1/8th thick aluminum

[NyCityKId]

Hello all,

Team 1601 is having a small dilemma. We're trying to figure out whether we should switch our arm material to .032 thick 2024-T3 aluminum from .125 thick 6061-T6 aluminum. We are using two window motors on the same shaft geared 1:2 and with the 6061 aluminum our arm is going to weigh rougly 10 lbs. Would the change to the thinner aluminum be worth the loss in strength?

[steelerborn]

I would say it really depends on how heavy your mechanism is on the end of the arm. Having a lot of weight dangling on the end, really increases the torque on the arm.

For instance we were using 1/8th inch aluminum sheet metal for our arm, after it was manufactured we noticed it was overkill. So we redesigned it to be 1/16th inch thickness. It decreased our arm weight by half, around 4-5 pounds total weight now. And by adding smart bends, and clever internal supports, our arm is just as sturdy.

Just test for your weak points and make a good solid design, if your using sheet metal remember that it gains strength with bends.

Hope this helps.

[AustinSchuh]

What size and shape tube are you using/proposing to use? A 1x1 will be a lot weaker than a 2" round tube.

[EricH]

What is your current projected robot weight? If you're looking at being under, you don't need to shed arm weight. If you're looking at being over, do the analysis. If you don't know how to do the analysis, say so--there are enough engineers on here that you'll get how to do the analysis and the analysis at the same time.

[Tristan Lall]

Remember that comparing theoretical static stresses against material strengths won't be sufficient to guard against complicated load cases—like the ones you're likely to see in an arm that's in motion (i.e. has momentum) and may well be pushing or pulling on things (or being pushed or pulled itself).

The thicker material will be more resistant to deformation, because of the increased stiffness (the modulus of elasticity is roughly constant for all aluminum alloys), so you'll be less likely to suffer a dent that causes the member to buckle.

Also, depending on the details of the planned design, you may find it more convenient to lighten the arm by cutting strategically-placed holes, rather than making it from lighter stock.

We'll need lots more information to give you concrete suggestions about your particular design.

[steelerborn]

Eric is right no point in making life more difficult, if you don't need to. Remember what is more beneficial redesigning an arm, or getting to practice for 3 more days?

[WizenedEE]

Originally Posted by steelerborn

Eric is right no point in making life more difficult, if you don't need to. Remember what is more beneficial redesigning an arm, or getting to practice for 3 more days?

Of course, you could change the question around to be, "What is more beneficial? Being lazy and not cutting down the weight on the arm and having it cut off at inspection or 3 days of drive practice?"

I would definitely run those calculations and cut it down if you have to.

[Mike Betts]

Originally Posted by NyCityKId

Hello all,

Team 1601 is having a small dilemma. We're trying to figure out whether we should switch our arm material to .032 thick 2024-T3 aluminum from .125 thick 6061-T6 aluminum. We are using two window motors on the same shaft geared 1:2 and with the 6061 aluminum our arm is going to weigh rougly 10 lbs. Would the change to the thinner aluminum be worth the loss in strength?

I'm not sure of the particulars of what or how your arm works but a ratio of 2:1 is pretty fast. Perhaps it is too fast to control? Changing gear ratios may be easier than building a new arm making it slower and having more torque all at the same time...

[Hawiian Cadder]

we are using 1/8 aluminum for everything, some members have nearly 75% of the materiel cut out with our CNC in a triangular pocket pattern to keep it light. we probably wont do this with the end manipulator however because we are 6 LBS less than the CAD says it should be. the reason if that we dont want weakness on something that will likely be beat on other things a lot.

[joek]

unless you have a very 80/20- and piston-heavy robot, it shouldn't be much of an issue with 1/8" aluminum. you can always swiss-cheese it later if need be

[ChrisH]

According to Machinery's handbook the yield stress for 6061-t6 is about 40ksi, ands 2024 is 57ksi, a 40% improvement. So it could be that you wind up with a stronger arm that is still lighter by changing alloys.

But to verify that I need the cross sectional dimensions of both the new material and the old.

I will say that .032 is pretty thin and easily dented. Those small dents can compromise the strength of the section. SO that is something to be considered as well.

The numbers on the trade off are pretty easy to run , but I need a few more of them

[Chris is me]

I would go with 1/16" wall AL before you cut holes in 1/8" or go all the way down to 1/32".

This is assuming you have a relatively simple arm...

[NyCityKId]

Originally Posted by AustinSchuh

What size and shape tube are you using/proposing to use? A 1x1 will be a lot weaker than a 2" round tube.

We aren't using tubing for the arm.

Originally Posted by EricH

What is your current projected robot weight? If you're looking at being under, you don't need to shed arm weight. If you're looking at being over, do the analysis. If you don't know how to do the analysis, say so--there are enough engineers on here that you'll get how to do the analysis and the analysis at the same time.

It's not neccessarily an analysis that we need as far as weight goes, it's more a time analysis. We are trying to decide if changing the material and redesigning the arm will cost us more time that actually machining each piece by hand (we don't have a CNC machine, or any other fancyequipment). we wanted to know if any other team had a similar issue in the past and could possibly provide a little help. As of right now there isn't a weight problem.

Originally Posted by Tristan Lall

Remember that comparing theoretical static stresses against material strengths won't be sufficient to guard against complicated load cases—like the ones you're likely to see in an arm that's in motion (i.e. has momentum) and may well be pushing or pulling on things (or being pushed or pulled itself).

The thicker material will be more resistant to deformation, because of the increased stiffness (the modulus of elasticity is roughly constant for all aluminum alloys), so you'll be less likely to suffer a dent that causes the member to buckle.

Also, depending on the details of the planned design, you may find it more convenient to lighten the arm by cutting strategically-placed holes, rather than making it from lighter stock.

We'll need lots more information to give you concrete suggestions about your particular design.

From the very begining we designed the part to have triangular lightening holes to form a built-in truss like structure. The problem is 6061-T6 at 1/8th thick is not easy to machine by hand.

Originally Posted by Mike Betts

I'm not sure of the particulars of what or how your arm works but a ratio of 2:1 is pretty fast. Perhaps it is too fast to control? Changing gear ratios may be easier than building a new arm making it slower and having more torque all at the same time...

This is a point i brought up in our design sessions. I wanted to make it a 4:1 ratio for torque purposes and pointed out that ~40 rpm might be too fast for an arm that will be traveling ~180 degrees either way.



I'm sorry if the original post was not clear but we mainly want to know whether its worth it to go back and redesign at this point in time based on other teams past experience. Also consider that we don't have a CNC machine or a machine shop sponsor. I apologize for the unclear post and thank you all for your time.

[DonRotolo]

Originally Posted by Chris is me

I would go with 1/16" wall AL before you cut holes in 1/8" or go all the way down to 1/32".

Re-quoted for truth. Going from .125 all the way down to .032 is a big change. You may be satisfied with .062 (1/16"), as this cuts the weight in half. If you do change alloys, you gain 40% in yield strength, so it's a net loss of 30% of the original strength for 50% of the weight. Might be worth it.

[NyCityKId]

Originally Posted by joek

unless you have a very 80/20- and piston-heavy robot, it shouldn't be much of an issue with 1/8" aluminum. you can always swiss-cheese it later if need be

In the begining we all made a promise: that we would be proud of our robot and that there would be no wood. Swiss cheesing would definitely dent our pride