Welded box sections vs CNC machined Uprights

[cdoc83]

Hi

I am looking to get an idea on the pros and cons of welded box uprights and CNC machined uprights? Any Ideas?

Thanks Chris

[Marshall Grice]

...you obviously haven't searched yet. there are volumes written on these forums about the exact same thing.

[Jersey Tom]

http://fsae.com/eve/forums/a/tpc/f/125607348/m/19710037821/p/2

Cmon, this is from yesterday...

[Z]

Chris started that other thread. I guess he is after a specific comparison between welded and machined.

FWIW, I remember Carroll Smith saying something like "Sure, the CNC'd aluminium is a lot easier to do, just throw it in the machine..., and it does look nice..., but the fabricated uprights, although they take more man-hours, are lighter, stiffer, stronger...". (Apologies to CS if I have misquoted him, but that was the gist of what he said as I remember. Maybe I remember it so well because it was exactly what I was thinking at the time...).

Also worth noting - all bolts used on aeroplanes must have rolled threads. No machined threads allowed, because the machining action tears up the crystal structure of the surface metal, making it prone to fatigue cracking. Rolling compacts the surface crystal structure making it stronger, like shot-peening. Cold-rolled sheet steel is made with a similar rolling process to rolled bolts, so has similar advantages over a machined surface. Of course you can machine, then polish, then shot-peen (a laborous and skilled process)...

Finally, it is hard to machine a hollow section. Most machined uprights are open sections, like an "H". Open sections are much weaker (more fexible) torsionally, and in one direction of bending, than closed sections like an "0". An upright doesn't necessarily see large torsional loads, or bending loads in all planes. But with a closed (boxed) section you are pretty safe from all "unforseen" loads - ie. the box is as strong as possible in all directions.

Z

[Chris Boyden]

Has anybody made uprights by bonding machined aluminum sections together to make a closed surface with a hollow section for weight loss?
WWU? All this talk of welding sounds expensive and tedious. The enclosure industry also makes an enourmous amount of extrusions that are very light, stiff, and relatively inexpensive. It may be something worth exploring. Although it wasn't a load bearing application, we bonded our sheet metal plenum to the runners with hysol, and it was one of the best things we could have done.....no welding to warp or blow through, no begging and pleading for services. I wish that we could have bonded the entire intake.

[kozak]

I think i remember WWU making something like this on their website.

[Chris Boyden]

Yep.

http://fsae.com/eve/forums/a/tpc/f/125607348/m/8731...10377911#23510377911 (http://fsae.com/eve/forums/a/tpc/f/125607348/m/87310573911/r/23510377911#23510377911)

[Z]

Originally posted by Chris Boyden:
All this talk of welding sounds expensive and tedious.
"expensive and tedious"?????

I've ranted on about this enough here and on the other thread, but one last plug for welding. http://fsae.com/groupee_common/emoticons/icon_smile.gif

I was taught to weld when I was about 11 years old. A one minute lesson - thanks Dad. Sometimes I wish I took some formal welding classes, but I have done a heck of a lot of welds, and none of the important ones have failed (finding the limit on non-critical stuff is a good way to learn what makes a good weld).

Welding is very CHEAP and EASY.

If you are going to go into mechanical engineering (which I guess most FSAE'rs are) then you really should get comfortable with welding - there is a lot of it about. As I said on the other thread, in industry I have found two types of engineers - those who can weld (to some degree), and those who are terrified of it. The non-welders usually wear neatly pressed white shirts, and never get out of the office. They also usually design elaborate, overly expensive stuff that breaks (honest)... http://fsae.com/groupee_common/emoticons/icon_smile.gif

Z

[Jersey Tom]

I'd generalize it more to, there are the engineers with working, hands-on knowledge of some manufacturing process (welding, machining, sheetmetal...), and there are those who don't.

The latter drive me up a wall. I co-run an instrument shop on campus and I can totally understand where all the friction comes from between machinists and engineers. Engineers on the whole come up with some dumb assed designs sometimes.

Machining aluminum is cheap and easy.

[Z]

Here (http://www.race-cars.com/carsales/other/1054227511/1054227511lh.htm) is a car that was done by someone who, I guess, didn't like welding. http://fsae.com/groupee_common/emoticons/icon_biggrin.gif

Z

[Travis Garrison]

Having done the bonded Al. ones and been on a team that's done the welded steel variety (not having done those myself), I'd say that simple CNC aluminum ones are probably a little easier on the whole, provided you have a proper 3 axis mill. The bonded aluminum uprights are probably what it's going to take in order to get a significant weight advantage over steel, but your ease of manufacture goes out the window.

