Advise for Galvanized Steel

[DELurker]

Originally Posted by jman4747

We recently got a local sheet metal company to donate their scrap metal to us. They have 3003 aluminum but not a large amounts. Most of what they can give us would be galvanized steel and I wanted to know what most people would use it for. I was also wondering about any suggestions on machining it and what other types of tooling to use as well as safety. From looking at the scrap I saw everything from angle to strips to sheets as big as 4'x4'.

Coming back to the original post...

Remember that steel is as strong as aluminum per pound. To accomplish the same task will require the same weight of either metal. With that said, aluminum is usually a better choice because (let's face it), we're building relatively low-weight robots compared to industry. Steel is (mostly) harder to machine than aluminum (slower feed rates, smaller cuts, etc).

For machining it, the machine is as important as the tooling. Although a drill press will work, I'd advise a mill for the improved control and precision. For safety, if you're not cooking it (see previous posts), it's (mostly) just like machining regular steel.

Personally, I'd advise my team against the galvanized steel in large quantities. We'd never go through enough of it to justify the hassle of storing it and machining it. Plus, we're always running close to the weight limits and steel would just make that worse.

[Ether]

Originally Posted by DELurker

Remember that steel is as strong as aluminum per pound.

Not true as a general proposition. Depends on the type of steel, and type of aluminum being compared.

[magnets]

Can you weld galvanized steel?

Yes.

Would I feel comfortable doing it around or with high school students?

No.

[magnets]

Originally Posted by DELurker

Coming back to the original post...

Remember that steel is as strong as aluminum per pound. To accomplish the same task will require the same weight of either metal.

Not true. Aluminum (as a generalization for FRC, you can find exceptions) has a significantly higher strength to weight than steel. In fact, for shafts/gears, your 7075 aluminum is both lighter and stronger that the common stainless steels places like andymark use. In terms of ultimate yield strength, aluminum is roughly 1/3 the weight and almost twice as strong.

[yash101]

I wouldn't say "significantly stronger" but yeah, it can be stronger per unit of weight.

[Ether]

Originally Posted by yash101

I wouldn't say "significantly stronger"

The difference is large enough to be noticed, for purposes of FRC use.

[DonRotolo]

Originally Posted by yash101

I don't get why not to put copper with steel. I connect copper to steel, once in a while!
By you saying "electrolysis," do you mean that it will break up water when you electrolyze it and place it inside water?

Um, we are talking about galvanized steel, so the copper would be coming into contact with zinc. Go look up what happens when copper and zinc are in contact with each other. Can you say... "battery"?

You can perform electrolysis with many materials, not just water.

Originally Posted by MrBasse

I always cringe when new people are jumped on for spreading misinformation... but I feel that this is a time for learning as well.

You can weld galvanized steel with no issues as long as you use proper equipment. You do not get zinc poisoning from the fumes.

Thank you MrBasse for the elaboration.

My concern was not the zinc fumes, but weld spatter. Glavanized spits out molten globs of metal a LOT more than other materials.

I did not say it couldn't be welded, just that this particular person should not try it.

I stand corrected.

[jman4747]

So if I have the tooling and can keep the weight down why not save the money? And if I get 3 times as much free sheet steel wouldn't it just be better to save the money and be smart about its use?

Basically what I'm getting at is it isn't as simple as Steel is denser or aluminum is less rigid. For instance at the beginning of last build I found that a welded steel version of the kit-bot frame we used would be several times more rigid and cheaper. It would only have been about 2-5lbs heavier and that robot was 98lbs at inspection. We didn't posses the tooling to work it quick enough yet so we scraped it.

Witch metal to use has to do with the circumstances surrounding the team and the build.

[Oblarg]

Originally Posted by jman4747

Witch metal to use has to do with the circumstances surrounding the team and the build.

This statement can be generalized to "material choices are context-dependent," or "design choices are context-dependent," or even "essentially all utility calculations are context-dependent." It is good to keep in mind the difference between a heuristic rule and a fundamental principal - general statements about "optimal materials for FRC" are quite clearly of the former sort, and you should thus interpret them as such.

[magnets]

Originally Posted by yash101

I wouldn't say "significantly stronger" but yeah, it can be stronger per unit of weight.

Well.... 7075 is twice as strong as certain steels, so yeah, it is "significantly stronger"

[DampRobot]

Originally Posted by magnets

Not true. Aluminum (as a generalization for FRC, you can find exceptions) has a significantly higher strength to weight than steel. In fact, for shafts/gears, your 7075 aluminum is both lighter and stronger that the common stainless steels places like andymark use. In terms of ultimate yield strength, aluminum is roughly 1/3 the weight and almost twice as strong.

It's also a more expensive. Something else to note is that steel generally is more stiff than aluminum per unit weight. When you're building something like a bridge, those two factors can be important.

The real reason aluminum is thought of as lighter (for FRC purposes) is that we rarely design on weight, we usually design on size. There's a lot of arbitrary standards in FRC (1/4" thick bearings, 20 DP gears, 2x1 tubing, etc) where stuff just needs to be that big to conform to a standard. Your WCD will be a ton harder to design if you choose .75x.75 steel tubing vs alu 2x1, for example. Also, for stuff like shafts, aluminum (especially 7075) usually has an adequate total yield strength, and since the shaft needs to be 1/2" hex, the alu will be a lot lighter than steel. In other places, we need certain plate thickenesses to get the right number of threads in a tapped hope. Again, steel will be heavier because we just care about overall thickness, not strength. Of course, if we designed all our parts to just be a certain strength, steel and aluminum would be a lot more comparable in terms of weight.

Sure, we could crazily pocket steel or change our FRC wide standards to conform to steel robots. But until then, the lower density of aluminum and comparable strength will make it seen lighter for FRC purposes.

