Well, yet another adventure! I am going to try casting a part in non ferrous metal (bronze, brass, etc) and would like to know if there is a Rhino 7 feature (shrinkage factor, etc) I can add to a model to scale it correctly, or do I have to scale layout lines before surfacing. The pockets introduce a pain when scaling, since I want them to get smaller when scaling up the model! (Can one scale up a pocket and have it get smaller, or do I have to scale down the pocket, then join to the scaled up other parts??) I’m looking at 1:48 shrinkage of the cast part.
It’s a hair over 2% on a 3.5" x .5" x.5" part and might be too knit picky, but thought I’d ask.
I might also need draft angle, but my plan right now is to take an original part, and see if I can squash it into the casting sand and remove it cleanly. I’m doing old school “pack the sand onto the pattern”, not even thinking about lost wax or investment casting, and I don’t have a 3D printer, although I could sub out the printing to a shop somewhere, if I don’t make a wooden pattern here.
The drawing is a half model and will be mirrored when ready. Rhino 7 Windows.
i am not aware of a shrink factor compensation. maybe there are some in the printer slicer software packages? i have practically no experience printing stuff sorry.
but if you already know the factor you can just model the geometry slightly bigger/thicker to compensate, or in your case it could maybe already work if you use OffsetSrf on the entire object.
i am not sure if it will shrink that even, scaling it entirely like that might not make sense. i also remember watching some youtube stuff about casting that they implement raisers that if i am not wrong bear reserve material for the shrinkage in the liquid stage which gets pulled in slightly during the cooling process.
Scaling the polysurface as I presented will not work. Scaling some larger, and some smaller, and then joining and trimming would work. The main body has to be larger, while pockets and holes have to become smaller. The casting should allow for machining, as well as shrinkage.
Are you sure that won’t work? Shrinkage factors are generally just that, an overall factor. Getting into modifying certain parts of the model because they will shrink differently may be beyond the precision of the process?
It does work, assuming you have the correct shrink factor for the particular alloy you will be casting
The holes and pockets will shrink at the same rate. All you have to do is scale the whole thing by the shrink factor. The material around the holes will shrink as the metal solidifies and the holes will become smaller on the casting.
Machining stock allowance is a separate issue. You need to add whatever material is required to allow for machining on the surfaces that will be machined. Draft on the vertical surfaces is also a separate issue that also requires modifying those surfaces. Your model is very simple so creating a model with the correct draft, machine stock and shrink should not take long.
Risers don’t prevent shrinkage overall. The risers are attached to the heaviest parts and feed molten metal as it solidifies to prevent localized shrinkage that would cause cavities and voids in the casting.
i made a simple model that should roughly outline why that can not be.
lets assume all the atoms realign and pull each other together at the same rate, thicker material will shrink more in relation meaning the rate at which thicker parts will shrink will be higher.
Sure there are differences but if you get into that mattering then a)you’re not likely to be able to figure that out before actually making it and/or b) you shouldn’t be counting on that level of precision for the process.
Shrink factors are just that, an overall factor. Anything I’ve done with a shrinkage factor has been just a straight scale.
is that maybe the reason why some rockets explode and others not? i am personally pretty allergic regarding reluctance and oversight, something i never understood why details can not be understood as integral parts of our existence. but hey does not matter when somebody dies, too many people on our blue pebble that seems so insignificant in the vast boundaries of infinite space.
Good grief dude I’m just telling you casting isn’t the most precise process in the world. Stop trying to look smart instead of actually helping. The other Jim agrees with me too, BTW. You start with an overall scale factor. If you need to do something more, you figure that out in prototyping. Buzz off.
I get the thing now about the holes shrinking - it’s because the entire object is shrinking. I think scaling will be fine. But I will definitely have to add in machining allowance. Most of my models go directly to CAM that generates code to machine a part from a rectangular blank, not a casting. There is no harm in making the casting larger. I’m working with bronze, brass, and aluminum, not precious metals or MIL-SPEC here.
Too much work to scale individual surfaces - this guitar bridge is not going to the moon, yeah?
I think your model supports the opposite of what you claim. The centers of 6 circles circle get scaled the same as two (like doing scale with the rigid option). If they all pull together at the same rate that implies a uniform shrink everywhere. As someone who has worked as a patternmaker for more than 40 years I can assure you that scaling the whole thing is how its done. In the old days patterns were built with every dimension scaled individually. But with a computer model you can scale every dimension in one step.
The problem with thicker versus thinner parts is that the solidification happens at different rates. Ideally to avoid problems you would want the inside to solidify before the outside, but that has the physics of how things cool working against it.
I worked in a pattern shop over 20 years ago, and never thought I’d care about this stuff. I ran a 25 HP CNC mill that machined aluminum castings. I was not involved with engineering the parts I cut.
its just how the command works…
if this one not the next one maybe 