Another example of bonded Al. uprights, I believe, is the recent Lister LMP car. There was an article in a Race Tech, or Racecar Eng on that car some time last year or the year before and they mentioned capping off the back side of a relatively standard CNC'd al. upright with a bonded on section...

The steel ones make much more sense for most teams, and I doubt you'll actually see much of a weight difference between welded steel, and your average CNC'd al upright.

As far as the cost report goes, the Al. ones will cost significantly less...again assuming you have a proper 3 axis mill. CNC'd aluminum parts are a joke in the cost report..take advantage of it if you can.

[CMURacing - Prometheus]

Originally posted by Travis Garrison:
CNC'd aluminum parts are a joke in the cost report..take advantage of it if you can.

by this, i assume you mean that there is no specified material removal rate, and so you can say that any given CNC'd part took 30 seconds, given a $1million mill?

we used (relatively) legitimate CNC times for our haas 3-axis...and our uprights ended up costing ~$300 per piece...

[Jersey Tom]

Well you can't get to ridiculous with it or they'll call you on it and you better be able to back it up.

But any competitive job shop, or automotive shop, is going to be running machine tools by Mori Seiki, Mazak, Matsuura, Okuma, etc.

Perfectly reasonable to say you're doing heavy peripheral milling, or even full slotting, pushing a half inch tool at 12000rpm and 180ipm. Perfectly reasonable that it will be set up with pallet changers, tool eye, touch probe...

[Travis Garrison]

Originally posted by CMURacing - Prometheus:


by this, i assume you mean that there is no specified material removal rate, and so you can say that any given CNC'd part took 30 seconds, given a $1million mill?


Pretty much http://fsae.com/groupee_common/emoticons/icon_smile.gif except you don't need an uber expensive mill...students just don't code aggressively enough...or they design overly complex parts

The fact that you don't have to specify any particular alloy of aluminum in the cost report, and that you don't have to factor in programming time, or fixture construction means that CNC'd parts come out costing much, much less than they would in the real world.

Fortunately Haas has an example posted http://fsae.com/groupee_common/emoticons/icon_smile.gif

http://www.haascnc.com/news/new_files/VMC/VF-2SS.wmv

looks like ~ a 8" x 4" x say 3" piece of stock

so thats 96 ci -> 9.36 lbs of Al -> $7.02 worth of stock (I'm sure we'd all like to find their supplier)

Machine time 3 min 22 sec or 0.056 hrs which at $70/hr comes to $3.93...if you want to pay the guy behind the machine (unecessary if you have a pallet loader and an automated work cell...but say you only want to buy the VF2) it would be $105 / hr which is still only $5.89.

So I get $12.91 / upright. Who is doing your cost report? http://fsae.com/groupee_common/emoticons/icon_wink.gif

Seriously when we were costing out the machined al bits for V35 up at WWU there were several situations were we said to ourselves..."we can't cost it this cheap, someone's gonna ask questions...tack on $50..."

That's what I mean by the cost report is a joke. The real cost of CNC work is in the programing, the fixture construction, and the first run that doesn't go so smoothly...if you don't have to pay for that, it's the greatest thing in the world.

[Jersey Tom]

I love how in the cost report $1300 worth of this particular alloy becomes $75.

[Chris Boyden]

"expensive and tedious"?????

Yes, expensive and tedious.

Our '04 car had sheet metal uprights...and really simple ones at that. By the time you add up the waterjet, bending, fitting, welding, heat treatment, machining, etc...it was way more work than the CNC'd aluminum parts....therefore....way less cost...in real world terms...not just the FSAE cost report.

Maybe if we outsourced all of those processes to China, it would be supercheap because labor costs wouldn't eat your lunch. Labor in FSAE can eat your time.-

I'm not afraid of welding one bit.....
I've welded oxy-acet...tig...mig...stick...
I'm not an expert.....I can lay a shitty bead as good as anybody.

I've got a stupid bracket that I inherited from a mechanical engineer that overdesigned it....1/4"x 3" plate SS, welded to a 1/4" 2" angle bracket that cost me $80 bucks to have fabricated by a "real world" shop. We changed the design so that it could be CNC punched out of 1/8" SS sheet and bent it, no welding, no machining = $20 bucks. That's because we did work with the fab shop, researched the processes he's cable of...etc. We reduced a 4 process part down to two. It's very important to understand fab processess.

So, it all depends on what you are trying to do.
Yes, I agree that welding can be cheap and easy...but for some things like Al welding, if you want it to turn out decently, takes skilled labor that costs money. Bonding parts can be done alot faster logistically speaking, which saves you time and money.

As with most things.....it depends on the application.

