Where to split this pattern?

Discussion in 'Pattern making' started by G3j, May 23, 2023.

  1. G3j

    G3j Lead

    I'm making a live steam locomotive. The next cast is to be the trailing wheels. The drawing is for the original 45" wheel, but my project is 1/8 scale, so just under 6" diameter.

    I can't figure out what the pattern should look like. The drawing shows the hub, rim and spokes drafted to the back side. That would imply the whole pattern would draw in one direction rather than being split. But then how to I get the shape of the spokes at the front, as well as the hub extending proud of the rim? It looks like at the plane of the rim I would need to pull in two different directions; the hub & rim vs the front of the spokes.

    The part will be cast iron in green sand. I've made a 3d printed pattern with no split as a test. Aside from the issue above, when I tried molding in green sand, the sand between the spokes pulled out. The gap is narrow and deep, so there isn't much to hold the sand in place when pulling the pattern. Can I do this in green sand, or do I need to use something more robust?

    Any advice?

    Attached Files:

  2. Al2O3

    Al2O3 Administrator Staff Member

    Hey G3j, welcome to the Home Foundry.

    If I'm reading the drawing correctly, the spokes are almost entirely drafted from one side, but, the spoke cross sections show a radius on the bottom side, so it would need to be parted where that radius meets the drafted part of spoke something like the red line below.

    45 Trailing Truck Wheels-compressed.jpg

    Now you could make a backer to do this but if you only want to cast a couple in green sand, the easiest way to do that is just sit the one-piece printed pattern on a board in the flask, pack/ram the whole thing, flip it over and cope down to the backside of the spoke fillets with a spoon to where they meet the drafted spoke (red line), apply parting compound, pack the backside, split the mold and remove the pattern. You'll need to decide if you want to core the axle hole or since it's small cast it solid.

    Many years ago, I had a complex part and was asking a master molder how to part it. He grabbed it out of my hand and did what described above in about 5 minutes. -Very humbling.

    HT1, Tops and Sawyer massey like this.
  3. HT1

    HT1 Silver Banner Member

    If you decide to "cope down" as kelly suggests only thing i would recommend is to clearly draw the parting line on the pattern , to help you know where to stop coping, and unless you intend to do A LOT of them his suggestion is spot on

    in the Navy we did alot of coping down one of the tricks is if you have all interchangeable flasks is to cope down once, then make all the Drags (or Copes ) using that one as a follow board , Im probably describing it poorly

    V/r HT1
    Tobho Mott likes this.
  4. G3j

    G3j Lead

    Thank you both for your replies and advice. I've not done "coping down" but I can see the value and understand how to do it. I've just done a test mold, focusing on the drag. Unfortunately, due to the long narrow channels between the spokes, the sand in that area pulled out when I tried pulling the pattern.
    Any suggestions to resolve this problem?
  5. Al2O3

    Al2O3 Administrator Staff Member

    How much draft do they have? Polish the surfaces to high luster, apply graphite, vibrate pattern before extraction. What kind of binder? Green sand? Might be a bit too wet if so. Short of that more draft.

    HT1 likes this.
  6. Tobho Mott

    Tobho Mott Gold Banner Member

    If all else fails, you could try pushing some flat head nails or twisting some screws into the sand in those gaps before you ram up the other half. It might help those fragile pieces of sand stay put when you draw the pattern rather than breaking off and coming with it.

  7. Melterskelter

    Melterskelter Gold Banner Member

    Those spokes should pull just fine. When I have similar problems I tend to think of secondary solutions, like gaggers, follow boards etc. But, what I have learned is that my pattern is trying to show me where I have not done a good job of maintaining draft, has rough spots, or other faults. Pracitcally invatriably, once I've gone back to the pattern with a strong directional light, bondo, 120 and 220 paper, wax, and graphite and given the pattern some love, it returns the favor. I have found that using Rustoleum White Specialty Lacquer on patterns reveals defects that are otherwise hard to see. This is a volatile-based lacquer that dries in minutes and has a high lustre not to be confused with other products also referred to as lacquers.

    To help with coping down (an excellent suggestion) I use a Sharpie to make a parting line on the pattern. That helps me see how far down to cope when I am digging sand away. Use plenty of talc and just a thin (can still see through it) layer of dry sand to ensure a perfect parting of cope from drag. Dry sand as an important auxillary parting agent has proven itself in my foundry.

    The other possible cause for them not pulling well is poor sand strength and inadequate rapping on the pattern. Use a draw spike to lightly tap in avertical direction while withdrawing the pattern. But, I would first look very closely at the pattern.

  8. Melterskelter

    Melterskelter Gold Banner Member

    All the "brave words" above aside, those fairly deep and narrowly spaced spokes may, indeed, require some sort of gagger as suggested by Tobho above. Getting them to draw is one thing. But getting them to hang in space while you flip the cope to mate up with drag could result in fallout of sand. So, yes, some sort of gagger along the line of headed nails that are most of the depth of the cope may be needed. Making them double-headed could make them even more effective---a head on each end with one head soldered ortigged or mabe the pointy end just bent at a right angle.

    I find sand falling out after an otherwise "perfect" draw to be really frustrating.

  9. Tops

    Tops Silver

    Wondering if HT1's tip of graphite and steel wool/scotch brite pad to the pattern would also help smooth it and identify potential trouble spots. If I mis-quoted the author of the tip, mia culpa!
    HT1 likes this.
  10. HT1

    HT1 Silver Banner Member

    old school parting powder was ground silica sand, obviously silicosis has made this a NoGo, but you can see why sand would work as a parting, BUT it HAS to be good sand, some shells or mica would cause terrible finish defects

    V/r HT1

    P.S. on a side note, I'm not seeing good draft on the OP's pattern, and pieces like that can be pure murder to pull. we used to do a similar part, and we used a 6 hand pull, two other people came in and held down the green sand cores while the molder puller the pattern
  11. Petee716

    Petee716 Gold Banner Member

    In my experience the answer is yes and yes, especially in the types of areas in question. I was working on a pattern yesterday where I wouldn’t have picked up on the flaws had I not used graphite and 0000 steel wool. The vertical surfaces of features that go through the pattern need very special care as several posters have pointed out above. Not ramming too hard is also important too.

    I’ve found that when coping down I need to keep in mind that I won’t have the same opportunity to “doctor” the cope as I do with the drag so I tend to favor the cope’s release when coping down if the parting line is at all in question. That sometimes leads to extra cleanup of the casting, but sometimes you’ve got to take the best deal you can get. That said, skipping a few minutes of extra work on a wooden or plastic pattern can cost hours of cleanup on a casting. I sometimes forget that fact and say f-it, and when I do I always pay the price later.

    A couple of alternative options might be to use sodium silicate molds which would still require coping down as described above but would make the molds less fragile, or else establish the parting line on your computer screen and reprint the pattern in two parts.

    Tops likes this.

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