Save time? Use aluminum extrusion kits!

[Tim Skloss]

I totally agree, Ken! Good point.

What you don't see is that we do put on panels around the sides (polycarbonate) and on on the bottom for a skid-plate. The drivetrain is mounted on a thick 1/4" aluminum plate that easily screws to the extrusion using the T nuts. This adds the stiffness you mentioned, although the ITEM extrusion joints don't flex much at all--it's good quality stuff.

All you need to make the plates is a saw to cut the aluminum plate and a drill to make the mounting holes for the motors, gearboxes and speed controllers. Drill more "speed holes" to reduce the weight.

The controls are mounted on a similar panel that goes in and out with four screws. It has been a very easy design to build, maintain and work around in.

Tim

[KenWittlief]

just wondering - which kit do you get? the pro (biggests one)?

and how do you account for the cost - include the whole kit price, or is there an easy way to pro-rate only the parts you use?

[Tim Skloss]

We got the Pro kit... Since most of the cost (90%+) is in the extrusion, you could divide the cost by the total number of feet in the kit. The pro kit comes with 60 METERS of extrusion, that comes to about $10 per meter... pretty cheap in the long run!

I'll try to get a sheet from ITEM that specifies the cost per part and post it here.

Tim

Originally Posted by KenWittlief

just wondering - which kit do you get? the pro (biggests one)?

and how do you account for the cost - include the whole kit price, or is there an easy way to pro-rate only the parts you use?

[Tim Skloss]

Whoops, my mistake:

not 60 METERS, 16x3= 48 METERS... Still works out to under $15 per meter.

Tim

Originally Posted by Tim Skloss

We got the Pro kit... Since most of the cost (90%+) is in the extrusion, you could divide the cost by the total number of feet in the kit. The pro kit comes with 60 METERS of extrusion, that comes to about $10 per meter... pretty cheap in the long run!

I'll try to get a sheet from ITEM that specifies the cost per part and post it here.

Tim

[kevin.li.rit]

We've been using the same stuff since we started. It durable and easy to build with. But stay away from those Zinc T-nuts They crack, stick the the brass. We had about a dozen cracked t-nuts by the end of Annapolis.

On another note we built our cart out of aluminium profile as well. And it rocks.

[Tim Skloss]

Here is how the pricing breaks down. I just called the number on the flyer and asked!

The 20x20 is $8.50 per meter
the 40x20 is $14.00 per meter
The fasteners are cents per piece, but I didn't get that info.

If you need the FIRST price for each part, just call Kristen at 888-729-4500 extension 234. She is in charge of the special FIRST pricing.

Also, look throught the ITEM catalog and they can get anything to your team in a day or two at reduced prices.

The catalog has lots of other stuff like fasteners that slide, pivot, etc.

[Stephen Kowski]

I'm wondering what all comes in the kit....well I can read the little advertisement, but all the teams I've been on have never use extruded aluminum....does anyone have a picture of a kit, or of some of the elements within the kit?

[Gui Cavalcanti]

So here's my question; what's the difference between this and 80/20? It looks like the same extrusion profile from these pictures, except 80/20 has a lighter version with four holes running down the length of the extrusion as well as a solid version.

[KenWittlief]

these have grooves that allow you to put fastners anywhere along the length - you hook them in and they will slide anywhere - also the end pieces are easy to use

its like a hi-tech erector set - except you are not limited by holes in the metal for placing attachements.

[Loren 930]

Hello Gui -

Hopefully I can answer your question regarding the differences in extrusion systems. My engineering company used many other brands in the past, but because we require performance and value, we settled on the "item" brand (not to be confused with the IPS brand - which is not and has never been the same as "item") extrusions in our shop. As Tim mentioned, there are wide differences in dimensional tolerances and alloy which in turn reflect on the strength of the extrusion. Equally important is the method of attachment - other systems have copied the item fasteners but they do not work unless you have a high strength extrusion to use them in. item has been in business for over 26 years and is the world sales leader in structural aluminum systems. So why is this important to you? It means you get a highly engineered and respected building system with very high level of engineering support. item is committed to staying at the top which is why they have decided to support FIRST teams with an outstanding value in the form of "kits" that contain the neccessary extrusions and fasteners to put together a modular and strong robot. Team 930 has used these kits exclusively for the past two years - and as anyone that has seen our robots will attest too - they are strong, rigid, and light and we have never had a fastener break or come loose. If you or anyone else reading this post has any questions about applying the system to your project, please contact me at loren@aes-wi.com.

Good luck this year - it is a great game!

Loren
engineering lead
Team 930
www.team930.com

[Loren 930]

Hello,

As a side note, I would like to clarify what size extrusion is in the kits. The extrusion size(s) - depending on which kit you use - has 20x20 and 40x20 size extrusions. This is the outside dimension size in millimeters. The weight (size 20x20) is 0.48kg/m (approx. .33lb / ft) and the 4th moment of inertia is 0.72cm4 - for those that want to do the calc's - which I highly recommend you do when comparing systems. The pullout strength for a fastener connected in the t-slots is 500N (112lbs.). More engineering information can be obtained at www.item-international.com or contact me at loren@aes-wi.com.

Hope this helps.

Loren
engineering lead
Team 930
www.team930.com

[Matt Adams]

If you're really looking for quality, then you want to go with Bosch 20 mm.