[Schumi_Jr]

Originally posted by Z:
Here (http://www.race-cars.com/carsales/other/1054227511/1054227511lh.htm) is a car that was done by someone who, I guess, didn't like welding. http://fsae.com/groupee_common/emoticons/icon_biggrin.gif

Z

Do you think he used enough fasteners? Bolted connections might be easier to do fatigue analysis on but their stiffness to weight is crap.

He must've worked in aerospace...

[RacingManiac]

We've had fabricated sheetsteel upright since 2002, we decided to go to a CNC Al design based on a member's design thesis last year(2005). It was machined in 2 halves, with self jiggle pins connects the halves together then welded. Effective became a closed box hollow upright. Sounds simple, it was a pain for our CNC sponsor(no CNC facility in the University) to make, and it took them forever with a particularly busy years for them. And effectively sets back our car by 1 month at least. Compare to 04's sheetsteel which took 2 weeks to make in-house.

We've gone back to sheetsteel this year.

[Jersey Tom]

..which is why I'm all about having your CNC-intensive parts designed by a student with considerable machine experience, and your weld-intensive parts designed by a student with considerable welding experience.

Concurrent engineering..

[CMURacing - Prometheus]

we (i) have always tried to use realistic numbers for the cost report. i didn't plan on continuing that this year, though, after i costed our engine coolant last year:

we used 1.5 gallons of dihydrogen monoxide engine coolant, obtained from the Pittsburgh Department of Public Works at $0.001 per gallon. PM me if you'd like a phone number to get in touch with them, they were a pretty reliable supplier and got the product to us right when we needed it.

as such, i maintain that no human actually reads the cost report, they just run it through a program that flags missing items.

[Mark Bacchetti]

Originally posted by Schumi_Jr:
He must've worked in aerospace...

Hahaha. Yeah, that design is DEFINITELY 'aerospace'd' out. http://fsae.com/groupee_common/emoticons/icon_biggrin.gif

Cal Poly Pomona has almost always used two tubes welded to a spindle or bearing holder. Its about as simple as it gets for welding uprights.

-Mark
Cal Poly Pomona Alumni

[BStoney]

Originally posted by Jersey Tom:
..which is why I'm all about having your CNC-intensive parts designed by a student with considerable machine experience, and your weld-intensive parts designed by a student with considerable welding experience.

Concurrent engineering..

Jersey Tom.... Couldn't have said thsi better myself. It's all about knowing how your part is going to be made when you design it. I am sure a lot of people wish they'd paid more attention in their manufacturing processes classes, or actually got their hands dirty at co-op, etc. Electives within most curriculums also allow for machining, welding, etc classes to count towards graduation.

[Z]

(Got some hard work to do... so I'm avoiding it by posting here again. http://fsae.com/groupee_common/emoticons/icon_smile.gif)

Went shopping for bicycles last weekend - Xmas, kids getting bigger... Huge range in prices, moderate range of fabrication methods, probably very small range of performance differences.

Anyway, most common fabrication method was Tig welded tubes, either steel or aluminium (couldn't find what Al. alloy - 6061???). Consistency of welds (generally very good) suggested that most welds were manual. "Made in China" stickers supported this. No more brazed-steel-tubes-in-gussets? Some carbonfibre frames only in high end ($$$) shops.

Otherwise quite a lot of forged steel and aluminium components. Did see an ally "fork-head" that looked forged but was actually two pressed ally halves welded together, then the fork arms welded in. Some components cut from ally extrusions - quite crappy looking. Wheel rims extruded ally, rolled into hoop, then welded? More plastic bits than there used to be, but bikes are still mostly metal.

And guess what? I didn't see a single "machined from billet" component (unless there was extensive post-polishing to make it look like a forged piece). These bikes are either extremely high volume sales (China), or medium volume (the moderately expensive ones). I guess all the metal swarf that has to be thrown away after CNC machining still makes a difference to the "bottom line". Forging is cheaper for larger volumes.

Also recently saw an agricultural water pump impellor that was fabricated from welded stainless steel sheet (~1mm thk). The welds were so fine and so perfect that they must have been done by machine - possibly laser?. The pump was quite cheap - similar cost to those with moulded plastic or cast iron impellors - which supports the idea that it was mass produced by machine.

Offhand, the only places I can think of where large CNC machined components are common are low volume applications, and/or markets where "image" is important - eg. motorsport, aftermarket car parts, etc. (Small CNC turned bits like hose fittings, etc., are common.)

Real FSAE is very low volume (ie. one car), but the hypothetical "build a 1000 cars a year" (or whatever it is in the rules?) is already getting up there in numbers. Any CNC supporters want to comment on what are realistic volumes for billet machined uprights?

