too long or unwieldy, the length could be artificially created by using a wire coil air resistor -- in fact that could be made as a variable resistor with an alligator clip. Let's say someone else wanted to do it with with .012 stainless fishing leader, or a guitar string -- what would the length be for that. etc.