Go to Post In the end, you get out of FIRST what you put in. - BlueLipstick [more]
Home
Go Back   Chief Delphi > FIRST > General Forum
CD-Media   CD-Spy  
portal register members calendar search Today's Posts Mark Forums Read FAQ rules

 
Reply
Thread Tools Rate Thread Display Modes
  #46   Spotlight this post!  
Unread 14-12-2004, 07:49
Andy Baker's Avatar Woodie Flowers Award
Andy Baker Andy Baker is offline
President, AndyMark, Inc.
FRC #3940 (CyberTooth)
Team Role: Engineer
 
Join Date: May 2001
Rookie Year: 1998
Location: Kokomo, Indiana
Posts: 3,425
Andy Baker has a reputation beyond reputeAndy Baker has a reputation beyond reputeAndy Baker has a reputation beyond reputeAndy Baker has a reputation beyond reputeAndy Baker has a reputation beyond reputeAndy Baker has a reputation beyond reputeAndy Baker has a reputation beyond reputeAndy Baker has a reputation beyond reputeAndy Baker has a reputation beyond reputeAndy Baker has a reputation beyond reputeAndy Baker has a reputation beyond repute
Send a message via AIM to Andy Baker
Re: Has any team or anybody made a 80% - 100% lexan bot?

Quote:
Originally Posted by JVN
Well if I remember correctly--
The tkats used that mean arm as a "stiffarm" to prevent teams from getting onto the puck. Perhaps they wanted a "flexy" arm that would give a little bit when it was under "stress". This would protect their gearboxes from damage.

Best guess...

John
Correct. Not only did this design protect gearboxes from damage, but the arm could withstand a significant deflection and then spring back into its original straightness. I remember a few times when the arm's polycarbonate box structure would buckle and bend at a 90 degree angle, and then it would spring back after the load was removed. Sure, there was some damage to the arm, but it did not fracture and we could easily repair it.



Many plastics have great elasticity, while most metals do not. Shown above and linked here is a stress strain curve for a high grade polycarbonate. As stress begins (lower left corner of the graph), the material goes through its elastic stage. Until the stress hits the material's Yield Point, the material can bounce back to its original structure. As the strain increases, the yield point is passes and the material is in the "plastic region". This is the area where you see the material being bent, and then maintaining that bent shape (not springing back to original form). Then, at the end, to the right is the Ultimate Strength point, where the material fractures.

All materials have these stress/strain curves, just like all motors have torque/speed curves. The main difference between plastics and metals is the distance between the yield point and the ultimate breaking point. With metals, the distance is short. With plastics, it is much longer. Therefore, choosing plastic (Lexan, PVC, etc.) as a construction member for a FIRST robotics design is a good choice, since these members see much deflection at times.

Andy B.
Reply With Quote
  #47   Spotlight this post!  
Unread 14-12-2004, 09:59
Max Lobovsky's Avatar
Max Lobovsky Max Lobovsky is offline
Fold em oval!
FRC #1257 (Parallel Universe)
Team Role: College Student
 
Join Date: Jan 2004
Rookie Year: 2004
Location: Scotch Plains, NJ
Posts: 1,026
Max Lobovsky has a brilliant futureMax Lobovsky has a brilliant futureMax Lobovsky has a brilliant futureMax Lobovsky has a brilliant futureMax Lobovsky has a brilliant futureMax Lobovsky has a brilliant futureMax Lobovsky has a brilliant futureMax Lobovsky has a brilliant futureMax Lobovsky has a brilliant futureMax Lobovsky has a brilliant futureMax Lobovsky has a brilliant future
Send a message via AIM to Max Lobovsky
Re: Has any team or anybody made a 80% - 100% lexan bot?

I have great respect for all the people to commit their time to FIRST and I have respect for the work they accomplish. That is in no way mutually exclusive to these same people not doing perfect work and having "significant design errors" (would we even be talking about any other kind of design error?).

When I was talking about design errors, I was referring to different requirements then you were. Not the requirements that are set by teams for themselves (the best teams probably nail all of these requirements consistently), but the requirements of the job. The requirement is simple: make a robot that wins. If it was possible to make a more robust robot, or save enough weight to add an aditional useful mechanism, etc, then I don't think the design was "correct". (I'm not speaking about the more general design as it is very complex to analyze wether a ball herder is better than a ball carrier or wether a hanging mechanism is worth the weight, etc, etc.)

The only thing I have been saying is that a very common design mistake is a bad choice in materials. As Mr. Baker pointed out very nicely, and as I didn't and don't disagree with, certain materials do have unique properties that aren't encompassed by the single number I was using for analysis.

If I have offended anyone, I apologize, but nothing was meant to be offensive, and I hope I have explained my intentions to you. This thread has gone way off topic, so if you really wish to continue this discussion, perhaps do it in private.
__________________
Learn, edit, inspire: The FIRSTwiki.
Team 1257


2005 NYC Regional - 2nd seed, Xerox Creativity Award, Autodesk Visualization Award
2005 Chesapeake Regional - Engineering Inspiration Award
2004 Chesapeake Regional - Rookie Inspiration award
2004 NJ Regional - Team Spirit Award

Last edited by Max Lobovsky : 14-12-2004 at 10:11.
Reply With Quote
  #48   Spotlight this post!  
Unread 14-12-2004, 13:54
Adam Y.'s Avatar
Adam Y. Adam Y. is offline
Adam Y.
no team (?????)
 