Z

[Denny Trimble]

Bikes - hey, that's my department!

Quick answers to question marks: most aluminum frames are 7005, which doesn't require post-weld heat treatment. Some nicer frames are 6061 and fully heat treated post-weld.

Modern high-end, proprietary steel alloys air-harden at the weld, leading to stronger, lighter, less expensive frames than the old lugged/brazed steel.

Extruded aluminum rims are the norm, half are welded and machined, the other half are pinned and glued.

CNC machined parts are definitely boutique items, but the volume is enough even on high end parts that forgings are used for crank arms and other large parts. These forgings are often finish-machined, sometimes on all surfaces, producing high-strength, pretty, expensive parts.

In the "boutique" category:
http://www.chrisking.com/hubs/hbs_20mm.html

It would be fun to play with forgings in FSAE, but the tooling expense is prohibitive.

[LCheung]

Real FSAE is very low volume (ie. one car), but the hypothetical "build a 1000 cars a year" (or whatever it is in the rules?) is already getting up there in numbers. Any CNC supporters want to comment on what are realistic volumes for billet machined uprights?

Any boxed steel welded upright supporters want to comment on what the realistic volumes for that? What about steel frames? Just look at Radical, how many cars do they make in a year?

I think the better question is what is the cost and time to make 1000 units.

When you're talking about bikes made in China you have to realize they're making things in volumes far larger than 1000, and many other methods of produciton come into play in terms of cost.

You have to look at the details of how eveyrthing is executed too. Is someone manually cutting out the panels of a box steel upright then cleaning and fitting them or is there a set pattern and custom made jigs already set up to make them?

If you're talking about sheer volume of a CNC upright. How big is your machine? Bed size? Horsepower? Max feed rate?

How many wheel manufacturers are able to make thousands of CNC wheel centers a year?

[Jersey Tom]

Well, while the competition's premise is to design a car that will be produced at however many it was per year (1000?), you can't realistically design your FSAE car to that.

If I was designing a production vehicle I'd take full advantage of a process that is geared towards mass production, be it screw machining, P/M, die casting, blanking, forging, or what have you. But the reality is, the FSAE car is 1 per year. No school can possibly front the thousands of dollars in tooling for the above. So you have to compromise to what YOU can do with the tools at your disposal, and optimize it for speed of production.

If you're asking what size CNC we have here...

In the shop I co-run, we use a Fadal 3016L.. 30x16x16" axis travel, 15hp spindle (peak), and a max feed on the order of a couple hundred ipm. I've never run faster than 100ipm. Not really practical for what we do.

I also have access to the CNC in the MechE shop here, with a is a Hurco VM1, 26x14x18" I think, 15hp spindle (peak), and also feeds on the order of a couple hundred ipm.

[LCheung]

Jersey Tom,

I agree with what you say. I was simply posing questions in response to Z's post.

Our FSAE cars are prototypes for a car that is to be produced at a 1000 unit per year rate. The protype itself does not need to be produced in the fashion that it will be in production.

But realisticlly what would you substitute a steel space frame with in production? You could easily substitute uprights with other methods of production, but you can't just say you'll substitute a steel frame with a carbon monocoque and not have that included in your prototype.

The whole reason why I talked about the 1000 units is because of Z's comment.

[Jersey Tom]

But realisticlly what would you substitute a steel space frame with in production?

Tube hydroforming and welding?

[LCheung]

Perhaps I don't understand tube hydroforming in very much depth, but don't you still have to start with steel tube? In which case what I stated previously still stands?

[Jersey Tom]

Hmm somehow I missed Z and Denny's posts. Sorry bout that. Should have been replying to theirs rather than yours Cheung...

I wouldn't say machined parts are only for low volume applications. Look at screw machines and CNC swiss turning. Unbelievably low cycle times. Though yea, it primarily dominates fairly medium-run parts and precision instruments.

Machined billet uprights, a realistic volume...

If you design it right and you have a couple good machines set up, especially with pallet changers, tool eyes and probing... ie lights-out manufacture, I think you could definately make enough parts for 1000 cars a year in a small job shop. Obviously other methods are better-suited for large-volume work, but CNC is not totally impractical.

Like I said earlier, and Denny alluded to, is you have to meet halfway between the 1-off approach (realistic and practical for us), and the 1000/year approach (premise of the competition). I'd love to run die cast or forged or P/M parts, but we sure as hell don't have the money for it. Even welded parts.. the way things are here right now its safer, more practical, and faster to use a machined design.

And dammit I have too much fun machining these things I aint given that up!

Edit-

And yea Leon I believe you do start with some sort of hollow steel section in hydroforming, and you still have to weld it up.