[DELurker]

Originally Posted by magnets

Not true. Aluminum (as a generalization for FRC, you can find exceptions) has a significantly higher strength to weight than steel. In fact, for shafts/gears, your 7075 aluminum is both lighter and stronger that the common stainless steels places like andymark use. In terms of ultimate yield strength, aluminum is roughly 1/3 the weight and almost twice as strong.

I disagree, although I will point out that stainless steel (302, 303, 304, 316) is a specific and (typically) less durable subset of steels (without getting into hardenable stainless such as 440C). The 3x price differential on stainless makes it less than desirable for teams to work with in the raw, aside from the additional tooling needed to properly work it. However, I will support my original contention with the following links and tables...

Reference #1
Reference #2

from reference #2...
........................................Aluminium. ................Steel
Density, ρ kg m−3...............2,700......................7 ,800
Young modulus, E N mm−2....70,000....................210,000
Shear modulus, G N mm−2.....27,000....................81,000
Poisson ratio, ν.....................0.33........................ 0.3

from matweb.com...
1018 steel versus 6061-T6 aluminum versus ASTM A526 galvanized steel

I could never bring myself to use aluminum for a shaft. It is too weak (comparably) to take that shock and torsion loads, as well as being more prone to unannounced catastrophic failure over steel (steel bends and then breaks ... aluminum has a much smaller deformation period before it snaps under the same loads). I'd rather use a steel shaft so that I could press-fit it into a bearing. The surface properties of aluminum just aren't good enough (at least for me) for rotating or direct-contact sliding.

[artdutra04]

Originally Posted by DELurker

...

from matweb.com...
1018 steel versus 6061-T6 aluminum versus ASTM A526 galvanized steel

I could never bring myself to use aluminum for a shaft. It is too weak (comparably) to take that shock and torsion loads, as well as being more prone to unannounced catastrophic failure over steel (steel bends and then breaks ... aluminum has a much smaller deformation period before it snaps under the same loads). I'd rather use a steel shaft so that I could press-fit it into a bearing. The surface properties of aluminum just aren't good enough (at least for me) for rotating or direct-contact sliding.

I would agree that 6061 alloy aluminum is a poor choice for shafts, but 2024 alloy aluminum and particularly 7075-T6 have excellent material properties that are competitive with many steel alloys. These two aluminum alloys are much harder and have significantly higher yield strengths than 6061.

I've been using 2024 and 7075 for shafts on FRC robots since I was in college in 2010, and have never had one fail. Plus, I absolutely love machining 7075 aluminum on mills and lathes. With good carbide tools, you can machine it like a hot knife through butter, as it does not have any of the galling issues that plague softer aluminum alloys. It's my second favorite material to machine after Delrin.

// Note: I didn't directly link to the Matweb pages for 2024 and 7075 since apparently the links are tied to cookies and 404 after a while.

[DELurker]

Originally Posted by artdutra04

I've been using 2024 and 7075 for shafts on FRC robots since I was in college in 2010, and have never had one fail. Plus, I absolutely love machining 7075 aluminum on mills and lathes. With good carbide tools, you can machine it like a hot knife through butter, as it does not have any of the galling issues that plague softer aluminum alloys. It's my second favorite material to machine after Delrin.

I will totally agree that aluminum, especially 7075, machines really well. However, we have a tough time getting anything but 6061 locally. One of our former mentors went out and got some cheap aluminum from a different alloy in the 6xxx series that machined like garbage. Regardless of speed, feed, or cutter age, it rolled instead of cutting. It reached a point where I would have preferred to be cutting gummy stainless (AL6XN).

However, I digress. We've bent a number of aluminum shafts and torn up the surface of others in bearings, so we're gunshy about going that route again. I'd be interested to know what installation and mounting techniques you use for the aluminum shafts.

Originally Posted by artdutra04

// Note: I didn't directly link to the Matweb pages for 2024 and 7075 since apparently the links are tied to cookies and 404 after a while.

Whoops. Didn't realize that. Sorry. The short form was that the materials listed matched the chart in the post for properties and that the galvanized was not significantly different from the 1018.

[magnets]

Originally Posted by DELurker

I disagree, although I will point out that stainless steel (302, 303, 304, 316) is a specific and (typically) less durable subset of steels (without getting into hardenable stainless such as 440C). The 3x price differential on stainless makes it less than desirable for teams to work with in the raw, aside from the additional tooling needed to properly work it. However, I will support my original contention with the following links and tables...

Reference #1
Reference #2

from reference #2...
........................................Aluminium. ................Steel
Density, ρ kg m−3...............2,700......................7 ,800
Young modulus, E N mm−2....70,000....................210,000
Shear modulus, G N mm−2.....27,000....................81,000
Poisson ratio, ν.....................0.33........................ 0.3

from matweb.com...
1018 steel versus 6061-T6 aluminum versus ASTM A526 galvanized steel

I could never bring myself to use aluminum for a shaft. It is too weak (comparably) to take that shock and torsion loads, as well as being more prone to unannounced catastrophic failure over steel (steel bends and then breaks ... aluminum has a much smaller deformation period before it snaps under the same loads). I'd rather use a steel shaft so that I could press-fit it into a bearing. The surface properties of aluminum just aren't good enough (at least for me) for rotating or direct-contact sliding.

You're not looking at the right alloys. AndyMark steel parts are 4140 stainless with a yield strength of 60200 psi, but 7075 aluminum (vex pro parts) have a yield strength of 78,000 psi. Also, I don't really follow the shaft argument. Any 3 CIM gearbox that I've ever seen uses an aluminum shaft. Same with our climber.