A little 'bake and shake' while humming Hank William's 'Hey Good Looking' since the #2 wax was too stiff to spread in the garage. Part left in covered warm pan and then wiped off followed by residual graphite from in the bag. Gating plan below, open to suggestions.
I’d suggest moving the gate to a corner. Pour slightly inclined with pour corner lowest, second intermediate, third highest. No vent needed. No riser needed, I think. patterns look good. Fingers crossed. Denis
Thanks Denis. I left out the part that I forgot to fill (3) 1/8" holes and covered them with 1/4" paper punch slugs and superglue, had to reset a dowel pin, and where the lacquer overspray on the pins almost cemented the halves together during one test fit...I will probably look things over a little better next time before shellac. The laser markings also show through about 5 layers of finish so I will need to adjust that for next time too.
Poured the pattern twice. First time got some shrinks, like the piece was soft clay and I grabbed it with my thumbs. Decided to enlarge the gate and add some dowel and hot melt glue 'gumdrops' where it shrank the first time, working fast while metal was heating. The gumdrops helped some but there is still shrink related to the gate- too much volume below part level? First piece edges and finish are nicer too, facing sand was from same pail and mull as first but backing sand was cooled and re-mulled from earlier. With the loss of edge definition perhaps too dry? Trying to tell myself it still is better than working for a living... PS: drilled and assembled as a mock-up. The little divots in the pattern we adequate for locating the drilled holes on this. I did not (!) plan out the hardware yet, looks like flat heads will be needed at least for the swing part.
I will comment on the gumdrop risers. I am surprised they had much positive effect given their small size. I understand the time pressure you were under. As an alternative modifying your pattern on the spot, you might consider using a small spoon or spatula and just carving out the sand for a riser. I have a pattern on which I occasionally neglect to place the risers before ramming the cope. When that happens I say some unpleasant things to myself about myself and grab the carving tool nearby and just carve them by hand. Takes less time than the warmup for the glue gun. Concerning the size: to work, a riser must be the last thing in the region to freeze, maintain a patent liquid channel to the part until the part is frozen, and be inclined to collapse and feed metal. Had your riser been maybe 1” in diameter and somewhat flat-topped (flat roof to promote collapse) and about an inch high, I would expect complete elimination of shrinkage. Here is one of those hand-carved riser I mentioned. Note it’s collapsed”roof.” It has been ground a bit and hardness tested—-thus the tiny dimples in the upper right. Overall size is about 1.5 inch diameter. But, as Bonz said, you have a couple of fine working parts, and more importantly, a whole lot of valuable experience. Denis
Yes, thanks, seeing it swing on the bench without the half pot of water careening off the stove is really nice. I have to admit that there were moments of doubt and frustration yesterday. I think my original hypothesis that the gate and sump/trap were causing issues should be more like the corners themselves are causing issues and the gates were in the corners. Some of the lessons learned: 1. Slow down and double/triple/4x check CAD to object before applying pattern shrink math and making patterns 2. Make sure pattern materials are adequate thickness for tools used 3. Finish filling and patching patterns before the last coat of paint 4. I am not fast enough to light the furnace and then ram up the flask 5. I need to increase mulled sand inventory and better condition used sand 6. I need to better understand the gating/cooling/shrinking mechanisms in castings 7. I maybe should have made the swings from flat 1/4" plate...
Thanks Denis. I need to get more comfortable carving sand. Good to know I am not the only person talking to themselves in the shop Is it that the corners are shrinking or the middles are, and then pulling from the corners...?
The areas with the highest mass/surface areas solidify last in general. The areas closest to the gate tend to solidify last but can be overruled by large mass/area segments. In this case the areas with the shrinks have the most mass/area with the worst shrink being near the gate. So, the narrower connecting arms froze first and then the center of the corner segments. That left those central corner segments obligated to make up for shrinkage related to solidification and cooling. They had nowhere to get metal other than to collapse one surface toward the other. Gravity increased the pull on the upper surfaces compared to lower. So they did what they had to do. It would not be amazing if the lower surface got pulled up a bit as well. But, "all things being equal" the upper surface will have the greater collapsing force. I had hoped that your patterns were thin enough so that they could smoothly contract vertically without noticable localized collapse---shrinks. "Thin" do not generally form shrinks. The above is my best understanding of the situation and might be correct. Or not... Denis
