machine#construction: patternmaking


Useful skill to have if you own a foundry.

*First, something be mindful of:

Castings will be stronger if the pattern is made with fillets (rounded corners) for several reasons. First and foremost, a sharp inner corner will concentrate the stress at a particular point - think: scoring a piece of glass before breaking it. This stress point can cause the weak, partially solidified casting to tear as it shrinks. This is traditionally prevented by smearing putty on the inner corners of the pattern.

Once cast, the metal loses heat and begins to solidify. The crystal grains nucleate and start their growth at the coldest point, which is usually the mould surface. These grains grow inward from this surface in a perpendicular direction, eventually meeting in the interior of the casting. There is a natural weakness at this meeting point and frequently small voids are caused by the reduction of volume during the freezing process. Grains growing inward from a curved surface tend to disperse this weakness throughout the casting. Grains growing inward from a sharp angle tend to meet along the plane bisecting the angle and leave the weakening voids in this plane. Hence sharp exterior angles should also be avoided. *1

How to Make the Patterns

These are only suggestions, not to be followed slavishly. Use your imagination!

*hand-built lostfoam patternmaking

1. cut the pieces out

  • use conventinal woodworking tools
    • design your part in a cad program, print out side front and top views, spray pattern with spray adhesive, cut out/drill along lines on printout with a bandsaw and drill press.
    • put an end mill or sanding drum in the drill press and pretend it's a milling machine. Move the foam billet around with your hands or against guides.
    • a disc or belt sander to remove large quantities of foam at once. it cuts down on the amount of dust formed since the foam is turned into dense powder instead of little foam bits
  • use hot wire cutter
    • i don't recommend using a hot wire cutter because it's inaccurate, leaves a crusty surface layer that can't be sanded off easily, smells bad, and gets stringy bits of melted foam everywhere. however, it is useful for certain things like making airplane wings and long "extrusions" like Dave Kush's HBLB's. It also cuts down on the amount of dust you have to deal with.

2. stick the pieces together

  • rubber cement (recommended)
    • Put a dab on, rub the two pieces together, take them apart and blow on them to speed up the drying, then when dry stick them back together for an instant bond. Make sure you stick it in the right place the first time! Requires fitting a more accurate joint than other methods, but it won't distort the joint either, since the glue film is so thin.
  • hot glue
    • put a light bead right on the joint, then smooth it out while it is still hot.
  • small bits of wire or toothpicks
  • PVA wood glue (Elmer's white glue)
    • Use cocktail sticks or weigh down to hold together. Allow to dry overnight. May not dry completely in the center of large joitns (i.e. laminating two sheets together) since the water is trapped inside.
  • Spray adhesive
    • tends to separate easily at the edges, makes a mess
  • double sided tape (not recommended)

3. add fillets

  • scotch tape
  • paste wax mixed with styrofoam-sawdust in a syringe
    • lay down a bead like with a caulk gun, then spread it out evenly with a ball on a stick.
    • recipe: 25% beeswax, 25% toilet bowl ring wax, 5% solvent, (I use Old English lemon oil wood polish,) optional 45% styrofoam sawdust to reduce gas generation
  • already carved into form with a ball end mill
  • hot glue gun glue

4. optionally add vents, sprues, risers, etc.

5. optionally apply some kind of coating

6. proceed to lostfoam casting

hotwire cutter links:

*CNC lostfoam patternmaking

Greg Jahnke says you can make a gingery shaper in 10 hours using a foam cutting CNC mill, since there's no scraping required after the patterns are cast. If it's true, (and it probably is) then it's definitely worth investigating cheap CNC systems.

*greensand patternmaking

ugh. i'm sure this section will get filled in some day.

*lostwax patternmaking

see wax for information on the lost wax process

*silicone molds for wax patterns

*1 thanks to bill williams for this metallurgical insight

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Last-modified: Thu, 14 Dec 2006 19:07:51 GMT (1489d)
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