Casting vs machining

[J. Schmidt]

I'm trying to weigh the pros/cons of casting aluminum parts vs machining them. I know aluminum castings are not as strong as machined billet pieces, but from my calculations doing a lost-foam casting us much cheeper (in the reports) than CNC machining the same part. It takes about the same amount of time to cast a part and the casting only needs minimal machining to get the final tolerances. Low cost is not a substitute for strength, duh, so lets not even go there. Is there anything else?

[J. Schmidt]

I'm trying to weigh the pros/cons of casting aluminum parts vs machining them. I know aluminum castings are not as strong as machined billet pieces, but from my calculations doing a lost-foam casting us much cheeper (in the reports) than CNC machining the same part. It takes about the same amount of time to cast a part and the casting only needs minimal machining to get the final tolerances. Low cost is not a substitute for strength, duh, so lets not even go there. Is there anything else?

[-]

Take a look at what resources you have. Do you have a CNC machine, or do you have a Casting company near by who would be willing to work with you? I have seen people make some pretty sick cast uprights only to have them take 5 times long to make since the company doing them was busy with more important work.

[Travis Garrison]

If you look carefully at the cost report, and don't base your CNC time on a 20 yr old, bridgeport based, 2.5 axis, punch card driven POS CNC parts are actually stupid cheap. I know that after talking with other student machinists, poor facilities and or overly conservative shop instructors tend to leave people with a warped idea of how long it takes to machine parts. The right machine and the right tool will make all the difference in the world. Factor in the lack of fixtures and programing costs and the cnc'd parts on your cost report should be dramaticaly cheaper than any shop would actually charge you to make them.

The above post makes an excellent point, do you have the ability to cast the parts you want?

Lastly, as far as design decisions go, I wouldn't place much weight on the cost report. Its probably not worth it to try to win that event outright, and sacraficing even a 1/4lb pound per corner to shave a small amount off the cost report is simply not worth it IMHO.

Travis Garrison

[Greg]

We had a cast intake component last year. It was a curved tube, with a slight taper to it, and an injector boss. It is basically impossible to machine in one piece, and would be a pain in the neck to weld, so we cast it.

[DontAsk]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by Travis:
If you look carefully at the cost report, and don't base your CNC time on a 20 yr old, bridgeport based, 2.5 axis, punch card driven POS CNC parts are actually stupid cheap. I know that after talking with other student machinists, poor facilities and or overly conservative shop instructors tend to leave people with a warped idea of how long it takes to machine parts. The right machine and the right tool will make all the difference in the world. Factor in the lack of fixtures and programing costs and the cnc'd parts on your cost report should be dramaticaly cheaper than any shop would actually charge you to make them.

The above post makes an excellent point, do you have the ability to cast the parts you want?

Lastly, as far as design decisions go, I wouldn't place much weight on the cost report. Its probably not worth it to try to win that event outright, and sacraficing even a 1/4lb pound per corner to shave a small amount off the cost report is simply not worth it IMHO.

Travis Garrison <HR></BLOCKQUOTE>

Laugh all you want, but my 20 Year Old 2.5 Axis Boss 4 works just fine, punch cards and all. Best of all it is in my garage and only cost me $2000 for a fully functional CNC machine.

Just add a laptop to feed it G-code and it will do fantasically complex programs. At least until I get around to upgrading the damn thing.

[MikeWaggoner at UW]

If you machine a part from good aluminum, FEA will be reasonable. If you cast a part, watch out for porosity, contamination, poor grain formation... and on. Casting is harder, especially for structural parts.

[Tim Heinemann]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by MikeWaggoner at UW:
If you machine a part from good aluminum, FEA will be reasonable. If you cast a part, watch out for porosity, contamination, poor grain formation... and on. Casting is harder, especially for structural parts. <HR></BLOCKQUOTE>

Casting is as good as machinig as long as both are done right and there are pros and cons on both. The equivalent of porosity, contamination, grain etc. on castings is e.g. notch effects on machined parts from the cutting tools. All this can be dealt with. If you happen to have the correct simulation tools (or workers with tons of experience or -at best- both) porosity and microstructure can be "tuned in" with the casting process and you get a good starting point on your FEA, too. BTW, the bar stock you machine your parts from has been cast too and generally the larger these profiles get the more their mechanical properties differ from the outside to the inside, even on "good aluminium".

Just my EUR 0.02 ;-)



Tim

[J. Schmidt]

I'm asking because of the facilities we have. UWP is one of the few universities in the country with an accredited foundry program (not within the engineering majors), taught my a metalagurical engineer who is more willing to help than the machine shop supervisor. We have the capability here to do some cool stuff with castings, our AFS (American Foundry Society) cast a bell for a Liberty ship out of an alloy they developed and made themselves. This and the Machine shop guy I have to work with is a dink, Duwe will agree with me. He's reluctant to help us and when he does he does a mediocre job. We resorted to making our own "key" for the shop to use between the hours of midnight and 2:15am. You didn't hear that from me though http://fsae.com/groupee_common/emoticons/icon_wink.gif.

So, cast parts can be just as strong as machined (depending on the alloy)? I've always been told that cast parts need to be heat treated for good strength.

[J. Schmidt]

Oh, we can even do cast iron if we want some really strong (but brittle and heavy) parts.

[B Hise]

J.Schmidt,

I think many of us have been involved in "key making" in our endeavors to build a car late at night. Just make sure you keep your "keys" safe ;-)

[-]

Schmidt,

I think you answered my question earlier. I am jealous that your school has a foundry, god knows that if I went to school there I would be casting all kinds of goodies. Assuming the metallurgist's cast parts usually come out without a large amount of porosity or contamination, I would do a cast part, otherwise you might want to think of welding or milling.

As for your question about heat treating cast parts, if your metallurgist is as good as you say he is, look no farther.

Goodluck

[Wham Bam]

Ah, you turkeys. Cost aside, the only advantage to casting one-offs is the intricate shapes you can produce. The grain structure of a cast part is crappy. All billet steel or aluminum you buy has been rolled, meaning it's been cold-worked. Without getting into the particulars, this increases the yield strength. You'll never get that grain refinement in a cast part.

So if you're making an engine head (ie ports that can't be machined) you'll definitely be heading for the foundry. If you're making uprights, you'd better have some wacky design with geometry that just isn't feasible for a machined part.

Of course, the really smart people out there are thinking "doesn't the modulus stay the same regardless of grain structure...?"

[J. Schmidt]

It seems that everyone agrees that cast parts can be just as strong as machined, if you know what you're doing. And the process we use is dependant on the time/facilities/experience available. All factors equal, though, machining is prefered.

From my experience foam is much cheeper to replace than billet if you make a mistake, so we will be sticking to castings where available. Thanks for the input.

http://us.f2.yahoofs.com/users/4074b96b_5067/bc/6d52/__sr_/2b2f.jpg?phd_MJBBT4L8iqFK
brake pedal before casting (minus riser, runners, and gates). The finished part was better than expected; just picture all the blue foam as aluminum (did I mentions lost-foam casting is cool?)

Oh, btw, this instructor I mentioned has helped out UW Madison's FSAE team in the past while getting his grad degree, Dr. Metzloff. If any of you were around several (7+?) years ago you may remember him.