Bosch Rexroth:
3m of 20x20 for $20.00 (no discount)
I = .67 cm^4, (.016 in^4)
W = .4 kg / m, (.269 lbs / ft)
Pullout = 1700 N, (382 lbs.)
Tensile Stress = 250 N / mm^2

Item:
3m of 20x20mm for $25.50 (discount)
I = 0.72 cm^4
Weight = 0.48kg/m (.33lb / ft)
Pullout = 500N (112lbs.)
Tensile Stress of Aluminum = 245 N / mm^2

I'll leave the choice up to you.

It's considerably lighter, just slightly less rigid, about 20% cheaper, and has better fasteners.

Hope this helps. Good luck boys and girls!

Matt

[Aignam]

I believe Raider Robotix's 2002 robot, Silver Scorpion, used extruded aluminum. That robot was the one that required the least maintenence of all. It practically never broke, or required much more maintenence than a change of battery in between matches.

[Tim Skloss]

The Bosch extrusion is NOT a superior product. A local business that builds industrial robots out of extrusion has completely quit using the Bosch because it is notorious for vibrating loose. And on a FIRST robot that is too risky.

Furthermore, some of the numbers you quoted are funny. The pullout strenght of 300+ lbs for the 20x20 is simply impossible according to the tensile strengh claimed. The material will fail and release the fastener before you get to 300 lbs.

The specifications claimed for the item extrusion are from actual test data posted by the manufacter. You will NOT see actual test data for the Bosch. Since in europe--where item originates--manufacturers claims must be absoluetly guaranteed, you will see very conservative estimates of structural strength. In our experience the material can be up to 3 times stronger than claimed in the catalog.

Which is better for a FIRST robot? You have to weigh strength and reliability against your pocketbook... But remember the free market ensures that "you get what you paid for".

[Matt Adams]

Originally Posted by Tim Skloss

The Bosch extrusion is NOT a superior product. A local business that builds industrial robots out of extrusion has completely quit using the Bosch because it is notorious for vibrating loose. And on a FIRST robot that is too risky.

I guess that I have a different opinion on something like this. Assuming that the robot needs to compete in 30 matches over it's life time, that's 1 hour of run time. I'd believe that the need of FIRST robots are significantly lower than industrial equipment, which is running for 8, 12 or even 24 hours spans. If this aluminum extrusion was so bad that a robot chasis could vibrate loose in a 2 minute round, I don't think that Bosch would be in business!

Originally Posted by Tim Skloss

Furthermore, some of the numbers you quoted are funny. The pullout strenght of 300+ lbs for the 20x20 is simply impossible according to the tensile strengh claimed. The material will fail and release the fastener before you get to 300 lbs.

Tim, since you're a practicing engineer, I assume you made a quick napkin calculation about the pullout strength before you claimed that my numbers (from the manufacturer) were 'funny'. I'd like to compare some numbers with you.

Let's go ahead and take the above 20mm extrusion piece, and pretend to load in a 15 mm long fastener, and assume that the bottom lip of the extrusion that holds the T nut in is 1.5 mm tall. That looks pretty close to scale. Then we'll assume a 1700 N load on the bottom pulling it out. We'll assume the fastener (a T-nut) does NOT span to the edge of the V cutout, but rather is a 1 mm short on both ends and there's some bending going on.

Assuming the T-nut is 15 mm long, and for the sake of arguement we'll even assume the piece of extrusion is only 15 mm long too, so you just need to just bend the flaps down to pull it out, and not shear the edges like you actually do, and which would multiple the strength SIGNIFICANTLY. We'll treat the bottom flaps as two cantilevered beams, just to be safe

The transverse shear stress is 3*V/2*A
V is the shear force.
A is the area.
V = (3*1700N)
A = (15mm * 1.5mm * 2 sides)
Transverse Shear Stress = 31.875 N/mm^2

Bending stress = M*y/I
M = (1.5mm*(1700N/2 sides))
y= (.75mm)
I = (15mm * 1.5mm^3/12)
That gives me a stress of 226.66 N/mm^2

Since there's transverse shear AND bending stress, Mohr's circle comes into play here, so we'll find that the max stress from Mohr's favorite shape is:
226.66/2 + sqrt( (226.66/2)^2 + (31.875)^2 ) = 231.05 N / mm^2

With the tensile strength of the aluminum used in this extrusion being equal to 250 N / mm^2... it appears there's a factor of saftey of about 1.1. We won't go into some sort energy-distorsion or modified mohr for failure, I think my point is adequate as-is.

Since there were so many conservative factors in this entire calculation, I think that the pull out strength listed by the manufacturer is very apropriate.

I would enjoy comparing calculations that you did that say these numbers are "funny." However, I'd appreciate if you'd include the shear stress needed to rip it out of the aluminum, since that would really be required to make the case.

Originally Posted by Tim Skloss

Since in europe--where item originates--manufacturers claims must be absoluetly guaranteed, you will see very conservative estimates of structural strength. In our experience the material can be up to 3 times stronger than claimed in the catalog.

Bosch is a Global company that originated in Germany... not to be too nit-picky.

Originally Posted by Tim Skloss

Which is better for a FIRST robot? You have to weigh strength and reliability against your pocketbook... But remember the free market ensures that "you get what you paid for".

I agree. The numbers make this easy.

Good luck everyone!!

Matt