Join Date: Mar 2002
Location: Long Island
Posts: 1,979
Adam Y. is a splendid one to beholdAdam Y. is a splendid one to beholdAdam Y. is a splendid one to beholdAdam Y. is a splendid one to beholdAdam Y. is a splendid one to beholdAdam Y. is a splendid one to beholdAdam Y. is a splendid one to behold
Send a message via AIM to Adam Y.
Re: Has any team or anybody made a 80% - 100% lexan bot?

I probably posted this before but Lexan/Polycarbonate has a pretty cool effect to spot wear and tear. Take two polarizing lens and rotate them so that you can't see any light coming out of them. Now look at a clear light source (glass, lexan, polycarbonate) and at points where there is stress you should be able to see rainbows/something.
__________________
If either a public officer or any one else saw a person attempting to cross a bridge which had been ascertained to be unsafe, and there were no time to warn him of his danger, they might seize him and turn him back without any real infringement of his liberty; for liberty consists in doing what one desires, and he does not desire to fall into the river. -Mill
Reply With Quote
  #49   Spotlight this post!  
Unread 14-12-2004, 14:24
Dave Campbell's Avatar
Dave Campbell Dave Campbell is offline
Miami Valley Planning Committee
FRC #0144 (ROCK Robotics)
Team Role: Teacher
 
Join Date: Dec 2003
Rookie Year: 1999
Location: Cincinnati
Posts: 187
Dave Campbell has a brilliant futureDave Campbell has a brilliant futureDave Campbell has a brilliant futureDave Campbell has a brilliant futureDave Campbell has a brilliant futureDave Campbell has a brilliant futureDave Campbell has a brilliant futureDave Campbell has a brilliant futureDave Campbell has a brilliant futureDave Campbell has a brilliant futureDave Campbell has a brilliant future
Re: Has any team or anybody made a 80% - 100% lexan bot?

Team 1038 did use the Lexan pictured in this thread. It wasn't acrylic. We only had minimal damage - a chunk taken out of a side plate during the 2003 IRI when a very strong and fast team tried to remove us from the top of the ramp. We also used 80% Lexan in 2002 when we were team #144. There may be some pics in the image galleries from that season. That machine was great. We like polycarb because it is workable for our students and it has many close characteristics to 6061 Aluminum. We also get it donated by a couple of our sponsors, therefore very affordable! One of our students (CD username crispyc) did a Strength of Materials project while studying at Miami University. I'll see if he has any of his research around. Good luck with your project Alex. I'll PM you if I find Crispyc, or his project.
__________________
"Great spirits have always encountered violent opposition from mediocre minds." - Albert Einstein

FRC Team #144 Advisor
Reply With Quote
  #50   Spotlight this post!  
Unread 15-12-2004, 12:18
Gary Dillard's Avatar Unsung FIRST Hero
Gary Dillard Gary Dillard is offline
Generator of Entropy
AKA: you know, the old bald guy
FRC #2973 (The Mad Rockers)
Team Role: Engineer
 
Join Date: Nov 2001
Rookie Year: 1998
Location: Huntsville, AL
Posts: 1,584
Gary Dillard has a reputation beyond reputeGary Dillard has a reputation beyond reputeGary Dillard has a reputation beyond reputeGary Dillard has a reputation beyond reputeGary Dillard has a reputation beyond reputeGary Dillard has a reputation beyond reputeGary Dillard has a reputation beyond reputeGary Dillard has a reputation beyond reputeGary Dillard has a reputation beyond reputeGary Dillard has a reputation beyond reputeGary Dillard has a reputation beyond repute
Send a message via AIM to Gary Dillard
Re: Has any team or anybody made a 80% - 100% lexan bot?

A few points about Lexan that we've learned over the years:

1) All polycarbonate is not created equal. We have tried cheaper forms of polycarbonate and found the the GE LEXAN brand name is definitely stronger / more rugged than others. There are different varieties of GE LEXAN as well, but in general they are all better than other manufacturers.

- Also, please don't confuse Lexan or polycarbonate with acrylic. You might as well make your robot out of glass if you go that route.

2) We started using Lexan when it was an unlimited allowable material because it made very nice assemblies. It is easy to machine and it makes nice square corner joint assemblies - we drill pilot holes through the edges using 3/8 inch thick Lexan and use self tapping drywall screws. You need to use Lithium grease or the screws will shear while torquing (use the low torque setting on your cordless drill when installing them), and don't use locktite because it heats up, expands and cracks/delaminates the lexan.

