to cast or to machine from the solid?

Hi, Back in December I inquired about making a ball vise. I have been practicing some simpler machining since, and now would like to begin.

I never did have any luck finding 7-10" diameter steel balls or even solid rod at scrap yards.

I can buy 6" rod stock from the campus machine shop for ~$60, or $1.10/lb I believe it was (although my calculator is predicting that it should be $48, so it might have been more...), and machine it into one hemisphere and one clipped-off cone shape. The hemisphere is doable; I have access to a CNC lathe. Off campus, inquiring about 8" diameter rod stock, they wanted $1.75/lb!

Alternatively, I can cast. I asked a local club, and they'll help me cast it for $1.20/lb if I provide the steel. I can find random scrap steel for $.10/lb. This at first had great appeal when I learned of the low cost of scrap steel, but it makes me recoil some to think that I'll end up paying more per pound because of casting costs! AND, coming out of the cupola, it will be cast iron, which while quite functional, is inferior to steel for this application. The two benefits of this would be that a sphere is 2/3 the volume of the smallest cylinder that will contain that sphere, so even though it's $1.30/lb and not $1.10/lb, it costs $.23/lb less to cast in a sphere than to purchase a solid cylinder and machine off 1/3 of its weight in making it a sphere. Also, while I still want to program the lathe to machine a sphere, it will have to machine off far less material, saving not an insignificant amount of work.

What do you advise? Cast an iron sphere, or machine from a solid steel cylinder, assuming the former to cost 23cents/lb less? I want a

100-lb vise (trust that I do intend to make a 4.5" vise from all scrap first, before investing in this), so this is expensive for me :-( Is it possible that, if I shop around, I may be able to have it cast for less than the $1.20/lb quoted me, or is that a bargain...?

thanks! -Bernard Arnest

Reply to
Bernard Arnest
Loading thread data ...

Oh, another factor-- this foundry is 1.5 hrs out of the way, a disincentive to save the $.23/lb in casting it. But if larger rod stock is more like $1.75/lb, and it seemed to be if I can't find it as scrap, casting in fact saves $.58/lb compared to that. And there is the coolness factor, since as this is a college club, I will be able to participate in the process :-) I hate intangibles, they make rational decisions more ambiguous....

I have no idea if their $1.00/lb fee (I said 1.20, because I'm supplying the 100 lbs of fire clay for the mold, so all together this is costing me $1.30/lb after I buy the clay and the scrap steel) just covers their costs in fuel and sand, as they are a college club and not a commercial foundry, or if they are making much of a profit at that fee.

Reply to
Bernard Arnest

Reply to
JR North

Are you sure it will be cast iron? It is very much possible and common to cast steel. It's been a while, but steel in = steel out, if nothing else is added (carbon).

Also, why would cast iron be inferior? Our dies slide on cast iron (NAAMS G2500 non-alloyed gray cast iron) against brass or hardened steel wear surfaces. You actually want dissimilar materials in a bearing situation or else you can run into problems with galling and seizing.

As far as your actual question, here's my WAG: You'll probably get more consistent material with steel bar stock. Castings can be porous, difficult to machine, have uneven hardness, etc.

Regards,

Robin

Reply to
Robin S.

If you have hollow spots in the casting which is almost guaranteed on this project. What have you saved if you need it cast several times. I would go with solid stock, since you already know the quality if the material before you start.

Reply to
Richard W.

Hey Bernard, it's Zack. What CNC late do you have access to that I don't know of? The one in the MIT Hobby Shop can't swing something that big, so unless you have something off campus or can get it done at Central Machine that doesn't look like much of an option. I'm not sure of the size of the manual lathes in Edgerton shop but you might be able to turn it with a set of outside jaws. There are a few options for turning the sphere on a manual lathe, none of them very appealing, but it can be done.

Reply to
zackbass

Hi,

Brian said that he would help me program one of the lathes in the Pappalardo lab. I think there may be a fee by the hour to use them, but to minimize that, there's no reason that I can't machine several facets to get it close and just have the CNC make it a smooth hemisphere. That said, I haven't seen them for months since the pre-orientation program in September, so let me know if you don't think they'll swing 50 lbs :-(

thanks! -Bernard

snipped-for-privacy@gmail.com wrote:

Reply to
Bernard Arnest

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.