Fabricating a frame with high accuracy

[stever95]

Claude:

We all understand your frustration, but your reactions here were childish. Thank you for getting on point, though..

Silverbolt:

I have had similar questions about what is appropriate in terms of error and gaps and the like. On the gaps - it's sort of up you. If your welder(s) can handle 1/4" gaps and still get a good structural weld, it's probably OK. I'm certainly no ASME certified welder, but for these race-cars that seems to be OK. The frames are usually overly strong, anyway.

Error is a tough problem. My justification for allowing up to 1/2" discrepancies between CAD and fab was that it didn't affect the strength of the chassis, suspension mounting, or ergo. It's a tough question to deal with, but my best advice is to ask your teammates how your fabrication methods might affect their systems.

Also - why SS? I'm very interested to find out.

[Silverbolt]

I have a doubt about the main hoop bracing. I want to know if the triangulation for the bracing is proper, or not?
Attachment 200

I think it should be something like this
232.jpg

[Claude Rouelle]

Silverbolt

I think it is only fair a) that you introduce your self b) that you answer the previous questions which were asked to you before you ask other questions in this forum.

[Claude Rouelle]

Silverbolt

I think it is only fair a) that you introduce your self b) that you answer the previous questions which were asked to you before you ask other questions in this forum.

[Silverbolt]

Sorry Claude, but I think I've already introduced myself. For your sake, We are OMR Racing, based in Chennai. We are participating in FSAE Italy this year. I am incharge of fabrication.

@Simactive: We are using SS 202 because it has a higher yield strength than Mild Steel. Also because of it's inherent anti-rust properties. We have simulated the frame on ANSYS and even did the suspension calculations before we designed the frame. However, we have no knowledge of building the frame with our own hands.

[Jay Lawrence]

Silverbolt,

I would not like to be manufacturing a frame from Stainless. It is harder on tools (there will typically be lots of notching and grinding to do) and harder to weld. If you really must use Stainless, you should probably be using an L grade (201L, 304L, etc.), and you will need specific filler wire etc. You also will not be able to use magnetic bench squares (these are extremely handy when welding up a steel frame).

In the earlier days of my team, we made a mock chassis from PVC pipe and sticky tape. This is a good head check, especially for a young team, to ensure everything will fit and that it looks sensible and passes rules. Then it's a matter of cutting up matching steel tubes and notching the ends so they fit together as neatly as possible (makes welding easier).

[MegaDeath]

I agree with what Jay said. As someone who cuts and welds stainless steel and a variety of carbon steels on a daily basis, I would highly discourage you from making a frame out of stainless. Especially if you are a new team who is inexperienced in manufacturing.

If you were making the frame out of stainless because the stainless was given to you for free and you would have to pay for carbon steel tubing, then I would understand you choosing electing to use stainless.

As for your point of 202 stainless having a higher tensile strength than mild steel, I'll ask this; Does it really matter? A properly designed frame under typical loading (i.e not being crashed) should not have any part near failure due to loading. If you are trying to increase stiffness and therefore decrease deflection of the frame by choosing a stronger material and not by designing a frame with proper load paths to decrease deflection, you are probably going to have problems elsewhere in the chassis/suspension/wheel setup leading to poor tire control (compliance in suspension components and wheel deflection immediately come to mind). I'm not saying that you shouldn't pay attention to the stiffness and strength of the frame, because you obviously should. But instead what I'm saying is that you should focus more on proper frame design than material stiffness.

With that being said, I will also note that the difference in modulus of elasticity between 202 and mild steel (1018?) is very small, less than 1.5%(depending where you take your materials data from). That takes me back to my previous point where if you're trying to increase frame stiffness by using a material that is only 1.5% stiffer, you could more than likely make up for that difference by implementing better load paths into your frame and therefore save yourself the trouble of having to build an entire frame out of stainless which does not sound like a fun undertaking for a rookie team.

Cheers, and good luck.

[Z]

I have to disagree with the "stainless steel is hard to work with" comments above.

I love the stuff! This is a result of using it a lot in the food industry, in a past life (mainly with 304 and 316). It welds beautifully (better than MS), and drilling and cutting is not much different to MS. Given that many FSAE students that I have seen don't know how to use a hacksaw properly (it is NOT a piece of sandpaper!), and apparently don't know how to sharpen a drill bit either, I would say that the problems are not with the type of steel, or with the tools (well, the problem is the "other" tools... ) .

Together with its easy-workability, SS just gets better looking the more you work it. You can cut-and-butt countless times, and the application of a linisher and buffing wheel has it looking better than ever. So you can easily modify the frame at any time. And no messy, smelly painting, with primers, and undercoats, and top-coats, and clear-coats..., and waiting, and fingerprints...

Errrrr.... on the other hand, given the OPs comments about "huge gaps", maybe a liberal dose of paint would help!

Of course, SS does cost quite a bit more than MS, about 5 x when I was using it. And it's about ~2 x more than chrome-moly, depending on suppliers. But if the SS is being donated as sponsorship, then I see no reason why not to use it.

Also on the issue of "tubing" vs "piping", there is a lot of high quality SS (and other alloy-steel) piping available, again mainly intended for the food and petro-chemical industries. Given the high pressures and unpleasant nature of the fluids these PIPES have to carry, I would say they are often of a much higher quality than the typical FSAE frame TUBES. IMO, there is nothing wrong with using these sorts of pipes for an FSAE frame.

So, do any "real racers" out there want to give a well-reasoned argument for why "tubes" are necessarily better than "pipes" for FSAE frames? Claude? Pat?

Z