3) An important point to understand when talking material choice is that for impact loading (like running into other robots or them running into you or your components) the imparted load is NOT INDEPENDANT of the material or construction of your robot. The kinetic energy input (1/2mv^2) is absorbed by the structure based on its spring rate; a very stiff structure will take a very high effective g-loading, where a soft structure will see a much lower load. So you could say for instance that aluminum is twice as strong as Lexan, but it the aluminum structure will see 10 g's and the lexan structure will only see 2 g's when they run into something, the lexan may be the better choice if you can tolerate the deflection. This is frequently the case with some mechanisms that don't require precision, and why PVC is a very good choice for them in many cases.

One other point about aluminum if you're making the comparison for a structure: 6061-T6 loses 2/3 of it's strength when welded. The Aluminum Association Handbook takes it from 35 ksi yield to 12 ksi yield in the heat affected zone (within 1 inch of the weld) so be careful.
__________________
Close enough to taste it, too far to reach it
Reply With Quote
  #51   Spotlight this post!  
Unread 15-12-2004, 15:43
Alaina's Avatar
Alaina Alaina is offline
J = Pf - Po
FRC #0814 (Megaforce Squadron)
Team Role: College Student
 
Join Date: Mar 2003
Rookie Year: 2003
Location: Napa, CA
Posts: 715
Alaina is a splendid one to beholdAlaina is a splendid one to beholdAlaina is a splendid one to beholdAlaina is a splendid one to beholdAlaina is a splendid one to beholdAlaina is a splendid one to beholdAlaina is a splendid one to behold
Send a message via AIM to Alaina
Re: Has any team or anybody made a 80% - 100% lexan bot?

Funny, there haven't really been any references to plywood in this thread.
For my team's first 2 years we used 3/8" plywood. Though in 2002 we had this large steel "knee breaker" apparatus that was used to grab ahold of goals. In 2003, however, the only metal you could see on the robot was the angled alluminum, 1/4-20 bolts and lock nuts used to hold the chassis together. It was sweet.
__________________
"It's cool to be a geek!" -- Dave Lavery
"It's all about the math." -- Mark Leon
Reply With Quote
  #52   Spotlight this post!  
Unread 15-12-2004, 16:55
ahecht's Avatar
ahecht ahecht is offline
'Luzer'
AKA: Zan
no team
Team Role: Alumni
 
Join Date: Dec 2001
Rookie Year: 2002
Location: Billerica, MA
Posts: 978
ahecht has a reputation beyond reputeahecht has a reputation beyond reputeahecht has a reputation beyond reputeahecht has a reputation beyond reputeahecht has a reputation beyond reputeahecht has a reputation beyond reputeahecht has a reputation beyond reputeahecht has a reputation beyond reputeahecht has a reputation beyond reputeahecht has a reputation beyond reputeahecht has a reputation beyond repute
Send a message via ICQ to ahecht Send a message via AIM to ahecht Send a message via Yahoo to ahecht
Re: Has any team or anybody made a 80% - 100% lexan bot?

Quote:
Originally Posted by Gary Dillard
2) We started using Lexan when it was an unlimited allowable material because it made very nice assemblies. It is easy to machine and it makes nice square corner joint assemblies - we drill pilot holes through the edges using 3/8 inch thick Lexan and use self tapping drywall screws. You need to use Lithium grease or the screws will shear while torquing (use the low torque setting on your cordless drill when installing them), and don't use locktite because it heats up, expands and cracks/delaminates the lexan.
While that would probably work quite well, I would highly recommend that anyone using lexan first glue the pieces together using an unthickened methylene chloride solvent glue. You can get this from any plastics supplier. It is applied using an applicator bottle that has a needle tip and uses capillary action to flow between the pieces of lexan. The solvent breaks down the plastic's surface, allowing the pieces to chemically weld themselves together. Edge preparation prior to glue application is of utmost importance. Your final seam will never be better than your beginning edge, so you may need to use a mill to make a flat edge if you are gluing a cut side.

Once the edges are glued, you can drill and tap them (or use self tapping screws if you are feeling lucky), and they will never come apart.
__________________
Zan Hecht

Scorekeeper: '05 Championship DaVinci Field/'10 WPI Regional
Co-Founder: WPI-EBOT Educational Robotics Program
Alumnus: WPI/Mass Academy Team #190
Alumnus (and founder): Oakwood Robotics Team #992


"Life is an odd numbered problem the answer isn't in the back of the book." — Anonymous WPI Student
Reply With Quote
Reply


Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
The Death of FIRST Anton Abaya General Forum 23 03-05-2006 17:18
How do you organize your team? NoodleKnight Team Organization 18 03-11-2005 22:57
Thank a Team with an i-Pod !!! OZ_341 General Forum 8 20-04-2004 23:57
Team 524's bot Amadkow Robot Showcase 2 27-02-2002 20:36
Novi Kickoff Seminar List (Tentative) Joe Johnson General Forum 1 05-12-2001 13:47


All times are GMT -5. The time now is 14:07.

The Chief Delphi Forums are sponsored by Innovation First International, Inc.


Powered by vBulletin® Version 3.6.4
Copyright ©2000 - 2017, Jelsoft Enterprises Ltd.
Copyright © Chief Delphi