Pattern Shrink and Draft Small Alu Part

Discussion in 'Pattern making' started by Tops, Mar 16, 2022.

  1. Tops

    Tops Silver Banner Member

    I may want to try this part in aluminum as a spare for my sailboat. I am a 'noob' at all of this...so some questions, please and thanks, before I go 'pound sand' figuratively and physically.

    -Figure on placing this into the drag so the flat bottom is on the table and pointed up to the cope while molding?
    -Add 2% overall for shrink?
    -Add draft? Percent?

    I can 3D print or CNC machine wood or foam for the pattern...or is this one where you hot melt glue cereal box cardboard to the three screw bosses and fill in the drilled holes and just take the mold off the original? Nothing rocket science about fit, horseshoe slides into an aluminum extrusion and the skinny part of the 'stabilizer' goes into a slot. Part has a few grind marks but none where it mates to the extrusion.

    BOOMEND_43_VER2 Drawing v3.png
    spot_s21_3dprint2.jpg
     
  2. Melterskelter

    Melterskelter Gold Banner Member

    That part looks like it is subject to loads from a halyard or outhaul. The original part was likely engineered taking into account that the casting would be properly heat treated for strength. The castings made using 356 and maybe crudely heat treated are generally weaker than commercial heat treated aluminum castings and are subject to fracture. So, if safety depends on the structural integrity of part, I think I might choose to make it out of bronze. Or, if I had CNC machine capability, I'd machine it out of a block of aluminum alloy suited to the task.

    Comments:
    I think your suggested orientation in the mold is correct if I understand it. Flat surface would be at the part line and the the drag would contain the splash basin and runner. The rest would be in the cope. The mold would fill from the flat surface up.

    I think I would use 2% draft. It could be done with less draft, but this is new territory for you and you might as well make life a little easier.

    It is likely to shrink about 1.5%

    I would use a single gate at the apex of the curved arc opposite the flat fin.

    I think a shrink defect is likely to occur at the apex of the horeshoe on the inside of the curve. Using a hot blind riser in this area might (likely) be needed.

    Denis
     
    Tops likes this.
  3. crazybillybob

    crazybillybob Silver Banner Member

    Your drawing notes the part being 158g (5oz) Al-mg. That could be 5000's series aluminum or it could be a 50-50 blend or even AL 32% Mg 68%. That's going to change the strength of the casting (Noted by Denis ) which seems to be a factor in the use of this item. The higher the Mg content the more the shrink (Mg shrinks about 4%). The more MG the stiffer the casting would be. Picking the "correct" one can be difficult.

    The other option is.... IS it a life saving device? Is it life critical (holding your butt up over an abyss). If it fails will it hurt anyone?
    If the answer to all of those are no, and the part is for your own use. Let her buck partner! cast in some good quality aluminum (recycled Engine heads, transmission case..etc) NOT BEER CANS. See how she works. If it starts to look beat up or stretched....Broken. Try it again in Bronze, or CNC it from a block of something. You made one, make another.

    Bruce
     
    Tops likes this.
  4. Tops

    Tops Silver Banner Member

    Thanks Guys!
    -Yes outhaul, 20' sailboat, not life-saving, failure means a small piece of metal tied to a line tied to a sail slotted into a boom.
    -Al-Mag note was from the sales literature from the boat. No other details on that alloy.
    -My current alu scrap is cutoffs from cast C250/MIC6/5083 alloy plate at work. 4-4.9% magnesium and 0.4-1% Manganese according to Wikipedia.
    Another picture to see if I understand the flask setup per Denis:
    sandcast_boomend.png
     
  5. Tops

    Tops Silver Banner Member

    Another picture, moved riser to heavy end of part: sandcast_boomend2.png
     
  6. Melterskelter

    Melterskelter Gold Banner Member

    Yup, this setup. Riser might want to abut the part at the part line between sprue and gate. That is the spot that will tend to freeze last. Just my opinion. Others may disagree. I would make the gate fairly heavy melding into riser to encourage the part to freeze before the gate/riser.

    Denis
     
    Tobho Mott, Tops and crazybillybob like this.

Share This Page